File: gawk.info, Node: Top, Next: Foreword3, Up: (dir)
General Introduction
********************
This file documents 'awk', a program that you can use to select
particular records in a file and perform operations upon them.
Copyright (C) 1989, 1991, 1992, 1993, 1996-2005, 2007, 2009-2018
Free Software Foundation, Inc.
This is Edition 4.2 of 'GAWK: Effective AWK Programming: A User's
Guide for GNU Awk', for the 4.2.1 (or later) version of the GNU
implementation of AWK.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "GNU General Public License", with the
Front-Cover Texts being "A GNU Manual", and with the Back-Cover Texts as
in (a) below. A copy of the license is included in the section entitled
"GNU Free Documentation License".
a. The FSF's Back-Cover Text is: "You have the freedom to copy and
modify this GNU manual."
* Menu:
* Foreword3:: Some nice words about this
Info file.
* Foreword4:: More nice words.
* Preface:: What this Info file is about; brief
history and acknowledgments.
* Getting Started:: A basic introduction to using
'awk'. How to run an 'awk'
program. Command-line syntax.
* Invoking Gawk:: How to run 'gawk'.
* Regexp:: All about matching things using regular
expressions.
* Reading Files:: How to read files and manipulate fields.
* Printing:: How to print using 'awk'. Describes
the 'print' and 'printf'
statements. Also describes redirection of
output.
* Expressions:: Expressions are the basic building blocks
of statements.
* Patterns and Actions:: Overviews of patterns and actions.
* Arrays:: The description and use of arrays. Also
includes array-oriented control statements.
* Functions:: Built-in and user-defined functions.
* Library Functions:: A Library of 'awk' Functions.
* Sample Programs:: Many 'awk' programs with complete
explanations.
* Advanced Features:: Stuff for advanced users, specific to
'gawk'.
* Internationalization:: Getting 'gawk' to speak your
language.
* Debugger:: The 'gawk' debugger.
* Arbitrary Precision Arithmetic:: Arbitrary precision arithmetic with
'gawk'.
* Dynamic Extensions:: Adding new built-in functions to
'gawk'.
* Language History:: The evolution of the 'awk'
language.
* Installation:: Installing 'gawk' under various
operating systems.
* Notes:: Notes about adding things to 'gawk'
and possible future work.
* Basic Concepts:: A very quick introduction to programming
concepts.
* Glossary:: An explanation of some unfamiliar terms.
* Copying:: Your right to copy and distribute
'gawk'.
* GNU Free Documentation License:: The license for this Info file.
* Index:: Concept and Variable Index.
* History:: The history of 'gawk' and
'awk'.
* Names:: What name to use to find
'awk'.
* This Manual:: Using this Info file. Includes
sample input files that you can use.
* Conventions:: Typographical Conventions.
* Manual History:: Brief history of the GNU project and
this Info file.
* How To Contribute:: Helping to save the world.
* Acknowledgments:: Acknowledgments.
* Running gawk:: How to run 'gawk' programs;
includes command-line syntax.
* One-shot:: Running a short throwaway
'awk' program.
* Read Terminal:: Using no input files (input from the
keyboard instead).
* Long:: Putting permanent 'awk'
programs in files.
* Executable Scripts:: Making self-contained 'awk'
programs.
* Comments:: Adding documentation to 'gawk'
programs.
* Quoting:: More discussion of shell quoting
issues.
* DOS Quoting:: Quoting in Windows Batch Files.
* Sample Data Files:: Sample data files for use in the
'awk' programs illustrated in
this Info file.
* Very Simple:: A very simple example.
* Two Rules:: A less simple one-line example using
two rules.
* More Complex:: A more complex example.
* Statements/Lines:: Subdividing or combining statements
into lines.
* Other Features:: Other Features of 'awk'.
* When:: When to use 'gawk' and when to
use other things.
* Intro Summary:: Summary of the introduction.
* Command Line:: How to run 'awk'.
* Options:: Command-line options and their
meanings.
* Other Arguments:: Input file names and variable
assignments.
* Naming Standard Input:: How to specify standard input with
other files.
* Environment Variables:: The environment variables
'gawk' uses.
* AWKPATH Variable:: Searching directories for
'awk' programs.
* AWKLIBPATH Variable:: Searching directories for
'awk' shared libraries.
* Other Environment Variables:: The environment variables.
* Exit Status:: 'gawk''s exit status.
* Include Files:: Including other files into your
program.
* Loading Shared Libraries:: Loading shared libraries into your
program.
* Obsolete:: Obsolete Options and/or features.
* Undocumented:: Undocumented Options and Features.
* Invoking Summary:: Invocation summary.
* Regexp Usage:: How to Use Regular Expressions.
* Escape Sequences:: How to write nonprinting characters.
* Regexp Operators:: Regular Expression Operators.
* Bracket Expressions:: What can go between '[...]'.
* Leftmost Longest:: How much text matches.
* Computed Regexps:: Using Dynamic Regexps.
* GNU Regexp Operators:: Operators specific to GNU software.
* Case-sensitivity:: How to do case-insensitive matching.
* Regexp Summary:: Regular expressions summary.
* Records:: Controlling how data is split into
records.
* awk split records:: How standard 'awk' splits
records.
* gawk split records:: How 'gawk' splits records.
* Fields:: An introduction to fields.
* Nonconstant Fields:: Nonconstant Field Numbers.
* Changing Fields:: Changing the Contents of a Field.
* Field Separators:: The field separator and how to change
it.
* Default Field Splitting:: How fields are normally separated.
* Regexp Field Splitting:: Using regexps as the field separator.
* Single Character Fields:: Making each character a separate
field.
* Command Line Field Separator:: Setting 'FS' from the command
line.
* Full Line Fields:: Making the full line be a single
field.
* Field Splitting Summary:: Some final points and a summary table.
* Constant Size:: Reading constant width data.
* Fixed width data:: Processing fixed-width data.
* Skipping intervening:: Skipping intervening fields.
* Allowing trailing data:: Capturing optional trailing data.
* Fields with fixed data:: Field values with fixed-width data.
* Splitting By Content:: Defining Fields By Content
* Testing field creation:: Checking how 'gawk' is
splitting records.
* Multiple Line:: Reading multiline records.
* Getline:: Reading files under explicit program
control using the 'getline'
function.
* Plain Getline:: Using 'getline' with no
arguments.
* Getline/Variable:: Using 'getline' into a variable.
* Getline/File:: Using 'getline' from a file.
* Getline/Variable/File:: Using 'getline' into a variable
from a file.
* Getline/Pipe:: Using 'getline' from a pipe.
* Getline/Variable/Pipe:: Using 'getline' into a variable
from a pipe.
* Getline/Coprocess:: Using 'getline' from a coprocess.
* Getline/Variable/Coprocess:: Using 'getline' into a variable
from a coprocess.
* Getline Notes:: Important things to know about
'getline'.
* Getline Summary:: Summary of 'getline' Variants.
* Read Timeout:: Reading input with a timeout.
* Retrying Input:: Retrying input after certain errors.
* Command-line directories:: What happens if you put a directory on
the command line.
* Input Summary:: Input summary.
* Input Exercises:: Exercises.
* Print:: The 'print' statement.
* Print Examples:: Simple examples of 'print'
statements.
* Output Separators:: The output separators and how to
change them.
* OFMT:: Controlling Numeric Output With
'print'.
* Printf:: The 'printf' statement.
* Basic Printf:: Syntax of the 'printf' statement.
* Control Letters:: Format-control letters.
* Format Modifiers:: Format-specification modifiers.
* Printf Examples:: Several examples.
* Redirection:: How to redirect output to multiple
files and pipes.
* Special FD:: Special files for I/O.
* Special Files:: File name interpretation in
'gawk'. 'gawk' allows
access to inherited file descriptors.
* Other Inherited Files:: Accessing other open files with
'gawk'.
* Special Network:: Special files for network
communications.
* Special Caveats:: Things to watch out for.
* Close Files And Pipes:: Closing Input and Output Files and
Pipes.
* Nonfatal:: Enabling Nonfatal Output.
* Output Summary:: Output summary.
* Output Exercises:: Exercises.
* Values:: Constants, Variables, and Regular
Expressions.
* Constants:: String, numeric and regexp constants.
* Scalar Constants:: Numeric and string constants.
* Nondecimal-numbers:: What are octal and hex numbers.
* Regexp Constants:: Regular Expression constants.
* Using Constant Regexps:: When and how to use a regexp constant.
* Standard Regexp Constants:: Regexp constants in standard
'awk'.
* Strong Regexp Constants:: Strongly typed regexp constants.
* Variables:: Variables give names to values for
later use.
* Using Variables:: Using variables in your programs.
* Assignment Options:: Setting variables on the command line
and a summary of command-line syntax.
This is an advanced method of input.
* Conversion:: The conversion of strings to numbers
and vice versa.
* Strings And Numbers:: How 'awk' Converts Between
Strings And Numbers.
* Locale influences conversions:: How the locale may affect conversions.
* All Operators:: 'gawk''s operators.
* Arithmetic Ops:: Arithmetic operations ('+',
'-', etc.)
* Concatenation:: Concatenating strings.
* Assignment Ops:: Changing the value of a variable or a
field.
* Increment Ops:: Incrementing the numeric value of a
variable.
* Truth Values and Conditions:: Testing for true and false.
* Truth Values:: What is "true" and what is
"false".
* Typing and Comparison:: How variables acquire types and how
this affects comparison of numbers and
strings with '<', etc.
* Variable Typing:: String type versus numeric type.
* Comparison Operators:: The comparison operators.
* POSIX String Comparison:: String comparison with POSIX rules.
* Boolean Ops:: Combining comparison expressions using
boolean operators '||' ("or"),
'&&' ("and") and '!'
("not").
* Conditional Exp:: Conditional expressions select between
two subexpressions under control of a
third subexpression.
* Function Calls:: A function call is an expression.
* Precedence:: How various operators nest.
* Locales:: How the locale affects things.
* Expressions Summary:: Expressions summary.
* Pattern Overview:: What goes into a pattern.
* Regexp Patterns:: Using regexps as patterns.
* Expression Patterns:: Any expression can be used as a
pattern.
* Ranges:: Pairs of patterns specify record
ranges.
* BEGIN/END:: Specifying initialization and cleanup
rules.
* Using BEGIN/END:: How and why to use BEGIN/END rules.
* I/O And BEGIN/END:: I/O issues in BEGIN/END rules.
* BEGINFILE/ENDFILE:: Two special patterns for advanced
control.
* Empty:: The empty pattern, which matches every
record.
* Using Shell Variables:: How to use shell variables with
'awk'.
* Action Overview:: What goes into an action.
* Statements:: Describes the various control
statements in detail.
* If Statement:: Conditionally execute some
'awk' statements.
* While Statement:: Loop until some condition is
satisfied.
* Do Statement:: Do specified action while looping
until some condition is satisfied.
* For Statement:: Another looping statement, that
provides initialization and increment
clauses.
* Switch Statement:: Switch/case evaluation for conditional
execution of statements based on a
value.
* Break Statement:: Immediately exit the innermost
enclosing loop.
* Continue Statement:: Skip to the end of the innermost
enclosing loop.
* Next Statement:: Stop processing the current input
record.
* Nextfile Statement:: Stop processing the current file.
* Exit Statement:: Stop execution of 'awk'.
* Built-in Variables:: Summarizes the predefined variables.
* User-modified:: Built-in variables that you change to
control 'awk'.
* Auto-set:: Built-in variables where 'awk'
gives you information.
* ARGC and ARGV:: Ways to use 'ARGC' and
'ARGV'.
* Pattern Action Summary:: Patterns and Actions summary.
* Array Basics:: The basics of arrays.
* Array Intro:: Introduction to Arrays
* Reference to Elements:: How to examine one element of an
array.
* Assigning Elements:: How to change an element of an array.
* Array Example:: Basic Example of an Array
* Scanning an Array:: A variation of the 'for'
statement. It loops through the
indices of an array's existing
elements.
* Controlling Scanning:: Controlling the order in which arrays
are scanned.
* Numeric Array Subscripts:: How to use numbers as subscripts in
'awk'.
* Uninitialized Subscripts:: Using Uninitialized variables as
subscripts.
* Delete:: The 'delete' statement removes an
element from an array.
* Multidimensional:: Emulating multidimensional arrays in
'awk'.
* Multiscanning:: Scanning multidimensional arrays.
* Arrays of Arrays:: True multidimensional arrays.
* Arrays Summary:: Summary of arrays.
* Built-in:: Summarizes the built-in functions.
* Calling Built-in:: How to call built-in functions.
* Numeric Functions:: Functions that work with numbers,
including 'int()', 'sin()'
and 'rand()'.
* String Functions:: Functions for string manipulation,
such as 'split()', 'match()'
and 'sprintf()'.
* Gory Details:: More than you want to know about
'\' and '&' with
'sub()', 'gsub()', and
'gensub()'.
* I/O Functions:: Functions for files and shell
commands.
* Time Functions:: Functions for dealing with timestamps.
* Bitwise Functions:: Functions for bitwise operations.
* Type Functions:: Functions for type information.
* I18N Functions:: Functions for string translation.
* User-defined:: Describes User-defined functions in
detail.
* Definition Syntax:: How to write definitions and what they
mean.
* Function Example:: An example function definition and
what it does.
* Function Caveats:: Things to watch out for.
* Calling A Function:: Don't use spaces.
* Variable Scope:: Controlling variable scope.
* Pass By Value/Reference:: Passing parameters.
* Return Statement:: Specifying the value a function
returns.
* Dynamic Typing:: How variable types can change at
runtime.
* Indirect Calls:: Choosing the function to call at
runtime.
* Functions Summary:: Summary of functions.
* Library Names:: How to best name private global
variables in library functions.
* General Functions:: Functions that are of general use.
* Strtonum Function:: A replacement for the built-in
'strtonum()' function.
* Assert Function:: A function for assertions in
'awk' programs.
* Round Function:: A function for rounding if
'sprintf()' does not do it
correctly.
* Cliff Random Function:: The Cliff Random Number Generator.
* Ordinal Functions:: Functions for using characters as
numbers and vice versa.
* Join Function:: A function to join an array into a
string.
* Getlocaltime Function:: A function to get formatted times.
* Readfile Function:: A function to read an entire file at
once.
* Shell Quoting:: A function to quote strings for the
shell.
* Data File Management:: Functions for managing command-line
data files.
* Filetrans Function:: A function for handling data file
transitions.
* Rewind Function:: A function for rereading the current
file.
* File Checking:: Checking that data files are readable.
* Empty Files:: Checking for zero-length files.
* Ignoring Assigns:: Treating assignments as file names.
* Getopt Function:: A function for processing command-line
arguments.
* Passwd Functions:: Functions for getting user
information.
* Group Functions:: Functions for getting group
information.
* Walking Arrays:: A function to walk arrays of arrays.
* Library Functions Summary:: Summary of library functions.
* Library Exercises:: Exercises.
* Running Examples:: How to run these examples.
* Clones:: Clones of common utilities.
* Cut Program:: The 'cut' utility.
* Egrep Program:: The 'egrep' utility.
* Id Program:: The 'id' utility.
* Split Program:: The 'split' utility.
* Tee Program:: The 'tee' utility.
* Uniq Program:: The 'uniq' utility.
* Wc Program:: The 'wc' utility.
* Miscellaneous Programs:: Some interesting 'awk'
programs.
* Dupword Program:: Finding duplicated words in a
document.
* Alarm Program:: An alarm clock.
* Translate Program:: A program similar to the 'tr'
utility.
* Labels Program:: Printing mailing labels.
* Word Sorting:: A program to produce a word usage
count.
* History Sorting:: Eliminating duplicate entries from a
history file.
* Extract Program:: Pulling out programs from Texinfo
source files.
* Simple Sed:: A Simple Stream Editor.
* Igawk Program:: A wrapper for 'awk' that
includes files.
* Anagram Program:: Finding anagrams from a dictionary.
* Signature Program:: People do amazing things with too much
time on their hands.
* Programs Summary:: Summary of programs.
* Programs Exercises:: Exercises.
* Nondecimal Data:: Allowing nondecimal input data.
* Array Sorting:: Facilities for controlling array
traversal and sorting arrays.
* Controlling Array Traversal:: How to use PROCINFO["sorted_in"].
* Array Sorting Functions:: How to use 'asort()' and
'asorti()'.
* Two-way I/O:: Two-way communications with another
process.
* TCP/IP Networking:: Using 'gawk' for network
programming.
* Profiling:: Profiling your 'awk' programs.
* Advanced Features Summary:: Summary of advanced features.
* I18N and L10N:: Internationalization and Localization.
* Explaining gettext:: How GNU 'gettext' works.
* Programmer i18n:: Features for the programmer.
* Translator i18n:: Features for the translator.
* String Extraction:: Extracting marked strings.
* Printf Ordering:: Rearranging 'printf' arguments.
* I18N Portability:: 'awk'-level portability
issues.
* I18N Example:: A simple i18n example.
* Gawk I18N:: 'gawk' is also
internationalized.
* I18N Summary:: Summary of I18N stuff.
* Debugging:: Introduction to 'gawk'
debugger.
* Debugging Concepts:: Debugging in General.
* Debugging Terms:: Additional Debugging Concepts.
* Awk Debugging:: Awk Debugging.
* Sample Debugging Session:: Sample debugging session.
* Debugger Invocation:: How to Start the Debugger.
* Finding The Bug:: Finding the Bug.
* List of Debugger Commands:: Main debugger commands.
* Breakpoint Control:: Control of Breakpoints.
* Debugger Execution Control:: Control of Execution.
* Viewing And Changing Data:: Viewing and Changing Data.
* Execution Stack:: Dealing with the Stack.
* Debugger Info:: Obtaining Information about the
Program and the Debugger State.
* Miscellaneous Debugger Commands:: Miscellaneous Commands.
* Readline Support:: Readline support.
* Limitations:: Limitations and future plans.
* Debugging Summary:: Debugging summary.
* Computer Arithmetic:: A quick intro to computer math.
* Math Definitions:: Defining terms used.
* MPFR features:: The MPFR features in 'gawk'.
* FP Math Caution:: Things to know.
* Inexactness of computations:: Floating point math is not exact.
* Inexact representation:: Numbers are not exactly represented.
* Comparing FP Values:: How to compare floating point values.
* Errors accumulate:: Errors get bigger as they go.
* Getting Accuracy:: Getting more accuracy takes some work.
* Try To Round:: Add digits and round.
* Setting precision:: How to set the precision.
* Setting the rounding mode:: How to set the rounding mode.
* Arbitrary Precision Integers:: Arbitrary Precision Integer Arithmetic
with 'gawk'.
* Checking for MPFR:: How to check if MPFR is available.
* POSIX Floating Point Problems:: Standards Versus Existing Practice.
* Floating point summary:: Summary of floating point discussion.
* Extension Intro:: What is an extension.
* Plugin License:: A note about licensing.
* Extension Mechanism Outline:: An outline of how it works.
* Extension API Description:: A full description of the API.
* Extension API Functions Introduction:: Introduction to the API functions.
* General Data Types:: The data types.
* Memory Allocation Functions:: Functions for allocating memory.
* Constructor Functions:: Functions for creating values.
* Registration Functions:: Functions to register things with
'gawk'.
* Extension Functions:: Registering extension functions.
* Exit Callback Functions:: Registering an exit callback.
* Extension Version String:: Registering a version string.
* Input Parsers:: Registering an input parser.
* Output Wrappers:: Registering an output wrapper.
* Two-way processors:: Registering a two-way processor.
* Printing Messages:: Functions for printing messages.
* Updating ERRNO:: Functions for updating 'ERRNO'.
* Requesting Values:: How to get a value.
* Accessing Parameters:: Functions for accessing parameters.
* Symbol Table Access:: Functions for accessing global
variables.
* Symbol table by name:: Accessing variables by name.
* Symbol table by cookie:: Accessing variables by "cookie".
* Cached values:: Creating and using cached values.
* Array Manipulation:: Functions for working with arrays.
* Array Data Types:: Data types for working with arrays.
* Array Functions:: Functions for working with arrays.
* Flattening Arrays:: How to flatten arrays.
* Creating Arrays:: How to create and populate arrays.
* Redirection API:: How to access and manipulate
redirections.
* Extension API Variables:: Variables provided by the API.
* Extension Versioning:: API Version information.
* Extension GMP/MPFR Versioning:: Version information about GMP and
MPFR.
* Extension API Informational Variables:: Variables providing information about
'gawk''s invocation.
* Extension API Boilerplate:: Boilerplate code for using the API.
* Changes from API V1:: Changes from V1 of the API.
* Finding Extensions:: How 'gawk' finds compiled
extensions.
* Extension Example:: Example C code for an extension.
* Internal File Description:: What the new functions will do.
* Internal File Ops:: The code for internal file operations.
* Using Internal File Ops:: How to use an external extension.
* Extension Samples:: The sample extensions that ship with
'gawk'.
* Extension Sample File Functions:: The file functions sample.
* Extension Sample Fnmatch:: An interface to 'fnmatch()'.
* Extension Sample Fork:: An interface to 'fork()' and
other process functions.
* Extension Sample Inplace:: Enabling in-place file editing.
* Extension Sample Ord:: Character to value to character
conversions.
* Extension Sample Readdir:: An interface to 'readdir()'.
* Extension Sample Revout:: Reversing output sample output
wrapper.
* Extension Sample Rev2way:: Reversing data sample two-way
processor.
* Extension Sample Read write array:: Serializing an array to a file.
* Extension Sample Readfile:: Reading an entire file into a string.
* Extension Sample Time:: An interface to 'gettimeofday()'
and 'sleep()'.
* Extension Sample API Tests:: Tests for the API.
* gawkextlib:: The 'gawkextlib' project.
* Extension summary:: Extension summary.
* Extension Exercises:: Exercises.
* V7/SVR3.1:: The major changes between V7 and
System V Release 3.1.
* SVR4:: Minor changes between System V
Releases 3.1 and 4.
* POSIX:: New features from the POSIX standard.
* BTL:: New features from Brian Kernighan's
version of 'awk'.
* POSIX/GNU:: The extensions in 'gawk' not
in POSIX 'awk'.
* Feature History:: The history of the features in
'gawk'.
* Common Extensions:: Common Extensions Summary.
* Ranges and Locales:: How locales used to affect regexp
ranges.
* Contributors:: The major contributors to
'gawk'.
* History summary:: History summary.
* Gawk Distribution:: What is in the 'gawk'
distribution.
* Getting:: How to get the distribution.
* Extracting:: How to extract the distribution.
* Distribution contents:: What is in the distribution.
* Unix Installation:: Installing 'gawk' under
various versions of Unix.
* Quick Installation:: Compiling 'gawk' under Unix.
* Shell Startup Files:: Shell convenience functions.
* Additional Configuration Options:: Other compile-time options.
* Configuration Philosophy:: How it's all supposed to work.
* Non-Unix Installation:: Installation on Other Operating
Systems.
* PC Installation:: Installing and Compiling
'gawk' on Microsoft Windows.
* PC Binary Installation:: Installing a prepared distribution.
* PC Compiling:: Compiling 'gawk' for
Windows32.
* PC Using:: Running 'gawk' on Windows32.
* Cygwin:: Building and running 'gawk'
for Cygwin.
* MSYS:: Using 'gawk' In The MSYS
Environment.
* VMS Installation:: Installing 'gawk' on VMS.
* VMS Compilation:: How to compile 'gawk' under
VMS.
* VMS Dynamic Extensions:: Compiling 'gawk' dynamic
extensions on VMS.
* VMS Installation Details:: How to install 'gawk' under
VMS.
* VMS Running:: How to run 'gawk' under VMS.
* VMS GNV:: The VMS GNV Project.
* VMS Old Gawk:: An old version comes with some VMS
systems.
* Bugs:: Reporting Problems and Bugs.
* Bug address:: Where to send reports to.
* Usenet:: Where not to send reports to.
* Maintainers:: Maintainers of non-*nix ports.
* Other Versions:: Other freely available 'awk'
implementations.
* Installation summary:: Summary of installation.
* Compatibility Mode:: How to disable certain 'gawk'
extensions.
* Additions:: Making Additions To 'gawk'.
* Accessing The Source:: Accessing the Git repository.
* Adding Code:: Adding code to the main body of
'gawk'.
* New Ports:: Porting 'gawk' to a new
operating system.
* Derived Files:: Why derived files are kept in the Git
repository.
* Future Extensions:: New features that may be implemented
one day.
* Implementation Limitations:: Some limitations of the
implementation.
* Extension Design:: Design notes about the extension API.
* Old Extension Problems:: Problems with the old mechanism.
* Extension New Mechanism Goals:: Goals for the new mechanism.
* Extension Other Design Decisions:: Some other design decisions.
* Extension Future Growth:: Some room for future growth.
* Old Extension Mechanism:: Some compatibility for old extensions.
* Notes summary:: Summary of implementation notes.
* Basic High Level:: The high level view.
* Basic Data Typing:: A very quick intro to data types.
To my parents, for their love, and for the wonderful example they set
for me.
To my wife Miriam, for making me complete. Thank you for building
your life together with me.
To our children Chana, Rivka, Nachum and Malka, for enrichening our
lives in innumerable ways.
File: gawk.info, Node: Foreword3, Next: Foreword4, Prev: Top, Up: Top
Foreword to the Third Edition
*****************************
Arnold Robbins and I are good friends. We were introduced in 1990 by
circumstances--and our favorite programming language, AWK. The
circumstances started a couple of years earlier. I was working at a new
job and noticed an unplugged Unix computer sitting in the corner. No
one knew how to use it, and neither did I. However, a couple of days
later, it was running, and I was 'root' and the one-and-only user. That
day, I began the transition from statistician to Unix programmer.
On one of many trips to the library or bookstore in search of books
on Unix, I found the gray AWK book, a.k.a. Alfred V. Aho, Brian W.
Kernighan, and Peter J. Weinberger's 'The AWK Programming Language'
(Addison-Wesley, 1988). 'awk''s simple programming paradigm--find a
pattern in the input and then perform an action--often reduced complex
or tedious data manipulations to a few lines of code. I was excited to
try my hand at programming in AWK.
Alas, the 'awk' on my computer was a limited version of the language
described in the gray book. I discovered that my computer had "old
'awk'" and the book described "new 'awk'." I learned that this was
typical; the old version refused to step aside or relinquish its name.
If a system had a new 'awk', it was invariably called 'nawk', and few
systems had it. The best way to get a new 'awk' was to 'ftp' the source
code for 'gawk' from 'prep.ai.mit.edu'. 'gawk' was a version of new
'awk' written by David Trueman and Arnold, and available under the GNU
General Public License.
(Incidentally, it's no longer difficult to find a new 'awk'. 'gawk'
ships with GNU/Linux, and you can download binaries or source code for
almost any system; my wife uses 'gawk' on her VMS box.)
My Unix system started out unplugged from the wall; it certainly was
not plugged into a network. So, oblivious to the existence of 'gawk'
and the Unix community in general, and desiring a new 'awk', I wrote my
own, called 'mawk'. Before I was finished, I knew about 'gawk', but it
was too late to stop, so I eventually posted to a 'comp.sources'
newsgroup.
A few days after my posting, I got a friendly email from Arnold
introducing himself. He suggested we share design and algorithms and
attached a draft of the POSIX standard so that I could update 'mawk' to
support language extensions added after publication of 'The AWK
Programming Language'.
Frankly, if our roles had been reversed, I would not have been so
open and we probably would have never met. I'm glad we did meet. He is
an AWK expert's AWK expert and a genuinely nice person. Arnold
contributes significant amounts of his expertise and time to the Free
Software Foundation.
This book is the 'gawk' reference manual, but at its core it is a
book about AWK programming that will appeal to a wide audience. It is a
definitive reference to the AWK language as defined by the 1987 Bell
Laboratories release and codified in the 1992 POSIX Utilities standard.
On the other hand, the novice AWK programmer can study a wealth of
practical programs that emphasize the power of AWK's basic idioms:
data-driven control flow, pattern matching with regular expressions, and
associative arrays. Those looking for something new can try out
'gawk''s interface to network protocols via special '/inet' files.
The programs in this book make clear that an AWK program is typically
much smaller and faster to develop than a counterpart written in C.
Consequently, there is often a payoff to prototyping an algorithm or
design in AWK to get it running quickly and expose problems early.
Often, the interpreted performance is adequate and the AWK prototype
becomes the product.
The new 'pgawk' (profiling 'gawk'), produces program execution
counts. I recently experimented with an algorithm that for n lines of
input, exhibited ~ C n^2 performance, while theory predicted ~ C n log n
behavior. A few minutes poring over the 'awkprof.out' profile
pinpointed the problem to a single line of code. 'pgawk' is a welcome
addition to my programmer's toolbox.
Arnold has distilled over a decade of experience writing and using
AWK programs, and developing 'gawk', into this book. If you use AWK or
want to learn how, then read this book.
Michael Brennan
Author of 'mawk'
March 2001
File: gawk.info, Node: Foreword4, Next: Preface, Prev: Foreword3, Up: Top
Foreword to the Fourth Edition
******************************
Some things don't change. Thirteen years ago I wrote: "If you use AWK
or want to learn how, then read this book." True then, and still true
today.
Learning to use a programming language is about more than mastering
the syntax. One needs to acquire an understanding of how to use the
features of the language to solve practical programming problems. A
focus of this book is many examples that show how to use AWK.
Some things do change. Our computers are much faster and have more
memory. Consequently, speed and storage inefficiencies of a high-level
language matter less. Prototyping in AWK and then rewriting in C for
performance reasons happens less, because more often the prototype is
fast enough.
Of course, there are computing operations that are best done in C or
C++. With 'gawk' 4.1 and later, you do not have to choose between
writing your program in AWK or in C/C++. You can write most of your
program in AWK and the aspects that require C/C++ capabilities can be
written in C/C++, and then the pieces glued together when the 'gawk'
module loads the C/C++ module as a dynamic plug-in. *note Dynamic
Extensions::, has all the details, and, as expected, many examples to
help you learn the ins and outs.
I enjoy programming in AWK and had fun (re)reading this book. I
think you will too.
Michael Brennan
Author of 'mawk'
October 2014
File: gawk.info, Node: Preface, Next: Getting Started, Prev: Foreword4, Up: Top
Preface
*******
Several kinds of tasks occur repeatedly when working with text files.
You might want to extract certain lines and discard the rest. Or you
may need to make changes wherever certain patterns appear, but leave the
rest of the file alone. Such jobs are often easy with 'awk'. The 'awk'
utility interprets a special-purpose programming language that makes it
easy to handle simple data-reformatting jobs.
The GNU implementation of 'awk' is called 'gawk'; if you invoke it
with the proper options or environment variables, it is fully compatible
with the POSIX(1) specification of the 'awk' language and with the Unix
version of 'awk' maintained by Brian Kernighan. This means that all
properly written 'awk' programs should work with 'gawk'. So most of the
time, we don't distinguish between 'gawk' and other 'awk'
implementations.
Using 'awk' you can:
* Manage small, personal databases
* Generate reports
* Validate data
* Produce indexes and perform other document-preparation tasks
* Experiment with algorithms that you can adapt later to other
computer languages
In addition, 'gawk' provides facilities that make it easy to:
* Extract bits and pieces of data for processing
* Sort data
* Perform simple network communications
* Profile and debug 'awk' programs
* Extend the language with functions written in C or C++
This Info file teaches you about the 'awk' language and how you can
use it effectively. You should already be familiar with basic system
commands, such as 'cat' and 'ls',(2) as well as basic shell facilities,
such as input/output (I/O) redirection and pipes.
Implementations of the 'awk' language are available for many
different computing environments. This Info file, while describing the
'awk' language in general, also describes the particular implementation
of 'awk' called 'gawk' (which stands for "GNU 'awk'"). 'gawk' runs on a
broad range of Unix systems, ranging from Intel-architecture PC-based
computers up through large-scale systems. 'gawk' has also been ported
to Mac OS X, Microsoft Windows (all versions), and OpenVMS.(3)
* Menu:
* History:: The history of 'gawk' and
'awk'.
* Names:: What name to use to find 'awk'.
* This Manual:: Using this Info file. Includes sample
input files that you can use.
* Conventions:: Typographical Conventions.
* Manual History:: Brief history of the GNU project and this
Info file.
* How To Contribute:: Helping to save the world.
* Acknowledgments:: Acknowledgments.
---------- Footnotes ----------
(1) The 2008 POSIX standard is accessible online at
<http://www.opengroup.org/onlinepubs/9699919799/>;.
(2) These utilities are available on POSIX-compliant systems, as well
as on traditional Unix-based systems. If you are using some other
operating system, you still need to be familiar with the ideas of I/O
redirection and pipes.
(3) Some other, obsolete systems to which 'gawk' was once ported are
no longer supported and the code for those systems has been removed.
File: gawk.info, Node: History, Next: Names, Up: Preface
History of 'awk' and 'gawk'
===========================
Recipe for a Programming Language
1 part 'egrep' 1 part 'snobol'
2 parts 'ed' 3 parts C
Blend all parts well using 'lex' and 'yacc'. Document minimally and
release.
After eight years, add another part 'egrep' and two more parts C.
Document very well and release.
The name 'awk' comes from the initials of its designers: Alfred V.
Aho, Peter J. Weinberger, and Brian W. Kernighan. The original version
of 'awk' was written in 1977 at AT&T Bell Laboratories. In 1985, a new
version made the programming language more powerful, introducing
user-defined functions, multiple input streams, and computed regular
expressions. This new version became widely available with Unix System
V Release 3.1 (1987). The version in System V Release 4 (1989) added
some new features and cleaned up the behavior in some of the "dark
corners" of the language. The specification for 'awk' in the POSIX
Command Language and Utilities standard further clarified the language.
Both the 'gawk' designers and the original 'awk' designers at Bell
Laboratories provided feedback for the POSIX specification.
Paul Rubin wrote 'gawk' in 1986. Jay Fenlason completed it, with
advice from Richard Stallman. John Woods contributed parts of the code
as well. In 1988 and 1989, David Trueman, with help from me, thoroughly
reworked 'gawk' for compatibility with the newer 'awk'. Circa 1994, I
became the primary maintainer. Current development focuses on bug
fixes, performance improvements, standards compliance, and,
occasionally, new features.
In May 1997, Ju"rgen Kahrs felt the need for network access from
'awk', and with a little help from me, set about adding features to do
this for 'gawk'. At that time, he also wrote the bulk of 'TCP/IP
Internetworking with 'gawk'' (a separate document, available as part of
the 'gawk' distribution). His code finally became part of the main
'gawk' distribution with 'gawk' version 3.1.
John Haque rewrote the 'gawk' internals, in the process providing an
'awk'-level debugger. This version became available as 'gawk' version
4.0 in 2011.
*Note Contributors:: for a full list of those who have made important
contributions to 'gawk'.
File: gawk.info, Node: Names, Next: This Manual, Prev: History, Up: Preface
A Rose by Any Other Name
========================
The 'awk' language has evolved over the years. Full details are
provided in *note Language History::. The language described in this
Info file is often referred to as "new 'awk'." By analogy, the original
version of 'awk' is referred to as "old 'awk'."
On most current systems, when you run the 'awk' utility you get some
version of new 'awk'.(1) If your system's standard 'awk' is the old
one, you will see something like this if you try the test program:
$ awk 1 /dev/null
error-> awk: syntax error near line 1
error-> awk: bailing out near line 1
In this case, you should find a version of new 'awk', or just install
'gawk'!
Throughout this Info file, whenever we refer to a language feature
that should be available in any complete implementation of POSIX 'awk',
we simply use the term 'awk'. When referring to a feature that is
specific to the GNU implementation, we use the term 'gawk'.
---------- Footnotes ----------
(1) Only Solaris systems still use an old 'awk' for the default 'awk'
utility. A more modern 'awk' lives in '/usr/xpg6/bin' on these systems.
File: gawk.info, Node: This Manual, Next: Conventions, Prev: Names, Up: Preface
Using This Book
===============
The term 'awk' refers to a particular program as well as to the language
you use to tell this program what to do. When we need to be careful, we
call the language "the 'awk' language," and the program "the 'awk'
utility." This Info file explains both how to write programs in the
'awk' language and how to run the 'awk' utility. The term "'awk'
program" refers to a program written by you in the 'awk' programming
language.
Primarily, this Info file explains the features of 'awk' as defined
in the POSIX standard. It does so in the context of the 'gawk'
implementation. While doing so, it also attempts to describe important
differences between 'gawk' and other 'awk' implementations.(1) Finally,
it notes any 'gawk' features that are not in the POSIX standard for
'awk'.
There are sidebars scattered throughout the Info file. They add a
more complete explanation of points that are relevant, but not likely to
be of interest on first reading. All appear in the index, under the
heading "sidebar."
Most of the time, the examples use complete 'awk' programs. Some of
the more advanced minor nodes show only the part of the 'awk' program
that illustrates the concept being described.
Although this Info file is aimed principally at people who have not
been exposed to 'awk', there is a lot of information here that even the
'awk' expert should find useful. In particular, the description of
POSIX 'awk' and the example programs in *note Library Functions::, and
in *note Sample Programs::, should be of interest.
This Info file is split into several parts, as follows:
* Part I describes the 'awk' language and the 'gawk' program in
detail. It starts with the basics, and continues through all of
the features of 'awk'. It contains the following chapters:
- *note Getting Started::, provides the essentials you need to
know to begin using 'awk'.
- *note Invoking Gawk::, describes how to run 'gawk', the
meaning of its command-line options, and how it finds 'awk'
program source files.
- *note Regexp::, introduces regular expressions in general, and
in particular the flavors supported by POSIX 'awk' and 'gawk'.
- *note Reading Files::, describes how 'awk' reads your data.
It introduces the concepts of records and fields, as well as
the 'getline' command. I/O redirection is first described
here. Network I/O is also briefly introduced here.
- *note Printing::, describes how 'awk' programs can produce
output with 'print' and 'printf'.
- *note Expressions::, describes expressions, which are the
basic building blocks for getting most things done in a
program.
- *note Patterns and Actions::, describes how to write patterns
for matching records, actions for doing something when a
record is matched, and the predefined variables 'awk' and
'gawk' use.
- *note Arrays::, covers 'awk''s one-and-only data structure:
the associative array. Deleting array elements and whole
arrays is described, as well as sorting arrays in 'gawk'. The
major node also describes how 'gawk' provides arrays of
arrays.
- *note Functions::, describes the built-in functions 'awk' and
'gawk' provide, as well as how to define your own functions.
It also discusses how 'gawk' lets you call functions
indirectly.
* Part II shows how to use 'awk' and 'gawk' for problem solving.
There is lots of code here for you to read and learn from. This
part contains the following chapters:
- *note Library Functions::, provides a number of functions
meant to be used from main 'awk' programs.
- *note Sample Programs::, provides many sample 'awk' programs.
Reading these two chapters allows you to see 'awk' solving real
problems.
* Part III focuses on features specific to 'gawk'. It contains the
following chapters:
- *note Advanced Features::, describes a number of advanced
features. Of particular note are the abilities to control the
order of array traversal, have two-way communications with
another process, perform TCP/IP networking, and profile your
'awk' programs.
- *note Internationalization::, describes special features for
translating program messages into different languages at
runtime.
- *note Debugger::, describes the 'gawk' debugger.
- *note Arbitrary Precision Arithmetic::, describes advanced
arithmetic facilities.
- *note Dynamic Extensions::, describes how to add new variables
and functions to 'gawk' by writing extensions in C or C++.
* Part IV provides the appendices, the Glossary, and two licenses
that cover the 'gawk' source code and this Info file, respectively.
It contains the following appendices:
- *note Language History::, describes how the 'awk' language has
evolved since its first release to the present. It also
describes how 'gawk' has acquired features over time.
- *note Installation::, describes how to get 'gawk', how to
compile it on POSIX-compatible systems, and how to compile and
use it on different non-POSIX systems. It also describes how
to report bugs in 'gawk' and where to get other freely
available 'awk' implementations.
- *note Notes::, describes how to disable 'gawk''s extensions,
as well as how to contribute new code to 'gawk', and some
possible future directions for 'gawk' development.
- *note Basic Concepts::, provides some very cursory background
material for those who are completely unfamiliar with computer
programming.
The *note Glossary::, defines most, if not all, of the
significant terms used throughout the Info file. If you find
terms that you aren't familiar with, try looking them up here.
- *note Copying::, and *note GNU Free Documentation License::,
present the licenses that cover the 'gawk' source code and
this Info file, respectively.
---------- Footnotes ----------
(1) All such differences appear in the index under the entry
"differences in 'awk' and 'gawk'."
File: gawk.info, Node: Conventions, Next: Manual History, Prev: This Manual, Up: Preface
Typographical Conventions
=========================
This Info file is written in Texinfo
(https://www.gnu.org/software/texinfo/), the GNU documentation
formatting language. A single Texinfo source file is used to produce
both the printed and online versions of the documentation. This minor
node briefly documents the typographical conventions used in Texinfo.
Examples you would type at the command line are preceded by the
common shell primary and secondary prompts, '$' and '>'. Input that you
type is shown 'like this'. Output from the command is preceded by the
glyph "-|". This typically represents the command's standard output.
Error messages and other output on the command's standard error are
preceded by the glyph "error->". For example:
$ echo hi on stdout
-| hi on stdout
$ echo hello on stderr 1>&2
error-> hello on stderr
Characters that you type at the keyboard look 'like this'. In
particular, there are special characters called "control characters."
These are characters that you type by holding down both the 'CONTROL'
key and another key, at the same time. For example, a 'Ctrl-d' is typed
by first pressing and holding the 'CONTROL' key, next pressing the 'd'
key, and finally releasing both keys.
For the sake of brevity, throughout this Info file, we refer to Brian
Kernighan's version of 'awk' as "BWK 'awk'." (*Note Other Versions::
for information on his and other versions.)
Dark Corners
------------
Dark corners are basically fractal--no matter how much you
illuminate, there's always a smaller but darker one.
-- _Brian Kernighan_
Until the POSIX standard (and 'GAWK: Effective AWK Programming'),
many features of 'awk' were either poorly documented or not documented
at all. Descriptions of such features (often called "dark corners") are
noted in this Info file with "(d.c.)." They also appear in the index
under the heading "dark corner."
But, as noted by the opening quote, any coverage of dark corners is
by definition incomplete.
Extensions to the standard 'awk' language that are supported by more
than one 'awk' implementation are marked "(c.e.)," and listed in the
index under "common extensions" and "extensions, common."
File: gawk.info, Node: Manual History, Next: How To Contribute, Prev: Conventions, Up: Preface
The GNU Project and This Book
=============================
The Free Software Foundation (FSF) is a nonprofit organization dedicated
to the production and distribution of freely distributable software. It
was founded by Richard M. Stallman, the author of the original Emacs
editor. GNU Emacs is the most widely used version of Emacs today.
The GNU(1) Project is an ongoing effort on the part of the Free
Software Foundation to create a complete, freely distributable,
POSIX-compliant computing environment. The FSF uses the GNU General
Public License (GPL) to ensure that its software's source code is always
available to the end user. A copy of the GPL is included for your
reference (*note Copying::). The GPL applies to the C language source
code for 'gawk'. To find out more about the FSF and the GNU Project
online, see the GNU Project's home page (https://www.gnu.org). This
Info file may also be read from GNU's website
(https://www.gnu.org/software/gawk/manual/).
A shell, an editor (Emacs), highly portable optimizing C, C++, and
Objective-C compilers, a symbolic debugger and dozens of large and small
utilities (such as 'gawk'), have all been completed and are freely
available. The GNU operating system kernel (the HURD), has been
released but remains in an early stage of development.
Until the GNU operating system is more fully developed, you should
consider using GNU/Linux, a freely distributable, Unix-like operating
system for Intel, Power Architecture, Sun SPARC, IBM S/390, and other
systems.(2) Many GNU/Linux distributions are available for download
from the Internet.
The Info file itself has gone through multiple previous editions.
Paul Rubin wrote the very first draft of 'The GAWK Manual'; it was
around 40 pages long. Diane Close and Richard Stallman improved it,
yielding a version that was around 90 pages and barely described the
original, "old" version of 'awk'.
I started working with that version in the fall of 1988. As work on
it progressed, the FSF published several preliminary versions (numbered
0.X). In 1996, edition 1.0 was released with 'gawk' 3.0.0. The FSF
published the first two editions under the title 'The GNU Awk User's
Guide'.
This edition maintains the basic structure of the previous editions.
For FSF edition 4.0, the content was thoroughly reviewed and updated.
All references to 'gawk' versions prior to 4.0 were removed. Of
significant note for that edition was the addition of *note Debugger::.
For FSF edition 4.2, the content has been reorganized into parts, and
the major new additions are *note Arbitrary Precision Arithmetic::, and
*note Dynamic Extensions::.
This Info file will undoubtedly continue to evolve. If you find an
error in the Info file, please report it! *Note Bugs:: for information
on submitting problem reports electronically.
---------- Footnotes ----------
(1) GNU stands for "GNU's Not Unix."
(2) The terminology "GNU/Linux" is explained in the *note Glossary::.
File: gawk.info, Node: How To Contribute, Next: Acknowledgments, Prev: Manual History, Up: Preface
How to Contribute
=================
As the maintainer of GNU 'awk', I once thought that I would be able to
manage a collection of publicly available 'awk' programs and I even
solicited contributions. Making things available on the Internet helps
keep the 'gawk' distribution down to manageable size.
The initial collection of material, such as it is, is still available
at <ftp://ftp.freefriends.org/arnold/Awkstuff>;.
In the hopes of doing something more broad, I acquired the
'awklang.org' domain. Late in 2017, a volunteer took on the task of
managing it.
If you have written an interesting 'awk' program, that you would like
to share with the rest of the world, please see <http://www.awklang.org>;
and use the "Contact" link.
If you have written a 'gawk' extension, please see *note
gawkextlib::.
File: gawk.info, Node: Acknowledgments, Prev: How To Contribute, Up: Preface
Acknowledgments
===============
The initial draft of 'The GAWK Manual' had the following
acknowledgments:
Many people need to be thanked for their assistance in producing
this manual. Jay Fenlason contributed many ideas and sample
programs. Richard Mlynarik and Robert Chassell gave helpful
comments on drafts of this manual. The paper 'A Supplemental
Document for AWK' by John W. Pierce of the Chemistry Department at
UC San Diego, pinpointed several issues relevant both to 'awk'
implementation and to this manual, that would otherwise have
escaped us.
I would like to acknowledge Richard M. Stallman, for his vision of a
better world and for his courage in founding the FSF and starting the
GNU Project.
Earlier editions of this Info file had the following
acknowledgements:
The following people (in alphabetical order) provided helpful
comments on various versions of this book: Rick Adams, Dr. Nelson
H.F. Beebe, Karl Berry, Dr. Michael Brennan, Rich Burridge, Claire
Cloutier, Diane Close, Scott Deifik, Christopher ("Topher") Eliot,
Jeffrey Friedl, Dr. Darrel Hankerson, Michal Jaegermann, Dr.
Richard J. LeBlanc, Michael Lijewski, Pat Rankin, Miriam Robbins,
Mary Sheehan, and Chuck Toporek.
Robert J. Chassell provided much valuable advice on the use of
Texinfo. He also deserves special thanks for convincing me _not_
to title this Info file 'How to Gawk Politely'. Karl Berry helped
significantly with the TeX part of Texinfo.
I would like to thank Marshall and Elaine Hartholz of Seattle and
Dr. Bert and Rita Schreiber of Detroit for large amounts of quiet
vacation time in their homes, which allowed me to make significant
progress on this Info file and on 'gawk' itself.
Phil Hughes of SSC contributed in a very important way by loaning
me his laptop GNU/Linux system, not once, but twice, which allowed
me to do a lot of work while away from home.
David Trueman deserves special credit; he has done a yeoman job of
evolving 'gawk' so that it performs well and without bugs.
Although he is no longer involved with 'gawk', working with him on
this project was a significant pleasure.
The intrepid members of the GNITS mailing list, and most notably
Ulrich Drepper, provided invaluable help and feedback for the
design of the internationalization features.
Chuck Toporek, Mary Sheehan, and Claire Cloutier of O'Reilly &
Associates contributed significant editorial help for this Info
file for the 3.1 release of 'gawk'.
Dr. Nelson Beebe, Andreas Buening, Dr. Manuel Collado, Antonio
Colombo, Stephen Davies, Scott Deifik, Akim Demaille, Daniel Richard G.,
Juan Manuel Guerrero, Darrel Hankerson, Michal Jaegermann, Ju"rgen
Kahrs, Stepan Kasal, John Malmberg, Dave Pitts, Chet Ramey, Pat Rankin,
Andrew Schorr, Corinna Vinschen, and Eli Zaretskii (in alphabetical
order) make up the current 'gawk' "crack portability team." Without
their hard work and help, 'gawk' would not be nearly the robust,
portable program it is today. It has been and continues to be a
pleasure working with this team of fine people.
Notable code and documentation contributions were made by a number of
people. *Note Contributors:: for the full list.
Thanks to Michael Brennan for the Forewords.
Thanks to Patrice Dumas for the new 'makeinfo' program. Thanks to
Karl Berry, who continues to work to keep the Texinfo markup language
sane.
Robert P.J. Day, Michael Brennan, and Brian Kernighan kindly acted as
reviewers for the 2015 edition of this Info file. Their feedback helped
improve the final work.
I would also like to thank Brian Kernighan for his invaluable
assistance during the testing and debugging of 'gawk', and for his
ongoing help and advice in clarifying numerous points about the
language. We could not have done nearly as good a job on either 'gawk'
or its documentation without his help.
Brian is in a class by himself as a programmer and technical author.
I have to thank him (yet again) for his ongoing friendship and for being
a role model to me for close to 30 years! Having him as a reviewer is
an exciting privilege. It has also been extremely humbling...
I must thank my wonderful wife, Miriam, for her patience through the
many versions of this project, for her proofreading, and for sharing me
with the computer. I would like to thank my parents for their love, and
for the grace with which they raised and educated me. Finally, I also
must acknowledge my gratitude to G-d, for the many opportunities He has
sent my way, as well as for the gifts He has given me with which to take
advantage of those opportunities.
Arnold Robbins
Nof Ayalon
Israel
February 2015
File: gawk.info, Node: Getting Started, Next: Invoking Gawk, Prev: Preface, Up: Top
1 Getting Started with 'awk'
****************************
The basic function of 'awk' is to search files for lines (or other units
of text) that contain certain patterns. When a line matches one of the
patterns, 'awk' performs specified actions on that line. 'awk'
continues to process input lines in this way until it reaches the end of
the input files.
Programs in 'awk' are different from programs in most other
languages, because 'awk' programs are "data driven" (i.e., you describe
the data you want to work with and then what to do when you find it).
Most other languages are "procedural"; you have to describe, in great
detail, every step the program should take. When working with
procedural languages, it is usually much harder to clearly describe the
data your program will process. For this reason, 'awk' programs are
often refreshingly easy to read and write.
When you run 'awk', you specify an 'awk' "program" that tells 'awk'
what to do. The program consists of a series of "rules" (it may also
contain "function definitions", an advanced feature that we will ignore
for now; *note User-defined::). Each rule specifies one pattern to
search for and one action to perform upon finding the pattern.
Syntactically, a rule consists of a "pattern" followed by an
"action". The action is enclosed in braces to separate it from the
pattern. Newlines usually separate rules. Therefore, an 'awk' program
looks like this:
PATTERN { ACTION }
PATTERN { ACTION }
...
* Menu:
* Running gawk:: How to run 'gawk' programs; includes
command-line syntax.
* Sample Data Files:: Sample data files for use in the 'awk'
programs illustrated in this Info file.
* Very Simple:: A very simple example.
* Two Rules:: A less simple one-line example using two
rules.
* More Complex:: A more complex example.
* Statements/Lines:: Subdividing or combining statements into
lines.
* Other Features:: Other Features of 'awk'.
* When:: When to use 'gawk' and when to use
other things.
* Intro Summary:: Summary of the introduction.
File: gawk.info, Node: Running gawk, Next: Sample Data Files, Up: Getting Started
1.1 How to Run 'awk' Programs
=============================
There are several ways to run an 'awk' program. If the program is
short, it is easiest to include it in the command that runs 'awk', like
this:
awk 'PROGRAM' INPUT-FILE1 INPUT-FILE2 ...
When the program is long, it is usually more convenient to put it in
a file and run it with a command like this:
awk -f PROGRAM-FILE INPUT-FILE1 INPUT-FILE2 ...
This minor node discusses both mechanisms, along with several
variations of each.
* Menu:
* One-shot:: Running a short throwaway 'awk'
program.
* Read Terminal:: Using no input files (input from the keyboard
instead).
* Long:: Putting permanent 'awk' programs in
files.
* Executable Scripts:: Making self-contained 'awk' programs.
* Comments:: Adding documentation to 'gawk'
programs.
* Quoting:: More discussion of shell quoting issues.
File: gawk.info, Node: One-shot, Next: Read Terminal, Up: Running gawk
1.1.1 One-Shot Throwaway 'awk' Programs
---------------------------------------
Once you are familiar with 'awk', you will often type in simple programs
the moment you want to use them. Then you can write the program as the
first argument of the 'awk' command, like this:
awk 'PROGRAM' INPUT-FILE1 INPUT-FILE2 ...
where PROGRAM consists of a series of patterns and actions, as described
earlier.
This command format instructs the "shell", or command interpreter, to
start 'awk' and use the PROGRAM to process records in the input file(s).
There are single quotes around PROGRAM so the shell won't interpret any
'awk' characters as special shell characters. The quotes also cause the
shell to treat all of PROGRAM as a single argument for 'awk', and allow
PROGRAM to be more than one line long.
This format is also useful for running short or medium-sized 'awk'
programs from shell scripts, because it avoids the need for a separate
file for the 'awk' program. A self-contained shell script is more
reliable because there are no other files to misplace.
Later in this chapter, in *note Very Simple::, we'll see examples of
several short, self-contained programs.
File: gawk.info, Node: Read Terminal, Next: Long, Prev: One-shot, Up: Running gawk
1.1.2 Running 'awk' Without Input Files
---------------------------------------
You can also run 'awk' without any input files. If you type the
following command line:
awk 'PROGRAM'
'awk' applies the PROGRAM to the "standard input", which usually means
whatever you type on the keyboard. This continues until you indicate
end-of-file by typing 'Ctrl-d'. (On non-POSIX operating systems, the
end-of-file character may be different.)
As an example, the following program prints a friendly piece of
advice (from Douglas Adams's 'The Hitchhiker's Guide to the Galaxy'), to
keep you from worrying about the complexities of computer programming:
$ awk 'BEGIN { print "Don\47t Panic!" }'
-| Don't Panic!
'awk' executes statements associated with 'BEGIN' before reading any
input. If there are no other statements in your program, as is the case
here, 'awk' just stops, instead of trying to read input it doesn't know
how to process. The '\47' is a magic way (explained later) of getting a
single quote into the program, without having to engage in ugly shell
quoting tricks.
NOTE: If you use Bash as your shell, you should execute the command
'set +H' before running this program interactively, to disable the
C shell-style command history, which treats '!' as a special
character. We recommend putting this command into your personal
startup file.
This next simple 'awk' program emulates the 'cat' utility; it copies
whatever you type on the keyboard to its standard output (why this works
is explained shortly):
$ awk '{ print }'
Now is the time for all good men
-| Now is the time for all good men
to come to the aid of their country.
-| to come to the aid of their country.
Four score and seven years ago, ...
-| Four score and seven years ago, ...
What, me worry?
-| What, me worry?
Ctrl-d
File: gawk.info, Node: Long, Next: Executable Scripts, Prev: Read Terminal, Up: Running gawk
1.1.3 Running Long Programs
---------------------------
Sometimes 'awk' programs are very long. In these cases, it is more
convenient to put the program into a separate file. In order to tell
'awk' to use that file for its program, you type:
awk -f SOURCE-FILE INPUT-FILE1 INPUT-FILE2 ...
The '-f' instructs the 'awk' utility to get the 'awk' program from
the file SOURCE-FILE (*note Options::). Any file name can be used for
SOURCE-FILE. For example, you could put the program:
BEGIN { print "Don't Panic!" }
into the file 'advice'. Then this command:
awk -f advice
does the same thing as this one:
awk 'BEGIN { print "Don\47t Panic!" }'
This was explained earlier (*note Read Terminal::). Note that you don't
usually need single quotes around the file name that you specify with
'-f', because most file names don't contain any of the shell's special
characters. Notice that in 'advice', the 'awk' program did not have
single quotes around it. The quotes are only needed for programs that
are provided on the 'awk' command line. (Also, placing the program in a
file allows us to use a literal single quote in the program text,
instead of the magic '\47'.)
If you want to clearly identify an 'awk' program file as such, you
can add the extension '.awk' to the file name. This doesn't affect the
execution of the 'awk' program but it does make "housekeeping" easier.
File: gawk.info, Node: Executable Scripts, Next: Comments, Prev: Long, Up: Running gawk
1.1.4 Executable 'awk' Programs
-------------------------------
Once you have learned 'awk', you may want to write self-contained 'awk'
scripts, using the '#!' script mechanism. You can do this on many
systems.(1) For example, you could update the file 'advice' to look
like this:
#! /bin/awk -f
BEGIN { print "Don't Panic!" }
After making this file executable (with the 'chmod' utility), simply
type 'advice' at the shell and the system arranges to run 'awk' as if
you had typed 'awk -f advice':
$ chmod +x advice
$ advice
-| Don't Panic!
(We assume you have the current directory in your shell's search path
variable [typically '$PATH']. If not, you may need to type './advice'
at the shell.)
Self-contained 'awk' scripts are useful when you want to write a
program that users can invoke without their having to know that the
program is written in 'awk'.
Understanding '#!'
'awk' is an "interpreted" language. This means that the 'awk'
utility reads your program and then processes your data according to the
instructions in your program. (This is different from a "compiled"
language such as C, where your program is first compiled into machine
code that is executed directly by your system's processor.) The 'awk'
utility is thus termed an "interpreter". Many modern languages are
interpreted.
The line beginning with '#!' lists the full file name of an
interpreter to run and a single optional initial command-line argument
to pass to that interpreter. The operating system then runs the
interpreter with the given argument and the full argument list of the
executed program. The first argument in the list is the full file name
of the 'awk' program. The rest of the argument list contains either
options to 'awk', or data files, or both. (Note that on many systems
'awk' may be found in '/usr/bin' instead of in '/bin'.)
Some systems limit the length of the interpreter name to 32
characters. Often, this can be dealt with by using a symbolic link.
You should not put more than one argument on the '#!' line after the
path to 'awk'. It does not work. The operating system treats the rest
of the line as a single argument and passes it to 'awk'. Doing this
leads to confusing behavior--most likely a usage diagnostic of some sort
from 'awk'.
Finally, the value of 'ARGV[0]' (*note Built-in Variables::) varies
depending upon your operating system. Some systems put 'awk' there,
some put the full pathname of 'awk' (such as '/bin/awk'), and some put
the name of your script ('advice'). (d.c.) Don't rely on the value of
'ARGV[0]' to provide your script name.
---------- Footnotes ----------
(1) The '#!' mechanism works on GNU/Linux systems, BSD-based systems,
and commercial Unix systems.
File: gawk.info, Node: Comments, Next: Quoting, Prev: Executable Scripts, Up: Running gawk
1.1.5 Comments in 'awk' Programs
--------------------------------
A "comment" is some text that is included in a program for the sake of
human readers; it is not really an executable part of the program.
Comments can explain what the program does and how it works. Nearly all
programming languages have provisions for comments, as programs are
typically hard to understand without them.
In the 'awk' language, a comment starts with the number sign
character ('#') and continues to the end of the line. The '#' does not
have to be the first character on the line. The 'awk' language ignores
the rest of a line following a number sign. For example, we could have
put the following into 'advice':
# This program prints a nice, friendly message. It helps
# keep novice users from being afraid of the computer.
BEGIN { print "Don't Panic!" }
You can put comment lines into keyboard-composed throwaway 'awk'
programs, but this usually isn't very useful; the purpose of a comment
is to help you or another person understand the program when reading it
at a later time.
CAUTION: As mentioned in *note One-shot::, you can enclose short to
medium-sized programs in single quotes, in order to keep your shell
scripts self-contained. When doing so, _don't_ put an apostrophe
(i.e., a single quote) into a comment (or anywhere else in your
program). The shell interprets the quote as the closing quote for
the entire program. As a result, usually the shell prints a
message about mismatched quotes, and if 'awk' actually runs, it
will probably print strange messages about syntax errors. For
example, look at the following:
$ awk 'BEGIN { print "hello" } # let's be cute'
>
The shell sees that the first two quotes match, and that a new
quoted object begins at the end of the command line. It therefore
prompts with the secondary prompt, waiting for more input. With
Unix 'awk', closing the quoted string produces this result:
$ awk '{ print "hello" } # let's be cute'
> '
error-> awk: can't open file be
error-> source line number 1
Putting a backslash before the single quote in 'let's' wouldn't
help, because backslashes are not special inside single quotes.
The next node describes the shell's quoting rules.
File: gawk.info, Node: Quoting, Prev: Comments, Up: Running gawk
1.1.6 Shell Quoting Issues
--------------------------
* Menu:
* DOS Quoting:: Quoting in Windows Batch Files.
For short to medium-length 'awk' programs, it is most convenient to
enter the program on the 'awk' command line. This is best done by
enclosing the entire program in single quotes. This is true whether you
are entering the program interactively at the shell prompt, or writing
it as part of a larger shell script:
awk 'PROGRAM TEXT' INPUT-FILE1 INPUT-FILE2 ...
Once you are working with the shell, it is helpful to have a basic
knowledge of shell quoting rules. The following rules apply only to
POSIX-compliant, Bourne-style shells (such as Bash, the GNU Bourne-Again
Shell). If you use the C shell, you're on your own.
Before diving into the rules, we introduce a concept that appears
throughout this Info file, which is that of the "null", or empty,
string.
The null string is character data that has no value. In other words,
it is empty. It is written in 'awk' programs like this: '""'. In the
shell, it can be written using single or double quotes: '""' or ''''.
Although the null string has no characters in it, it does exist. For
example, consider this command:
$ echo ""
Here, the 'echo' utility receives a single argument, even though that
argument has no characters in it. In the rest of this Info file, we use
the terms "null string" and "empty string" interchangeably. Now, on to
the quoting rules:
* Quoted items can be concatenated with nonquoted items as well as
with other quoted items. The shell turns everything into one
argument for the command.
* Preceding any single character with a backslash ('\') quotes that
character. The shell removes the backslash and passes the quoted
character on to the command.
* Single quotes protect everything between the opening and closing
quotes. The shell does no interpretation of the quoted text,
passing it on verbatim to the command. It is _impossible_ to embed
a single quote inside single-quoted text. Refer back to *note
Comments:: for an example of what happens if you try.
* Double quotes protect most things between the opening and closing
quotes. The shell does at least variable and command substitution
on the quoted text. Different shells may do additional kinds of
processing on double-quoted text.
Because certain characters within double-quoted text are processed
by the shell, they must be "escaped" within the text. Of note are
the characters '$', '`', '\', and '"', all of which must be
preceded by a backslash within double-quoted text if they are to be
passed on literally to the program. (The leading backslash is
stripped first.) Thus, the example seen in *note Read Terminal:::
awk 'BEGIN { print "Don\47t Panic!" }'
could instead be written this way:
$ awk "BEGIN { print \"Don't Panic!\" }"
-| Don't Panic!
Note that the single quote is not special within double quotes.
* Null strings are removed when they occur as part of a non-null
command-line argument, while explicit null objects are kept. For
example, to specify that the field separator 'FS' should be set to
the null string, use:
awk -F "" 'PROGRAM' FILES # correct
Don't use this:
awk -F"" 'PROGRAM' FILES # wrong!
In the second case, 'awk' attempts to use the text of the program
as the value of 'FS', and the first file name as the text of the
program! This results in syntax errors at best, and confusing
behavior at worst.
Mixing single and double quotes is difficult. You have to resort to
shell quoting tricks, like this:
$ awk 'BEGIN { print "Here is a single quote <'"'"'>" }'
-| Here is a single quote <'>
This program consists of three concatenated quoted strings. The first
and the third are single-quoted, and the second is double-quoted.
This can be "simplified" to:
$ awk 'BEGIN { print "Here is a single quote <'\''>" }'
-| Here is a single quote <'>
Judge for yourself which of these two is the more readable.
Another option is to use double quotes, escaping the embedded,
'awk'-level double quotes:
$ awk "BEGIN { print \"Here is a single quote <'>\" }"
-| Here is a single quote <'>
This option is also painful, because double quotes, backslashes, and
dollar signs are very common in more advanced 'awk' programs.
A third option is to use the octal escape sequence equivalents (*note
Escape Sequences::) for the single- and double-quote characters, like
so:
$ awk 'BEGIN { print "Here is a single quote <\47>" }'
-| Here is a single quote <'>
$ awk 'BEGIN { print "Here is a double quote <\42>" }'
-| Here is a double quote <">
This works nicely, but you should comment clearly what the escapes mean.
A fourth option is to use command-line variable assignment, like
this:
$ awk -v sq="'" 'BEGIN { print "Here is a single quote <" sq ">" }'
-| Here is a single quote <'>
(Here, the two string constants and the value of 'sq' are
concatenated into a single string that is printed by 'print'.)
If you really need both single and double quotes in your 'awk'
program, it is probably best to move it into a separate file, where the
shell won't be part of the picture and you can say what you mean.
File: gawk.info, Node: DOS Quoting, Up: Quoting
1.1.6.1 Quoting in MS-Windows Batch Files
.........................................
Although this Info file generally only worries about POSIX systems and
the POSIX shell, the following issue arises often enough for many users
that it is worth addressing.
The "shells" on Microsoft Windows systems use the double-quote
character for quoting, and make it difficult or impossible to include an
escaped double-quote character in a command-line script. The following
example, courtesy of Jeroen Brink, shows how to escape the double quotes
from this one liner script that prints all lines in a file surrounded by
double quotes:
{ print "\"" $0 "\"" }
In an MS-Windows command-line the one-liner script above may be passed
as follows:
gawk "{ print \"\042\" $0 \"\042\" }" FILE
In this example the '\042' is the octal code for a double-quote;
'gawk' converts it into a real double-quote for output by the 'print'
statement.
In MS-Windows escaping double-quotes is a little tricky because you
use backslashes to escape double-quotes, but backslashes themselves are
not escaped in the usual way; indeed they are either duplicated or not,
depending upon whether there is a subsequent double-quote. The
MS-Windows rule for double-quoting a string is the following:
1. For each double quote in the original string, let N be the number
of backslash(es) before it, N might be zero. Replace these N
backslash(es) by 2*N+1 backslash(es)
2. Let N be the number of backslash(es) tailing the original string, N
might be zero. Replace these N backslash(es) by 2*N backslash(es)
3. Surround the resulting string by double-quotes.
So to double-quote the one-liner script '{ print "\"" $0 "\"" }' from
the previous example you would do it this way:
gawk "{ print \"\\\"\" $0 \"\\\"\" }" FILE
However, the use of '\042' instead of '\\\"' is also possible and easier
to read, because backslashes that are not followed by a double-quote
don't need duplication.
File: gawk.info, Node: Sample Data Files, Next: Very Simple, Prev: Running gawk, Up: Getting Started
1.2 Data files for the Examples
===============================
Many of the examples in this Info file take their input from two sample
data files. The first, 'mail-list', represents a list of peoples' names
together with their email addresses and information about those people.
The second data file, called 'inventory-shipped', contains information
about monthly shipments. In both files, each line is considered to be
one "record".
In 'mail-list', each record contains the name of a person, his/her
phone number, his/her email address, and a code for his/her relationship
with the author of the list. The columns are aligned using spaces. An
'A' in the last column means that the person is an acquaintance. An 'F'
in the last column means that the person is a friend. An 'R' means that
the person is a relative:
Amelia 555-5553 amelia.zodiacusque AT gmail.com F
Anthony 555-3412 anthony.asserturo AT hotmail.com A
Becky 555-7685 becky.algebrarum AT gmail.com A
Bill 555-1675 bill.drowning AT hotmail.com A
Broderick 555-0542 broderick.aliquotiens AT yahoo.com R
Camilla 555-2912 camilla.infusarum AT skynet.be R
Fabius 555-1234 fabius.undevicesimus AT ucb.edu F
Julie 555-6699 julie.perscrutabor AT skeeve.com F
Martin 555-6480 martin.codicibus AT hotmail.com A
Samuel 555-3430 samuel.lanceolis AT shu.edu A
Jean-Paul 555-2127 jeanpaul.campanorum AT nyu.edu R
The data file 'inventory-shipped' represents information about
shipments during the year. Each record contains the month, the number
of green crates shipped, the number of red boxes shipped, the number of
orange bags shipped, and the number of blue packages shipped,
respectively. There are 16 entries, covering the 12 months of last year
and the first four months of the current year. An empty line separates
the data for the two years:
Jan 13 25 15 115
Feb 15 32 24 226
Mar 15 24 34 228
Apr 31 52 63 420
May 16 34 29 208
Jun 31 42 75 492
Jul 24 34 67 436
Aug 15 34 47 316
Sep 13 55 37 277
Oct 29 54 68 525
Nov 20 87 82 577
Dec 17 35 61 401
Jan 21 36 64 620
Feb 26 58 80 652
Mar 24 75 70 495
Apr 21 70 74 514
The sample files are included in the 'gawk' distribution, in the
directory 'awklib/eg/data'.
File: gawk.info, Node: Very Simple, Next: Two Rules, Prev: Sample Data Files, Up: Getting Started
1.3 Some Simple Examples
========================
The following command runs a simple 'awk' program that searches the
input file 'mail-list' for the character string 'li' (a grouping of
characters is usually called a "string"; the term "string" is based on
similar usage in English, such as "a string of pearls" or "a string of
cars in a train"):
awk '/li/ { print $0 }' mail-list
When lines containing 'li' are found, they are printed because
'print $0' means print the current line. (Just 'print' by itself means
the same thing, so we could have written that instead.)
You will notice that slashes ('/') surround the string 'li' in the
'awk' program. The slashes indicate that 'li' is the pattern to search
for. This type of pattern is called a "regular expression", which is
covered in more detail later (*note Regexp::). The pattern is allowed
to match parts of words. There are single quotes around the 'awk'
program so that the shell won't interpret any of it as special shell
characters.
Here is what this program prints:
$ awk '/li/ { print $0 }' mail-list
-| Amelia 555-5553 amelia.zodiacusque AT gmail.com F
-| Broderick 555-0542 broderick.aliquotiens AT yahoo.com R
-| Julie 555-6699 julie.perscrutabor AT skeeve.com F
-| Samuel 555-3430 samuel.lanceolis AT shu.edu A
In an 'awk' rule, either the pattern or the action can be omitted,
but not both. If the pattern is omitted, then the action is performed
for _every_ input line. If the action is omitted, the default action is
to print all lines that match the pattern.
Thus, we could leave out the action (the 'print' statement and the
braces) in the previous example and the result would be the same: 'awk'
prints all lines matching the pattern 'li'. By comparison, omitting the
'print' statement but retaining the braces makes an empty action that
does nothing (i.e., no lines are printed).
Many practical 'awk' programs are just a line or two long. Following
is a collection of useful, short programs to get you started. Some of
these programs contain constructs that haven't been covered yet. (The
description of the program will give you a good idea of what is going
on, but you'll need to read the rest of the Info file to become an 'awk'
expert!) Most of the examples use a data file named 'data'. This is
just a placeholder; if you use these programs yourself, substitute your
own file names for 'data'. For future reference, note that there is
often more than one way to do things in 'awk'. At some point, you may
want to look back at these examples and see if you can come up with
different ways to do the same things shown here:
* Print every line that is longer than 80 characters:
awk 'length($0) > 80' data
The sole rule has a relational expression as its pattern and has no
action--so it uses the default action, printing the record.
* Print the length of the longest input line:
awk '{ if (length($0) > max) max = length($0) }
END { print max }' data
The code associated with 'END' executes after all input has been
read; it's the other side of the coin to 'BEGIN'.
* Print the length of the longest line in 'data':
expand data | awk '{ if (x < length($0)) x = length($0) }
END { print "maximum line length is " x }'
This example differs slightly from the previous one: the input is
processed by the 'expand' utility to change TABs into spaces, so
the widths compared are actually the right-margin columns, as
opposed to the number of input characters on each line.
* Print every line that has at least one field:
awk 'NF > 0' data
This is an easy way to delete blank lines from a file (or rather,
to create a new file similar to the old file but from which the
blank lines have been removed).
* Print seven random numbers from 0 to 100, inclusive:
awk 'BEGIN { for (i = 1; i <= 7; i++)
print int(101 * rand()) }'
* Print the total number of bytes used by FILES:
ls -l FILES | awk '{ x += $5 }
END { print "total bytes: " x }'
* Print the total number of kilobytes used by FILES:
ls -l FILES | awk '{ x += $5 }
END { print "total K-bytes:", x / 1024 }'
* Print a sorted list of the login names of all users:
awk -F: '{ print $1 }' /etc/passwd | sort
* Count the lines in a file:
awk 'END { print NR }' data
* Print the even-numbered lines in the data file:
awk 'NR % 2 == 0' data
If you used the expression 'NR % 2 == 1' instead, the program would
print the odd-numbered lines.
File: gawk.info, Node: Two Rules, Next: More Complex, Prev: Very Simple, Up: Getting Started
1.4 An Example with Two Rules
=============================
The 'awk' utility reads the input files one line at a time. For each
line, 'awk' tries the patterns of each rule. If several patterns match,
then several actions execute in the order in which they appear in the
'awk' program. If no patterns match, then no actions run.
After processing all the rules that match the line (and perhaps there
are none), 'awk' reads the next line. (However, *note Next Statement::
and also *note Nextfile Statement::.) This continues until the program
reaches the end of the file. For example, the following 'awk' program
contains two rules:
/12/ { print $0 }
/21/ { print $0 }
The first rule has the string '12' as the pattern and 'print $0' as the
action. The second rule has the string '21' as the pattern and also has
'print $0' as the action. Each rule's action is enclosed in its own
pair of braces.
This program prints every line that contains the string '12' _or_ the
string '21'. If a line contains both strings, it is printed twice, once
by each rule.
This is what happens if we run this program on our two sample data
files, 'mail-list' and 'inventory-shipped':
$ awk '/12/ { print $0 }
> /21/ { print $0 }' mail-list inventory-shipped
-| Anthony 555-3412 anthony.asserturo AT hotmail.com A
-| Camilla 555-2912 camilla.infusarum AT skynet.be R
-| Fabius 555-1234 fabius.undevicesimus AT ucb.edu F
-| Jean-Paul 555-2127 jeanpaul.campanorum AT nyu.edu R
-| Jean-Paul 555-2127 jeanpaul.campanorum AT nyu.edu R
-| Jan 21 36 64 620
-| Apr 21 70 74 514
Note how the line beginning with 'Jean-Paul' in 'mail-list' was printed
twice, once for each rule.
File: gawk.info, Node: More Complex, Next: Statements/Lines, Prev: Two Rules, Up: Getting Started
1.5 A More Complex Example
==========================
Now that we've mastered some simple tasks, let's look at what typical
'awk' programs do. This example shows how 'awk' can be used to
summarize, select, and rearrange the output of another utility. It uses
features that haven't been covered yet, so don't worry if you don't
understand all the details:
ls -l | awk '$6 == "Nov" { sum += $5 }
END { print sum }'
This command prints the total number of bytes in all the files in the
current directory that were last modified in November (of any year).
The 'ls -l' part of this example is a system command that gives you a
listing of the files in a directory, including each file's size and the
date the file was last modified. Its output looks like this:
-rw-r--r-- 1 arnold user 1933 Nov 7 13:05 Makefile
-rw-r--r-- 1 arnold user 10809 Nov 7 13:03 awk.h
-rw-r--r-- 1 arnold user 983 Apr 13 12:14 awk.tab.h
-rw-r--r-- 1 arnold user 31869 Jun 15 12:20 awkgram.y
-rw-r--r-- 1 arnold user 22414 Nov 7 13:03 awk1.c
-rw-r--r-- 1 arnold user 37455 Nov 7 13:03 awk2.c
-rw-r--r-- 1 arnold user 27511 Dec 9 13:07 awk3.c
-rw-r--r-- 1 arnold user 7989 Nov 7 13:03 awk4.c
The first field contains read-write permissions, the second field
contains the number of links to the file, and the third field identifies
the file's owner. The fourth field identifies the file's group. The
fifth field contains the file's size in bytes. The sixth, seventh, and
eighth fields contain the month, day, and time, respectively, that the
file was last modified. Finally, the ninth field contains the file
name.
The '$6 == "Nov"' in our 'awk' program is an expression that tests
whether the sixth field of the output from 'ls -l' matches the string
'Nov'. Each time a line has the string 'Nov' for its sixth field, 'awk'
performs the action 'sum += $5'. This adds the fifth field (the file's
size) to the variable 'sum'. As a result, when 'awk' has finished
reading all the input lines, 'sum' is the total of the sizes of the
files whose lines matched the pattern. (This works because 'awk'
variables are automatically initialized to zero.)
After the last line of output from 'ls' has been processed, the 'END'
rule executes and prints the value of 'sum'. In this example, the value
of 'sum' is 80600.
These more advanced 'awk' techniques are covered in later minor nodes
(*note Action Overview::). Before you can move on to more advanced
'awk' programming, you have to know how 'awk' interprets your input and
displays your output. By manipulating fields and using 'print'
statements, you can produce some very useful and impressive-looking
reports.
File: gawk.info, Node: Statements/Lines, Next: Other Features, Prev: More Complex, Up: Getting Started
1.6 'awk' Statements Versus Lines
=================================
Most often, each line in an 'awk' program is a separate statement or
separate rule, like this:
awk '/12/ { print $0 }
/21/ { print $0 }' mail-list inventory-shipped
However, 'gawk' ignores newlines after any of the following symbols
and keywords:
, { ? : || && do else
A newline at any other point is considered the end of the statement.(1)
If you would like to split a single statement into two lines at a
point where a newline would terminate it, you can "continue" it by
ending the first line with a backslash character ('\'). The backslash
must be the final character on the line in order to be recognized as a
continuation character. A backslash is allowed anywhere in the
statement, even in the middle of a string or regular expression. For
example:
awk '/This regular expression is too long, so continue it\
on the next line/ { print $1 }'
We have generally not used backslash continuation in our sample
programs. 'gawk' places no limit on the length of a line, so backslash
continuation is never strictly necessary; it just makes programs more
readable. For this same reason, as well as for clarity, we have kept
most statements short in the programs presented throughout the Info
file. Backslash continuation is most useful when your 'awk' program is
in a separate source file instead of entered from the command line. You
should also note that many 'awk' implementations are more particular
about where you may use backslash continuation. For example, they may
not allow you to split a string constant using backslash continuation.
Thus, for maximum portability of your 'awk' programs, it is best not to
split your lines in the middle of a regular expression or a string.
CAUTION: _Backslash continuation does not work as described with
the C shell._ It works for 'awk' programs in files and for
one-shot programs, _provided_ you are using a POSIX-compliant
shell, such as the Unix Bourne shell or Bash. But the C shell
behaves differently! There you must use two backslashes in a row,
followed by a newline. Note also that when using the C shell,
_every_ newline in your 'awk' program must be escaped with a
backslash. To illustrate:
% awk 'BEGIN { \
? print \\
? "hello, world" \
? }'
-| hello, world
Here, the '%' and '?' are the C shell's primary and secondary
prompts, analogous to the standard shell's '$' and '>'.
Compare the previous example to how it is done with a
POSIX-compliant shell:
$ awk 'BEGIN {
> print \
> "hello, world"
> }'
-| hello, world
'awk' is a line-oriented language. Each rule's action has to begin
on the same line as the pattern. To have the pattern and action on
separate lines, you _must_ use backslash continuation; there is no other
option.
Another thing to keep in mind is that backslash continuation and
comments do not mix. As soon as 'awk' sees the '#' that starts a
comment, it ignores _everything_ on the rest of the line. For example:
$ gawk 'BEGIN { print "dont panic" # a friendly \
> BEGIN rule
> }'
error-> gawk: cmd. line:2: BEGIN rule
error-> gawk: cmd. line:2: ^ syntax error
In this case, it looks like the backslash would continue the comment
onto the next line. However, the backslash-newline combination is never
even noticed because it is "hidden" inside the comment. Thus, the
'BEGIN' is noted as a syntax error.
When 'awk' statements within one rule are short, you might want to
put more than one of them on a line. This is accomplished by separating
the statements with a semicolon (';'). This also applies to the rules
themselves. Thus, the program shown at the start of this minor node
could also be written this way:
/12/ { print $0 } ; /21/ { print $0 }
NOTE: The requirement that states that rules on the same line must
be separated with a semicolon was not in the original 'awk'
language; it was added for consistency with the treatment of
statements within an action.
---------- Footnotes ----------
(1) The '?' and ':' referred to here is the three-operand conditional
expression described in *note Conditional Exp::. Splitting lines after
'?' and ':' is a minor 'gawk' extension; if '--posix' is specified
(*note Options::), then this extension is disabled.
File: gawk.info, Node: Other Features, Next: When, Prev: Statements/Lines, Up: Getting Started
1.7 Other Features of 'awk'
===========================
The 'awk' language provides a number of predefined, or "built-in",
variables that your programs can use to get information from 'awk'.
There are other variables your program can set as well to control how
'awk' processes your data.
In addition, 'awk' provides a number of built-in functions for doing
common computational and string-related operations. 'gawk' provides
built-in functions for working with timestamps, performing bit
manipulation, for runtime string translation (internationalization),
determining the type of a variable, and array sorting.
As we develop our presentation of the 'awk' language, we will
introduce most of the variables and many of the functions. They are
described systematically in *note Built-in Variables:: and in *note
Built-in::.
File: gawk.info, Node: When, Next: Intro Summary, Prev: Other Features, Up: Getting Started
1.8 When to Use 'awk'
=====================
Now that you've seen some of what 'awk' can do, you might wonder how
'awk' could be useful for you. By using utility programs, advanced
patterns, field separators, arithmetic statements, and other selection
criteria, you can produce much more complex output. The 'awk' language
is very useful for producing reports from large amounts of raw data,
such as summarizing information from the output of other utility
programs like 'ls'. (*Note More Complex::.)
Programs written with 'awk' are usually much smaller than they would
be in other languages. This makes 'awk' programs easy to compose and
use. Often, 'awk' programs can be quickly composed at your keyboard,
used once, and thrown away. Because 'awk' programs are interpreted, you
can avoid the (usually lengthy) compilation part of the typical
edit-compile-test-debug cycle of software development.
Complex programs have been written in 'awk', including a complete
retargetable assembler for eight-bit microprocessors (*note Glossary::,
for more information), and a microcode assembler for a special-purpose
Prolog computer. The original 'awk''s capabilities were strained by
tasks of such complexity, but modern versions are more capable.
If you find yourself writing 'awk' scripts of more than, say, a few
hundred lines, you might consider using a different programming
language. The shell is good at string and pattern matching; in
addition, it allows powerful use of the system utilities. Python offers
a nice balance between high-level ease of programming and access to
system facilities.(1)
---------- Footnotes ----------
(1) Other popular scripting languages include Ruby and Perl.
File: gawk.info, Node: Intro Summary, Prev: When, Up: Getting Started
1.9 Summary
===========
* Programs in 'awk' consist of PATTERN-ACTION pairs.
* An ACTION without a PATTERN always runs. The default ACTION for a
pattern without one is '{ print $0 }'.
* Use either 'awk 'PROGRAM' FILES' or 'awk -f PROGRAM-FILE FILES' to
run 'awk'.
* You may use the special '#!' header line to create 'awk' programs
that are directly executable.
* Comments in 'awk' programs start with '#' and continue to the end
of the same line.
* Be aware of quoting issues when writing 'awk' programs as part of a
larger shell script (or MS-Windows batch file).
* You may use backslash continuation to continue a source line.
Lines are automatically continued after a comma, open brace,
question mark, colon, '||', '&&', 'do', and 'else'.
File: gawk.info, Node: Invoking Gawk, Next: Regexp, Prev: Getting Started, Up: Top
2 Running 'awk' and 'gawk'
**************************
This major node covers how to run 'awk', both POSIX-standard and
'gawk'-specific command-line options, and what 'awk' and 'gawk' do with
nonoption arguments. It then proceeds to cover how 'gawk' searches for
source files, reading standard input along with other files, 'gawk''s
environment variables, 'gawk''s exit status, using include files, and
obsolete and undocumented options and/or features.
Many of the options and features described here are discussed in more
detail later in the Info file; feel free to skip over things in this
major node that don't interest you right now.
* Menu:
* Command Line:: How to run 'awk'.
* Options:: Command-line options and their meanings.
* Other Arguments:: Input file names and variable assignments.
* Naming Standard Input:: How to specify standard input with other
files.
* Environment Variables:: The environment variables 'gawk' uses.
* Exit Status:: 'gawk''s exit status.
* Include Files:: Including other files into your program.
* Loading Shared Libraries:: Loading shared libraries into your program.
* Obsolete:: Obsolete Options and/or features.
* Undocumented:: Undocumented Options and Features.
* Invoking Summary:: Invocation summary.
File: gawk.info, Node: Command Line, Next: Options, Up: Invoking Gawk
2.1 Invoking 'awk'
==================
There are two ways to run 'awk'--with an explicit program or with one or
more program files. Here are templates for both of them; items enclosed
in [...] in these templates are optional:
'awk' [OPTIONS] '-f' PROGFILE ['--'] FILE ...
'awk' [OPTIONS] ['--'] ''PROGRAM'' FILE ...
In addition to traditional one-letter POSIX-style options, 'gawk'
also supports GNU long options.
It is possible to invoke 'awk' with an empty program:
awk '' datafile1 datafile2
Doing so makes little sense, though; 'awk' exits silently when given an
empty program. (d.c.) If '--lint' has been specified on the command
line, 'gawk' issues a warning that the program is empty.
File: gawk.info, Node: Options, Next: Other Arguments, Prev: Command Line, Up: Invoking Gawk
2.2 Command-Line Options
========================
Options begin with a dash and consist of a single character. GNU-style
long options consist of two dashes and a keyword. The keyword can be
abbreviated, as long as the abbreviation allows the option to be
uniquely identified. If the option takes an argument, either the
keyword is immediately followed by an equals sign ('=') and the
argument's value, or the keyword and the argument's value are separated
by whitespace. If a particular option with a value is given more than
once, it is the last value that counts.
Each long option for 'gawk' has a corresponding POSIX-style short
option. The long and short options are interchangeable in all contexts.
The following list describes options mandated by the POSIX standard:
'-F FS'
'--field-separator FS'
Set the 'FS' variable to FS (*note Field Separators::).
'-f SOURCE-FILE'
'--file SOURCE-FILE'
Read the 'awk' program source from SOURCE-FILE instead of in the
first nonoption argument. This option may be given multiple times;
the 'awk' program consists of the concatenation of the contents of
each specified SOURCE-FILE.
'-v VAR=VAL'
'--assign VAR=VAL'
Set the variable VAR to the value VAL _before_ execution of the
program begins. Such variable values are available inside the
'BEGIN' rule (*note Other Arguments::).
The '-v' option can only set one variable, but it can be used more
than once, setting another variable each time, like this: 'awk
-v foo=1 -v bar=2 ...'.
CAUTION: Using '-v' to set the values of the built-in
variables may lead to surprising results. 'awk' will reset
the values of those variables as it needs to, possibly
ignoring any initial value you may have given.
'-W GAWK-OPT'
Provide an implementation-specific option. This is the POSIX
convention for providing implementation-specific options. These
options also have corresponding GNU-style long options. Note that
the long options may be abbreviated, as long as the abbreviations
remain unique. The full list of 'gawk'-specific options is
provided next.
'--'
Signal the end of the command-line options. The following
arguments are not treated as options even if they begin with '-'.
This interpretation of '--' follows the POSIX argument parsing
conventions.
This is useful if you have file names that start with '-', or in
shell scripts, if you have file names that will be specified by the
user that could start with '-'. It is also useful for passing
options on to the 'awk' program; see *note Getopt Function::.
The following list describes 'gawk'-specific options:
'-b'
'--characters-as-bytes'
Cause 'gawk' to treat all input data as single-byte characters. In
addition, all output written with 'print' or 'printf' is treated as
single-byte characters.
Normally, 'gawk' follows the POSIX standard and attempts to process
its input data according to the current locale (*note Locales::).
This can often involve converting multibyte characters into wide
characters (internally), and can lead to problems or confusion if
the input data does not contain valid multibyte characters. This
option is an easy way to tell 'gawk', "Hands off my data!"
'-c'
'--traditional'
Specify "compatibility mode", in which the GNU extensions to the
'awk' language are disabled, so that 'gawk' behaves just like BWK
'awk'. *Note POSIX/GNU::, which summarizes the extensions. Also
see *note Compatibility Mode::.
'-C'
'--copyright'
Print the short version of the General Public License and then
exit.
'-d'[FILE]
'--dump-variables'['='FILE]
Print a sorted list of global variables, their types, and final
values to FILE. If no FILE is provided, print this list to a file
named 'awkvars.out' in the current directory. No space is allowed
between the '-d' and FILE, if FILE is supplied.
Having a list of all global variables is a good way to look for
typographical errors in your programs. You would also use this
option if you have a large program with a lot of functions, and you
want to be sure that your functions don't inadvertently use global
variables that you meant to be local. (This is a particularly easy
mistake to make with simple variable names like 'i', 'j', etc.)
'-D'[FILE]
'--debug'['='FILE]
Enable debugging of 'awk' programs (*note Debugging::). By
default, the debugger reads commands interactively from the
keyboard (standard input). The optional FILE argument allows you
to specify a file with a list of commands for the debugger to
execute noninteractively. No space is allowed between the '-D' and
FILE, if FILE is supplied.
'-e' PROGRAM-TEXT
'--source' PROGRAM-TEXT
Provide program source code in the PROGRAM-TEXT. This option
allows you to mix source code in files with source code that you
enter on the command line. This is particularly useful when you
have library functions that you want to use from your command-line
programs (*note AWKPATH Variable::).
Note that 'gawk' treats each string as if it ended with a newline
character (even if it doesn't). This makes building the total
program easier.
CAUTION: At the moment, there is no requirement that each
PROGRAM-TEXT be a full syntactic unit. I.e., the following
currently works:
$ gawk -e 'BEGIN { a = 5 ;' -e 'print a }'
-| 5
However, this could change in the future, so it's not a good
idea to rely upon this feature.
'-E' FILE
'--exec' FILE
Similar to '-f', read 'awk' program text from FILE. There are two
differences from '-f':
* This option terminates option processing; anything else on the
command line is passed on directly to the 'awk' program.
* Command-line variable assignments of the form 'VAR=VALUE' are
disallowed.
This option is particularly necessary for World Wide Web CGI
applications that pass arguments through the URL; using this option
prevents a malicious (or other) user from passing in options,
assignments, or 'awk' source code (via '-e') to the CGI
application.(1) This option should be used with '#!' scripts
(*note Executable Scripts::), like so:
#! /usr/local/bin/gawk -E
AWK PROGRAM HERE ...
'-g'
'--gen-pot'
Analyze the source program and generate a GNU 'gettext' portable
object template file on standard output for all string constants
that have been marked for translation. *Note
Internationalization::, for information about this option.
'-h'
'--help'
Print a "usage" message summarizing the short- and long-style
options that 'gawk' accepts and then exit.
'-i' SOURCE-FILE
'--include' SOURCE-FILE
Read an 'awk' source library from SOURCE-FILE. This option is
completely equivalent to using the '@include' directive inside your
program. It is very similar to the '-f' option, but there are two
important differences. First, when '-i' is used, the program
source is not loaded if it has been previously loaded, whereas with
'-f', 'gawk' always loads the file. Second, because this option is
intended to be used with code libraries, 'gawk' does not recognize
such files as constituting main program input. Thus, after
processing an '-i' argument, 'gawk' still expects to find the main
source code via the '-f' option or on the command line.
'-l' EXT
'--load' EXT
Load a dynamic extension named EXT. Extensions are stored as
system shared libraries. This option searches for the library
using the 'AWKLIBPATH' environment variable. The correct library
suffix for your platform will be supplied by default, so it need
not be specified in the extension name. The extension
initialization routine should be named 'dl_load()'. An alternative
is to use the '@load' keyword inside the program to load a shared
library. This advanced feature is described in detail in *note
Dynamic Extensions::.
'-L'[VALUE]
'--lint'['='VALUE]
Warn about constructs that are dubious or nonportable to other
'awk' implementations. No space is allowed between the '-L' and
VALUE, if VALUE is supplied. Some warnings are issued when 'gawk'
first reads your program. Others are issued at runtime, as your
program executes. With an optional argument of 'fatal', lint
warnings become fatal errors. This may be drastic, but its use
will certainly encourage the development of cleaner 'awk' programs.
With an optional argument of 'invalid', only warnings about things
that are actually invalid are issued. (This is not fully
implemented yet.)
Some warnings are only printed once, even if the dubious constructs
they warn about occur multiple times in your 'awk' program. Thus,
when eliminating problems pointed out by '--lint', you should take
care to search for all occurrences of each inappropriate construct.
As 'awk' programs are usually short, doing so is not burdensome.
'-M'
'--bignum'
Select arbitrary-precision arithmetic on numbers. This option has
no effect if 'gawk' is not compiled to use the GNU MPFR and MP
libraries (*note Arbitrary Precision Arithmetic::).
'-n'
'--non-decimal-data'
Enable automatic interpretation of octal and hexadecimal values in
input data (*note Nondecimal Data::).
CAUTION: This option can severely break old programs. Use
with care. Also note that this option may disappear in a
future version of 'gawk'.
'-N'
'--use-lc-numeric'
Force the use of the locale's decimal point character when parsing
numeric input data (*note Locales::).
'-o'[FILE]
'--pretty-print'['='FILE]
Enable pretty-printing of 'awk' programs. Implies '--no-optimize'.
By default, the output program is created in a file named
'awkprof.out' (*note Profiling::). The optional FILE argument
allows you to specify a different file name for the output. No
space is allowed between the '-o' and FILE, if FILE is supplied.
NOTE: In the past, this option would also execute your
program. This is no longer the case.
'-O'
'--optimize'
Enable 'gawk''s default optimizations on the internal
representation of the program. At the moment, this includes simple
constant folding and tail recursion elimination in function calls.
These optimizations are enabled by default. This option remains
primarily for backwards compatibility. However, it may be used to
cancel the effect of an earlier '-s' option (see later in this
list).
'-p'[FILE]
'--profile'['='FILE]
Enable profiling of 'awk' programs (*note Profiling::). Implies
'--no-optimize'. By default, profiles are created in a file named
'awkprof.out'. The optional FILE argument allows you to specify a
different file name for the profile file. No space is allowed
between the '-p' and FILE, if FILE is supplied.
The profile contains execution counts for each statement in the
program in the left margin, and function call counts for each
function.
'-P'
'--posix'
Operate in strict POSIX mode. This disables all 'gawk' extensions
(just like '--traditional') and disables all extensions not allowed
by POSIX. *Note Common Extensions:: for a summary of the extensions
in 'gawk' that are disabled by this option. Also, the following
additional restrictions apply:
* Newlines are not allowed after '?' or ':' (*note Conditional
Exp::).
* Specifying '-Ft' on the command line does not set the value of
'FS' to be a single TAB character (*note Field Separators::).
* The locale's decimal point character is used for parsing input
data (*note Locales::).
If you supply both '--traditional' and '--posix' on the command
line, '--posix' takes precedence. 'gawk' issues a warning if both
options are supplied.
'-r'
'--re-interval'
Allow interval expressions (*note Regexp Operators::) in regexps.
This is now 'gawk''s default behavior. Nevertheless, this option
remains (both for backward compatibility and for use in combination
with '--traditional').
'-s'
'--no-optimize'
Disable 'gawk''s default optimizations on the internal
representation of the program.
'-S'
'--sandbox'
Disable the 'system()' function, input redirections with 'getline',
output redirections with 'print' and 'printf', and dynamic
extensions. This is particularly useful when you want to run 'awk'
scripts from questionable sources and need to make sure the scripts
can't access your system (other than the specified input data
file).
'-t'
'--lint-old'
Warn about constructs that are not available in the original
version of 'awk' from Version 7 Unix (*note V7/SVR3.1::).
'-V'
'--version'
Print version information for this particular copy of 'gawk'. This
allows you to determine if your copy of 'gawk' is up to date with
respect to whatever the Free Software Foundation is currently
distributing. It is also useful for bug reports (*note Bugs::).
As long as program text has been supplied, any other options are
flagged as invalid with a warning message but are otherwise ignored.
In compatibility mode, as a special case, if the value of FS supplied
to the '-F' option is 't', then 'FS' is set to the TAB character
('"\t"'). This is true only for '--traditional' and not for '--posix'
(*note Field Separators::).
The '-f' option may be used more than once on the command line. If
it is, 'awk' reads its program source from all of the named files, as if
they had been concatenated together into one big file. This is useful
for creating libraries of 'awk' functions. These functions can be
written once and then retrieved from a standard place, instead of having
to be included in each individual program. The '-i' option is similar
in this regard. (As mentioned in *note Definition Syntax::, function
names must be unique.)
With standard 'awk', library functions can still be used, even if the
program is entered at the keyboard, by specifying '-f /dev/tty'. After
typing your program, type 'Ctrl-d' (the end-of-file character) to
terminate it. (You may also use '-f -' to read program source from the
standard input, but then you will not be able to also use the standard
input as a source of data.)
Because it is clumsy using the standard 'awk' mechanisms to mix
source file and command-line 'awk' programs, 'gawk' provides the '-e'
option. This does not require you to preempt the standard input for
your source code; it allows you to easily mix command-line and library
source code (*note AWKPATH Variable::). As with '-f', the '-e' and '-i'
options may also be used multiple times on the command line.
If no '-f' or '-e' option is specified, then 'gawk' uses the first
nonoption command-line argument as the text of the program source code.
If the environment variable 'POSIXLY_CORRECT' exists, then 'gawk'
behaves in strict POSIX mode, exactly as if you had supplied '--posix'.
Many GNU programs look for this environment variable to suppress
extensions that conflict with POSIX, but 'gawk' behaves differently: it
suppresses all extensions, even those that do not conflict with POSIX,
and behaves in strict POSIX mode. If '--lint' is supplied on the
command line and 'gawk' turns on POSIX mode because of
'POSIXLY_CORRECT', then it issues a warning message indicating that
POSIX mode is in effect. You would typically set this variable in your
shell's startup file. For a Bourne-compatible shell (such as Bash), you
would add these lines to the '.profile' file in your home directory:
POSIXLY_CORRECT=true
export POSIXLY_CORRECT
For a C shell-compatible shell,(2) you would add this line to the
'.login' file in your home directory:
setenv POSIXLY_CORRECT true
Having 'POSIXLY_CORRECT' set is not recommended for daily use, but it
is good for testing the portability of your programs to other
environments.
---------- Footnotes ----------
(1) For more detail, please see Section 4.4 of RFC 3875
(http://www.ietf.org/rfc/rfc3875). Also see the explanatory note sent
to the 'gawk' bug mailing list
(https://lists.gnu.org/archive/html/bug-gawk/2014-11/msg00022.html).
(2) Not recommended.
File: gawk.info, Node: Other Arguments, Next: Naming Standard Input, Prev: Options, Up: Invoking Gawk
2.3 Other Command-Line Arguments
================================
Any additional arguments on the command line are normally treated as
input files to be processed in the order specified. However, an
argument that has the form 'VAR=VALUE', assigns the value VALUE to the
variable VAR--it does not specify a file at all. (See *note Assignment
Options::.) In the following example, COUNT=1 is a variable assignment,
not a file name:
awk -f program.awk file1 count=1 file2
All the command-line arguments are made available to your 'awk'
program in the 'ARGV' array (*note Built-in Variables::). Command-line
options and the program text (if present) are omitted from 'ARGV'. All
other arguments, including variable assignments, are included. As each
element of 'ARGV' is processed, 'gawk' sets 'ARGIND' to the index in
'ARGV' of the current element.
Changing 'ARGC' and 'ARGV' in your 'awk' program lets you control how
'awk' processes the input files; this is described in more detail in
*note ARGC and ARGV::.
The distinction between file name arguments and variable-assignment
arguments is made when 'awk' is about to open the next input file. At
that point in execution, it checks the file name to see whether it is
really a variable assignment; if so, 'awk' sets the variable instead of
reading a file.
Therefore, the variables actually receive the given values after all
previously specified files have been read. In particular, the values of
variables assigned in this fashion are _not_ available inside a 'BEGIN'
rule (*note BEGIN/END::), because such rules are run before 'awk' begins
scanning the argument list.
The variable values given on the command line are processed for
escape sequences (*note Escape Sequences::). (d.c.)
In some very early implementations of 'awk', when a variable
assignment occurred before any file names, the assignment would happen
_before_ the 'BEGIN' rule was executed. 'awk''s behavior was thus
inconsistent; some command-line assignments were available inside the
'BEGIN' rule, while others were not. Unfortunately, some applications
came to depend upon this "feature." When 'awk' was changed to be more
consistent, the '-v' option was added to accommodate applications that
depended upon the old behavior.
The variable assignment feature is most useful for assigning to
variables such as 'RS', 'OFS', and 'ORS', which control input and output
formats, before scanning the data files. It is also useful for
controlling state if multiple passes are needed over a data file. For
example:
awk 'pass == 1 { PASS 1 STUFF }
pass == 2 { PASS 2 STUFF }' pass=1 mydata pass=2 mydata
Given the variable assignment feature, the '-F' option for setting
the value of 'FS' is not strictly necessary. It remains for historical
compatibility.
File: gawk.info, Node: Naming Standard Input, Next: Environment Variables, Prev: Other Arguments, Up: Invoking Gawk
2.4 Naming Standard Input
=========================
Often, you may wish to read standard input together with other files.
For example, you may wish to read one file, read standard input coming
from a pipe, and then read another file.
The way to name the standard input, with all versions of 'awk', is to
use a single, standalone minus sign or dash, '-'. For example:
SOME_COMMAND | awk -f myprog.awk file1 - file2
Here, 'awk' first reads 'file1', then it reads the output of
SOME_COMMAND, and finally it reads 'file2'.
You may also use '"-"' to name standard input when reading files with
'getline' (*note Getline/File::).
In addition, 'gawk' allows you to specify the special file name
'/dev/stdin', both on the command line and with 'getline'. Some other
versions of 'awk' also support this, but it is not standard. (Some
operating systems provide a '/dev/stdin' file in the filesystem;
however, 'gawk' always processes this file name itself.)
File: gawk.info, Node: Environment Variables, Next: Exit Status, Prev: Naming Standard Input, Up: Invoking Gawk
2.5 The Environment Variables 'gawk' Uses
=========================================
A number of environment variables influence how 'gawk' behaves.
* Menu:
* AWKPATH Variable:: Searching directories for 'awk'
programs.
* AWKLIBPATH Variable:: Searching directories for 'awk' shared
libraries.
* Other Environment Variables:: The environment variables.
File: gawk.info, Node: AWKPATH Variable, Next: AWKLIBPATH Variable, Up: Environment Variables
2.5.1 The 'AWKPATH' Environment Variable
----------------------------------------
The previous minor node described how 'awk' program files can be named
on the command line with the '-f' option. In most 'awk'
implementations, you must supply a precise pathname for each program
file, unless the file is in the current directory. But with 'gawk', if
the file name supplied to the '-f' or '-i' options does not contain a
directory separator '/', then 'gawk' searches a list of directories
(called the "search path") one by one, looking for a file with the
specified name.
The search path is a string consisting of directory names separated
by colons.(1) 'gawk' gets its search path from the 'AWKPATH'
environment variable. If that variable does not exist, or if it has an
empty value, 'gawk' uses a default path (described shortly).
The search path feature is particularly helpful for building
libraries of useful 'awk' functions. The library files can be placed in
a standard directory in the default path and then specified on the
command line with a short file name. Otherwise, you would have to type
the full file name for each file.
By using the '-i' or '-f' options, your command-line 'awk' programs
can use facilities in 'awk' library files (*note Library Functions::).
Path searching is not done if 'gawk' is in compatibility mode. This is
true for both '--traditional' and '--posix'. *Note Options::.
If the source code file is not found after the initial search, the
path is searched again after adding the suffix '.awk' to the file name.
'gawk''s path search mechanism is similar to the shell's. (See 'The
Bourne-Again SHell manual' (https://www.gnu.org/software/bash/manual/).)
It treats a null entry in the path as indicating the current directory.
(A null entry is indicated by starting or ending the path with a colon
or by placing two colons next to each other ['::'].)
NOTE: To include the current directory in the path, either place
'.' as an entry in the path or write a null entry in the path.
Different past versions of 'gawk' would also look explicitly in the
current directory, either before or after the path search. As of
version 4.1.2, this no longer happens; if you wish to look in the
current directory, you must include '.' either as a separate entry
or as a null entry in the search path.
The default value for 'AWKPATH' is '.:/usr/local/share/awk'.(2)
Since '.' is included at the beginning, 'gawk' searches first in the
current directory and then in '/usr/local/share/awk'. In practice, this
means that you will rarely need to change the value of 'AWKPATH'.
*Note Shell Startup Files::, for information on functions that help
to manipulate the 'AWKPATH' variable.
'gawk' places the value of the search path that it used into
'ENVIRON["AWKPATH"]'. This provides access to the actual search path
value from within an 'awk' program.
Although you can change 'ENVIRON["AWKPATH"]' within your 'awk'
program, this has no effect on the running program's behavior. This
makes sense: the 'AWKPATH' environment variable is used to find the
program source files. Once your program is running, all the files have
been found, and 'gawk' no longer needs to use 'AWKPATH'.
---------- Footnotes ----------
(1) Semicolons on MS-Windows.
(2) Your version of 'gawk' may use a different directory; it will
depend upon how 'gawk' was built and installed. The actual directory is
the value of '$(datadir)' generated when 'gawk' was configured. You
probably don't need to worry about this, though.
File: gawk.info, Node: AWKLIBPATH Variable, Next: Other Environment Variables, Prev: AWKPATH Variable, Up: Environment Variables
2.5.2 The 'AWKLIBPATH' Environment Variable
-------------------------------------------
The 'AWKLIBPATH' environment variable is similar to the 'AWKPATH'
variable, but it is used to search for loadable extensions (stored as
system shared libraries) specified with the '-l' option rather than for
source files. If the extension is not found, the path is searched again
after adding the appropriate shared library suffix for the platform.
For example, on GNU/Linux systems, the suffix '.so' is used. The search
path specified is also used for extensions loaded via the '@load'
keyword (*note Loading Shared Libraries::).
If 'AWKLIBPATH' does not exist in the environment, or if it has an
empty value, 'gawk' uses a default path; this is typically
'/usr/local/lib/gawk', although it can vary depending upon how 'gawk'
was built.
*Note Shell Startup Files::, for information on functions that help
to manipulate the 'AWKLIBPATH' variable.
'gawk' places the value of the search path that it used into
'ENVIRON["AWKLIBPATH"]'. This provides access to the actual search path
value from within an 'awk' program.
Although you can change 'ENVIRON["AWKLIBPATH"]' within your 'awk'
program, this has no effect on the running program's behavior. This
makes sense: the 'AWKLIBPATH' environment variable is used to find any
requested extensions, and they are loaded before the program starts to
run. Once your program is running, all the extensions have been found,
and 'gawk' no longer needs to use 'AWKLIBPATH'.
File: gawk.info, Node: Other Environment Variables, Prev: AWKLIBPATH Variable, Up: Environment Variables
2.5.3 Other Environment Variables
---------------------------------
A number of other environment variables affect 'gawk''s behavior, but
they are more specialized. Those in the following list are meant to be
used by regular users:
'GAWK_MSEC_SLEEP'
Specifies the interval between connection retries, in milliseconds.
On systems that do not support the 'usleep()' system call, the
value is rounded up to an integral number of seconds.
'GAWK_READ_TIMEOUT'
Specifies the time, in milliseconds, for 'gawk' to wait for input
before returning with an error. *Note Read Timeout::.
'GAWK_SOCK_RETRIES'
Controls the number of times 'gawk' attempts to retry a two-way
TCP/IP (socket) connection before giving up. *Note TCP/IP
Networking::. Note that when nonfatal I/O is enabled (*note
Nonfatal::), 'gawk' only tries to open a TCP/IP socket once.
'POSIXLY_CORRECT'
Causes 'gawk' to switch to POSIX-compatibility mode, disabling all
traditional and GNU extensions. *Note Options::.
The environment variables in the following list are meant for use by
the 'gawk' developers for testing and tuning. They are subject to
change. The variables are:
'AWKBUFSIZE'
This variable only affects 'gawk' on POSIX-compliant systems. With
a value of 'exact', 'gawk' uses the size of each input file as the
size of the memory buffer to allocate for I/O. Otherwise, the value
should be a number, and 'gawk' uses that number as the size of the
buffer to allocate. (When this variable is not set, 'gawk' uses
the smaller of the file's size and the "default" blocksize, which
is usually the filesystem's I/O blocksize.)
'AWK_HASH'
If this variable exists with a value of 'gst', 'gawk' switches to
using the hash function from GNU Smalltalk for managing arrays.
This function may be marginally faster than the standard function.
'AWKREADFUNC'
If this variable exists, 'gawk' switches to reading source files
one line at a time, instead of reading in blocks. This exists for
debugging problems on filesystems on non-POSIX operating systems
where I/O is performed in records, not in blocks.
'GAWK_MSG_SRC'
If this variable exists, 'gawk' includes the file name and line
number within the 'gawk' source code from which warning and/or
fatal messages are generated. Its purpose is to help isolate the
source of a message, as there are multiple places that produce the
same warning or error message.
'GAWK_LOCALE_DIR'
Specifies the location of compiled message object files for 'gawk'
itself. This is passed to the 'bindtextdomain()' function when
'gawk' starts up.
'GAWK_NO_DFA'
If this variable exists, 'gawk' does not use the DFA regexp matcher
for "does it match" kinds of tests. This can cause 'gawk' to be
slower. Its purpose is to help isolate differences between the two
regexp matchers that 'gawk' uses internally. (There aren't
supposed to be differences, but occasionally theory and practice
don't coordinate with each other.)
'GAWK_STACKSIZE'
This specifies the amount by which 'gawk' should grow its internal
evaluation stack, when needed.
'INT_CHAIN_MAX'
This specifies intended maximum number of items 'gawk' will
maintain on a hash chain for managing arrays indexed by integers.
'STR_CHAIN_MAX'
This specifies intended maximum number of items 'gawk' will
maintain on a hash chain for managing arrays indexed by strings.
'TIDYMEM'
If this variable exists, 'gawk' uses the 'mtrace()' library calls
from the GNU C library to help track down possible memory leaks.
File: gawk.info, Node: Exit Status, Next: Include Files, Prev: Environment Variables, Up: Invoking Gawk
2.6 'gawk''s Exit Status
========================
If the 'exit' statement is used with a value (*note Exit Statement::),
then 'gawk' exits with the numeric value given to it.
Otherwise, if there were no problems during execution, 'gawk' exits
with the value of the C constant 'EXIT_SUCCESS'. This is usually zero.
If an error occurs, 'gawk' exits with the value of the C constant
'EXIT_FAILURE'. This is usually one.
If 'gawk' exits because of a fatal error, the exit status is two. On
non-POSIX systems, this value may be mapped to 'EXIT_FAILURE'.
File: gawk.info, Node: Include Files, Next: Loading Shared Libraries, Prev: Exit Status, Up: Invoking Gawk
2.7 Including Other Files into Your Program
===========================================
This minor node describes a feature that is specific to 'gawk'.
The '@include' keyword can be used to read external 'awk' source
files. This gives you the ability to split large 'awk' source files
into smaller, more manageable pieces, and also lets you reuse common
'awk' code from various 'awk' scripts. In other words, you can group
together 'awk' functions used to carry out specific tasks into external
files. These files can be used just like function libraries, using the
'@include' keyword in conjunction with the 'AWKPATH' environment
variable. Note that source files may also be included using the '-i'
option.
Let's see an example. We'll start with two (trivial) 'awk' scripts,
namely 'test1' and 'test2'. Here is the 'test1' script:
BEGIN {
print "This is script test1."
}
and here is 'test2':
@include "test1"
BEGIN {
print "This is script test2."
}
Running 'gawk' with 'test2' produces the following result:
$ gawk -f test2
-| This is script test1.
-| This is script test2.
'gawk' runs the 'test2' script, which includes 'test1' using the
'@include' keyword. So, to include external 'awk' source files, you
just use '@include' followed by the name of the file to be included,
enclosed in double quotes.
NOTE: Keep in mind that this is a language construct and the file
name cannot be a string variable, but rather just a literal string
constant in double quotes.
The files to be included may be nested; e.g., given a third script,
namely 'test3':
@include "test2"
BEGIN {
print "This is script test3."
}
Running 'gawk' with the 'test3' script produces the following results:
$ gawk -f test3
-| This is script test1.
-| This is script test2.
-| This is script test3.
The file name can, of course, be a pathname. For example:
@include "../io_funcs"
and:
@include "/usr/awklib/network"
are both valid. The 'AWKPATH' environment variable can be of great
value when using '@include'. The same rules for the use of the
'AWKPATH' variable in command-line file searches (*note AWKPATH
Variable::) apply to '@include' also.
This is very helpful in constructing 'gawk' function libraries. If
you have a large script with useful, general-purpose 'awk' functions,
you can break it down into library files and put those files in a
special directory. You can then include those "libraries," either by
using the full pathnames of the files, or by setting the 'AWKPATH'
environment variable accordingly and then using '@include' with just the
file part of the full pathname. Of course, you can keep library files
in more than one directory; the more complex the working environment is,
the more directories you may need to organize the files to be included.
Given the ability to specify multiple '-f' options, the '@include'
mechanism is not strictly necessary. However, the '@include' keyword
can help you in constructing self-contained 'gawk' programs, thus
reducing the need for writing complex and tedious command lines. In
particular, '@include' is very useful for writing CGI scripts to be run
from web pages.
The rules for finding a source file described in *note AWKPATH
Variable:: also apply to files loaded with '@include'.
File: gawk.info, Node: Loading Shared Libraries, Next: Obsolete, Prev: Include Files, Up: Invoking Gawk
2.8 Loading Dynamic Extensions into Your Program
================================================
This minor node describes a feature that is specific to 'gawk'.
The '@load' keyword can be used to read external 'awk' extensions
(stored as system shared libraries). This allows you to link in
compiled code that may offer superior performance and/or give you access
to extended capabilities not supported by the 'awk' language. The
'AWKLIBPATH' variable is used to search for the extension. Using
'@load' is completely equivalent to using the '-l' command-line option.
If the extension is not initially found in 'AWKLIBPATH', another
search is conducted after appending the platform's default shared
library suffix to the file name. For example, on GNU/Linux systems, the
suffix '.so' is used:
$ gawk '@load "ordchr"; BEGIN {print chr(65)}'
-| A
This is equivalent to the following example:
$ gawk -lordchr 'BEGIN {print chr(65)}'
-| A
For command-line usage, the '-l' option is more convenient, but '@load'
is useful for embedding inside an 'awk' source file that requires access
to an extension.
*note Dynamic Extensions::, describes how to write extensions (in C
or C++) that can be loaded with either '@load' or the '-l' option. It
also describes the 'ordchr' extension.
File: gawk.info, Node: Obsolete, Next: Undocumented, Prev: Loading Shared Libraries, Up: Invoking Gawk
2.9 Obsolete Options and/or Features
====================================
This minor node describes features and/or command-line options from
previous releases of 'gawk' that either are not available in the current
version or are still supported but deprecated (meaning that they will
_not_ be in the next release).
The process-related special files '/dev/pid', '/dev/ppid',
'/dev/pgrpid', and '/dev/user' were deprecated in 'gawk' 3.1, but still
worked. As of version 4.0, they are no longer interpreted specially by
'gawk'. (Use 'PROCINFO' instead; see *note Auto-set::.)
File: gawk.info, Node: Undocumented, Next: Invoking Summary, Prev: Obsolete, Up: Invoking Gawk
2.10 Undocumented Options and Features
======================================
Use the Source, Luke!
-- _Obi-Wan_
This minor node intentionally left blank.
File: gawk.info, Node: Invoking Summary, Prev: Undocumented, Up: Invoking Gawk
2.11 Summary
============
* Use either 'awk 'PROGRAM' FILES' or 'awk -f PROGRAM-FILE FILES' to
run 'awk'.
* The three standard options for all versions of 'awk' are '-f',
'-F', and '-v'. 'gawk' supplies these and many others, as well as
corresponding GNU-style long options.
* Nonoption command-line arguments are usually treated as file names,
unless they have the form 'VAR=VALUE', in which case they are taken
as variable assignments to be performed at that point in processing
the input.
* All nonoption command-line arguments, excluding the program text,
are placed in the 'ARGV' array. Adjusting 'ARGC' and 'ARGV'
affects how 'awk' processes input.
* You can use a single minus sign ('-') to refer to standard input on
the command line. 'gawk' also lets you use the special file name
'/dev/stdin'.
* 'gawk' pays attention to a number of environment variables.
'AWKPATH', 'AWKLIBPATH', and 'POSIXLY_CORRECT' are the most
important ones.
* 'gawk''s exit status conveys information to the program that
invoked it. Use the 'exit' statement from within an 'awk' program
to set the exit status.
* 'gawk' allows you to include other 'awk' source files into your
program using the '@include' statement and/or the '-i' and '-f'
command-line options.
* 'gawk' allows you to load additional functions written in C or C++
using the '@load' statement and/or the '-l' option. (This advanced
feature is described later, in *note Dynamic Extensions::.)
File: gawk.info, Node: Regexp, Next: Reading Files, Prev: Invoking Gawk, Up: Top
3 Regular Expressions
*********************
A "regular expression", or "regexp", is a way of describing a set of
strings. Because regular expressions are such a fundamental part of
'awk' programming, their format and use deserve a separate major node.
A regular expression enclosed in slashes ('/') is an 'awk' pattern
that matches every input record whose text belongs to that set. The
simplest regular expression is a sequence of letters, numbers, or both.
Such a regexp matches any string that contains that sequence. Thus, the
regexp 'foo' matches any string containing 'foo'. Thus, the pattern
'/foo/' matches any input record containing the three adjacent
characters 'foo' _anywhere_ in the record. Other kinds of regexps let
you specify more complicated classes of strings.
* Menu:
* Regexp Usage:: How to Use Regular Expressions.
* Escape Sequences:: How to write nonprinting characters.
* Regexp Operators:: Regular Expression Operators.
* Bracket Expressions:: What can go between '[...]'.
* Leftmost Longest:: How much text matches.
* Computed Regexps:: Using Dynamic Regexps.
* GNU Regexp Operators:: Operators specific to GNU software.
* Case-sensitivity:: How to do case-insensitive matching.
* Regexp Summary:: Regular expressions summary.
File: gawk.info, Node: Regexp Usage, Next: Escape Sequences, Up: Regexp
3.1 How to Use Regular Expressions
==================================
A regular expression can be used as a pattern by enclosing it in
slashes. Then the regular expression is tested against the entire text
of each record. (Normally, it only needs to match some part of the text
in order to succeed.) For example, the following prints the second
field of each record where the string 'li' appears anywhere in the
record:
$ awk '/li/ { print $2 }' mail-list
-| 555-5553
-| 555-0542
-| 555-6699
-| 555-3430
Regular expressions can also be used in matching expressions. These
expressions allow you to specify the string to match against; it need
not be the entire current input record. The two operators '~' and '!~'
perform regular expression comparisons. Expressions using these
operators can be used as patterns, or in 'if', 'while', 'for', and 'do'
statements. (*Note Statements::.) For example, the following is true
if the expression EXP (taken as a string) matches REGEXP:
EXP ~ /REGEXP/
This example matches, or selects, all input records with the uppercase
letter 'J' somewhere in the first field:
$ awk '$1 ~ /J/' inventory-shipped
-| Jan 13 25 15 115
-| Jun 31 42 75 492
-| Jul 24 34 67 436
-| Jan 21 36 64 620
So does this:
awk '{ if ($1 ~ /J/) print }' inventory-shipped
This next example is true if the expression EXP (taken as a character
string) does _not_ match REGEXP:
EXP !~ /REGEXP/
The following example matches, or selects, all input records whose
first field _does not_ contain the uppercase letter 'J':
$ awk '$1 !~ /J/' inventory-shipped
-| Feb 15 32 24 226
-| Mar 15 24 34 228
-| Apr 31 52 63 420
-| May 16 34 29 208
...
When a regexp is enclosed in slashes, such as '/foo/', we call it a
"regexp constant", much like '5.27' is a numeric constant and '"foo"' is
a string constant.
File: gawk.info, Node: Escape Sequences, Next: Regexp Operators, Prev: Regexp Usage, Up: Regexp
3.2 Escape Sequences
====================
Some characters cannot be included literally in string constants
('"foo"') or regexp constants ('/foo/'). Instead, they should be
represented with "escape sequences", which are character sequences
beginning with a backslash ('\'). One use of an escape sequence is to
include a double-quote character in a string constant. Because a plain
double quote ends the string, you must use '\"' to represent an actual
double-quote character as a part of the string. For example:
$ awk 'BEGIN { print "He said \"hi!\" to her." }'
-| He said "hi!" to her.
The backslash character itself is another character that cannot be
included normally; you must write '\\' to put one backslash in the
string or regexp. Thus, the string whose contents are the two
characters '"' and '\' must be written '"\"\\"'.
Other escape sequences represent unprintable characters such as TAB
or newline. There is nothing to stop you from entering most unprintable
characters directly in a string constant or regexp constant, but they
may look ugly.
The following list presents all the escape sequences used in 'awk'
and what they represent. Unless noted otherwise, all these escape
sequences apply to both string constants and regexp constants:
'\\'
A literal backslash, '\'.
'\a'
The "alert" character, 'Ctrl-g', ASCII code 7 (BEL). (This often
makes some sort of audible noise.)
'\b'
Backspace, 'Ctrl-h', ASCII code 8 (BS).
'\f'
Formfeed, 'Ctrl-l', ASCII code 12 (FF).
'\n'
Newline, 'Ctrl-j', ASCII code 10 (LF).
'\r'
Carriage return, 'Ctrl-m', ASCII code 13 (CR).
'\t'
Horizontal TAB, 'Ctrl-i', ASCII code 9 (HT).
'\v'
Vertical TAB, 'Ctrl-k', ASCII code 11 (VT).
'\NNN'
The octal value NNN, where NNN stands for 1 to 3 digits between '0'
and '7'. For example, the code for the ASCII ESC (escape)
character is '\033'.
'\xHH...'
The hexadecimal value HH, where HH stands for a sequence of
hexadecimal digits ('0'-'9', and either 'A'-'F' or 'a'-'f'). A
maximum of two digts are allowed after the '\x'. Any further
hexadecimal digits are treated as simple letters or numbers.
(c.e.) (The '\x' escape sequence is not allowed in POSIX awk.)
CAUTION: In ISO C, the escape sequence continues until the
first nonhexadecimal digit is seen. For many years, 'gawk'
would continue incorporating hexadecimal digits into the value
until a non-hexadecimal digit or the end of the string was
encountered. However, using more than two hexadecimal digits
produced undefined results. As of version 4.2, only two
digits are processed.
'\/'
A literal slash (necessary for regexp constants only). This
sequence is used when you want to write a regexp constant that
contains a slash (such as '/.*:\/home\/[[:alnum:]]+:.*/'; the
'[[:alnum:]]' notation is discussed in *note Bracket
Expressions::). Because the regexp is delimited by slashes, you
need to escape any slash that is part of the pattern, in order to
tell 'awk' to keep processing the rest of the regexp.
'\"'
A literal double quote (necessary for string constants only). This
sequence is used when you want to write a string constant that
contains a double quote (such as '"He said \"hi!\" to her."').
Because the string is delimited by double quotes, you need to
escape any quote that is part of the string, in order to tell 'awk'
to keep processing the rest of the string.
In 'gawk', a number of additional two-character sequences that begin
with a backslash have special meaning in regexps. *Note GNU Regexp
Operators::.
In a regexp, a backslash before any character that is not in the
previous list and not listed in *note GNU Regexp Operators:: means that
the next character should be taken literally, even if it would normally
be a regexp operator. For example, '/a\+b/' matches the three
characters 'a+b'.
For complete portability, do not use a backslash before any character
not shown in the previous list or that is not an operator.
Backslash Before Regular Characters
If you place a backslash in a string constant before something that
is not one of the characters previously listed, POSIX 'awk' purposely
leaves what happens as undefined. There are two choices:
Strip the backslash out
This is what BWK 'awk' and 'gawk' both do. For example, '"a\qc"'
is the same as '"aqc"'. (Because this is such an easy bug both to
introduce and to miss, 'gawk' warns you about it.) Consider 'FS =
"[ \t]+\|[ \t]+"' to use vertical bars surrounded by whitespace as
the field separator. There should be two backslashes in the
string: 'FS = "[ \t]+\\|[ \t]+"'.)
Leave the backslash alone
Some other 'awk' implementations do this. In such implementations,
typing '"a\qc"' is the same as typing '"a\\qc"'.
To summarize:
* The escape sequences in the preceding list are always processed
first, for both string constants and regexp constants. This
happens very early, as soon as 'awk' reads your program.
* 'gawk' processes both regexp constants and dynamic regexps (*note
Computed Regexps::), for the special operators listed in *note GNU
Regexp Operators::.
* A backslash before any other character means to treat that
character literally.
Escape Sequences for Metacharacters
Suppose you use an octal or hexadecimal escape to represent a regexp
metacharacter. (See *note Regexp Operators::.) Does 'awk' treat the
character as a literal character or as a regexp operator?
Historically, such characters were taken literally. (d.c.) However,
the POSIX standard indicates that they should be treated as real
metacharacters, which is what 'gawk' does. In compatibility mode (*note
Options::), 'gawk' treats the characters represented by octal and
hexadecimal escape sequences literally when used in regexp constants.
Thus, '/a\52b/' is equivalent to '/a\*b/'.
File: gawk.info, Node: Regexp Operators, Next: Bracket Expressions, Prev: Escape Sequences, Up: Regexp
3.3 Regular Expression Operators
================================
You can combine regular expressions with special characters, called
"regular expression operators" or "metacharacters", to increase the
power and versatility of regular expressions.
The escape sequences described in *note Escape Sequences:: are valid
inside a regexp. They are introduced by a '\' and are recognized and
converted into corresponding real characters as the very first step in
processing regexps.
Here is a list of metacharacters. All characters that are not escape
sequences and that are not listed here stand for themselves:
'\'
This suppresses the special meaning of a character when matching.
For example, '\$' matches the character '$'.
'^'
This matches the beginning of a string. '^@chapter' matches
'@chapter' at the beginning of a string, for example, and can be
used to identify chapter beginnings in Texinfo source files. The
'^' is known as an "anchor", because it anchors the pattern to
match only at the beginning of the string.
It is important to realize that '^' does not match the beginning of
a line (the point right after a '\n' newline character) embedded in
a string. The condition is not true in the following example:
if ("line1\nLINE 2" ~ /^L/) ...
'$'
This is similar to '^', but it matches only at the end of a string.
For example, 'p$' matches a record that ends with a 'p'. The '$'
is an anchor and does not match the end of a line (the point right
before a '\n' newline character) embedded in a string. The
condition in the following example is not true:
if ("line1\nLINE 2" ~ /1$/) ...
'.' (period)
This matches any single character, _including_ the newline
character. For example, '.P' matches any single character followed
by a 'P' in a string. Using concatenation, we can make a regular
expression such as 'U.A', which matches any three-character
sequence that begins with 'U' and ends with 'A'.
In strict POSIX mode (*note Options::), '.' does not match the NUL
character, which is a character with all bits equal to zero.
Otherwise, NUL is just another character. Other versions of 'awk'
may not be able to match the NUL character.
'['...']'
This is called a "bracket expression".(1) It matches any _one_ of
the characters that are enclosed in the square brackets. For
example, '[MVX]' matches any one of the characters 'M', 'V', or 'X'
in a string. A full discussion of what can be inside the square
brackets of a bracket expression is given in *note Bracket
Expressions::.
'[^'...']'
This is a "complemented bracket expression". The first character
after the '[' _must_ be a '^'. It matches any characters _except_
those in the square brackets. For example, '[^awk]' matches any
character that is not an 'a', 'w', or 'k'.
'|'
This is the "alternation operator" and it is used to specify
alternatives. The '|' has the lowest precedence of all the regular
expression operators. For example, '^P|[aeiouy]' matches any
string that matches either '^P' or '[aeiouy]'. This means it
matches any string that starts with 'P' or contains (anywhere
within it) a lowercase English vowel.
The alternation applies to the largest possible regexps on either
side.
'('...')'
Parentheses are used for grouping in regular expressions, as in
arithmetic. They can be used to concatenate regular expressions
containing the alternation operator, '|'. For example,
'@(samp|code)\{[^}]+\}' matches both '@code{foo}' and '@samp{bar}'.
(These are Texinfo formatting control sequences. The '+' is
explained further on in this list.)
'*'
This symbol means that the preceding regular expression should be
repeated as many times as necessary to find a match. For example,
'ph*' applies the '*' symbol to the preceding 'h' and looks for
matches of one 'p' followed by any number of 'h's. This also
matches just 'p' if no 'h's are present.
There are two subtle points to understand about how '*' works.
First, the '*' applies only to the single preceding regular
expression component (e.g., in 'ph*', it applies just to the 'h').
To cause '*' to apply to a larger subexpression, use parentheses:
'(ph)*' matches 'ph', 'phph', 'phphph', and so on.
Second, '*' finds as many repetitions as possible. If the text to
be matched is 'phhhhhhhhhhhhhhooey', 'ph*' matches all of the 'h's.
'+'
This symbol is similar to '*', except that the preceding expression
must be matched at least once. This means that 'wh+y' would match
'why' and 'whhy', but not 'wy', whereas 'wh*y' would match all
three.
'?'
This symbol is similar to '*', except that the preceding expression
can be matched either once or not at all. For example, 'fe?d'
matches 'fed' and 'fd', but nothing else.
'{'N'}'
'{'N',}'
'{'N','M'}'
One or two numbers inside braces denote an "interval expression".
If there is one number in the braces, the preceding regexp is
repeated N times. If there are two numbers separated by a comma,
the preceding regexp is repeated N to M times. If there is one
number followed by a comma, then the preceding regexp is repeated
at least N times:
'wh{3}y'
Matches 'whhhy', but not 'why' or 'whhhhy'.
'wh{3,5}y'
Matches 'whhhy', 'whhhhy', or 'whhhhhy' only.
'wh{2,}y'
Matches 'whhy', 'whhhy', and so on.
Interval expressions were not traditionally available in 'awk'.
They were added as part of the POSIX standard to make 'awk' and
'egrep' consistent with each other.
Initially, because old programs may use '{' and '}' in regexp
constants, 'gawk' did _not_ match interval expressions in regexps.
However, beginning with version 4.0, 'gawk' does match interval
expressions by default. This is because compatibility with POSIX
has become more important to most 'gawk' users than compatibility
with old programs.
For programs that use '{' and '}' in regexp constants, it is good
practice to always escape them with a backslash. Then the regexp
constants are valid and work the way you want them to, using any
version of 'awk'.(2)
Finally, when '{' and '}' appear in regexp constants in a way that
cannot be interpreted as an interval expression (such as '/q{a}/'),
then they stand for themselves.
In regular expressions, the '*', '+', and '?' operators, as well as
the braces '{' and '}', have the highest precedence, followed by
concatenation, and finally by '|'. As in arithmetic, parentheses can
change how operators are grouped.
In POSIX 'awk' and 'gawk', the '*', '+', and '?' operators stand for
themselves when there is nothing in the regexp that precedes them. For
example, '/+/' matches a literal plus sign. However, many other
versions of 'awk' treat such a usage as a syntax error.
If 'gawk' is in compatibility mode (*note Options::), interval
expressions are not available in regular expressions.
---------- Footnotes ----------
(1) In other literature, you may see a bracket expression referred to
as either a "character set", a "character class", or a "character list".
(2) Use two backslashes if you're using a string constant with a
regexp operator or function.
File: gawk.info, Node: Bracket Expressions, Next: Leftmost Longest, Prev: Regexp Operators, Up: Regexp
3.4 Using Bracket Expressions
=============================
As mentioned earlier, a bracket expression matches any character among
those listed between the opening and closing square brackets.
Within a bracket expression, a "range expression" consists of two
characters separated by a hyphen. It matches any single character that
sorts between the two characters, based upon the system's native
character set. For example, '[0-9]' is equivalent to '[0123456789]'.
(See *note Ranges and Locales:: for an explanation of how the POSIX
standard and 'gawk' have changed over time. This is mainly of
historical interest.)
With the increasing popularity of the Unicode character standard
(http://www.unicode.org), there is an additional wrinkle to consider.
Octal and hexadecimal escape sequences inside bracket expressions are
taken to represent only single-byte characters (characters whose values
fit within the range 0-256). To match a range of characters where the
endpoints of the range are larger than 256, enter the multibyte
encodings of the characters directly.
To include one of the characters '\', ']', '-', or '^' in a bracket
expression, put a '\' in front of it. For example:
[d\]]
matches either 'd' or ']'. Additionally, if you place ']' right after
the opening '[', the closing bracket is treated as one of the characters
to be matched.
The treatment of '\' in bracket expressions is compatible with other
'awk' implementations and is also mandated by POSIX. The regular
expressions in 'awk' are a superset of the POSIX specification for
Extended Regular Expressions (EREs). POSIX EREs are based on the
regular expressions accepted by the traditional 'egrep' utility.
"Character classes" are a feature introduced in the POSIX standard.
A character class is a special notation for describing lists of
characters that have a specific attribute, but the actual characters can
vary from country to country and/or from character set to character set.
For example, the notion of what is an alphabetic character differs
between the United States and France.
A character class is only valid in a regexp _inside_ the brackets of
a bracket expression. Character classes consist of '[:', a keyword
denoting the class, and ':]'. *note Table 3.1: table-char-classes.
lists the character classes defined by the POSIX standard.
Class Meaning
--------------------------------------------------------------------------
'[:alnum:]' Alphanumeric characters
'[:alpha:]' Alphabetic characters
'[:blank:]' Space and TAB characters
'[:cntrl:]' Control characters
'[:digit:]' Numeric characters
'[:graph:]' Characters that are both printable and visible (a space is
printable but not visible, whereas an 'a' is both)
'[:lower:]' Lowercase alphabetic characters
'[:print:]' Printable characters (characters that are not control
characters)
'[:punct:]' Punctuation characters (characters that are not letters,
digits, control characters, or space characters)
'[:space:]' Space characters (such as space, TAB, and formfeed, to name
a few)
'[:upper:]' Uppercase alphabetic characters
'[:xdigit:]'Characters that are hexadecimal digits
Table 3.1: POSIX character classes
For example, before the POSIX standard, you had to write
'/[A-Za-z0-9]/' to match alphanumeric characters. If your character set
had other alphabetic characters in it, this would not match them. With
the POSIX character classes, you can write '/[[:alnum:]]/' to match the
alphabetic and numeric characters in your character set.
Some utilities that match regular expressions provide a nonstandard
'[:ascii:]' character class; 'awk' does not. However, you can simulate
such a construct using '[\x00-\x7F]'. This matches all values
numerically between zero and 127, which is the defined range of the
ASCII character set. Use a complemented character list ('[^\x00-\x7F]')
to match any single-byte characters that are not in the ASCII range.
Two additional special sequences can appear in bracket expressions.
These apply to non-ASCII character sets, which can have single symbols
(called "collating elements") that are represented with more than one
character. They can also have several characters that are equivalent
for "collating", or sorting, purposes. (For example, in French, a plain
"e" and a grave-accented "e`" are equivalent.) These sequences are:
Collating symbols
Multicharacter collating elements enclosed between '[.' and '.]'.
For example, if 'ch' is a collating element, then '[[.ch.]]' is a
regexp that matches this collating element, whereas '[ch]' is a
regexp that matches either 'c' or 'h'.
Equivalence classes
Locale-specific names for a list of characters that are equal. The
name is enclosed between '[=' and '=]'. For example, the name 'e'
might be used to represent all of "e," "e^," "e`," and "e'." In
this case, '[[=e=]]' is a regexp that matches any of 'e', 'e^',
'e'', or 'e`'.
These features are very valuable in non-English-speaking locales.
CAUTION: The library functions that 'gawk' uses for regular
expression matching currently recognize only POSIX character
classes; they do not recognize collating symbols or equivalence
classes.
Inside a bracket expression, an opening bracket ('[') that does not
start a character class, collating element or equivalence class is taken
literally. This is also true of '.' and '*'.
File: gawk.info, Node: Leftmost Longest, Next: Computed Regexps, Prev: Bracket Expressions, Up: Regexp
3.5 How Much Text Matches?
==========================
Consider the following:
echo aaaabcd | awk '{ sub(/a+/, "<A>"); print }'
This example uses the 'sub()' function to make a change to the input
record. ('sub()' replaces the first instance of any text matched by the
first argument with the string provided as the second argument; *note
String Functions::.) Here, the regexp '/a+/' indicates "one or more 'a'
characters," and the replacement text is '<A>'.
The input contains four 'a' characters. 'awk' (and POSIX) regular
expressions always match the leftmost, _longest_ sequence of input
characters that can match. Thus, all four 'a' characters are replaced
with '<A>' in this example:
$ echo aaaabcd | awk '{ sub(/a+/, "<A>"); print }'
-| <A>bcd
For simple match/no-match tests, this is not so important. But when
doing text matching and substitutions with the 'match()', 'sub()',
'gsub()', and 'gensub()' functions, it is very important. *Note String
Functions::, for more information on these functions. Understanding
this principle is also important for regexp-based record and field
splitting (*note Records::, and also *note Field Separators::).
File: gawk.info, Node: Computed Regexps, Next: GNU Regexp Operators, Prev: Leftmost Longest, Up: Regexp
3.6 Using Dynamic Regexps
=========================
The righthand side of a '~' or '!~' operator need not be a regexp
constant (i.e., a string of characters between slashes). It may be any
expression. The expression is evaluated and converted to a string if
necessary; the contents of the string are then used as the regexp. A
regexp computed in this way is called a "dynamic regexp" or a "computed
regexp":
BEGIN { digits_regexp = "[[:digit:]]+" }
$0 ~ digits_regexp { print }
This sets 'digits_regexp' to a regexp that describes one or more digits,
and tests whether the input record matches this regexp.
NOTE: When using the '~' and '!~' operators, be aware that there is
a difference between a regexp constant enclosed in slashes and a
string constant enclosed in double quotes. If you are going to use
a string constant, you have to understand that the string is, in
essence, scanned _twice_: the first time when 'awk' reads your
program, and the second time when it goes to match the string on
the lefthand side of the operator with the pattern on the right.
This is true of any string-valued expression (such as
'digits_regexp', shown in the previous example), not just string
constants.
What difference does it make if the string is scanned twice? The
answer has to do with escape sequences, and particularly with
backslashes. To get a backslash into a regular expression inside a
string, you have to type two backslashes.
For example, '/\*/' is a regexp constant for a literal '*'. Only one
backslash is needed. To do the same thing with a string, you have to
type '"\\*"'. The first backslash escapes the second one so that the
string actually contains the two characters '\' and '*'.
Given that you can use both regexp and string constants to describe
regular expressions, which should you use? The answer is "regexp
constants," for several reasons:
* String constants are more complicated to write and more difficult
to read. Using regexp constants makes your programs less
error-prone. Not understanding the difference between the two
kinds of constants is a common source of errors.
* It is more efficient to use regexp constants. 'awk' can note that
you have supplied a regexp and store it internally in a form that
makes pattern matching more efficient. When using a string
constant, 'awk' must first convert the string into this internal
form and then perform the pattern matching.
* Using regexp constants is better form; it shows clearly that you
intend a regexp match.
Using '\n' in Bracket Expressions of Dynamic Regexps
Some older versions of 'awk' do not allow the newline character to be
used inside a bracket expression for a dynamic regexp:
$ awk '$0 ~ "[ \t\n]"'
error-> awk: newline in character class [
error-> ]...
error-> source line number 1
error-> context is
error-> $0 ~ "[ >>> \t\n]" <<<
But a newline in a regexp constant works with no problem:
$ awk '$0 ~ /[ \t\n]/'
here is a sample line
-| here is a sample line
Ctrl-d
'gawk' does not have this problem, and it isn't likely to occur often
in practice, but it's worth noting for future reference.
File: gawk.info, Node: GNU Regexp Operators, Next: Case-sensitivity, Prev: Computed Regexps, Up: Regexp
3.7 'gawk'-Specific Regexp Operators
====================================
GNU software that deals with regular expressions provides a number of
additional regexp operators. These operators are described in this
minor node and are specific to 'gawk'; they are not available in other
'awk' implementations. Most of the additional operators deal with word
matching. For our purposes, a "word" is a sequence of one or more
letters, digits, or underscores ('_'):
'\s'
Matches any whitespace character. Think of it as shorthand for
'[[:space:]]'.
'\S'
Matches any character that is not whitespace. Think of it as
shorthand for '[^[:space:]]'.
'\w'
Matches any word-constituent character--that is, it matches any
letter, digit, or underscore. Think of it as shorthand for
'[[:alnum:]_]'.
'\W'
Matches any character that is not word-constituent. Think of it as
shorthand for '[^[:alnum:]_]'.
'\<'
Matches the empty string at the beginning of a word. For example,
'/\<away/' matches 'away' but not 'stowaway'.
'\>'
Matches the empty string at the end of a word. For example,
'/stow\>/' matches 'stow' but not 'stowaway'.
'\y'
Matches the empty string at either the beginning or the end of a
word (i.e., the word boundar*y*). For example, '\yballs?\y'
matches either 'ball' or 'balls', as a separate word.
'\B'
Matches the empty string that occurs between two word-constituent
characters. For example, '/\Brat\B/' matches 'crate', but it does
not match 'dirty rat'. '\B' is essentially the opposite of '\y'.
There are two other operators that work on buffers. In Emacs, a
"buffer" is, naturally, an Emacs buffer. Other GNU programs, including
'gawk', consider the entire string to match as the buffer. The
operators are:
'\`'
Matches the empty string at the beginning of a buffer (string)
'\''
Matches the empty string at the end of a buffer (string)
Because '^' and '$' always work in terms of the beginning and end of
strings, these operators don't add any new capabilities for 'awk'. They
are provided for compatibility with other GNU software.
In other GNU software, the word-boundary operator is '\b'. However,
that conflicts with the 'awk' language's definition of '\b' as
backspace, so 'gawk' uses a different letter. An alternative method
would have been to require two backslashes in the GNU operators, but
this was deemed too confusing. The current method of using '\y' for the
GNU '\b' appears to be the lesser of two evils.
The various command-line options (*note Options::) control how 'gawk'
interprets characters in regexps:
No options
In the default case, 'gawk' provides all the facilities of POSIX
regexps and the GNU regexp operators described in *note Regexp
Operators::.
'--posix'
Match only POSIX regexps; the GNU operators are not special (e.g.,
'\w' matches a literal 'w'). Interval expressions are allowed.
'--traditional'
Match traditional Unix 'awk' regexps. The GNU operators are not
special, and interval expressions are not available. Because BWK
'awk' supports them, the POSIX character classes ('[[:alnum:]]',
etc.) are available. Characters described by octal and
hexadecimal escape sequences are treated literally, even if they
represent regexp metacharacters.
'--re-interval'
Allow interval expressions in regexps, if '--traditional' has been
provided. Otherwise, interval expressions are available by
default.
File: gawk.info, Node: Case-sensitivity, Next: Regexp Summary, Prev: GNU Regexp Operators, Up: Regexp
3.8 Case Sensitivity in Matching
================================
Case is normally significant in regular expressions, both when matching
ordinary characters (i.e., not metacharacters) and inside bracket
expressions. Thus, a 'w' in a regular expression matches only a
lowercase 'w' and not an uppercase 'W'.
The simplest way to do a case-independent match is to use a bracket
expression--for example, '[Ww]'. However, this can be cumbersome if you
need to use it often, and it can make the regular expressions harder to
read. There are two alternatives that you might prefer.
One way to perform a case-insensitive match at a particular point in
the program is to convert the data to a single case, using the
'tolower()' or 'toupper()' built-in string functions (which we haven't
discussed yet; *note String Functions::). For example:
tolower($1) ~ /foo/ { ... }
converts the first field to lowercase before matching against it. This
works in any POSIX-compliant 'awk'.
Another method, specific to 'gawk', is to set the variable
'IGNORECASE' to a nonzero value (*note Built-in Variables::). When
'IGNORECASE' is not zero, _all_ regexp and string operations ignore
case.
Changing the value of 'IGNORECASE' dynamically controls the case
sensitivity of the program as it runs. Case is significant by default
because 'IGNORECASE' (like most variables) is initialized to zero:
x = "aB"
if (x ~ /ab/) ... # this test will fail
IGNORECASE = 1
if (x ~ /ab/) ... # now it will succeed
In general, you cannot use 'IGNORECASE' to make certain rules case
insensitive and other rules case sensitive, as there is no
straightforward way to set 'IGNORECASE' just for the pattern of a
particular rule.(1) To do this, use either bracket expressions or
'tolower()'. However, one thing you can do with 'IGNORECASE' only is
dynamically turn case sensitivity on or off for all the rules at once.
'IGNORECASE' can be set on the command line or in a 'BEGIN' rule
(*note Other Arguments::; also *note Using BEGIN/END::). Setting
'IGNORECASE' from the command line is a way to make a program case
insensitive without having to edit it.
In multibyte locales, the equivalences between upper- and lowercase
characters are tested based on the wide-character values of the locale's
character set. Otherwise, the characters are tested based on the
ISO-8859-1 (ISO Latin-1) character set. This character set is a
superset of the traditional 128 ASCII characters, which also provides a
number of characters suitable for use with European languages.(2)
The value of 'IGNORECASE' has no effect if 'gawk' is in compatibility
mode (*note Options::). Case is always significant in compatibility
mode.
---------- Footnotes ----------
(1) Experienced C and C++ programmers will note that it is possible,
using something like 'IGNORECASE = 1 && /foObAr/ { ... }' and
'IGNORECASE = 0 || /foobar/ { ... }'. However, this is somewhat obscure
and we don't recommend it.
(2) If you don't understand this, don't worry about it; it just means
that 'gawk' does the right thing.
File: gawk.info, Node: Regexp Summary, Prev: Case-sensitivity, Up: Regexp
3.9 Summary
===========
* Regular expressions describe sets of strings to be matched. In
'awk', regular expression constants are written enclosed between
slashes: '/'...'/'.
* Regexp constants may be used standalone in patterns and in
conditional expressions, or as part of matching expressions using
the '~' and '!~' operators.
* Escape sequences let you represent nonprintable characters and also
let you represent regexp metacharacters as literal characters to be
matched.
* Regexp operators provide grouping, alternation, and repetition.
* Bracket expressions give you a shorthand for specifying sets of
characters that can match at a particular point in a regexp.
Within bracket expressions, POSIX character classes let you specify
certain groups of characters in a locale-independent fashion.
* Regular expressions match the leftmost longest text in the string
being matched. This matters for cases where you need to know the
extent of the match, such as for text substitution and when the
record separator is a regexp.
* Matching expressions may use dynamic regexps (i.e., string values
treated as regular expressions).
* 'gawk''s 'IGNORECASE' variable lets you control the case
sensitivity of regexp matching. In other 'awk' versions, use
'tolower()' or 'toupper()'.
File: gawk.info, Node: Reading Files, Next: Printing, Prev: Regexp, Up: Top
4 Reading Input Files
*********************
In the typical 'awk' program, 'awk' reads all input either from the
standard input (by default, this is the keyboard, but often it is a pipe
from another command) or from files whose names you specify on the 'awk'
command line. If you specify input files, 'awk' reads them in order,
processing all the data from one before going on to the next. The name
of the current input file can be found in the predefined variable
'FILENAME' (*note Built-in Variables::).
The input is read in units called "records", and is processed by the
rules of your program one record at a time. By default, each record is
one line. Each record is automatically split into chunks called
"fields". This makes it more convenient for programs to work on the
parts of a record.
On rare occasions, you may need to use the 'getline' command. The
'getline' command is valuable both because it can do explicit input from
any number of files, and because the files used with it do not have to
be named on the 'awk' command line (*note Getline::).
* Menu:
* Records:: Controlling how data is split into records.
* Fields:: An introduction to fields.
* Nonconstant Fields:: Nonconstant Field Numbers.
* Changing Fields:: Changing the Contents of a Field.
* Field Separators:: The field separator and how to change it.
* Constant Size:: Reading constant width data.
* Splitting By Content:: Defining Fields By Content
* Testing field creation:: Checking how 'gawk' is splitting
records.
* Multiple Line:: Reading multiline records.
* Getline:: Reading files under explicit program control
using the 'getline' function.
* Read Timeout:: Reading input with a timeout.
* Retrying Input:: Retrying input after certain errors.
* Command-line directories:: What happens if you put a directory on the
command line.
* Input Summary:: Input summary.
* Input Exercises:: Exercises.
File: gawk.info, Node: Records, Next: Fields, Up: Reading Files
4.1 How Input Is Split into Records
===================================
'awk' divides the input for your program into records and fields. It
keeps track of the number of records that have been read so far from the
current input file. This value is stored in a predefined variable
called 'FNR', which is reset to zero every time a new file is started.
Another predefined variable, 'NR', records the total number of input
records read so far from all data files. It starts at zero, but is
never automatically reset to zero.
* Menu:
* awk split records:: How standard 'awk' splits records.
* gawk split records:: How 'gawk' splits records.
File: gawk.info, Node: awk split records, Next: gawk split records, Up: Records
4.1.1 Record Splitting with Standard 'awk'
------------------------------------------
Records are separated by a character called the "record separator". By
default, the record separator is the newline character. This is why
records are, by default, single lines. To use a different character for
the record separator, simply assign that character to the predefined
variable 'RS'.
Like any other variable, the value of 'RS' can be changed in the
'awk' program with the assignment operator, '=' (*note Assignment
Ops::). The new record-separator character should be enclosed in
quotation marks, which indicate a string constant. Often, the right
time to do this is at the beginning of execution, before any input is
processed, so that the very first record is read with the proper
separator. To do this, use the special 'BEGIN' pattern (*note
BEGIN/END::). For example:
awk 'BEGIN { RS = "u" }
{ print $0 }' mail-list
changes the value of 'RS' to 'u', before reading any input. The new
value is a string whose first character is the letter "u"; as a result,
records are separated by the letter "u". Then the input file is read,
and the second rule in the 'awk' program (the action with no pattern)
prints each record. Because each 'print' statement adds a newline at
the end of its output, this 'awk' program copies the input with each 'u'
changed to a newline. Here are the results of running the program on
'mail-list':
$ awk 'BEGIN { RS = "u" }
> { print $0 }' mail-list
-| Amelia 555-5553 amelia.zodiac
-| sq
-| e AT gmail.com F
-| Anthony 555-3412 anthony.assert
-| ro AT hotmail.com A
-| Becky 555-7685 becky.algebrar
-| m AT gmail.com A
-| Bill 555-1675 bill.drowning AT hotmail.com A
-| Broderick 555-0542 broderick.aliq
-| otiens AT yahoo.com R
-| Camilla 555-2912 camilla.inf
-| sar
-| m AT skynet.be R
-| Fabi
-| s 555-1234 fabi
-| s.
-| ndevicesim
-| s@
-| cb.ed
-| F
-| J
-| lie 555-6699 j
-| lie.perscr
-| tabor AT skeeve.com F
-| Martin 555-6480 martin.codicib
-| s AT hotmail.com A
-| Sam
-| el 555-3430 sam
-| el.lanceolis@sh
-| .ed
-| A
-| Jean-Pa
-| l 555-2127 jeanpa
-| l.campanor
-| m@ny
-| .ed
-| R
-|
Note that the entry for the name 'Bill' is not split. In the original
data file (*note Sample Data Files::), the line looks like this:
Bill 555-1675 bill.drowning AT hotmail.com A
It contains no 'u', so there is no reason to split the record, unlike
the others, which each have one or more occurrences of the 'u'. In
fact, this record is treated as part of the previous record; the newline
separating them in the output is the original newline in the data file,
not the one added by 'awk' when it printed the record!
Another way to change the record separator is on the command line,
using the variable-assignment feature (*note Other Arguments::):
awk '{ print $0 }' RS="u" mail-list
This sets 'RS' to 'u' before processing 'mail-list'.
Using an alphabetic character such as 'u' for the record separator is
highly likely to produce strange results. Using an unusual character
such as '/' is more likely to produce correct behavior in the majority
of cases, but there are no guarantees. The moral is: Know Your Data.
When using regular characters as the record separator, there is one
unusual case that occurs when 'gawk' is being fully POSIX-compliant
(*note Options::). Then, the following (extreme) pipeline prints a
surprising '1':
$ echo | gawk --posix 'BEGIN { RS = "a" } ; { print NF }'
-| 1
There is one field, consisting of a newline. The value of the
built-in variable 'NF' is the number of fields in the current record.
(In the normal case, 'gawk' treats the newline as whitespace, printing
'0' as the result. Most other versions of 'awk' also act this way.)
Reaching the end of an input file terminates the current input
record, even if the last character in the file is not the character in
'RS'. (d.c.)
The empty string '""' (a string without any characters) has a special
meaning as the value of 'RS'. It means that records are separated by
one or more blank lines and nothing else. *Note Multiple Line:: for
more details.
If you change the value of 'RS' in the middle of an 'awk' run, the
new value is used to delimit subsequent records, but the record
currently being processed, as well as records already processed, are not
affected.
After the end of the record has been determined, 'gawk' sets the
variable 'RT' to the text in the input that matched 'RS'.
File: gawk.info, Node: gawk split records, Prev: awk split records, Up: Records
4.1.2 Record Splitting with 'gawk'
----------------------------------
When using 'gawk', the value of 'RS' is not limited to a one-character
string. It can be any regular expression (*note Regexp::). (c.e.) In
general, each record ends at the next string that matches the regular
expression; the next record starts at the end of the matching string.
This general rule is actually at work in the usual case, where 'RS'
contains just a newline: a record ends at the beginning of the next
matching string (the next newline in the input), and the following
record starts just after the end of this string (at the first character
of the following line). The newline, because it matches 'RS', is not
part of either record.
When 'RS' is a single character, 'RT' contains the same single
character. However, when 'RS' is a regular expression, 'RT' contains
the actual input text that matched the regular expression.
If the input file ends without any text matching 'RS', 'gawk' sets
'RT' to the null string.
The following example illustrates both of these features. It sets
'RS' equal to a regular expression that matches either a newline or a
series of one or more uppercase letters with optional leading and/or
trailing whitespace:
$ echo record 1 AAAA record 2 BBBB record 3 |
> gawk 'BEGIN { RS = "\n|( *[[:upper:]]+ *)" }
> { print "Record =", $0,"and RT = [" RT "]" }'
-| Record = record 1 and RT = [ AAAA ]
-| Record = record 2 and RT = [ BBBB ]
-| Record = record 3 and RT = [
-| ]
The square brackets delineate the contents of 'RT', letting you see the
leading and trailing whitespace. The final value of 'RT' is a newline.
*Note Simple Sed:: for a more useful example of 'RS' as a regexp and
'RT'.
If you set 'RS' to a regular expression that allows optional trailing
text, such as 'RS = "abc(XYZ)?"', it is possible, due to implementation
constraints, that 'gawk' may match the leading part of the regular
expression, but not the trailing part, particularly if the input text
that could match the trailing part is fairly long. 'gawk' attempts to
avoid this problem, but currently, there's no guarantee that this will
never happen.
NOTE: Remember that in 'awk', the '^' and '$' anchor metacharacters
match the beginning and end of a _string_, and not the beginning
and end of a _line_. As a result, something like 'RS =
"^[[:upper:]]"' can only match at the beginning of a file. This is
because 'gawk' views the input file as one long string that happens
to contain newline characters. It is thus best to avoid anchor
metacharacters in the value of 'RS'.
The use of 'RS' as a regular expression and the 'RT' variable are
'gawk' extensions; they are not available in compatibility mode (*note
Options::). In compatibility mode, only the first character of the
value of 'RS' determines the end of the record.
'RS = "\0"' Is Not Portable
There are times when you might want to treat an entire data file as a
single record. The only way to make this happen is to give 'RS' a value
that you know doesn't occur in the input file. This is hard to do in a
general way, such that a program always works for arbitrary input files.
You might think that for text files, the NUL character, which
consists of a character with all bits equal to zero, is a good value to
use for 'RS' in this case:
BEGIN { RS = "\0" } # whole file becomes one record?
'gawk' in fact accepts this, and uses the NUL character for the
record separator. This works for certain special files, such as
'/proc/environ' on GNU/Linux systems, where the NUL character is in fact
the record separator. However, this usage is _not_ portable to most
other 'awk' implementations.
Almost all other 'awk' implementations(1) store strings internally as
C-style strings. C strings use the NUL character as the string
terminator. In effect, this means that 'RS = "\0"' is the same as 'RS =
""'. (d.c.)
It happens that recent versions of 'mawk' can use the NUL character
as a record separator. However, this is a special case: 'mawk' does not
allow embedded NUL characters in strings. (This may change in a future
version of 'mawk'.)
*Note Readfile Function:: for an interesting way to read whole files.
If you are using 'gawk', see *note Extension Sample Readfile:: for
another option.
---------- Footnotes ----------
(1) At least that we know about.
File: gawk.info, Node: Fields, Next: Nonconstant Fields, Prev: Records, Up: Reading Files
4.2 Examining Fields
====================
When 'awk' reads an input record, the record is automatically "parsed"
or separated by the 'awk' utility into chunks called "fields". By
default, fields are separated by "whitespace", like words in a line.
Whitespace in 'awk' means any string of one or more spaces, TABs, or
newlines; other characters that are considered whitespace by other
languages (such as formfeed, vertical tab, etc.) are _not_ considered
whitespace by 'awk'.
The purpose of fields is to make it more convenient for you to refer
to these pieces of the record. You don't have to use them--you can
operate on the whole record if you want--but fields are what make simple
'awk' programs so powerful.
You use a dollar sign ('$') to refer to a field in an 'awk' program,
followed by the number of the field you want. Thus, '$1' refers to the
first field, '$2' to the second, and so on. (Unlike in the Unix shells,
the field numbers are not limited to single digits. '$127' is the 127th
field in the record.) For example, suppose the following is a line of
input:
This seems like a pretty nice example.
Here the first field, or '$1', is 'This', the second field, or '$2', is
'seems', and so on. Note that the last field, '$7', is 'example.'.
Because there is no space between the 'e' and the '.', the period is
considered part of the seventh field.
'NF' is a predefined variable whose value is the number of fields in
the current record. 'awk' automatically updates the value of 'NF' each
time it reads a record. No matter how many fields there are, the last
field in a record can be represented by '$NF'. So, '$NF' is the same as
'$7', which is 'example.'. If you try to reference a field beyond the
last one (such as '$8' when the record has only seven fields), you get
the empty string. (If used in a numeric operation, you get zero.)
The use of '$0', which looks like a reference to the "zeroth" field,
is a special case: it represents the whole input record. Use it when
you are not interested in specific fields. Here are some more examples:
$ awk '$1 ~ /li/ { print $0 }' mail-list
-| Amelia 555-5553 amelia.zodiacusque AT gmail.com F
-| Julie 555-6699 julie.perscrutabor AT skeeve.com F
This example prints each record in the file 'mail-list' whose first
field contains the string 'li'.
By contrast, the following example looks for 'li' in _the entire
record_ and prints the first and last fields for each matching input
record:
$ awk '/li/ { print $1, $NF }' mail-list
-| Amelia F
-| Broderick R
-| Julie F
-| Samuel A
File: gawk.info, Node: Nonconstant Fields, Next: Changing Fields, Prev: Fields, Up: Reading Files
4.3 Nonconstant Field Numbers
=============================
A field number need not be a constant. Any expression in the 'awk'
language can be used after a '$' to refer to a field. The value of the
expression specifies the field number. If the value is a string, rather
than a number, it is converted to a number. Consider this example:
awk '{ print $NR }'
Recall that 'NR' is the number of records read so far: one in the first
record, two in the second, and so on. So this example prints the first
field of the first record, the second field of the second record, and so
on. For the twentieth record, field number 20 is printed; most likely,
the record has fewer than 20 fields, so this prints a blank line. Here
is another example of using expressions as field numbers:
awk '{ print $(2*2) }' mail-list
'awk' evaluates the expression '(2*2)' and uses its value as the
number of the field to print. The '*' represents multiplication, so the
expression '2*2' evaluates to four. The parentheses are used so that
the multiplication is done before the '$' operation; they are necessary
whenever there is a binary operator(1) in the field-number expression.
This example, then, prints the type of relationship (the fourth field)
for every line of the file 'mail-list'. (All of the 'awk' operators are
listed, in order of decreasing precedence, in *note Precedence::.)
If the field number you compute is zero, you get the entire record.
Thus, '$(2-2)' has the same value as '$0'. Negative field numbers are
not allowed; trying to reference one usually terminates the program.
(The POSIX standard does not define what happens when you reference a
negative field number. 'gawk' notices this and terminates your program.
Other 'awk' implementations may behave differently.)
As mentioned in *note Fields::, 'awk' stores the current record's
number of fields in the built-in variable 'NF' (also *note Built-in
Variables::). Thus, the expression '$NF' is not a special feature--it
is the direct consequence of evaluating 'NF' and using its value as a
field number.
---------- Footnotes ----------
(1) A "binary operator", such as '*' for multiplication, is one that
takes two operands. The distinction is required because 'awk' also has
unary (one-operand) and ternary (three-operand) operators.
File: gawk.info, Node: Changing Fields, Next: Field Separators, Prev: Nonconstant Fields, Up: Reading Files
4.4 Changing the Contents of a Field
====================================
The contents of a field, as seen by 'awk', can be changed within an
'awk' program; this changes what 'awk' perceives as the current input
record. (The actual input is untouched; 'awk' _never_ modifies the
input file.) Consider the following example and its output:
$ awk '{ nboxes = $3 ; $3 = $3 - 10
> print nboxes, $3 }' inventory-shipped
-| 25 15
-| 32 22
-| 24 14
...
The program first saves the original value of field three in the
variable 'nboxes'. The '-' sign represents subtraction, so this program
reassigns field three, '$3', as the original value of field three minus
ten: '$3 - 10'. (*Note Arithmetic Ops::.) Then it prints the original
and new values for field three. (Someone in the warehouse made a
consistent mistake while inventorying the red boxes.)
For this to work, the text in '$3' must make sense as a number; the
string of characters must be converted to a number for the computer to
do arithmetic on it. The number resulting from the subtraction is
converted back to a string of characters that then becomes field three.
*Note Conversion::.
When the value of a field is changed (as perceived by 'awk'), the
text of the input record is recalculated to contain the new field where
the old one was. In other words, '$0' changes to reflect the altered
field. Thus, this program prints a copy of the input file, with 10
subtracted from the second field of each line:
$ awk '{ $2 = $2 - 10; print $0 }' inventory-shipped
-| Jan 3 25 15 115
-| Feb 5 32 24 226
-| Mar 5 24 34 228
...
It is also possible to assign contents to fields that are out of
range. For example:
$ awk '{ $6 = ($5 + $4 + $3 + $2)
> print $6 }' inventory-shipped
-| 168
-| 297
-| 301
...
We've just created '$6', whose value is the sum of fields '$2', '$3',
'$4', and '$5'. The '+' sign represents addition. For the file
'inventory-shipped', '$6' represents the total number of parcels shipped
for a particular month.
Creating a new field changes 'awk''s internal copy of the current
input record, which is the value of '$0'. Thus, if you do 'print $0'
after adding a field, the record printed includes the new field, with
the appropriate number of field separators between it and the previously
existing fields.
This recomputation affects and is affected by 'NF' (the number of
fields; *note Fields::). For example, the value of 'NF' is set to the
number of the highest field you create. The exact format of '$0' is
also affected by a feature that has not been discussed yet: the "output
field separator", 'OFS', used to separate the fields (*note Output
Separators::).
Note, however, that merely _referencing_ an out-of-range field does
_not_ change the value of either '$0' or 'NF'. Referencing an
out-of-range field only produces an empty string. For example:
if ($(NF+1) != "")
print "can't happen"
else
print "everything is normal"
should print 'everything is normal', because 'NF+1' is certain to be out
of range. (*Note If Statement:: for more information about 'awk''s
'if-else' statements. *Note Typing and Comparison:: for more
information about the '!=' operator.)
It is important to note that making an assignment to an existing
field changes the value of '$0' but does not change the value of 'NF',
even when you assign the empty string to a field. For example:
$ echo a b c d | awk '{ OFS = ":"; $2 = ""
> print $0; print NF }'
-| a::c:d
-| 4
The field is still there; it just has an empty value, delimited by the
two colons between 'a' and 'c'. This example shows what happens if you
create a new field:
$ echo a b c d | awk '{ OFS = ":"; $2 = ""; $6 = "new"
> print $0; print NF }'
-| a::c:d::new
-| 6
The intervening field, '$5', is created with an empty value (indicated
by the second pair of adjacent colons), and 'NF' is updated with the
value six.
Decrementing 'NF' throws away the values of the fields after the new
value of 'NF' and recomputes '$0'. (d.c.) Here is an example:
$ echo a b c d e f | awk '{ print "NF =", NF;
> NF = 3; print $0 }'
-| NF = 6
-| a b c
CAUTION: Some versions of 'awk' don't rebuild '$0' when 'NF' is
decremented.
Finally, there are times when it is convenient to force 'awk' to
rebuild the entire record, using the current values of the fields and
'OFS'. To do this, use the seemingly innocuous assignment:
$1 = $1 # force record to be reconstituted
print $0 # or whatever else with $0
This forces 'awk' to rebuild the record. It does help to add a comment,
as we've shown here.
There is a flip side to the relationship between '$0' and the fields.
Any assignment to '$0' causes the record to be reparsed into fields
using the _current_ value of 'FS'. This also applies to any built-in
function that updates '$0', such as 'sub()' and 'gsub()' (*note String
Functions::).
Understanding '$0'
It is important to remember that '$0' is the _full_ record, exactly
as it was read from the input. This includes any leading or trailing
whitespace, and the exact whitespace (or other characters) that
separates the fields.
It is a common error to try to change the field separators in a
record simply by setting 'FS' and 'OFS', and then expecting a plain
'print' or 'print $0' to print the modified record.
But this does not work, because nothing was done to change the record
itself. Instead, you must force the record to be rebuilt, typically
with a statement such as '$1 = $1', as described earlier.
File: gawk.info, Node: Field Separators, Next: Constant Size, Prev: Changing Fields, Up: Reading Files
4.5 Specifying How Fields Are Separated
=======================================
* Menu:
* Default Field Splitting:: How fields are normally separated.
* Regexp Field Splitting:: Using regexps as the field separator.
* Single Character Fields:: Making each character a separate field.
* Command Line Field Separator:: Setting 'FS' from the command line.
* Full Line Fields:: Making the full line be a single field.
* Field Splitting Summary:: Some final points and a summary table.
The "field separator", which is either a single character or a regular
expression, controls the way 'awk' splits an input record into fields.
'awk' scans the input record for character sequences that match the
separator; the fields themselves are the text between the matches.
In the examples that follow, we use the bullet symbol (*) to
represent spaces in the output. If the field separator is 'oo', then
the following line:
moo goo gai pan
is split into three fields: 'm', '*g', and '*gai*pan'. Note the leading
spaces in the values of the second and third fields.
The field separator is represented by the predefined variable 'FS'.
Shell programmers take note: 'awk' does _not_ use the name 'IFS' that is
used by the POSIX-compliant shells (such as the Unix Bourne shell, 'sh',
or Bash).
The value of 'FS' can be changed in the 'awk' program with the
assignment operator, '=' (*note Assignment Ops::). Often, the right
time to do this is at the beginning of execution before any input has
been processed, so that the very first record is read with the proper
separator. To do this, use the special 'BEGIN' pattern (*note
BEGIN/END::). For example, here we set the value of 'FS' to the string
'","':
awk 'BEGIN { FS = "," } ; { print $2 }'
Given the input line:
John Q. Smith, 29 Oak St., Walamazoo, MI 42139
this 'awk' program extracts and prints the string '*29*Oak*St.'.
Sometimes the input data contains separator characters that don't
separate fields the way you thought they would. For instance, the
person's name in the example we just used might have a title or suffix
attached, such as:
John Q. Smith, LXIX, 29 Oak St., Walamazoo, MI 42139
The same program would extract '*LXIX' instead of '*29*Oak*St.'. If you
were expecting the program to print the address, you would be surprised.
The moral is to choose your data layout and separator characters
carefully to prevent such problems. (If the data is not in a form that
is easy to process, perhaps you can massage it first with a separate
'awk' program.)
File: gawk.info, Node: Default Field Splitting, Next: Regexp Field Splitting, Up: Field Separators
4.5.1 Whitespace Normally Separates Fields
------------------------------------------
Fields are normally separated by whitespace sequences (spaces, TABs, and
newlines), not by single spaces. Two spaces in a row do not delimit an
empty field. The default value of the field separator 'FS' is a string
containing a single space, '" "'. If 'awk' interpreted this value in
the usual way, each space character would separate fields, so two spaces
in a row would make an empty field between them. The reason this does
not happen is that a single space as the value of 'FS' is a special
case--it is taken to specify the default manner of delimiting fields.
If 'FS' is any other single character, such as '","', then each
occurrence of that character separates two fields. Two consecutive
occurrences delimit an empty field. If the character occurs at the
beginning or the end of the line, that too delimits an empty field. The
space character is the only single character that does not follow these
rules.
File: gawk.info, Node: Regexp Field Splitting, Next: Single Character Fields, Prev: Default Field Splitting, Up: Field Separators
4.5.2 Using Regular Expressions to Separate Fields
--------------------------------------------------
The previous node discussed the use of single characters or simple
strings as the value of 'FS'. More generally, the value of 'FS' may be
a string containing any regular expression. In this case, each match in
the record for the regular expression separates fields. For example,
the assignment:
FS = ", \t"
makes every area of an input line that consists of a comma followed by a
space and a TAB into a field separator. ('\t' is an "escape sequence"
that stands for a TAB; *note Escape Sequences::, for the complete list
of similar escape sequences.)
For a less trivial example of a regular expression, try using single
spaces to separate fields the way single commas are used. 'FS' can be
set to '"[ ]"' (left bracket, space, right bracket). This regular
expression matches a single space and nothing else (*note Regexp::).
There is an important difference between the two cases of 'FS = " "'
(a single space) and 'FS = "[ \t\n]+"' (a regular expression matching
one or more spaces, TABs, or newlines). For both values of 'FS', fields
are separated by "runs" (multiple adjacent occurrences) of spaces, TABs,
and/or newlines. However, when the value of 'FS' is '" "', 'awk' first
strips leading and trailing whitespace from the record and then decides
where the fields are. For example, the following pipeline prints 'b':
$ echo ' a b c d ' | awk '{ print $2 }'
-| b
However, this pipeline prints 'a' (note the extra spaces around each
letter):
$ echo ' a b c d ' | awk 'BEGIN { FS = "[ \t\n]+" }
> { print $2 }'
-| a
In this case, the first field is null, or empty.
The stripping of leading and trailing whitespace also comes into play
whenever '$0' is recomputed. For instance, study this pipeline:
$ echo ' a b c d' | awk '{ print; $2 = $2; print }'
-| a b c d
-| a b c d
The first 'print' statement prints the record as it was read, with
leading whitespace intact. The assignment to '$2' rebuilds '$0' by
concatenating '$1' through '$NF' together, separated by the value of
'OFS' (which is a space by default). Because the leading whitespace was
ignored when finding '$1', it is not part of the new '$0'. Finally, the
last 'print' statement prints the new '$0'.
There is an additional subtlety to be aware of when using regular
expressions for field splitting. It is not well specified in the POSIX
standard, or anywhere else, what '^' means when splitting fields. Does
the '^' match only at the beginning of the entire record? Or is each
field separator a new string? It turns out that different 'awk'
versions answer this question differently, and you should not rely on
any specific behavior in your programs. (d.c.)
As a point of information, BWK 'awk' allows '^' to match only at the
beginning of the record. 'gawk' also works this way. For example:
$ echo 'xxAA xxBxx C' |
> gawk -F '(^x+)|( +)' '{ for (i = 1; i <= NF; i++)
> printf "-->%s<--\n", $i }'
-| --><--
-| -->AA<--
-| -->xxBxx<--
-| -->C<--
File: gawk.info, Node: Single Character Fields, Next: Command Line Field Separator, Prev: Regexp Field Splitting, Up: Field Separators
4.5.3 Making Each Character a Separate Field
--------------------------------------------
There are times when you may want to examine each character of a record
separately. This can be done in 'gawk' by simply assigning the null
string ('""') to 'FS'. (c.e.) In this case, each individual character
in the record becomes a separate field. For example:
$ echo a b | gawk 'BEGIN { FS = "" }
> {
> for (i = 1; i <= NF; i = i + 1)
> print "Field", i, "is", $i
> }'
-| Field 1 is a
-| Field 2 is
-| Field 3 is b
Traditionally, the behavior of 'FS' equal to '""' was not defined.
In this case, most versions of Unix 'awk' simply treat the entire record
as only having one field. (d.c.) In compatibility mode (*note
Options::), if 'FS' is the null string, then 'gawk' also behaves this
way.
File: gawk.info, Node: Command Line Field Separator, Next: Full Line Fields, Prev: Single Character Fields, Up: Field Separators
4.5.4 Setting 'FS' from the Command Line
----------------------------------------
'FS' can be set on the command line. Use the '-F' option to do so. For
example:
awk -F, 'PROGRAM' INPUT-FILES
sets 'FS' to the ',' character. Notice that the option uses an
uppercase 'F' instead of a lowercase 'f'. The latter option ('-f')
specifies a file containing an 'awk' program.
The value used for the argument to '-F' is processed in exactly the
same way as assignments to the predefined variable 'FS'. Any special
characters in the field separator must be escaped appropriately. For
example, to use a '\' as the field separator on the command line, you
would have to type:
# same as FS = "\\"
awk -F\\\\ '...' files ...
Because '\' is used for quoting in the shell, 'awk' sees '-F\\'. Then
'awk' processes the '\\' for escape characters (*note Escape
Sequences::), finally yielding a single '\' to use for the field
separator.
As a special case, in compatibility mode (*note Options::), if the
argument to '-F' is 't', then 'FS' is set to the TAB character. If you
type '-F\t' at the shell, without any quotes, the '\' gets deleted, so
'awk' figures that you really want your fields to be separated with TABs
and not 't's. Use '-v FS="t"' or '-F"[t]"' on the command line if you
really do want to separate your fields with 't's. Use '-F '\t'' when
not in compatibility mode to specify that TABs separate fields.
As an example, let's use an 'awk' program file called 'edu.awk' that
contains the pattern '/edu/' and the action 'print $1':
/edu/ { print $1 }
Let's also set 'FS' to be the '-' character and run the program on
the file 'mail-list'. The following command prints a list of the names
of the people that work at or attend a university, and the first three
digits of their phone numbers:
$ awk -F- -f edu.awk mail-list
-| Fabius 555
-| Samuel 555
-| Jean
Note the third line of output. The third line in the original file
looked like this:
Jean-Paul 555-2127 jeanpaul.campanorum AT nyu.edu R
The '-' as part of the person's name was used as the field separator,
instead of the '-' in the phone number that was originally intended.
This demonstrates why you have to be careful in choosing your field and
record separators.
Perhaps the most common use of a single character as the field
separator occurs when processing the Unix system password file. On many
Unix systems, each user has a separate entry in the system password
file, with one line per user. The information in these lines is
separated by colons. The first field is the user's login name and the
second is the user's encrypted or shadow password. (A shadow password
is indicated by the presence of a single 'x' in the second field.) A
password file entry might look like this:
arnold:x:2076:10:Arnold Robbins:/home/arnold:/bin/bash
The following program searches the system password file and prints
the entries for users whose full name is not indicated:
awk -F: '$5 == ""' /etc/passwd
File: gawk.info, Node: Full Line Fields, Next: Field Splitting Summary, Prev: Command Line Field Separator, Up: Field Separators
4.5.5 Making the Full Line Be a Single Field
--------------------------------------------
Occasionally, it's useful to treat the whole input line as a single
field. This can be done easily and portably simply by setting 'FS' to
'"\n"' (a newline):(1)
awk -F'\n' 'PROGRAM' FILES ...
When you do this, '$1' is the same as '$0'.
Changing 'FS' Does Not Affect the Fields
According to the POSIX standard, 'awk' is supposed to behave as if
each record is split into fields at the time it is read. In particular,
this means that if you change the value of 'FS' after a record is read,
the values of the fields (i.e., how they were split) should reflect the
old value of 'FS', not the new one.
However, many older implementations of 'awk' do not work this way.
Instead, they defer splitting the fields until a field is actually
referenced. The fields are split using the _current_ value of 'FS'!
(d.c.) This behavior can be difficult to diagnose. The following
example illustrates the difference between the two methods:
sed 1q /etc/passwd | awk '{ FS = ":" ; print $1 }'
which usually prints:
root
on an incorrect implementation of 'awk', while 'gawk' prints the full
first line of the file, something like:
root:x:0:0:Root:/:
(The 'sed'(2) command prints just the first line of '/etc/passwd'.)
---------- Footnotes ----------
(1) Thanks to Andrew Schorr for this tip.
(2) The 'sed' utility is a "stream editor." Its behavior is also
defined by the POSIX standard.
File: gawk.info, Node: Field Splitting Summary, Prev: Full Line Fields, Up: Field Separators
4.5.6 Field-Splitting Summary
-----------------------------
It is important to remember that when you assign a string constant as
the value of 'FS', it undergoes normal 'awk' string processing. For
example, with Unix 'awk' and 'gawk', the assignment 'FS = "\.."' assigns
the character string '".."' to 'FS' (the backslash is stripped). This
creates a regexp meaning "fields are separated by occurrences of any two
characters." If instead you want fields to be separated by a literal
period followed by any single character, use 'FS = "\\.."'.
The following list summarizes how fields are split, based on the
value of 'FS' ('==' means "is equal to"):
'FS == " "'
Fields are separated by runs of whitespace. Leading and trailing
whitespace are ignored. This is the default.
'FS == ANY OTHER SINGLE CHARACTER'
Fields are separated by each occurrence of the character. Multiple
successive occurrences delimit empty fields, as do leading and
trailing occurrences. The character can even be a regexp
metacharacter; it does not need to be escaped.
'FS == REGEXP'
Fields are separated by occurrences of characters that match
REGEXP. Leading and trailing matches of REGEXP delimit empty
fields.
'FS == ""'
Each individual character in the record becomes a separate field.
(This is a common extension; it is not specified by the POSIX
standard.)
'FS' and 'IGNORECASE'
The 'IGNORECASE' variable (*note User-modified::) affects field
splitting _only_ when the value of 'FS' is a regexp. It has no effect
when 'FS' is a single character, even if that character is a letter.
Thus, in the following code:
FS = "c"
IGNORECASE = 1
$0 = "aCa"
print $1
The output is 'aCa'. If you really want to split fields on an
alphabetic character while ignoring case, use a regexp that will do it
for you (e.g., 'FS = "[c]"'). In this case, 'IGNORECASE' will take
effect.
File: gawk.info, Node: Constant Size, Next: Splitting By Content, Prev: Field Separators, Up: Reading Files
4.6 Reading Fixed-Width Data
============================
This minor node discusses an advanced feature of 'gawk'. If you are a
novice 'awk' user, you might want to skip it on the first reading.
'gawk' provides a facility for dealing with fixed-width fields with
no distinctive field separator. We discuss this feature in the
following nodes.
* Menu:
* Fixed width data:: Processing fixed-width data.
* Skipping intervening:: Skipping intervening fields.
* Allowing trailing data:: Capturing optional trailing data.
* Fields with fixed data:: Field values with fixed-width data.
File: gawk.info, Node: Fixed width data, Next: Skipping intervening, Up: Constant Size
4.6.1 Processing Fixed-Width Data
---------------------------------
An example of fixed-width data would be the input for old Fortran
programs where numbers are run together, or the output of programs that
did not anticipate the use of their output as input for other programs.
An example of the latter is a table where all the columns are lined
up by the use of a variable number of spaces and _empty fields are just
spaces_. Clearly, 'awk''s normal field splitting based on 'FS' does not
work well in this case. Although a portable 'awk' program can use a
series of 'substr()' calls on '$0' (*note String Functions::), this is
awkward and inefficient for a large number of fields.
The splitting of an input record into fixed-width fields is specified
by assigning a string containing space-separated numbers to the built-in
variable 'FIELDWIDTHS'. Each number specifies the width of the field,
_including_ columns between fields. If you want to ignore the columns
between fields, you can specify the width as a separate field that is
subsequently ignored. It is a fatal error to supply a field width that
has a negative value.
The following data is the output of the Unix 'w' utility. It is
useful to illustrate the use of 'FIELDWIDTHS':
10:06pm up 21 days, 14:04, 23 users
User tty login idle JCPU PCPU what
hzuo ttyV0 8:58pm 9 5 vi p24.tex
hzang ttyV3 6:37pm 50 -csh
eklye ttyV5 9:53pm 7 1 em thes.tex
dportein ttyV6 8:17pm 1:47 -csh
gierd ttyD3 10:00pm 1 elm
dave ttyD4 9:47pm 4 4 w
brent ttyp0 26Jun91 4:46 26:46 4:41 bash
dave ttyq4 26Jun9115days 46 46 wnewmail
The following program takes this input, converts the idle time to
number of seconds, and prints out the first two fields and the
calculated idle time:
BEGIN { FIELDWIDTHS = "9 6 10 6 7 7 35" }
NR > 2 {
idle = $4
sub(/^ +/, "", idle) # strip leading spaces
if (idle == "")
idle = 0
if (idle ~ /:/) { # hh:mm
split(idle, t, ":")
idle = t[1] * 60 + t[2]
}
if (idle ~ /days/)
idle *= 24 * 60 * 60
print $1, $2, idle
}
NOTE: The preceding program uses a number of 'awk' features that
haven't been introduced yet.
Running the program on the data produces the following results:
hzuo ttyV0 0
hzang ttyV3 50
eklye ttyV5 0
dportein ttyV6 107
gierd ttyD3 1
dave ttyD4 0
brent ttyp0 286
dave ttyq4 1296000
Another (possibly more practical) example of fixed-width input data
is the input from a deck of balloting cards. In some parts of the
United States, voters mark their choices by punching holes in computer
cards. These cards are then processed to count the votes for any
particular candidate or on any particular issue. Because a voter may
choose not to vote on some issue, any column on the card may be empty.
An 'awk' program for processing such data could use the 'FIELDWIDTHS'
feature to simplify reading the data. (Of course, getting 'gawk' to run
on a system with card readers is another story!)
File: gawk.info, Node: Skipping intervening, Next: Allowing trailing data, Prev: Fixed width data, Up: Constant Size
4.6.2 Skipping Intervening Fields
---------------------------------
Starting in version 4.2, each field width may optionally be preceded by
a colon-separated value specifying the number of characters to skip
before the field starts. Thus, the preceding program could be rewritten
to specify 'FIELDWIDTHS' like so:
BEGIN { FIELDWIDTHS = "8 1:5 4:7 6 1:6 1:6 2:33" }
This strips away some of the white space separating the fields. With
such a change, the program produces the following results:
hzang ttyV3 50
eklye ttyV5 0
dportein ttyV6 107
gierd ttyD3 1
dave ttyD4 0
brent ttyp0 286
dave ttyq4 1296000
File: gawk.info, Node: Allowing trailing data, Next: Fields with fixed data, Prev: Skipping intervening, Up: Constant Size
4.6.3 Capturing Optional Trailing Data
--------------------------------------
There are times when fixed-width data may be followed by additional data
that has no fixed length. Such data may or may not be present, but if
it is, it should be possible to get at it from an 'awk' program.
Starting with version 4.2, in order to provide a way to say "anything
else in the record after the defined fields," 'gawk' allows you to add a
final '*' character to the value of 'FIELDWIDTHS'. There can only be
one such character, and it must be the final non-whitespace character in
'FIELDWIDTHS'. For example:
$ cat fw.awk Show the program
-| BEGIN { FIELDWIDTHS = "2 2 *" }
-| { print NF, $1, $2, $3 }
$ cat fw.in Show sample input
-| 1234abcdefghi
$ gawk -f fw.awk fw.in Run the program
-| 3 12 34 abcdefghi
File: gawk.info, Node: Fields with fixed data, Prev: Allowing trailing data, Up: Constant Size
4.6.4 Field Values With Fixed-Width Data
----------------------------------------
So far, so good. But what happens if there isn't as much data as there
should be based on the contents of 'FIELDWIDTHS'? Or, what happens if
there is more data than expected?
For many years, what happens in these cases was not well defined.
Starting with version 4.2, the rules are as follows:
Enough data for some fields
For example, if 'FIELDWIDTHS' is set to '"2 3 4"' and the input
record is 'aabbb'. In this case, 'NF' is set to two.
Not enough data for a field
For example, if 'FIELDWIDTHS' is set to '"2 3 4"' and the input
record is 'aab'. In this case, 'NF' is set to two and '$2' has the
value '"b"'. The idea is that even though there aren't as many
characters as were expected, there are some, so the data should be
made available to the program.
Too much data
For example, if 'FIELDWIDTHS' is set to '"2 3 4"' and the input
record is 'aabbbccccddd'. In this case, 'NF' is set to three and
the extra characters ('ddd') are ignored. If you want 'gawk' to
capture the extra characters, supply a final '*' in the value of
'FIELDWIDTHS'.
Too much data, but with '*' supplied
For example, if 'FIELDWIDTHS' is set to '"2 3 4 *"' and the input
record is 'aabbbccccddd'. In this case, 'NF' is set to four, and
'$4' has the value '"ddd"'.
File: gawk.info, Node: Splitting By Content, Next: Testing field creation, Prev: Constant Size, Up: Reading Files
4.7 Defining Fields by Content
==============================
This minor node discusses an advanced feature of 'gawk'. If you are a
novice 'awk' user, you might want to skip it on the first reading.
Normally, when using 'FS', 'gawk' defines the fields as the parts of
the record that occur in between each field separator. In other words,
'FS' defines what a field _is not_, instead of what a field _is_.
However, there are times when you really want to define the fields by
what they are, and not by what they are not.
The most notorious such case is so-called "comma-separated values"
(CSV) data. Many spreadsheet programs, for example, can export their
data into text files, where each record is terminated with a newline,
and fields are separated by commas. If commas only separated the data,
there wouldn't be an issue. The problem comes when one of the fields
contains an _embedded_ comma. In such cases, most programs embed the
field in double quotes.(1) So, we might have data like this:
Robbins,Arnold,"1234 A Pretty Street, NE",MyTown,MyState,12345-6789,USA
The 'FPAT' variable offers a solution for cases like this. The value
of 'FPAT' should be a string that provides a regular expression. This
regular expression describes the contents of each field.
In the case of CSV data as presented here, each field is either
"anything that is not a comma," or "a double quote, anything that is not
a double quote, and a closing double quote." If written as a regular
expression constant (*note Regexp::), we would have
'/([^,]+)|("[^"]+")/'. Writing this as a string requires us to escape
the double quotes, leading to:
FPAT = "([^,]+)|(\"[^\"]+\")"
Putting this to use, here is a simple program to parse the data:
BEGIN {
FPAT = "([^,]+)|(\"[^\"]+\")"
}
{
print "NF = ", NF
for (i = 1; i <= NF; i++) {
printf("$%d = <%s>\n", i, $i)
}
}
When run, we get the following:
$ gawk -f simple-csv.awk addresses.csv
NF = 7
$1 = <Robbins>
$2 = <Arnold>
$3 = <"1234 A Pretty Street, NE">
$4 = <MyTown>
$5 = <MyState>
$6 = <12345-6789>
$7 = <USA>
Note the embedded comma in the value of '$3'.
A straightforward improvement when processing CSV data of this sort
would be to remove the quotes when they occur, with something like this:
if (substr($i, 1, 1) == "\"") {
len = length($i)
$i = substr($i, 2, len - 2) # Get text within the two quotes
}
As with 'FS', the 'IGNORECASE' variable (*note User-modified::)
affects field splitting with 'FPAT'.
Assigning a value to 'FPAT' overrides field splitting with 'FS' and
with 'FIELDWIDTHS'.
NOTE: Some programs export CSV data that contains embedded newlines
between the double quotes. 'gawk' provides no way to deal with
this. Even though a formal specification for CSV data exists,
there isn't much more to be done; the 'FPAT' mechanism provides an
elegant solution for the majority of cases, and the 'gawk'
developers are satisfied with that.
As written, the regexp used for 'FPAT' requires that each field
contain at least one character. A straightforward modification
(changing the first '+' to '*') allows fields to be empty:
FPAT = "([^,]*)|(\"[^\"]+\")"
Finally, the 'patsplit()' function makes the same functionality
available for splitting regular strings (*note String Functions::).
---------- Footnotes ----------
(1) The CSV format lacked a formal standard definition for many
years. RFC 4180 (http://www.ietf.org/rfc/rfc4180.txt) standardizes the
most common practices.
File: gawk.info, Node: Testing field creation, Next: Multiple Line, Prev: Splitting By Content, Up: Reading Files
4.8 Checking How 'gawk' Is Splitting Records
============================================
As we've seen, 'gawk' provides three independent methods to split input
records into fields. The mechanism used is based on which of the three
variables--'FS', 'FIELDWIDTHS', or 'FPAT'--was last assigned to. In
addition, an API input parser may choose to override the record parsing
mechanism; please refer to *note Input Parsers:: for further information
about this feature.
To restore normal field splitting after using 'FIELDWIDTHS' and/or
'FPAT', simply assign a value to 'FS'. You can use 'FS = FS' to do
this, without having to know the current value of 'FS'.
In order to tell which kind of field splitting is in effect, use
'PROCINFO["FS"]' (*note Auto-set::). The value is '"FS"' if regular
field splitting is being used, '"FIELDWIDTHS"' if fixed-width field
splitting is being used, or '"FPAT"' if content-based field splitting is
being used:
if (PROCINFO["FS"] == "FS")
REGULAR FIELD SPLITTING ...
else if (PROCINFO["FS"] == "FIELDWIDTHS")
FIXED-WIDTH FIELD SPLITTING ...
else if (PROCINFO["FS"] == "FPAT")
CONTENT-BASED FIELD SPLITTING ...
else
API INPUT PARSER FIELD SPLITTING ... (advanced feature)
This information is useful when writing a function that needs to
temporarily change 'FS' or 'FIELDWIDTHS', read some records, and then
restore the original settings (*note Passwd Functions:: for an example
of such a function).
File: gawk.info, Node: Multiple Line, Next: Getline, Prev: Testing field creation, Up: Reading Files
4.9 Multiple-Line Records
=========================
In some databases, a single line cannot conveniently hold all the
information in one entry. In such cases, you can use multiline records.
The first step in doing this is to choose your data format.
One technique is to use an unusual character or string to separate
records. For example, you could use the formfeed character (written
'\f' in 'awk', as in C) to separate them, making each record a page of
the file. To do this, just set the variable 'RS' to '"\f"' (a string
containing the formfeed character). Any other character could equally
well be used, as long as it won't be part of the data in a record.
Another technique is to have blank lines separate records. By a
special dispensation, an empty string as the value of 'RS' indicates
that records are separated by one or more blank lines. When 'RS' is set
to the empty string, each record always ends at the first blank line
encountered. The next record doesn't start until the first nonblank
line that follows. No matter how many blank lines appear in a row, they
all act as one record separator. (Blank lines must be completely empty;
lines that contain only whitespace do not count.)
You can achieve the same effect as 'RS = ""' by assigning the string
'"\n\n+"' to 'RS'. This regexp matches the newline at the end of the
record and one or more blank lines after the record. In addition, a
regular expression always matches the longest possible sequence when
there is a choice (*note Leftmost Longest::). So, the next record
doesn't start until the first nonblank line that follows--no matter how
many blank lines appear in a row, they are considered one record
separator.
However, there is an important difference between 'RS = ""' and 'RS =
"\n\n+"'. In the first case, leading newlines in the input data file
are ignored, and if a file ends without extra blank lines after the last
record, the final newline is removed from the record. In the second
case, this special processing is not done. (d.c.)
Now that the input is separated into records, the second step is to
separate the fields in the records. One way to do this is to divide
each of the lines into fields in the normal manner. This happens by
default as the result of a special feature. When 'RS' is set to the
empty string _and_ 'FS' is set to a single character, the newline
character _always_ acts as a field separator. This is in addition to
whatever field separations result from 'FS'.(1)
The original motivation for this special exception was probably to
provide useful behavior in the default case (i.e., 'FS' is equal to
'" "'). This feature can be a problem if you really don't want the
newline character to separate fields, because there is no way to prevent
it. However, you can work around this by using the 'split()' function
to break up the record manually (*note String Functions::). If you have
a single-character field separator, you can work around the special
feature in a different way, by making 'FS' into a regexp for that single
character. For example, if the field separator is a percent character,
instead of 'FS = "%"', use 'FS = "[%]"'.
Another way to separate fields is to put each field on a separate
line: to do this, just set the variable 'FS' to the string '"\n"'.
(This single-character separator matches a single newline.) A practical
example of a data file organized this way might be a mailing list, where
blank lines separate the entries. Consider a mailing list in a file
named 'addresses', which looks like this:
Jane Doe
123 Main Street
Anywhere, SE 12345-6789
John Smith
456 Tree-lined Avenue
Smallville, MW 98765-4321
...
A simple program to process this file is as follows:
# addrs.awk --- simple mailing list program
# Records are separated by blank lines.
# Each line is one field.
BEGIN { RS = "" ; FS = "\n" }
{
print "Name is:", $1
print "Address is:", $2
print "City and State are:", $3
print ""
}
Running the program produces the following output:
$ awk -f addrs.awk addresses
-| Name is: Jane Doe
-| Address is: 123 Main Street
-| City and State are: Anywhere, SE 12345-6789
-|
-| Name is: John Smith
-| Address is: 456 Tree-lined Avenue
-| City and State are: Smallville, MW 98765-4321
-|
...
*Note Labels Program:: for a more realistic program dealing with
address lists. The following list summarizes how records are split,
based on the value of 'RS'. ('==' means "is equal to.")
'RS == "\n"'
Records are separated by the newline character ('\n'). In effect,
every line in the data file is a separate record, including blank
lines. This is the default.
'RS == ANY SINGLE CHARACTER'
Records are separated by each occurrence of the character.
Multiple successive occurrences delimit empty records.
'RS == ""'
Records are separated by runs of blank lines. When 'FS' is a
single character, then the newline character always serves as a
field separator, in addition to whatever value 'FS' may have.
Leading and trailing newlines in a file are ignored.
'RS == REGEXP'
Records are separated by occurrences of characters that match
REGEXP. Leading and trailing matches of REGEXP delimit empty
records. (This is a 'gawk' extension; it is not specified by the
POSIX standard.)
If not in compatibility mode (*note Options::), 'gawk' sets 'RT' to
the input text that matched the value specified by 'RS'. But if the
input file ended without any text that matches 'RS', then 'gawk' sets
'RT' to the null string.
---------- Footnotes ----------
(1) When 'FS' is the null string ('""') or a regexp, this special
feature of 'RS' does not apply. It does apply to the default field
separator of a single space: 'FS = " "'.
File: gawk.info, Node: Getline, Next: Read Timeout, Prev: Multiple Line, Up: Reading Files
4.10 Explicit Input with 'getline'
==================================
So far we have been getting our input data from 'awk''s main input
stream--either the standard input (usually your keyboard, sometimes the
output from another program) or the files specified on the command line.
The 'awk' language has a special built-in command called 'getline' that
can be used to read input under your explicit control.
The 'getline' command is used in several different ways and should
_not_ be used by beginners. The examples that follow the explanation of
the 'getline' command include material that has not been covered yet.
Therefore, come back and study the 'getline' command _after_ you have
reviewed the rest of this Info file and have a good knowledge of how
'awk' works.
The 'getline' command returns 1 if it finds a record and 0 if it
encounters the end of the file. If there is some error in getting a
record, such as a file that cannot be opened, then 'getline' returns -1.
In this case, 'gawk' sets the variable 'ERRNO' to a string describing
the error that occurred.
If 'ERRNO' indicates that the I/O operation may be retried, and
'PROCINFO["INPUT", "RETRY"]' is set, then 'getline' returns -2 instead
of -1, and further calls to 'getline' may be attempted. *Note Retrying
Input:: for further information about this feature.
In the following examples, COMMAND stands for a string value that
represents a shell command.
NOTE: When '--sandbox' is specified (*note Options::), reading
lines from files, pipes, and coprocesses is disabled.
* Menu:
* Plain Getline:: Using 'getline' with no arguments.
* Getline/Variable:: Using 'getline' into a variable.
* Getline/File:: Using 'getline' from a file.
* Getline/Variable/File:: Using 'getline' into a variable from a
file.
* Getline/Pipe:: Using 'getline' from a pipe.
* Getline/Variable/Pipe:: Using 'getline' into a variable from a
pipe.
* Getline/Coprocess:: Using 'getline' from a coprocess.
* Getline/Variable/Coprocess:: Using 'getline' into a variable from a
coprocess.
* Getline Notes:: Important things to know about 'getline'.
* Getline Summary:: Summary of 'getline' Variants.
File: gawk.info, Node: Plain Getline, Next: Getline/Variable, Up: Getline
4.10.1 Using 'getline' with No Arguments
----------------------------------------
The 'getline' command can be used without arguments to read input from
the current input file. All it does in this case is read the next input
record and split it up into fields. This is useful if you've finished
processing the current record, but want to do some special processing on
the next record _right now_. For example:
# Remove text between /* and */, inclusive
{
if ((i = index($0, "/*")) != 0) {
out = substr($0, 1, i - 1) # leading part of the string
rest = substr($0, i + 2) # ... */ ...
j = index(rest, "*/") # is */ in trailing part?
if (j > 0) {
rest = substr(rest, j + 2) # remove comment
} else {
while (j == 0) {
# get more text
if (getline <= 0) {
print("unexpected EOF or error:", ERRNO) > "/dev/stderr"
exit
}
# build up the line using string concatenation
rest = rest $0
j = index(rest, "*/") # is */ in trailing part?
if (j != 0) {
rest = substr(rest, j + 2)
break
}
}
}
# build up the output line using string concatenation
$0 = out rest
}
print $0
}
This 'awk' program deletes C-style comments ('/* ... */') from the
input. It uses a number of features we haven't covered yet, including
string concatenation (*note Concatenation::) and the 'index()' and
'substr()' built-in functions (*note String Functions::). By replacing
the 'print $0' with other statements, you could perform more complicated
processing on the decommented input, such as searching for matches of a
regular expression. (This program has a subtle problem--it does not
work if one comment ends and another begins on the same line.)
This form of the 'getline' command sets 'NF', 'NR', 'FNR', 'RT', and
the value of '$0'.
NOTE: The new value of '$0' is used to test the patterns of any
subsequent rules. The original value of '$0' that triggered the
rule that executed 'getline' is lost. By contrast, the 'next'
statement reads a new record but immediately begins processing it
normally, starting with the first rule in the program. *Note Next
Statement::.
File: gawk.info, Node: Getline/Variable, Next: Getline/File, Prev: Plain Getline, Up: Getline
4.10.2 Using 'getline' into a Variable
--------------------------------------
You can use 'getline VAR' to read the next record from 'awk''s input
into the variable VAR. No other processing is done. For example,
suppose the next line is a comment or a special string, and you want to
read it without triggering any rules. This form of 'getline' allows you
to read that line and store it in a variable so that the main
read-a-line-and-check-each-rule loop of 'awk' never sees it. The
following example swaps every two lines of input:
{
if ((getline tmp) > 0) {
print tmp
print $0
} else
print $0
}
It takes the following list:
wan
tew
free
phore
and produces these results:
tew
wan
phore
free
The 'getline' command used in this way sets only the variables 'NR',
'FNR', and 'RT' (and, of course, VAR). The record is not split into
fields, so the values of the fields (including '$0') and the value of
'NF' do not change.
File: gawk.info, Node: Getline/File, Next: Getline/Variable/File, Prev: Getline/Variable, Up: Getline
4.10.3 Using 'getline' from a File
----------------------------------
Use 'getline < FILE' to read the next record from FILE. Here, FILE is a
string-valued expression that specifies the file name. '< FILE' is
called a "redirection" because it directs input to come from a different
place. For example, the following program reads its input record from
the file 'secondary.input' when it encounters a first field with a value
equal to 10 in the current input file:
{
if ($1 == 10) {
getline < "secondary.input"
print
} else
print
}
Because the main input stream is not used, the values of 'NR' and
'FNR' are not changed. However, the record it reads is split into
fields in the normal manner, so the values of '$0' and the other fields
are changed, resulting in a new value of 'NF'. 'RT' is also set.
According to POSIX, 'getline < EXPRESSION' is ambiguous if EXPRESSION
contains unparenthesized operators other than '$'; for example, 'getline
< dir "/" file' is ambiguous because the concatenation operator (not
discussed yet; *note Concatenation::) is not parenthesized. You should
write it as 'getline < (dir "/" file)' if you want your program to be
portable to all 'awk' implementations.
File: gawk.info, Node: Getline/Variable/File, Next: Getline/Pipe, Prev: Getline/File, Up: Getline
4.10.4 Using 'getline' into a Variable from a File
--------------------------------------------------
Use 'getline VAR < FILE' to read input from the file FILE, and put it in
the variable VAR. As earlier, FILE is a string-valued expression that
specifies the file from which to read.
In this version of 'getline', none of the predefined variables are
changed and the record is not split into fields. The only variable
changed is VAR.(1) For example, the following program copies all the
input files to the output, except for records that say
'@include FILENAME'. Such a record is replaced by the contents of the
file FILENAME:
{
if (NF == 2 && $1 == "@include") {
while ((getline line < $2) > 0)
print line
close($2)
} else
print
}
Note here how the name of the extra input file is not built into the
program; it is taken directly from the data, specifically from the
second field on the '@include' line.
The 'close()' function is called to ensure that if two identical
'@include' lines appear in the input, the entire specified file is
included twice. *Note Close Files And Pipes::.
One deficiency of this program is that it does not process nested
'@include' statements (i.e., '@include' statements in included files)
the way a true macro preprocessor would. *Note Igawk Program:: for a
program that does handle nested '@include' statements.
---------- Footnotes ----------
(1) This is not quite true. 'RT' could be changed if 'RS' is a
regular expression.
File: gawk.info, Node: Getline/Pipe, Next: Getline/Variable/Pipe, Prev: Getline/Variable/File, Up: Getline
4.10.5 Using 'getline' from a Pipe
----------------------------------
Omniscience has much to recommend it. Failing that, attention to
details would be useful.
-- _Brian Kernighan_
The output of a command can also be piped into 'getline', using
'COMMAND | getline'. In this case, the string COMMAND is run as a shell
command and its output is piped into 'awk' to be used as input. This
form of 'getline' reads one record at a time from the pipe. For
example, the following program copies its input to its output, except
for lines that begin with '@execute', which are replaced by the output
produced by running the rest of the line as a shell command:
{
if ($1 == "@execute") {
tmp = substr($0, 10) # Remove "@execute"
while ((tmp | getline) > 0)
print
close(tmp)
} else
print
}
The 'close()' function is called to ensure that if two identical
'@execute' lines appear in the input, the command is run for each one.
*Note Close Files And Pipes::. Given the input:
foo
bar
baz
@execute who
bletch
the program might produce:
foo
bar
baz
arnold ttyv0 Jul 13 14:22
miriam ttyp0 Jul 13 14:23 (murphy:0)
bill ttyp1 Jul 13 14:23 (murphy:0)
bletch
Notice that this program ran the command 'who' and printed the result.
(If you try this program yourself, you will of course get different
results, depending upon who is logged in on your system.)
This variation of 'getline' splits the record into fields, sets the
value of 'NF', and recomputes the value of '$0'. The values of 'NR' and
'FNR' are not changed. 'RT' is set.
According to POSIX, 'EXPRESSION | getline' is ambiguous if EXPRESSION
contains unparenthesized operators other than '$'--for example, '"echo "
"date" | getline' is ambiguous because the concatenation operator is not
parenthesized. You should write it as '("echo " "date") | getline' if
you want your program to be portable to all 'awk' implementations.
NOTE: Unfortunately, 'gawk' has not been consistent in its
treatment of a construct like '"echo " "date" | getline'. Most
versions, including the current version, treat it at as '("echo "
"date") | getline'. (This is also how BWK 'awk' behaves.) Some
versions instead treat it as '"echo " ("date" | getline)'. (This
is how 'mawk' behaves.) In short, _always_ use explicit
parentheses, and then you won't have to worry.
File: gawk.info, Node: Getline/Variable/Pipe, Next: Getline/Coprocess, Prev: Getline/Pipe, Up: Getline
4.10.6 Using 'getline' into a Variable from a Pipe
--------------------------------------------------
When you use 'COMMAND | getline VAR', the output of COMMAND is sent
through a pipe to 'getline' and into the variable VAR. For example, the
following program reads the current date and time into the variable
'current_time', using the 'date' utility, and then prints it:
BEGIN {
"date" | getline current_time
close("date")
print "Report printed on " current_time
}
In this version of 'getline', none of the predefined variables are
changed and the record is not split into fields. However, 'RT' is set.
According to POSIX, 'EXPRESSION | getline VAR' is ambiguous if
EXPRESSION contains unparenthesized operators other than '$'; for
example, '"echo " "date" | getline VAR' is ambiguous because the
concatenation operator is not parenthesized. You should write it as
'("echo " "date") | getline VAR' if you want your program to be portable
to other 'awk' implementations.
File: gawk.info, Node: Getline/Coprocess, Next: Getline/Variable/Coprocess, Prev: Getline/Variable/Pipe, Up: Getline
4.10.7 Using 'getline' from a Coprocess
---------------------------------------
Reading input into 'getline' from a pipe is a one-way operation. The
command that is started with 'COMMAND | getline' only sends data _to_
your 'awk' program.
On occasion, you might want to send data to another program for
processing and then read the results back. 'gawk' allows you to start a
"coprocess", with which two-way communications are possible. This is
done with the '|&' operator. Typically, you write data to the coprocess
first and then read the results back, as shown in the following:
print "SOME QUERY" |& "db_server"
"db_server" |& getline
which sends a query to 'db_server' and then reads the results.
The values of 'NR' and 'FNR' are not changed, because the main input
stream is not used. However, the record is split into fields in the
normal manner, thus changing the values of '$0', of the other fields,
and of 'NF' and 'RT'.
Coprocesses are an advanced feature. They are discussed here only
because this is the minor node on 'getline'. *Note Two-way I/O::, where
coprocesses are discussed in more detail.
File: gawk.info, Node: Getline/Variable/Coprocess, Next: Getline Notes, Prev: Getline/Coprocess, Up: Getline
4.10.8 Using 'getline' into a Variable from a Coprocess
-------------------------------------------------------
When you use 'COMMAND |& getline VAR', the output from the coprocess
COMMAND is sent through a two-way pipe to 'getline' and into the
variable VAR.
In this version of 'getline', none of the predefined variables are
changed and the record is not split into fields. The only variable
changed is VAR. However, 'RT' is set.
Coprocesses are an advanced feature. They are discussed here only
because this is the minor node on 'getline'. *Note Two-way I/O::, where
coprocesses are discussed in more detail.
File: gawk.info, Node: Getline Notes, Next: Getline Summary, Prev: Getline/Variable/Coprocess, Up: Getline
4.10.9 Points to Remember About 'getline'
-----------------------------------------
Here are some miscellaneous points about 'getline' that you should bear
in mind:
* When 'getline' changes the value of '$0' and 'NF', 'awk' does _not_
automatically jump to the start of the program and start testing
the new record against every pattern. However, the new record is
tested against any subsequent rules.
* Some very old 'awk' implementations limit the number of pipelines
that an 'awk' program may have open to just one. In 'gawk', there
is no such limit. You can open as many pipelines (and coprocesses)
as the underlying operating system permits.
* An interesting side effect occurs if you use 'getline' without a
redirection inside a 'BEGIN' rule. Because an unredirected
'getline' reads from the command-line data files, the first
'getline' command causes 'awk' to set the value of 'FILENAME'.
Normally, 'FILENAME' does not have a value inside 'BEGIN' rules,
because you have not yet started to process the command-line data
files. (d.c.) (See *note BEGIN/END::; also *note Auto-set::.)
* Using 'FILENAME' with 'getline' ('getline < FILENAME') is likely to
be a source of confusion. 'awk' opens a separate input stream from
the current input file. However, by not using a variable, '$0' and
'NF' are still updated. If you're doing this, it's probably by
accident, and you should reconsider what it is you're trying to
accomplish.
* *note Getline Summary::, presents a table summarizing the 'getline'
variants and which variables they can affect. It is worth noting
that those variants that do not use redirection can cause
'FILENAME' to be updated if they cause 'awk' to start reading a new
input file.
* If the variable being assigned is an expression with side effects,
different versions of 'awk' behave differently upon encountering
end-of-file. Some versions don't evaluate the expression; many
versions (including 'gawk') do. Here is an example, courtesy of
Duncan Moore:
BEGIN {
system("echo 1 > f")
while ((getline a[++c] < "f") > 0) { }
print c
}
Here, the side effect is the '++c'. Is 'c' incremented if
end-of-file is encountered before the element in 'a' is assigned?
'gawk' treats 'getline' like a function call, and evaluates the
expression 'a[++c]' before attempting to read from 'f'. However,
some versions of 'awk' only evaluate the expression once they know
that there is a string value to be assigned.
File: gawk.info, Node: Getline Summary, Prev: Getline Notes, Up: Getline
4.10.10 Summary of 'getline' Variants
-------------------------------------
*note Table 4.1: table-getline-variants. summarizes the eight variants
of 'getline', listing which predefined variables are set by each one,
and whether the variant is standard or a 'gawk' extension. Note: for
each variant, 'gawk' sets the 'RT' predefined variable.
Variant Effect 'awk' / 'gawk'
-------------------------------------------------------------------------
'getline' Sets '$0', 'NF', 'FNR', 'awk'
'NR', and 'RT'
'getline' VAR Sets VAR, 'FNR', 'NR', 'awk'
and 'RT'
'getline <' FILE Sets '$0', 'NF', and 'RT' 'awk'
'getline VAR < FILE' Sets VAR and 'RT' 'awk'
COMMAND '| getline' Sets '$0', 'NF', and 'RT' 'awk'
COMMAND '| getline' Sets VAR and 'RT' 'awk'
VAR
COMMAND '|& getline' Sets '$0', 'NF', and 'RT' 'gawk'
COMMAND '|& getline' Sets VAR and 'RT' 'gawk'
VAR
Table 4.1: 'getline' variants and what they set
File: gawk.info, Node: Read Timeout, Next: Retrying Input, Prev: Getline, Up: Reading Files
4.11 Reading Input with a Timeout
=================================
This minor node describes a feature that is specific to 'gawk'.
You may specify a timeout in milliseconds for reading input from the
keyboard, a pipe, or two-way communication, including TCP/IP sockets.
This can be done on a per-input, per-command, or per-connection basis,
by setting a special element in the 'PROCINFO' array (*note Auto-set::):
PROCINFO["input_name", "READ_TIMEOUT"] = TIMEOUT IN MILLISECONDS
When set, this causes 'gawk' to time out and return failure if no
data is available to read within the specified timeout period. For
example, a TCP client can decide to give up on receiving any response
from the server after a certain amount of time:
Service = "/inet/tcp/0/localhost/daytime"
PROCINFO[Service, "READ_TIMEOUT"] = 100
if ((Service |& getline) > 0)
print $0
else if (ERRNO != "")
print ERRNO
Here is how to read interactively from the user(1) without waiting
for more than five seconds:
PROCINFO["/dev/stdin", "READ_TIMEOUT"] = 5000
while ((getline < "/dev/stdin") > 0)
print $0
'gawk' terminates the read operation if input does not arrive after
waiting for the timeout period, returns failure, and sets 'ERRNO' to an
appropriate string value. A negative or zero value for the timeout is
the same as specifying no timeout at all.
A timeout can also be set for reading from the keyboard in the
implicit loop that reads input records and matches them against
patterns, like so:
$ gawk 'BEGIN { PROCINFO["-", "READ_TIMEOUT"] = 5000 }
> { print "You entered: " $0 }'
gawk
-| You entered: gawk
In this case, failure to respond within five seconds results in the
following error message:
error-> gawk: cmd. line:2: (FILENAME=- FNR=1) fatal: error reading input file `-': Connection timed out
The timeout can be set or changed at any time, and will take effect
on the next attempt to read from the input device. In the following
example, we start with a timeout value of one second, and progressively
reduce it by one-tenth of a second until we wait indefinitely for the
input to arrive:
PROCINFO[Service, "READ_TIMEOUT"] = 1000
while ((Service |& getline) > 0) {
print $0
PROCINFO[Service, "READ_TIMEOUT"] -= 100
}
NOTE: You should not assume that the read operation will block
exactly after the tenth record has been printed. It is possible
that 'gawk' will read and buffer more than one record's worth of
data the first time. Because of this, changing the value of
timeout like in the preceding example is not very useful.
If the 'PROCINFO' element is not present and the 'GAWK_READ_TIMEOUT'
environment variable exists, 'gawk' uses its value to initialize the
timeout value. The exclusive use of the environment variable to specify
timeout has the disadvantage of not being able to control it on a
per-command or per-connection basis.
'gawk' considers a timeout event to be an error even though the
attempt to read from the underlying device may succeed in a later
attempt. This is a limitation, and it also means that you cannot use
this to multiplex input from two or more sources. *Note Retrying
Input:: for a way to enable later I/O attempts to succeed.
Assigning a timeout value prevents read operations from blocking
indefinitely. But bear in mind that there are other ways 'gawk' can
stall waiting for an input device to be ready. A network client can
sometimes take a long time to establish a connection before it can start
reading any data, or the attempt to open a FIFO special file for reading
can block indefinitely until some other process opens it for writing.
---------- Footnotes ----------
(1) This assumes that standard input is the keyboard.
File: gawk.info, Node: Retrying Input, Next: Command-line directories, Prev: Read Timeout, Up: Reading Files
4.12 Retrying Reads After Certain Input Errors
==============================================
This minor node describes a feature that is specific to 'gawk'.
When 'gawk' encounters an error while reading input, by default
'getline' returns -1, and subsequent attempts to read from that file
result in an end-of-file indication. However, you may optionally
instruct 'gawk' to allow I/O to be retried when certain errors are
encountered by setting a special element in the 'PROCINFO' array (*note
Auto-set::):
PROCINFO["INPUT_NAME", "RETRY"] = 1
When this element exists, 'gawk' checks the value of the system (C
language) 'errno' variable when an I/O error occurs. If 'errno'
indicates a subsequent I/O attempt may succeed, 'getline' instead
returns -2 and further calls to 'getline' may succeed. This applies to
the 'errno' values 'EAGAIN', 'EWOULDBLOCK', 'EINTR', or 'ETIMEDOUT'.
This feature is useful in conjunction with 'PROCINFO["INPUT_NAME",
"READ_TIMEOUT"]' or situations where a file descriptor has been
configured to behave in a non-blocking fashion.
File: gawk.info, Node: Command-line directories, Next: Input Summary, Prev: Retrying Input, Up: Reading Files
4.13 Directories on the Command Line
====================================
According to the POSIX standard, files named on the 'awk' command line
must be text files; it is a fatal error if they are not. Most versions
of 'awk' treat a directory on the command line as a fatal error.
By default, 'gawk' produces a warning for a directory on the command
line, but otherwise ignores it. This makes it easier to use shell
wildcards with your 'awk' program:
$ gawk -f whizprog.awk * Directories could kill this program
If either of the '--posix' or '--traditional' options is given, then
'gawk' reverts to treating a directory on the command line as a fatal
error.
*Note Extension Sample Readdir:: for a way to treat directories as
usable data from an 'awk' program.
File: gawk.info, Node: Input Summary, Next: Input Exercises, Prev: Command-line directories, Up: Reading Files
4.14 Summary
============
* Input is split into records based on the value of 'RS'. The
possibilities are as follows:
Value of 'RS' Records are split on 'awk' / 'gawk'
...
---------------------------------------------------------------------------
Any single That character 'awk'
character
The empty string Runs of two or more 'awk'
('""') newlines
A regexp Text that matches the 'gawk'
regexp
* 'FNR' indicates how many records have been read from the current
input file; 'NR' indicates how many records have been read in
total.
* 'gawk' sets 'RT' to the text matched by 'RS'.
* After splitting the input into records, 'awk' further splits the
records into individual fields, named '$1', '$2', and so on. '$0'
is the whole record, and 'NF' indicates how many fields there are.
The default way to split fields is between whitespace characters.
* Fields may be referenced using a variable, as in '$NF'. Fields may
also be assigned values, which causes the value of '$0' to be
recomputed when it is later referenced. Assigning to a field with
a number greater than 'NF' creates the field and rebuilds the
record, using 'OFS' to separate the fields. Incrementing 'NF' does
the same thing. Decrementing 'NF' throws away fields and rebuilds
the record.
* Field splitting is more complicated than record splitting:
Field separator value Fields are split ... 'awk' /
'gawk'
---------------------------------------------------------------------------
'FS == " "' On runs of whitespace 'awk'
'FS == ANY SINGLE On that character 'awk'
CHARACTER'
'FS == REGEXP' On text matching the regexp 'awk'
'FS == ""' Such that each individual 'gawk'
character is a separate
field
'FIELDWIDTHS == LIST OF Based on character position 'gawk'
COLUMNS'
'FPAT == REGEXP' On the text surrounding 'gawk'
text matching the regexp
* Using 'FS = "\n"' causes the entire record to be a single field
(assuming that newlines separate records).
* 'FS' may be set from the command line using the '-F' option. This
can also be done using command-line variable assignment.
* Use 'PROCINFO["FS"]' to see how fields are being split.
* Use 'getline' in its various forms to read additional records from
the default input stream, from a file, or from a pipe or coprocess.
* Use 'PROCINFO[FILE, "READ_TIMEOUT"]' to cause reads to time out for
FILE.
* Directories on the command line are fatal for standard 'awk';
'gawk' ignores them if not in POSIX mode.
File: gawk.info, Node: Input Exercises, Prev: Input Summary, Up: Reading Files
4.15 Exercises
==============
1. Using the 'FIELDWIDTHS' variable (*note Constant Size::), write a
program to read election data, where each record represents one
voter's votes. Come up with a way to define which columns are
associated with each ballot item, and print the total votes,
including abstentions, for each item.
2. *note Plain Getline::, presented a program to remove C-style
comments ('/* ... */') from the input. That program does not work
if one comment ends on one line and another one starts later on the
same line. That can be fixed by making one simple change. What is
it?
File: gawk.info, Node: Printing, Next: Expressions, Prev: Reading Files, Up: Top
5 Printing Output
*****************
One of the most common programming actions is to "print", or output,
some or all of the input. Use the 'print' statement for simple output,
and the 'printf' statement for fancier formatting. The 'print'
statement is not limited when computing _which_ values to print.
However, with two exceptions, you cannot specify _how_ to print
them--how many columns, whether to use exponential notation or not, and
so on. (For the exceptions, *note Output Separators:: and *note
OFMT::.) For printing with specifications, you need the 'printf'
statement (*note Printf::).
Besides basic and formatted printing, this major node also covers I/O
redirections to files and pipes, introduces the special file names that
'gawk' processes internally, and discusses the 'close()' built-in
function.
* Menu:
* Print:: The 'print' statement.
* Print Examples:: Simple examples of 'print' statements.
* Output Separators:: The output separators and how to change them.
* OFMT:: Controlling Numeric Output With 'print'.
* Printf:: The 'printf' statement.
* Redirection:: How to redirect output to multiple files and
pipes.
* Special FD:: Special files for I/O.
* Special Files:: File name interpretation in 'gawk'.
'gawk' allows access to inherited file
descriptors.
* Close Files And Pipes:: Closing Input and Output Files and Pipes.
* Nonfatal:: Enabling Nonfatal Output.
* Output Summary:: Output summary.
* Output Exercises:: Exercises.
File: gawk.info, Node: Print, Next: Print Examples, Up: Printing
5.1 The 'print' Statement
=========================
Use the 'print' statement to produce output with simple, standardized
formatting. You specify only the strings or numbers to print, in a list
separated by commas. They are output, separated by single spaces,
followed by a newline. The statement looks like this:
print ITEM1, ITEM2, ...
The entire list of items may be optionally enclosed in parentheses. The
parentheses are necessary if any of the item expressions uses the '>'
relational operator; otherwise it could be confused with an output
redirection (*note Redirection::).
The items to print can be constant strings or numbers, fields of the
current record (such as '$1'), variables, or any 'awk' expression.
Numeric values are converted to strings and then printed.
The simple statement 'print' with no items is equivalent to 'print
$0': it prints the entire current record. To print a blank line, use
'print ""'. To print a fixed piece of text, use a string constant, such
as '"Don't Panic"', as one item. If you forget to use the double-quote
characters, your text is taken as an 'awk' expression, and you will
probably get an error. Keep in mind that a space is printed between any
two items.
Note that the 'print' statement is a statement and not an
expression--you can't use it in the pattern part of a pattern-action
statement, for example.
File: gawk.info, Node: Print Examples, Next: Output Separators, Prev: Print, Up: Printing
5.2 'print' Statement Examples
==============================
Each 'print' statement makes at least one line of output. However, it
isn't limited to only one line. If an item value is a string containing
a newline, the newline is output along with the rest of the string. A
single 'print' statement can make any number of lines this way.
The following is an example of printing a string that contains
embedded newlines (the '\n' is an escape sequence, used to represent the
newline character; *note Escape Sequences::):
$ awk 'BEGIN { print "line one\nline two\nline three" }'
-| line one
-| line two
-| line three
The next example, which is run on the 'inventory-shipped' file,
prints the first two fields of each input record, with a space between
them:
$ awk '{ print $1, $2 }' inventory-shipped
-| Jan 13
-| Feb 15
-| Mar 15
...
A common mistake in using the 'print' statement is to omit the comma
between two items. This often has the effect of making the items run
together in the output, with no space. The reason for this is that
juxtaposing two string expressions in 'awk' means to concatenate them.
Here is the same program, without the comma:
$ awk '{ print $1 $2 }' inventory-shipped
-| Jan13
-| Feb15
-| Mar15
...
To someone unfamiliar with the 'inventory-shipped' file, neither
example's output makes much sense. A heading line at the beginning
would make it clearer. Let's add some headings to our table of months
('$1') and green crates shipped ('$2'). We do this using a 'BEGIN' rule
(*note BEGIN/END::) so that the headings are only printed once:
awk 'BEGIN { print "Month Crates"
print "----- ------" }
{ print $1, $2 }' inventory-shipped
When run, the program prints the following:
Month Crates
----- ------
Jan 13
Feb 15
Mar 15
...
The only problem, however, is that the headings and the table data don't
line up! We can fix this by printing some spaces between the two
fields:
awk 'BEGIN { print "Month Crates"
print "----- ------" }
{ print $1, " ", $2 }' inventory-shipped
Lining up columns this way can get pretty complicated when there are
many columns to fix. Counting spaces for two or three columns is
simple, but any more than this can take up a lot of time. This is why
the 'printf' statement was created (*note Printf::); one of its
specialties is lining up columns of data.
NOTE: You can continue either a 'print' or 'printf' statement
simply by putting a newline after any comma (*note
Statements/Lines::).
File: gawk.info, Node: Output Separators, Next: OFMT, Prev: Print Examples, Up: Printing
5.3 Output Separators
=====================
As mentioned previously, a 'print' statement contains a list of items
separated by commas. In the output, the items are normally separated by
single spaces. However, this doesn't need to be the case; a single
space is simply the default. Any string of characters may be used as
the "output field separator" by setting the predefined variable 'OFS'.
The initial value of this variable is the string '" "' (i.e., a single
space).
The output from an entire 'print' statement is called an "output
record". Each 'print' statement outputs one output record, and then
outputs a string called the "output record separator" (or 'ORS'). The
initial value of 'ORS' is the string '"\n"' (i.e., a newline character).
Thus, each 'print' statement normally makes a separate line.
In order to change how output fields and records are separated,
assign new values to the variables 'OFS' and 'ORS'. The usual place to
do this is in the 'BEGIN' rule (*note BEGIN/END::), so that it happens
before any input is processed. It can also be done with assignments on
the command line, before the names of the input files, or using the '-v'
command-line option (*note Options::). The following example prints the
first and second fields of each input record, separated by a semicolon,
with a blank line added after each newline:
$ awk 'BEGIN { OFS = ";"; ORS = "\n\n" }
> { print $1, $2 }' mail-list
-| Amelia;555-5553
-|
-| Anthony;555-3412
-|
-| Becky;555-7685
-|
-| Bill;555-1675
-|
-| Broderick;555-0542
-|
-| Camilla;555-2912
-|
-| Fabius;555-1234
-|
-| Julie;555-6699
-|
-| Martin;555-6480
-|
-| Samuel;555-3430
-|
-| Jean-Paul;555-2127
-|
If the value of 'ORS' does not contain a newline, the program's
output runs together on a single line.
File: gawk.info, Node: OFMT, Next: Printf, Prev: Output Separators, Up: Printing
5.4 Controlling Numeric Output with 'print'
===========================================
When printing numeric values with the 'print' statement, 'awk'
internally converts each number to a string of characters and prints
that string. 'awk' uses the 'sprintf()' function to do this conversion
(*note String Functions::). For now, it suffices to say that the
'sprintf()' function accepts a "format specification" that tells it how
to format numbers (or strings), and that there are a number of different
ways in which numbers can be formatted. The different format
specifications are discussed more fully in *note Control Letters::.
The predefined variable 'OFMT' contains the format specification that
'print' uses with 'sprintf()' when it wants to convert a number to a
string for printing. The default value of 'OFMT' is '"%.6g"'. The way
'print' prints numbers can be changed by supplying a different format
specification for the value of 'OFMT', as shown in the following
example:
$ awk 'BEGIN {
> OFMT = "%.0f" # print numbers as integers (rounds)
> print 17.23, 17.54 }'
-| 17 18
According to the POSIX standard, 'awk''s behavior is undefined if 'OFMT'
contains anything but a floating-point conversion specification. (d.c.)
File: gawk.info, Node: Printf, Next: Redirection, Prev: OFMT, Up: Printing
5.5 Using 'printf' Statements for Fancier Printing
==================================================
For more precise control over the output format than what is provided by
'print', use 'printf'. With 'printf' you can specify the width to use
for each item, as well as various formatting choices for numbers (such
as what output base to use, whether to print an exponent, whether to
print a sign, and how many digits to print after the decimal point).
* Menu:
* Basic Printf:: Syntax of the 'printf' statement.
* Control Letters:: Format-control letters.
* Format Modifiers:: Format-specification modifiers.
* Printf Examples:: Several examples.
File: gawk.info, Node: Basic Printf, Next: Control Letters, Up: Printf
5.5.1 Introduction to the 'printf' Statement
--------------------------------------------
A simple 'printf' statement looks like this:
printf FORMAT, ITEM1, ITEM2, ...
As for 'print', the entire list of arguments may optionally be enclosed
in parentheses. Here too, the parentheses are necessary if any of the
item expressions uses the '>' relational operator; otherwise, it can be
confused with an output redirection (*note Redirection::).
The difference between 'printf' and 'print' is the FORMAT argument.
This is an expression whose value is taken as a string; it specifies how
to output each of the other arguments. It is called the "format
string".
The format string is very similar to that in the ISO C library
function 'printf()'. Most of FORMAT is text to output verbatim.
Scattered among this text are "format specifiers"--one per item. Each
format specifier says to output the next item in the argument list at
that place in the format.
The 'printf' statement does not automatically append a newline to its
output. It outputs only what the format string specifies. So if a
newline is needed, you must include one in the format string. The
output separator variables 'OFS' and 'ORS' have no effect on 'printf'
statements. For example:
$ awk 'BEGIN {
> ORS = "\nOUCH!\n"; OFS = "+"
> msg = "Don\47t Panic!"
> printf "%s\n", msg
> }'
-| Don't Panic!
Here, neither the '+' nor the 'OUCH!' appears in the output message.
File: gawk.info, Node: Control Letters, Next: Format Modifiers, Prev: Basic Printf, Up: Printf
5.5.2 Format-Control Letters
----------------------------
A format specifier starts with the character '%' and ends with a
"format-control letter"--it tells the 'printf' statement how to output
one item. The format-control letter specifies what _kind_ of value to
print. The rest of the format specifier is made up of optional
"modifiers" that control _how_ to print the value, such as the field
width. Here is a list of the format-control letters:
'%c'
Print a number as a character; thus, 'printf "%c", 65' outputs the
letter 'A'. The output for a string value is the first character
of the string.
NOTE: The POSIX standard says the first character of a string
is printed. In locales with multibyte characters, 'gawk'
attempts to convert the leading bytes of the string into a
valid wide character and then to print the multibyte encoding
of that character. Similarly, when printing a numeric value,
'gawk' allows the value to be within the numeric range of
values that can be held in a wide character. If the
conversion to multibyte encoding fails, 'gawk' uses the low
eight bits of the value as the character to print.
Other 'awk' versions generally restrict themselves to printing
the first byte of a string or to numeric values within the
range of a single byte (0-255). (d.c.)
'%d', '%i'
Print a decimal integer. The two control letters are equivalent.
(The '%i' specification is for compatibility with ISO C.)
'%e', '%E'
Print a number in scientific (exponential) notation. For example:
printf "%4.3e\n", 1950
prints '1.950e+03', with a total of four significant figures, three
of which follow the decimal point. (The '4.3' represents two
modifiers, discussed in the next node.) '%E' uses 'E' instead of
'e' in the output.
'%f'
Print a number in floating-point notation. For example:
printf "%4.3f", 1950
prints '1950.000', with a total of four significant figures, three
of which follow the decimal point. (The '4.3' represents two
modifiers, discussed in the next node.)
On systems supporting IEEE 754 floating-point format, values
representing negative infinity are formatted as '-inf' or
'-infinity', and positive infinity as 'inf' or 'infinity'. The
special "not a number" value formats as '-nan' or 'nan' (*note Math
Definitions::).
'%F'
Like '%f', but the infinity and "not a number" values are spelled
using uppercase letters.
The '%F' format is a POSIX extension to ISO C; not all systems
support it. On those that don't, 'gawk' uses '%f' instead.
'%g', '%G'
Print a number in either scientific notation or in floating-point
notation, whichever uses fewer characters; if the result is printed
in scientific notation, '%G' uses 'E' instead of 'e'.
'%o'
Print an unsigned octal integer (*note Nondecimal-numbers::).
'%s'
Print a string.
'%u'
Print an unsigned decimal integer. (This format is of marginal
use, because all numbers in 'awk' are floating point; it is
provided primarily for compatibility with C.)
'%x', '%X'
Print an unsigned hexadecimal integer; '%X' uses the letters 'A'
through 'F' instead of 'a' through 'f' (*note
Nondecimal-numbers::).
'%%'
Print a single '%'. This does not consume an argument and it
ignores any modifiers.
NOTE: When using the integer format-control letters for values that
are outside the range of the widest C integer type, 'gawk' switches
to the '%g' format specifier. If '--lint' is provided on the
command line (*note Options::), 'gawk' warns about this. Other
versions of 'awk' may print invalid values or do something else
entirely. (d.c.)
File: gawk.info, Node: Format Modifiers, Next: Printf Examples, Prev: Control Letters, Up: Printf
5.5.3 Modifiers for 'printf' Formats
------------------------------------
A format specification can also include "modifiers" that can control how
much of the item's value is printed, as well as how much space it gets.
The modifiers come between the '%' and the format-control letter. We
use the bullet symbol "*" in the following examples to represent spaces
in the output. Here are the possible modifiers, in the order in which
they may appear:
'N$'
An integer constant followed by a '$' is a "positional specifier".
Normally, format specifications are applied to arguments in the
order given in the format string. With a positional specifier, the
format specification is applied to a specific argument, instead of
what would be the next argument in the list. Positional specifiers
begin counting with one. Thus:
printf "%s %s\n", "don't", "panic"
printf "%2$s %1$s\n", "panic", "don't"
prints the famous friendly message twice.
At first glance, this feature doesn't seem to be of much use. It
is in fact a 'gawk' extension, intended for use in translating
messages at runtime. *Note Printf Ordering::, which describes how
and why to use positional specifiers. For now, we ignore them.
'-' (Minus)
The minus sign, used before the width modifier (see later on in
this list), says to left-justify the argument within its specified
width. Normally, the argument is printed right-justified in the
specified width. Thus:
printf "%-4s", "foo"
prints 'foo*'.
SPACE
For numeric conversions, prefix positive values with a space and
negative values with a minus sign.
'+'
The plus sign, used before the width modifier (see later on in this
list), says to always supply a sign for numeric conversions, even
if the data to format is positive. The '+' overrides the space
modifier.
'#'
Use an "alternative form" for certain control letters. For '%o',
supply a leading zero. For '%x' and '%X', supply a leading '0x' or
'0X' for a nonzero result. For '%e', '%E', '%f', and '%F', the
result always contains a decimal point. For '%g' and '%G',
trailing zeros are not removed from the result.
'0'
A leading '0' (zero) acts as a flag indicating that output should
be padded with zeros instead of spaces. This applies only to the
numeric output formats. This flag only has an effect when the
field width is wider than the value to print.
'''
A single quote or apostrophe character is a POSIX extension to ISO
C. It indicates that the integer part of a floating-point value, or
the entire part of an integer decimal value, should have a
thousands-separator character in it. This only works in locales
that support such characters. For example:
$ cat thousands.awk Show source program
-| BEGIN { printf "%'d\n", 1234567 }
$ LC_ALL=C gawk -f thousands.awk
-| 1234567 Results in "C" locale
$ LC_ALL=en_US.UTF-8 gawk -f thousands.awk
-| 1,234,567 Results in US English UTF locale
For more information about locales and internationalization issues,
see *note Locales::.
NOTE: The ''' flag is a nice feature, but its use complicates
things: it becomes difficult to use it in command-line
programs. For information on appropriate quoting tricks, see
*note Quoting::.
WIDTH
This is a number specifying the desired minimum width of a field.
Inserting any number between the '%' sign and the format-control
character forces the field to expand to this width. The default
way to do this is to pad with spaces on the left. For example:
printf "%4s", "foo"
prints '*foo'.
The value of WIDTH is a minimum width, not a maximum. If the item
value requires more than WIDTH characters, it can be as wide as
necessary. Thus, the following:
printf "%4s", "foobar"
prints 'foobar'.
Preceding the WIDTH with a minus sign causes the output to be
padded with spaces on the right, instead of on the left.
'.PREC'
A period followed by an integer constant specifies the precision to
use when printing. The meaning of the precision varies by control
letter:
'%d', '%i', '%o', '%u', '%x', '%X'
Minimum number of digits to print.
'%e', '%E', '%f', '%F'
Number of digits to the right of the decimal point.
'%g', '%G'
Maximum number of significant digits.
'%s'
Maximum number of characters from the string that should
print.
Thus, the following:
printf "%.4s", "foobar"
prints 'foob'.
The C library 'printf''s dynamic WIDTH and PREC capability (e.g.,
'"%*.*s"') is supported. Instead of supplying explicit WIDTH and/or
PREC values in the format string, they are passed in the argument list.
For example:
w = 5
p = 3
s = "abcdefg"
printf "%*.*s\n", w, p, s
is exactly equivalent to:
s = "abcdefg"
printf "%5.3s\n", s
Both programs output '**abc'. Earlier versions of 'awk' did not support
this capability. If you must use such a version, you may simulate this
feature by using concatenation to build up the format string, like so:
w = 5
p = 3
s = "abcdefg"
printf "%" w "." p "s\n", s
This is not particularly easy to read, but it does work.
C programmers may be used to supplying additional modifiers ('h',
'j', 'l', 'L', 't', and 'z') in 'printf' format strings. These are not
valid in 'awk'. Most 'awk' implementations silently ignore them. If
'--lint' is provided on the command line (*note Options::), 'gawk' warns
about their use. If '--posix' is supplied, their use is a fatal error.
File: gawk.info, Node: Printf Examples, Prev: Format Modifiers, Up: Printf
5.5.4 Examples Using 'printf'
-----------------------------
The following simple example shows how to use 'printf' to make an
aligned table:
awk '{ printf "%-10s %s\n", $1, $2 }' mail-list
This command prints the names of the people ('$1') in the file
'mail-list' as a string of 10 characters that are left-justified. It
also prints the phone numbers ('$2') next on the line. This produces an
aligned two-column table of names and phone numbers, as shown here:
$ awk '{ printf "%-10s %s\n", $1, $2 }' mail-list
-| Amelia 555-5553
-| Anthony 555-3412
-| Becky 555-7685
-| Bill 555-1675
-| Broderick 555-0542
-| Camilla 555-2912
-| Fabius 555-1234
-| Julie 555-6699
-| Martin 555-6480
-| Samuel 555-3430
-| Jean-Paul 555-2127
In this case, the phone numbers had to be printed as strings because
the numbers are separated by dashes. Printing the phone numbers as
numbers would have produced just the first three digits: '555'. This
would have been pretty confusing.
It wasn't necessary to specify a width for the phone numbers because
they are last on their lines. They don't need to have spaces after
them.
The table could be made to look even nicer by adding headings to the
tops of the columns. This is done using a 'BEGIN' rule (*note
BEGIN/END::) so that the headers are only printed once, at the beginning
of the 'awk' program:
awk 'BEGIN { print "Name Number"
print "---- ------" }
{ printf "%-10s %s\n", $1, $2 }' mail-list
The preceding example mixes 'print' and 'printf' statements in the
same program. Using just 'printf' statements can produce the same
results:
awk 'BEGIN { printf "%-10s %s\n", "Name", "Number"
printf "%-10s %s\n", "----", "------" }
{ printf "%-10s %s\n", $1, $2 }' mail-list
Printing each column heading with the same format specification used for
the column elements ensures that the headings are aligned just like the
columns.
The fact that the same format specification is used three times can
be emphasized by storing it in a variable, like this:
awk 'BEGIN { format = "%-10s %s\n"
printf format, "Name", "Number"
printf format, "----", "------" }
{ printf format, $1, $2 }' mail-list
File: gawk.info, Node: Redirection, Next: Special FD, Prev: Printf, Up: Printing
5.6 Redirecting Output of 'print' and 'printf'
==============================================
So far, the output from 'print' and 'printf' has gone to the standard
output, usually the screen. Both 'print' and 'printf' can also send
their output to other places. This is called "redirection".
NOTE: When '--sandbox' is specified (*note Options::), redirecting
output to files, pipes, and coprocesses is disabled.
A redirection appears after the 'print' or 'printf' statement.
Redirections in 'awk' are written just like redirections in shell
commands, except that they are written inside the 'awk' program.
There are four forms of output redirection: output to a file, output
appended to a file, output through a pipe to another command, and output
to a coprocess. We show them all for the 'print' statement, but they
work identically for 'printf':
'print ITEMS > OUTPUT-FILE'
This redirection prints the items into the output file named
OUTPUT-FILE. The file name OUTPUT-FILE can be any expression. Its
value is changed to a string and then used as a file name (*note
Expressions::).
When this type of redirection is used, the OUTPUT-FILE is erased
before the first output is written to it. Subsequent writes to the
same OUTPUT-FILE do not erase OUTPUT-FILE, but append to it. (This
is different from how you use redirections in shell scripts.) If
OUTPUT-FILE does not exist, it is created. For example, here is
how an 'awk' program can write a list of peoples' names to one file
named 'name-list', and a list of phone numbers to another file
named 'phone-list':
$ awk '{ print $2 > "phone-list"
> print $1 > "name-list" }' mail-list
$ cat phone-list
-| 555-5553
-| 555-3412
...
$ cat name-list
-| Amelia
-| Anthony
...
Each output file contains one name or number per line.
'print ITEMS >> OUTPUT-FILE'
This redirection prints the items into the preexisting output file
named OUTPUT-FILE. The difference between this and the single-'>'
redirection is that the old contents (if any) of OUTPUT-FILE are
not erased. Instead, the 'awk' output is appended to the file. If
OUTPUT-FILE does not exist, then it is created.
'print ITEMS | COMMAND'
It is possible to send output to another program through a pipe
instead of into a file. This redirection opens a pipe to COMMAND,
and writes the values of ITEMS through this pipe to another process
created to execute COMMAND.
The redirection argument COMMAND is actually an 'awk' expression.
Its value is converted to a string whose contents give the shell
command to be run. For example, the following produces two files,
one unsorted list of peoples' names, and one list sorted in reverse
alphabetical order:
awk '{ print $1 > "names.unsorted"
command = "sort -r > names.sorted"
print $1 | command }' mail-list
The unsorted list is written with an ordinary redirection, while
the sorted list is written by piping through the 'sort' utility.
The next example uses redirection to mail a message to the mailing
list 'bug-system'. This might be useful when trouble is
encountered in an 'awk' script run periodically for system
maintenance:
report = "mail bug-system"
print("Awk script failed:", $0) | report
print("at record number", FNR, "of", FILENAME) | report
close(report)
The 'close()' function is called here because it's a good idea to
close the pipe as soon as all the intended output has been sent to
it. *Note Close Files And Pipes:: for more information.
This example also illustrates the use of a variable to represent a
FILE or COMMAND--it is not necessary to always use a string
constant. Using a variable is generally a good idea, because (if
you mean to refer to that same file or command) 'awk' requires that
the string value be written identically every time.
'print ITEMS |& COMMAND'
This redirection prints the items to the input of COMMAND. The
difference between this and the single-'|' redirection is that the
output from COMMAND can be read with 'getline'. Thus, COMMAND is a
"coprocess", which works together with but is subsidiary to the
'awk' program.
This feature is a 'gawk' extension, and is not available in POSIX
'awk'. *Note Getline/Coprocess::, for a brief discussion. *Note
Two-way I/O::, for a more complete discussion.
Redirecting output using '>', '>>', '|', or '|&' asks the system to
open a file, pipe, or coprocess only if the particular FILE or COMMAND
you specify has not already been written to by your program or if it has
been closed since it was last written to.
It is a common error to use '>' redirection for the first 'print' to
a file, and then to use '>>' for subsequent output:
# clear the file
print "Don't panic" > "guide.txt"
...
# append
print "Avoid improbability generators" >> "guide.txt"
This is indeed how redirections must be used from the shell. But in
'awk', it isn't necessary. In this kind of case, a program should use
'>' for all the 'print' statements, because the output file is only
opened once. (It happens that if you mix '>' and '>>' output is
produced in the expected order. However, mixing the operators for the
same file is definitely poor style, and is confusing to readers of your
program.)
Many older 'awk' implementations limit the number of pipelines that
an 'awk' program may have open to just one! In 'gawk', there is no such
limit. 'gawk' allows a program to open as many pipelines as the
underlying operating system permits.
Piping into 'sh'
A particularly powerful way to use redirection is to build command
lines and pipe them into the shell, 'sh'. For example, suppose you have
a list of files brought over from a system where all the file names are
stored in uppercase, and you wish to rename them to have names in all
lowercase. The following program is both simple and efficient:
{ printf("mv %s %s\n", $0, tolower($0)) | "sh" }
END { close("sh") }
The 'tolower()' function returns its argument string with all
uppercase characters converted to lowercase (*note String Functions::).
The program builds up a list of command lines, using the 'mv' utility to
rename the files. It then sends the list to the shell for execution.
*Note Shell Quoting:: for a function that can help in generating
command lines to be fed to the shell.
File: gawk.info, Node: Special FD, Next: Special Files, Prev: Redirection, Up: Printing
5.7 Special Files for Standard Preopened Data Streams
=====================================================
Running programs conventionally have three input and output streams
already available to them for reading and writing. These are known as
the "standard input", "standard output", and "standard error output".
These open streams (and any other open files or pipes) are often
referred to by the technical term "file descriptors".
These streams are, by default, connected to your keyboard and screen,
but they are often redirected with the shell, via the '<', '<<', '>',
'>>', '>&', and '|' operators. Standard error is typically used for
writing error messages; the reason there are two separate streams,
standard output and standard error, is so that they can be redirected
separately.
In traditional implementations of 'awk', the only way to write an
error message to standard error in an 'awk' program is as follows:
print "Serious error detected!" | "cat 1>&2"
This works by opening a pipeline to a shell command that can access the
standard error stream that it inherits from the 'awk' process. This is
far from elegant, and it also requires a separate process. So people
writing 'awk' programs often don't do this. Instead, they send the
error messages to the screen, like this:
print "Serious error detected!" > "/dev/tty"
('/dev/tty' is a special file supplied by the operating system that is
connected to your keyboard and screen. It represents the "terminal,"(1)
which on modern systems is a keyboard and screen, not a serial console.)
This generally has the same effect, but not always: although the
standard error stream is usually the screen, it can be redirected; when
that happens, writing to the screen is not correct. In fact, if 'awk'
is run from a background job, it may not have a terminal at all. Then
opening '/dev/tty' fails.
'gawk', BWK 'awk', and 'mawk' provide special file names for
accessing the three standard streams. If the file name matches one of
these special names when 'gawk' (or one of the others) redirects input
or output, then it directly uses the descriptor that the file name
stands for. These special file names work for all operating systems
that 'gawk' has been ported to, not just those that are POSIX-compliant:
'/dev/stdin'
The standard input (file descriptor 0).
'/dev/stdout'
The standard output (file descriptor 1).
'/dev/stderr'
The standard error output (file descriptor 2).
With these facilities, the proper way to write an error message then
becomes:
print "Serious error detected!" > "/dev/stderr"
Note the use of quotes around the file name. Like with any other
redirection, the value must be a string. It is a common error to omit
the quotes, which leads to confusing results.
'gawk' does not treat these file names as special when in
POSIX-compatibility mode. However, because BWK 'awk' supports them,
'gawk' does support them even when invoked with the '--traditional'
option (*note Options::).
---------- Footnotes ----------
(1) The "tty" in '/dev/tty' stands for "Teletype," a serial terminal.
File: gawk.info, Node: Special Files, Next: Close Files And Pipes, Prev: Special FD, Up: Printing
5.8 Special File names in 'gawk'
================================
Besides access to standard input, standard output, and standard error,
'gawk' provides access to any open file descriptor. Additionally, there
are special file names reserved for TCP/IP networking.
* Menu:
* Other Inherited Files:: Accessing other open files with
'gawk'.
* Special Network:: Special files for network communications.
* Special Caveats:: Things to watch out for.
File: gawk.info, Node: Other Inherited Files, Next: Special Network, Up: Special Files
5.8.1 Accessing Other Open Files with 'gawk'
--------------------------------------------
Besides the '/dev/stdin', '/dev/stdout', and '/dev/stderr' special file
names mentioned earlier, 'gawk' provides syntax for accessing any other
inherited open file:
'/dev/fd/N'
The file associated with file descriptor N. Such a file must be
opened by the program initiating the 'awk' execution (typically the
shell). Unless special pains are taken in the shell from which
'gawk' is invoked, only descriptors 0, 1, and 2 are available.
The file names '/dev/stdin', '/dev/stdout', and '/dev/stderr' are
essentially aliases for '/dev/fd/0', '/dev/fd/1', and '/dev/fd/2',
respectively. However, those names are more self-explanatory.
Note that using 'close()' on a file name of the form '"/dev/fd/N"',
for file descriptor numbers above two, does actually close the given
file descriptor.
File: gawk.info, Node: Special Network, Next: Special Caveats, Prev: Other Inherited Files, Up: Special Files
5.8.2 Special Files for Network Communications
----------------------------------------------
'gawk' programs can open a two-way TCP/IP connection, acting as either a
client or a server. This is done using a special file name of the form:
/NET-TYPE/PROTOCOL/LOCAL-PORT/REMOTE-HOST/REMOTE-PORT
The NET-TYPE is one of 'inet', 'inet4', or 'inet6'. The PROTOCOL is
one of 'tcp' or 'udp', and the other fields represent the other
essential pieces of information for making a networking connection.
These file names are used with the '|&' operator for communicating with
a coprocess (*note Two-way I/O::). This is an advanced feature,
mentioned here only for completeness. Full discussion is delayed until
*note TCP/IP Networking::.
File: gawk.info, Node: Special Caveats, Prev: Special Network, Up: Special Files
5.8.3 Special File name Caveats
-------------------------------
Here are some things to bear in mind when using the special file names
that 'gawk' provides:
* Recognition of the file names for the three standard preopened
files is disabled only in POSIX mode.
* Recognition of the other special file names is disabled if 'gawk'
is in compatibility mode (either '--traditional' or '--posix';
*note Options::).
* 'gawk' _always_ interprets these special file names. For example,
using '/dev/fd/4' for output actually writes on file descriptor 4,
and not on a new file descriptor that is 'dup()'ed from file
descriptor 4. Most of the time this does not matter; however, it
is important to _not_ close any of the files related to file
descriptors 0, 1, and 2. Doing so results in unpredictable
behavior.
File: gawk.info, Node: Close Files And Pipes, Next: Nonfatal, Prev: Special Files, Up: Printing
5.9 Closing Input and Output Redirections
=========================================
If the same file name or the same shell command is used with 'getline'
more than once during the execution of an 'awk' program (*note
Getline::), the file is opened (or the command is executed) the first
time only. At that time, the first record of input is read from that
file or command. The next time the same file or command is used with
'getline', another record is read from it, and so on.
Similarly, when a file or pipe is opened for output, 'awk' remembers
the file name or command associated with it, and subsequent writes to
the same file or command are appended to the previous writes. The file
or pipe stays open until 'awk' exits.
This implies that special steps are necessary in order to read the
same file again from the beginning, or to rerun a shell command (rather
than reading more output from the same command). The 'close()' function
makes these things possible:
close(FILENAME)
or:
close(COMMAND)
The argument FILENAME or COMMAND can be any expression. Its value
must _exactly_ match the string that was used to open the file or start
the command (spaces and other "irrelevant" characters included). For
example, if you open a pipe with this:
"sort -r names" | getline foo
then you must close it with this:
close("sort -r names")
Once this function call is executed, the next 'getline' from that
file or command, or the next 'print' or 'printf' to that file or
command, reopens the file or reruns the command. Because the expression
that you use to close a file or pipeline must exactly match the
expression used to open the file or run the command, it is good practice
to use a variable to store the file name or command. The previous
example becomes the following:
sortcom = "sort -r names"
sortcom | getline foo
...
close(sortcom)
This helps avoid hard-to-find typographical errors in your 'awk'
programs. Here are some of the reasons for closing an output file:
* To write a file and read it back later on in the same 'awk'
program. Close the file after writing it, then begin reading it
with 'getline'.
* To write numerous files, successively, in the same 'awk' program.
If the files aren't closed, eventually 'awk' may exceed a system
limit on the number of open files in one process. It is best to
close each one when the program has finished writing it.
* To make a command finish. When output is redirected through a
pipe, the command reading the pipe normally continues to try to
read input as long as the pipe is open. Often this means the
command cannot really do its work until the pipe is closed. For
example, if output is redirected to the 'mail' program, the message
is not actually sent until the pipe is closed.
* To run the same program a second time, with the same arguments.
This is not the same thing as giving more input to the first run!
For example, suppose a program pipes output to the 'mail' program.
If it outputs several lines redirected to this pipe without closing
it, they make a single message of several lines. By contrast, if
the program closes the pipe after each line of output, then each
line makes a separate message.
If you use more files than the system allows you to have open, 'gawk'
attempts to multiplex the available open files among your data files.
'gawk''s ability to do this depends upon the facilities of your
operating system, so it may not always work. It is therefore both good
practice and good portability advice to always use 'close()' on your
files when you are done with them. In fact, if you are using a lot of
pipes, it is essential that you close commands when done. For example,
consider something like this:
{
...
command = ("grep " $1 " /some/file | my_prog -q " $3)
while ((command | getline) > 0) {
PROCESS OUTPUT OF command
}
# need close(command) here
}
This example creates a new pipeline based on data in _each_ record.
Without the call to 'close()' indicated in the comment, 'awk' creates
child processes to run the commands, until it eventually runs out of
file descriptors for more pipelines.
Even though each command has finished (as indicated by the
end-of-file return status from 'getline'), the child process is not
terminated;(1) more importantly, the file descriptor for the pipe is not
closed and released until 'close()' is called or 'awk' exits.
'close()' silently does nothing if given an argument that does not
represent a file, pipe, or coprocess that was opened with a redirection.
In such a case, it returns a negative value, indicating an error. In
addition, 'gawk' sets 'ERRNO' to a string indicating the error.
Note also that 'close(FILENAME)' has no "magic" effects on the
implicit loop that reads through the files named on the command line.
It is, more likely, a close of a file that was never opened with a
redirection, so 'awk' silently does nothing, except return a negative
value.
When using the '|&' operator to communicate with a coprocess, it is
occasionally useful to be able to close one end of the two-way pipe
without closing the other. This is done by supplying a second argument
to 'close()'. As in any other call to 'close()', the first argument is
the name of the command or special file used to start the coprocess.
The second argument should be a string, with either of the values '"to"'
or '"from"'. Case does not matter. As this is an advanced feature,
discussion is delayed until *note Two-way I/O::, which describes it in
more detail and gives an example.
Using 'close()''s Return Value
In many older versions of Unix 'awk', the 'close()' function is
actually a statement. (d.c.) It is a syntax error to try and use the
return value from 'close()':
command = "..."
command | getline info
retval = close(command) # syntax error in many Unix awks
'gawk' treats 'close()' as a function. The return value is -1 if the
argument names something that was never opened with a redirection, or if
there is a system problem closing the file or process. In these cases,
'gawk' sets the predefined variable 'ERRNO' to a string describing the
problem.
In 'gawk', starting with version 4.2, when closing a pipe or
coprocess (input or output), the return value is the exit status of the
command, as described in *note Table 5.1:
table-close-pipe-return-values.(2) Otherwise, it is the return value
from the system's 'close()' or 'fclose()' C functions when closing input
or output files, respectively. This value is zero if the close
succeeds, or -1 if it fails.
Situation Return value from 'close()'
--------------------------------------------------------------------------
Normal exit of command Command's exit status
Death by signal of command 256 + number of murderous signal
Death by signal of command with 512 + number of murderous signal
core dump
Some kind of error -1
Table 5.1: Return values from 'close()' of a pipe
The POSIX standard is very vague; it says that 'close()' returns zero
on success and a nonzero value otherwise. In general, different
implementations vary in what they report when closing pipes; thus, the
return value cannot be used portably. (d.c.) In POSIX mode (*note
Options::), 'gawk' just returns zero when closing a pipe.
---------- Footnotes ----------
(1) The technical terminology is rather morbid. The finished child
is called a "zombie," and cleaning up after it is referred to as
"reaping."
(2) Prior to version 4.2, the return value from closing a pipe or
co-process was the full 16-bit exit value as defined by the 'wait()'
system call.
File: gawk.info, Node: Nonfatal, Next: Output Summary, Prev: Close Files And Pipes, Up: Printing
5.10 Enabling Nonfatal Output
=============================
This minor node describes a 'gawk'-specific feature.
In standard 'awk', output with 'print' or 'printf' to a nonexistent
file, or some other I/O error (such as filling up the disk) is a fatal
error.
$ gawk 'BEGIN { print "hi" > "/no/such/file" }'
error-> gawk: cmd. line:1: fatal: can't redirect to `/no/such/file' (No
error-> such file or directory)
'gawk' makes it possible to detect that an error has occurred,
allowing you to possibly recover from the error, or at least print an
error message of your choosing before exiting. You can do this in one
of two ways:
* For all output files, by assigning any value to
'PROCINFO["NONFATAL"]'.
* On a per-file basis, by assigning any value to 'PROCINFO[FILENAME,
"NONFATAL"]'. Here, FILENAME is the name of the file to which you
wish output to be nonfatal.
Once you have enabled nonfatal output, you must check 'ERRNO' after
every relevant 'print' or 'printf' statement to see if something went
wrong. It is also a good idea to initialize 'ERRNO' to zero before
attempting the output. For example:
$ gawk '
> BEGIN {
> PROCINFO["NONFATAL"] = 1
> ERRNO = 0
> print "hi" > "/no/such/file"
> if (ERRNO) {
> print("Output failed:", ERRNO) > "/dev/stderr"
> exit 1
> }
> }'
error-> Output failed: No such file or directory
Here, 'gawk' did not produce a fatal error; instead it let the 'awk'
program code detect the problem and handle it.
This mechanism works also for standard output and standard error.
For standard output, you may use 'PROCINFO["-", "NONFATAL"]' or
'PROCINFO["/dev/stdout", "NONFATAL"]'. For standard error, use
'PROCINFO["/dev/stderr", "NONFATAL"]'.
When attempting to open a TCP/IP socket (*note TCP/IP Networking::),
'gawk' tries multiple times. The 'GAWK_SOCK_RETRIES' environment
variable (*note Other Environment Variables::) allows you to override
'gawk''s builtin default number of attempts. However, once nonfatal I/O
is enabled for a given socket, 'gawk' only retries once, relying on
'awk'-level code to notice that there was a problem.
File: gawk.info, Node: Output Summary, Next: Output Exercises, Prev: Nonfatal, Up: Printing
5.11 Summary
============
* The 'print' statement prints comma-separated expressions. Each
expression is separated by the value of 'OFS' and terminated by the
value of 'ORS'. 'OFMT' provides the conversion format for numeric
values for the 'print' statement.
* The 'printf' statement provides finer-grained control over output,
with format-control letters for different data types and various
flags that modify the behavior of the format-control letters.
* Output from both 'print' and 'printf' may be redirected to files,
pipes, and coprocesses.
* 'gawk' provides special file names for access to standard input,
output, and error, and for network communications.
* Use 'close()' to close open file, pipe, and coprocess redirections.
For coprocesses, it is possible to close only one direction of the
communications.
* Normally errors with 'print' or 'printf' are fatal. 'gawk' lets
you make output errors be nonfatal either for all files or on a
per-file basis. You must then check for errors after every
relevant output statement.
File: gawk.info, Node: Output Exercises, Prev: Output Summary, Up: Printing
5.12 Exercises
==============
1. Rewrite the program:
awk 'BEGIN { print "Month Crates"
print "----- ------" }
{ print $1, " ", $2 }' inventory-shipped
from *note Output Separators::, by using a new value of 'OFS'.
2. Use the 'printf' statement to line up the headings and table data
for the 'inventory-shipped' example that was covered in *note
Print::.
3. What happens if you forget the double quotes when redirecting
output, as follows:
BEGIN { print "Serious error detected!" > /dev/stderr }
File: gawk.info, Node: Expressions, Next: Patterns and Actions, Prev: Printing, Up: Top
6 Expressions
*************
Expressions are the basic building blocks of 'awk' patterns and actions.
An expression evaluates to a value that you can print, test, or pass to
a function. Additionally, an expression can assign a new value to a
variable or a field by using an assignment operator.
An expression can serve as a pattern or action statement on its own.
Most other kinds of statements contain one or more expressions that
specify the data on which to operate. As in other languages,
expressions in 'awk' can include variables, array references, constants,
and function calls, as well as combinations of these with various
operators.
* Menu:
* Values:: Constants, Variables, and Regular Expressions.
* All Operators:: 'gawk''s operators.
* Truth Values and Conditions:: Testing for true and false.
* Function Calls:: A function call is an expression.
* Precedence:: How various operators nest.
* Locales:: How the locale affects things.
* Expressions Summary:: Expressions summary.
File: gawk.info, Node: Values, Next: All Operators, Up: Expressions
6.1 Constants, Variables, and Conversions
=========================================
Expressions are built up from values and the operations performed upon
them. This minor node describes the elementary objects that provide the
values used in expressions.
* Menu:
* Constants:: String, numeric and regexp constants.
* Using Constant Regexps:: When and how to use a regexp constant.
* Variables:: Variables give names to values for later use.
* Conversion:: The conversion of strings to numbers and vice
versa.
File: gawk.info, Node: Constants, Next: Using Constant Regexps, Up: Values
6.1.1 Constant Expressions
--------------------------
The simplest type of expression is the "constant", which always has the
same value. There are three types of constants: numeric, string, and
regular expression.
Each is used in the appropriate context when you need a data value
that isn't going to change. Numeric constants can have different forms,
but are internally stored in an identical manner.
* Menu:
* Scalar Constants:: Numeric and string constants.
* Nondecimal-numbers:: What are octal and hex numbers.
* Regexp Constants:: Regular Expression constants.
File: gawk.info, Node: Scalar Constants, Next: Nondecimal-numbers, Up: Constants
6.1.1.1 Numeric and String Constants
....................................
A "numeric constant" stands for a number. This number can be an
integer, a decimal fraction, or a number in scientific (exponential)
notation.(1) Here are some examples of numeric constants that all have
the same value:
105
1.05e+2
1050e-1
A "string constant" consists of a sequence of characters enclosed in
double quotation marks. For example:
"parrot"
represents the string whose contents are 'parrot'. Strings in 'gawk'
can be of any length, and they can contain any of the possible eight-bit
ASCII characters, including ASCII NUL (character code zero). Other
'awk' implementations may have difficulty with some character codes.
---------- Footnotes ----------
(1) The internal representation of all numbers, including integers,
uses double-precision floating-point numbers. On most modern systems,
these are in IEEE 754 standard format. *Note Arbitrary Precision
Arithmetic::, for much more information.
File: gawk.info, Node: Nondecimal-numbers, Next: Regexp Constants, Prev: Scalar Constants, Up: Constants
6.1.1.2 Octal and Hexadecimal Numbers
.....................................
In 'awk', all numbers are in decimal (i.e., base 10). Many other
programming languages allow you to specify numbers in other bases, often
octal (base 8) and hexadecimal (base 16). In octal, the numbers go 0,
1, 2, 3, 4, 5, 6, 7, 10, 11, 12, and so on. Just as '11' in decimal is
1 times 10 plus 1, so '11' in octal is 1 times 8 plus 1. This equals 9
in decimal. In hexadecimal, there are 16 digits. Because the everyday
decimal number system only has ten digits ('0'-'9'), the letters 'a'
through 'f' represent the rest. (Case in the letters is usually
irrelevant; hexadecimal 'a' and 'A' have the same value.) Thus, '11' in
hexadecimal is 1 times 16 plus 1, which equals 17 in decimal.
Just by looking at plain '11', you can't tell what base it's in. So,
in C, C++, and other languages derived from C, there is a special
notation to signify the base. Octal numbers start with a leading '0',
and hexadecimal numbers start with a leading '0x' or '0X':
'11'
Decimal value 11
'011'
Octal 11, decimal value 9
'0x11'
Hexadecimal 11, decimal value 17
This example shows the difference:
$ gawk 'BEGIN { printf "%d, %d, %d\n", 011, 11, 0x11 }'
-| 9, 11, 17
Being able to use octal and hexadecimal constants in your programs is
most useful when working with data that cannot be represented
conveniently as characters or as regular numbers, such as binary data of
various sorts.
'gawk' allows the use of octal and hexadecimal constants in your
program text. However, such numbers in the input data are not treated
differently; doing so by default would break old programs. (If you
really need to do this, use the '--non-decimal-data' command-line
option; *note Nondecimal Data::.) If you have octal or hexadecimal
data, you can use the 'strtonum()' function (*note String Functions::)
to convert the data into a number. Most of the time, you will want to
use octal or hexadecimal constants when working with the built-in
bit-manipulation functions; see *note Bitwise Functions:: for more
information.
Unlike in some early C implementations, '8' and '9' are not valid in
octal constants. For example, 'gawk' treats '018' as decimal 18:
$ gawk 'BEGIN { print "021 is", 021 ; print 018 }'
-| 021 is 17
-| 18
Octal and hexadecimal source code constants are a 'gawk' extension.
If 'gawk' is in compatibility mode (*note Options::), they are not
available.
A Constant's Base Does Not Affect Its Value
Once a numeric constant has been converted internally into a number,
'gawk' no longer remembers what the original form of the constant was;
the internal value is always used. This has particular consequences for
conversion of numbers to strings:
$ gawk 'BEGIN { printf "0x11 is <%s>\n", 0x11 }'
-| 0x11 is <17>
File: gawk.info, Node: Regexp Constants, Prev: Nondecimal-numbers, Up: Constants
6.1.1.3 Regular Expression Constants
....................................
A "regexp constant" is a regular expression description enclosed in
slashes, such as '/^beginning and end$/'. Most regexps used in 'awk'
programs are constant, but the '~' and '!~' matching operators can also
match computed or dynamic regexps (which are typically just ordinary
strings or variables that contain a regexp, but could be more complex
expressions).
File: gawk.info, Node: Using Constant Regexps, Next: Variables, Prev: Constants, Up: Values
6.1.2 Using Regular Expression Constants
----------------------------------------
Regular expression constants consist of text describing a regular
expression enclosed in slashes (such as '/the +answer/'). This minor
node describes how such constants work in POSIX 'awk' and 'gawk', and
then goes on to describe "strongly typed regexp constants", which are a
'gawk' extension.
* Menu:
* Standard Regexp Constants:: Regexp constants in standard 'awk'.
* Strong Regexp Constants:: Strongly typed regexp constants.
File: gawk.info, Node: Standard Regexp Constants, Next: Strong Regexp Constants, Up: Using Constant Regexps
6.1.2.1 Standard Regular Expression Constants
.............................................
When used on the righthand side of the '~' or '!~' operators, a regexp
constant merely stands for the regexp that is to be matched. However,
regexp constants (such as '/foo/') may be used like simple expressions.
When a regexp constant appears by itself, it has the same meaning as if
it appeared in a pattern (i.e., '($0 ~ /foo/)'). (d.c.) *Note
Expression Patterns::. This means that the following two code segments:
if ($0 ~ /barfly/ || $0 ~ /camelot/)
print "found"
and:
if (/barfly/ || /camelot/)
print "found"
are exactly equivalent. One rather bizarre consequence of this rule is
that the following Boolean expression is valid, but does not do what its
author probably intended:
# Note that /foo/ is on the left of the ~
if (/foo/ ~ $1) print "found foo"
This code is "obviously" testing '$1' for a match against the regexp
'/foo/'. But in fact, the expression '/foo/ ~ $1' really means '($0 ~
/foo/) ~ $1'. In other words, first match the input record against the
regexp '/foo/'. The result is either zero or one, depending upon the
success or failure of the match. That result is then matched against
the first field in the record. Because it is unlikely that you would
ever really want to make this kind of test, 'gawk' issues a warning when
it sees this construct in a program. Another consequence of this rule
is that the assignment statement:
matches = /foo/
assigns either zero or one to the variable 'matches', depending upon the
contents of the current input record.
Constant regular expressions are also used as the first argument for
the 'gensub()', 'sub()', and 'gsub()' functions, as the second argument
of the 'match()' function, and as the third argument of the 'split()'
and 'patsplit()' functions (*note String Functions::). Modern
implementations of 'awk', including 'gawk', allow the third argument of
'split()' to be a regexp constant, but some older implementations do
not. (d.c.) Because some built-in functions accept regexp constants as
arguments, confusion can arise when attempting to use regexp constants
as arguments to user-defined functions (*note User-defined::). For
example:
function mysub(pat, repl, str, global)
{
if (global)
gsub(pat, repl, str)
else
sub(pat, repl, str)
return str
}
{
...
text = "hi! hi yourself!"
mysub(/hi/, "howdy", text, 1)
...
}
In this example, the programmer wants to pass a regexp constant to
the user-defined function 'mysub()', which in turn passes it on to
either 'sub()' or 'gsub()'. However, what really happens is that the
'pat' parameter is assigned a value of either one or zero, depending
upon whether or not '$0' matches '/hi/'. 'gawk' issues a warning when
it sees a regexp constant used as a parameter to a user-defined
function, because passing a truth value in this way is probably not what
was intended.
File: gawk.info, Node: Strong Regexp Constants, Prev: Standard Regexp Constants, Up: Using Constant Regexps
6.1.2.2 Strongly Typed Regexp Constants
.......................................
This minor node describes a 'gawk'-specific feature.
As we saw in the previous minor node, regexp constants ('/.../') hold
a strange position in the 'awk' language. In most contexts, they act
like an expression: '$0 ~ /.../'. In other contexts, they denote only a
regexp to be matched. In no case are they really a "first class
citizen" of the language. That is, you cannot define a scalar variable
whose type is "regexp" in the same sense that you can define a variable
to be a number or a string:
num = 42 Numeric variable
str = "hi" String variable
re = /foo/ Wrong! re is the result of $0 ~ /foo/
For a number of more advanced use cases, it would be nice to have
regexp constants that are "strongly typed"; in other words, that denote
a regexp useful for matching, and not an expression.
'gawk' provides this feature. A strongly typed regexp constant looks
almost like a regular regexp constant, except that it is preceded by an
'@' sign:
re = @/foo/ Regexp variable
Strongly typed regexp constants _cannot_ be used everywhere that a
regular regexp constant can, because this would make the language even
more confusing. Instead, you may use them only in certain contexts:
* On the righthand side of the '~' and '!~' operators: 'some_var ~
@/foo/' (*note Regexp Usage::).
* In the 'case' part of a 'switch' statement (*note Switch
Statement::).
* As an argument to one of the built-in functions that accept regexp
constants: 'gensub()', 'gsub()', 'match()', 'patsplit()',
'split()', and 'sub()' (*note String Functions::).
* As a parameter in a call to a user-defined function (*note
User-defined::).
* On the righthand side of an assignment to a variable: 'some_var =
@/foo/'. In this case, the type of 'some_var' is regexp.
Additionally, 'some_var' can be used with '~' and '!~', passed to
one of the built-in functions listed above, or passed as a
parameter to a user-defined function.
You may use the 'typeof()' built-in function (*note Type Functions::)
to determine if a variable or function parameter is a regexp variable.
The true power of this feature comes from the ability to create
variables that have regexp type. Such variables can be passed on to
user-defined functions, without the confusing aspects of computed
regular expressions created from strings or string constants. They may
also be passed through indirect function calls (*note Indirect Calls::)
and on to the built-in functions that accept regexp constants.
When used in numeric conversions, strongly typed regexp variables
convert to zero. When used in string conversions, they convert to the
string value of the original regexp text.
File: gawk.info, Node: Variables, Next: Conversion, Prev: Using Constant Regexps, Up: Values
6.1.3 Variables
---------------
"Variables" are ways of storing values at one point in your program for
use later in another part of your program. They can be manipulated
entirely within the program text, and they can also be assigned values
on the 'awk' command line.
* Menu:
* Using Variables:: Using variables in your programs.
* Assignment Options:: Setting variables on the command line and a
summary of command-line syntax. This is an
advanced method of input.
File: gawk.info, Node: Using Variables, Next: Assignment Options, Up: Variables
6.1.3.1 Using Variables in a Program
....................................
Variables let you give names to values and refer to them later.
Variables have already been used in many of the examples. The name of a
variable must be a sequence of letters, digits, or underscores, and it
may not begin with a digit. Here, a "letter" is any one of the 52
upper- and lowercase English letters. Other characters that may be
defined as letters in non-English locales are not valid in variable
names. Case is significant in variable names; 'a' and 'A' are distinct
variables.
A variable name is a valid expression by itself; it represents the
variable's current value. Variables are given new values with
"assignment operators", "increment operators", and "decrement operators"
(*note Assignment Ops::). In addition, the 'sub()' and 'gsub()'
functions can change a variable's value, and the 'match()', 'split()',
and 'patsplit()' functions can change the contents of their array
parameters (*note String Functions::).
A few variables have special built-in meanings, such as 'FS' (the
field separator) and 'NF' (the number of fields in the current input
record). *Note Built-in Variables:: for a list of the predefined
variables. These predefined variables can be used and assigned just
like all other variables, but their values are also used or changed
automatically by 'awk'. All predefined variables' names are entirely
uppercase.
Variables in 'awk' can be assigned either numeric or string values.
The kind of value a variable holds can change over the life of a
program. By default, variables are initialized to the empty string,
which is zero if converted to a number. There is no need to explicitly
initialize a variable in 'awk', which is what you would do in C and in
most other traditional languages.
File: gawk.info, Node: Assignment Options, Prev: Using Variables, Up: Variables
6.1.3.2 Assigning Variables on the Command Line
...............................................
Any 'awk' variable can be set by including a "variable assignment" among
the arguments on the command line when 'awk' is invoked (*note Other
Arguments::). Such an assignment has the following form:
VARIABLE=TEXT
With it, a variable is set either at the beginning of the 'awk' run or
in between input files. When the assignment is preceded with the '-v'
option, as in the following:
-v VARIABLE=TEXT
the variable is set at the very beginning, even before the 'BEGIN' rules
execute. The '-v' option and its assignment must precede all the file
name arguments, as well as the program text. (*Note Options:: for more
information about the '-v' option.) Otherwise, the variable assignment
is performed at a time determined by its position among the input file
arguments--after the processing of the preceding input file argument.
For example:
awk '{ print $n }' n=4 inventory-shipped n=2 mail-list
prints the value of field number 'n' for all input records. Before the
first file is read, the command line sets the variable 'n' equal to
four. This causes the fourth field to be printed in lines from
'inventory-shipped'. After the first file has finished, but before the
second file is started, 'n' is set to two, so that the second field is
printed in lines from 'mail-list':
$ awk '{ print $n }' n=4 inventory-shipped n=2 mail-list
-| 15
-| 24
...
-| 555-5553
-| 555-3412
...
Command-line arguments are made available for explicit examination by
the 'awk' program in the 'ARGV' array (*note ARGC and ARGV::). 'awk'
processes the values of command-line assignments for escape sequences
(*note Escape Sequences::). (d.c.)
File: gawk.info, Node: Conversion, Prev: Variables, Up: Values
6.1.4 Conversion of Strings and Numbers
---------------------------------------
Number-to-string and string-to-number conversion are generally
straightforward. There can be subtleties to be aware of; this minor
node discusses this important facet of 'awk'.
* Menu:
* Strings And Numbers:: How 'awk' Converts Between Strings And
Numbers.
* Locale influences conversions:: How the locale may affect conversions.
File: gawk.info, Node: Strings And Numbers, Next: Locale influences conversions, Up: Conversion
6.1.4.1 How 'awk' Converts Between Strings and Numbers
......................................................
Strings are converted to numbers and numbers are converted to strings,
if the context of the 'awk' program demands it. For example, if the
value of either 'foo' or 'bar' in the expression 'foo + bar' happens to
be a string, it is converted to a number before the addition is
performed. If numeric values appear in string concatenation, they are
converted to strings. Consider the following:
two = 2; three = 3
print (two three) + 4
This prints the (numeric) value 27. The numeric values of the variables
'two' and 'three' are converted to strings and concatenated together.
The resulting string is converted back to the number 23, to which 4 is
then added.
If, for some reason, you need to force a number to be converted to a
string, concatenate that number with the empty string, '""'. To force a
string to be converted to a number, add zero to that string. A string
is converted to a number by interpreting any numeric prefix of the
string as numerals: '"2.5"' converts to 2.5, '"1e3"' converts to 1,000,
and '"25fix"' has a numeric value of 25. Strings that can't be
interpreted as valid numbers convert to zero.
The exact manner in which numbers are converted into strings is
controlled by the 'awk' predefined variable 'CONVFMT' (*note Built-in
Variables::). Numbers are converted using the 'sprintf()' function with
'CONVFMT' as the format specifier (*note String Functions::).
'CONVFMT''s default value is '"%.6g"', which creates a value with at
most six significant digits. For some applications, you might want to
change it to specify more precision. On most modern machines, 17 digits
is usually enough to capture a floating-point number's value exactly.(1)
Strange results can occur if you set 'CONVFMT' to a string that
doesn't tell 'sprintf()' how to format floating-point numbers in a
useful way. For example, if you forget the '%' in the format, 'awk'
converts all numbers to the same constant string.
As a special case, if a number is an integer, then the result of
converting it to a string is _always_ an integer, no matter what the
value of 'CONVFMT' may be. Given the following code fragment:
CONVFMT = "%2.2f"
a = 12
b = a ""
'b' has the value '"12"', not '"12.00"'. (d.c.)
Pre-POSIX 'awk' Used 'OFMT' for String Conversion
Prior to the POSIX standard, 'awk' used the value of 'OFMT' for
converting numbers to strings. 'OFMT' specifies the output format to
use when printing numbers with 'print'. 'CONVFMT' was introduced in
order to separate the semantics of conversion from the semantics of
printing. Both 'CONVFMT' and 'OFMT' have the same default value:
'"%.6g"'. In the vast majority of cases, old 'awk' programs do not
change their behavior. *Note Print:: for more information on the
'print' statement.
---------- Footnotes ----------
(1) Pathological cases can require up to 752 digits (!), but we doubt
that you need to worry about this.
File: gawk.info, Node: Locale influences conversions, Prev: Strings And Numbers, Up: Conversion
6.1.4.2 Locales Can Influence Conversion
........................................
Where you are can matter when it comes to converting between numbers and
strings. The local character set and language--the "locale"--can affect
numeric formats. In particular, for 'awk' programs, it affects the
decimal point character and the thousands-separator character. The
'"C"' locale, and most English-language locales, use the period
character ('.') as the decimal point and don't have a thousands
separator. However, many (if not most) European and non-English locales
use the comma (',') as the decimal point character. European locales
often use either a space or a period as the thousands separator, if they
have one.
The POSIX standard says that 'awk' always uses the period as the
decimal point when reading the 'awk' program source code, and for
command-line variable assignments (*note Other Arguments::). However,
when interpreting input data, for 'print' and 'printf' output, and for
number-to-string conversion, the local decimal point character is used.
(d.c.) In all cases, numbers in source code and in input data cannot
have a thousands separator. Here are some examples indicating the
difference in behavior, on a GNU/Linux system:
$ export POSIXLY_CORRECT=1 Force POSIX behavior
$ gawk 'BEGIN { printf "%g\n", 3.1415927 }'
-| 3.14159
$ LC_ALL=en_DK.utf-8 gawk 'BEGIN { printf "%g\n", 3.1415927 }'
-| 3,14159
$ echo 4,321 | gawk '{ print $1 + 1 }'
-| 5
$ echo 4,321 | LC_ALL=en_DK.utf-8 gawk '{ print $1 + 1 }'
-| 5,321
The 'en_DK.utf-8' locale is for English in Denmark, where the comma acts
as the decimal point separator. In the normal '"C"' locale, 'gawk'
treats '4,321' as 4, while in the Danish locale, it's treated as the
full number including the fractional part, 4.321.
Some earlier versions of 'gawk' fully complied with this aspect of
the standard. However, many users in non-English locales complained
about this behavior, because their data used a period as the decimal
point, so the default behavior was restored to use a period as the
decimal point character. You can use the '--use-lc-numeric' option
(*note Options::) to force 'gawk' to use the locale's decimal point
character. ('gawk' also uses the locale's decimal point character when
in POSIX mode, either via '--posix' or the 'POSIXLY_CORRECT' environment
variable, as shown previously.)
*note Table 6.1: table-locale-affects. describes the cases in which
the locale's decimal point character is used and when a period is used.
Some of these features have not been described yet.
Feature Default '--posix' or
'--use-lc-numeric'
------------------------------------------------------------
'%'g' Use locale Use locale
'%g' Use period Use locale
Input Use period Use locale
'strtonum()'Use period Use locale
Table 6.1: Locale decimal point versus a period
Finally, modern-day formal standards and the IEEE standard
floating-point representation can have an unusual but important effect
on the way 'gawk' converts some special string values to numbers. The
details are presented in *note POSIX Floating Point Problems::.
File: gawk.info, Node: All Operators, Next: Truth Values and Conditions, Prev: Values, Up: Expressions
6.2 Operators: Doing Something with Values
==========================================
This minor node introduces the "operators" that make use of the values
provided by constants and variables.
* Menu:
* Arithmetic Ops:: Arithmetic operations ('+', '-',
etc.)
* Concatenation:: Concatenating strings.
* Assignment Ops:: Changing the value of a variable or a field.
* Increment Ops:: Incrementing the numeric value of a variable.
File: gawk.info, Node: Arithmetic Ops, Next: Concatenation, Up: All Operators
6.2.1 Arithmetic Operators
--------------------------
The 'awk' language uses the common arithmetic operators when evaluating
expressions. All of these arithmetic operators follow normal precedence
rules and work as you would expect them to.
The following example uses a file named 'grades', which contains a
list of student names as well as three test scores per student (it's a
small class):
Pat 100 97 58
Sandy 84 72 93
Chris 72 92 89
This program takes the file 'grades' and prints the average of the
scores:
$ awk '{ sum = $2 + $3 + $4 ; avg = sum / 3
> print $1, avg }' grades
-| Pat 85
-| Sandy 83
-| Chris 84.3333
The following list provides the arithmetic operators in 'awk', in
order from the highest precedence to the lowest:
'X ^ Y'
'X ** Y'
Exponentiation; X raised to the Y power. '2 ^ 3' has the value
eight; the character sequence '**' is equivalent to '^'. (c.e.)
'- X'
Negation.
'+ X'
Unary plus; the expression is converted to a number.
'X * Y'
Multiplication.
'X / Y'
Division; because all numbers in 'awk' are floating-point numbers,
the result is _not_ rounded to an integer--'3 / 4' has the value
0.75. (It is a common mistake, especially for C programmers, to
forget that _all_ numbers in 'awk' are floating point, and that
division of integer-looking constants produces a real number, not
an integer.)
'X % Y'
Remainder; further discussion is provided in the text, just after
this list.
'X + Y'
Addition.
'X - Y'
Subtraction.
Unary plus and minus have the same precedence, the multiplication
operators all have the same precedence, and addition and subtraction
have the same precedence.
When computing the remainder of 'X % Y', the quotient is rounded
toward zero to an integer and multiplied by Y. This result is
subtracted from X; this operation is sometimes known as "trunc-mod."
The following relation always holds:
b * int(a / b) + (a % b) == a
One possibly undesirable effect of this definition of remainder is
that 'X % Y' is negative if X is negative. Thus:
-17 % 8 = -1
In other 'awk' implementations, the signedness of the remainder may
be machine-dependent.
NOTE: The POSIX standard only specifies the use of '^' for
exponentiation. For maximum portability, do not use the '**'
operator.
File: gawk.info, Node: Concatenation, Next: Assignment Ops, Prev: Arithmetic Ops, Up: All Operators
6.2.2 String Concatenation
--------------------------
It seemed like a good idea at the time.
-- _Brian Kernighan_
There is only one string operation: concatenation. It does not have
a specific operator to represent it. Instead, concatenation is
performed by writing expressions next to one another, with no operator.
For example:
$ awk '{ print "Field number one: " $1 }' mail-list
-| Field number one: Amelia
-| Field number one: Anthony
...
Without the space in the string constant after the ':', the line runs
together. For example:
$ awk '{ print "Field number one:" $1 }' mail-list
-| Field number one:Amelia
-| Field number one:Anthony
...
Because string concatenation does not have an explicit operator, it
is often necessary to ensure that it happens at the right time by using
parentheses to enclose the items to concatenate. For example, you might
expect that the following code fragment concatenates 'file' and 'name':
file = "file"
name = "name"
print "something meaningful" > file name
This produces a syntax error with some versions of Unix 'awk'.(1) It is
necessary to use the following:
print "something meaningful" > (file name)
Parentheses should be used around concatenation in all but the most
common contexts, such as on the righthand side of '='. Be careful about
the kinds of expressions used in string concatenation. In particular,
the order of evaluation of expressions used for concatenation is
undefined in the 'awk' language. Consider this example:
BEGIN {
a = "don't"
print (a " " (a = "panic"))
}
It is not defined whether the second assignment to 'a' happens before or
after the value of 'a' is retrieved for producing the concatenated
value. The result could be either 'don't panic', or 'panic panic'.
The precedence of concatenation, when mixed with other operators, is
often counter-intuitive. Consider this example:
$ awk 'BEGIN { print -12 " " -24 }'
-| -12-24
This "obviously" is concatenating -12, a space, and -24. But where
did the space disappear to? The answer lies in the combination of
operator precedences and 'awk''s automatic conversion rules. To get the
desired result, write the program this way:
$ awk 'BEGIN { print -12 " " (-24) }'
-| -12 -24
This forces 'awk' to treat the '-' on the '-24' as unary. Otherwise,
it's parsed as follows:
-12 ('" "' - 24)
=> -12 (0 - 24)
=> -12 (-24)
=> -12-24
As mentioned earlier, when mixing concatenation with other operators,
_parenthesize_. Otherwise, you're never quite sure what you'll get.
---------- Footnotes ----------
(1) It happens that BWK 'awk', 'gawk', and 'mawk' all "get it right,"
but you should not rely on this.
File: gawk.info, Node: Assignment Ops, Next: Increment Ops, Prev: Concatenation, Up: All Operators
6.2.3 Assignment Expressions
----------------------------
An "assignment" is an expression that stores a (usually different) value
into a variable. For example, let's assign the value one to the
variable 'z':
z = 1
After this expression is executed, the variable 'z' has the value
one. Whatever old value 'z' had before the assignment is forgotten.
Assignments can also store string values. For example, the following
stores the value '"this food is good"' in the variable 'message':
thing = "food"
predicate = "good"
message = "this " thing " is " predicate
This also illustrates string concatenation. The '=' sign is called an
"assignment operator". It is the simplest assignment operator because
the value of the righthand operand is stored unchanged. Most operators
(addition, concatenation, and so on) have no effect except to compute a
value. If the value isn't used, there's no reason to use the operator.
An assignment operator is different; it does produce a value, but even
if you ignore it, the assignment still makes itself felt through the
alteration of the variable. We call this a "side effect".
The lefthand operand of an assignment need not be a variable (*note
Variables::); it can also be a field (*note Changing Fields::) or an
array element (*note Arrays::). These are all called "lvalues", which
means they can appear on the lefthand side of an assignment operator.
The righthand operand may be any expression; it produces the new value
that the assignment stores in the specified variable, field, or array
element. (Such values are called "rvalues".)
It is important to note that variables do _not_ have permanent types.
A variable's type is simply the type of whatever value was last assigned
to it. In the following program fragment, the variable 'foo' has a
numeric value at first, and a string value later on:
foo = 1
print foo
foo = "bar"
print foo
When the second assignment gives 'foo' a string value, the fact that it
previously had a numeric value is forgotten.
String values that do not begin with a digit have a numeric value of
zero. After executing the following code, the value of 'foo' is five:
foo = "a string"
foo = foo + 5
NOTE: Using a variable as a number and then later as a string can
be confusing and is poor programming style. The previous two
examples illustrate how 'awk' works, _not_ how you should write
your programs!
An assignment is an expression, so it has a value--the same value
that is assigned. Thus, 'z = 1' is an expression with the value one.
One consequence of this is that you can write multiple assignments
together, such as:
x = y = z = 5
This example stores the value five in all three variables ('x', 'y', and
'z'). It does so because the value of 'z = 5', which is five, is stored
into 'y' and then the value of 'y = z = 5', which is five, is stored
into 'x'.
Assignments may be used anywhere an expression is called for. For
example, it is valid to write 'x != (y = 1)' to set 'y' to one, and then
test whether 'x' equals one. But this style tends to make programs hard
to read; such nesting of assignments should be avoided, except perhaps
in a one-shot program.
Aside from '=', there are several other assignment operators that do
arithmetic with the old value of the variable. For example, the
operator '+=' computes a new value by adding the righthand value to the
old value of the variable. Thus, the following assignment adds five to
the value of 'foo':
foo += 5
This is equivalent to the following:
foo = foo + 5
Use whichever makes the meaning of your program clearer.
There are situations where using '+=' (or any assignment operator) is
_not_ the same as simply repeating the lefthand operand in the righthand
expression. For example:
# Thanks to Pat Rankin for this example
BEGIN {
foo[rand()] += 5
for (x in foo)
print x, foo[x]
bar[rand()] = bar[rand()] + 5
for (x in bar)
print x, bar[x]
}
The indices of 'bar' are practically guaranteed to be different, because
'rand()' returns different values each time it is called. (Arrays and
the 'rand()' function haven't been covered yet. *Note Arrays::, and
*note Numeric Functions:: for more information.) This example
illustrates an important fact about assignment operators: the lefthand
expression is only evaluated _once_.
It is up to the implementation as to which expression is evaluated
first, the lefthand or the righthand. Consider this example:
i = 1
a[i += 2] = i + 1
The value of 'a[3]' could be either two or four.
*note Table 6.2: table-assign-ops. lists the arithmetic assignment
operators. In each case, the righthand operand is an expression whose
value is converted to a number.
Operator Effect
--------------------------------------------------------------------------
LVALUE '+=' Add INCREMENT to the value of LVALUE.
INCREMENT
LVALUE '-=' Subtract DECREMENT from the value of LVALUE.
DECREMENT
LVALUE '*=' Multiply the value of LVALUE by COEFFICIENT.
COEFFICIENT
LVALUE '/=' DIVISOR Divide the value of LVALUE by DIVISOR.
LVALUE '%=' MODULUS Set LVALUE to its remainder by MODULUS.
LVALUE '^=' POWER Raise LVALUE to the power POWER.
LVALUE '**=' POWER Raise LVALUE to the power POWER. (c.e.)
Table 6.2: Arithmetic assignment operators
NOTE: Only the '^=' operator is specified by POSIX. For maximum
portability, do not use the '**=' operator.
Syntactic Ambiguities Between '/=' and Regular Expressions
There is a syntactic ambiguity between the '/=' assignment operator
and regexp constants whose first character is an '='. (d.c.) This is
most notable in some commercial 'awk' versions. For example:
$ awk /==/ /dev/null
error-> awk: syntax error at source line 1
error-> context is
error-> >>> /= <<<
error-> awk: bailing out at source line 1
A workaround is:
awk '/[=]=/' /dev/null
'gawk' does not have this problem; BWK 'awk' and 'mawk' also do not.
File: gawk.info, Node: Increment Ops, Prev: Assignment Ops, Up: All Operators
6.2.4 Increment and Decrement Operators
---------------------------------------
"Increment" and "decrement operators" increase or decrease the value of
a variable by one. An assignment operator can do the same thing, so the
increment operators add no power to the 'awk' language; however, they
are convenient abbreviations for very common operations.
The operator used for adding one is written '++'. It can be used to
increment a variable either before or after taking its value. To
"pre-increment" a variable 'v', write '++v'. This adds one to the value
of 'v'--that new value is also the value of the expression. (The
assignment expression 'v += 1' is completely equivalent.) Writing the
'++' after the variable specifies "post-increment". This increments the
variable value just the same; the difference is that the value of the
increment expression itself is the variable's _old_ value. Thus, if
'foo' has the value four, then the expression 'foo++' has the value
four, but it changes the value of 'foo' to five. In other words, the
operator returns the old value of the variable, but with the side effect
of incrementing it.
The post-increment 'foo++' is nearly the same as writing '(foo += 1)
- 1'. It is not perfectly equivalent because all numbers in 'awk' are
floating point--in floating point, 'foo + 1 - 1' does not necessarily
equal 'foo'. But the difference is minute as long as you stick to
numbers that are fairly small (less than 10e12).
Fields and array elements are incremented just like variables. (Use
'$(i++)' when you want to do a field reference and a variable increment
at the same time. The parentheses are necessary because of the
precedence of the field reference operator '$'.)
The decrement operator '--' works just like '++', except that it
subtracts one instead of adding it. As with '++', it can be used before
the lvalue to pre-decrement or after it to post-decrement. Following is
a summary of increment and decrement expressions:
'++LVALUE'
Increment LVALUE, returning the new value as the value of the
expression.
'LVALUE++'
Increment LVALUE, returning the _old_ value of LVALUE as the value
of the expression.
'--LVALUE'
Decrement LVALUE, returning the new value as the value of the
expression. (This expression is like '++LVALUE', but instead of
adding, it subtracts.)
'LVALUE--'
Decrement LVALUE, returning the _old_ value of LVALUE as the value
of the expression. (This expression is like 'LVALUE++', but
instead of adding, it subtracts.)
Operator Evaluation Order
Doctor, it hurts when I do this!
Then don't do that!
-- _Groucho Marx_
What happens for something like the following?
b = 6
print b += b++
Or something even stranger?
b = 6
b += ++b + b++
print b
In other words, when do the various side effects prescribed by the
postfix operators ('b++') take effect? When side effects happen is
"implementation-defined". In other words, it is up to the particular
version of 'awk'. The result for the first example may be 12 or 13, and
for the second, it may be 22 or 23.
In short, doing things like this is not recommended and definitely
not anything that you can rely upon for portability. You should avoid
such things in your own programs.
File: gawk.info, Node: Truth Values and Conditions, Next: Function Calls, Prev: All Operators, Up: Expressions
6.3 Truth Values and Conditions
===============================
In certain contexts, expression values also serve as "truth values";
i.e., they determine what should happen next as the program runs. This
minor node describes how 'awk' defines "true" and "false" and how values
are compared.
* Menu:
* Truth Values:: What is "true" and what is "false".
* Typing and Comparison:: How variables acquire types and how this
affects comparison of numbers and strings with
'<', etc.
* Boolean Ops:: Combining comparison expressions using boolean
operators '||' ("or"), '&&'
("and") and '!' ("not").
* Conditional Exp:: Conditional expressions select between two
subexpressions under control of a third
subexpression.
File: gawk.info, Node: Truth Values, Next: Typing and Comparison, Up: Truth Values and Conditions
6.3.1 True and False in 'awk'
-----------------------------
Many programming languages have a special representation for the
concepts of "true" and "false." Such languages usually use the special
constants 'true' and 'false', or perhaps their uppercase equivalents.
However, 'awk' is different. It borrows a very simple concept of true
and false from C. In 'awk', any nonzero numeric value _or_ any nonempty
string value is true. Any other value (zero or the null string, '""')
is false. The following program prints 'A strange truth value' three
times:
BEGIN {
if (3.1415927)
print "A strange truth value"
if ("Four Score And Seven Years Ago")
print "A strange truth value"
if (j = 57)
print "A strange truth value"
}
There is a surprising consequence of the "nonzero or non-null" rule:
the string constant '"0"' is actually true, because it is non-null.
(d.c.)
File: gawk.info, Node: Typing and Comparison, Next: Boolean Ops, Prev: Truth Values, Up: Truth Values and Conditions
6.3.2 Variable Typing and Comparison Expressions
------------------------------------------------
The Guide is definitive. Reality is frequently inaccurate.
-- _Douglas Adams, 'The Hitchhiker's Guide to the Galaxy'_
Unlike in other programming languages, in 'awk' variables do not have
a fixed type. Instead, they can be either a number or a string,
depending upon the value that is assigned to them. We look now at how
variables are typed, and how 'awk' compares variables.
* Menu:
* Variable Typing:: String type versus numeric type.
* Comparison Operators:: The comparison operators.
* POSIX String Comparison:: String comparison with POSIX rules.
File: gawk.info, Node: Variable Typing, Next: Comparison Operators, Up: Typing and Comparison
6.3.2.1 String Type versus Numeric Type
.......................................
Scalar objects in 'awk' (variables, array elements, and fields) are
_dynamically_ typed. This means their type can change as the program
runs, from "untyped" before any use,(1) to string or number, and then
from string to number or number to string, as the program progresses.
('gawk' also provides regexp-typed scalars, but let's ignore that for
now; *note Strong Regexp Constants::.)
You can't do much with untyped variables, other than tell that they
are untyped. The following program tests 'a' against '""' and '0'; the
test succeeds when 'a' has never been assigned a value. It also uses
the built-in 'typeof()' function (not presented yet; *note Type
Functions::) to show 'a''s type:
$ gawk 'BEGIN { print (a == "" && a == 0 ?
> "a is untyped" : "a has a type!") ; print typeof(a) }'
-| a is untyped
-| unassigned
A scalar has numeric type when assigned a numeric value, such as from
a numeric constant, or from another scalar with numeric type:
$ gawk 'BEGIN { a = 42 ; print typeof(a)
> b = a ; print typeof(b) }'
number
number
Similarly, a scalar has string type when assigned a string value,
such as from a string constant, or from another scalar with string type:
$ gawk 'BEGIN { a = "forty two" ; print typeof(a)
> b = a ; print typeof(b) }'
string
string
So far, this is all simple and straightforward. What happens,
though, when 'awk' has to process data from a user? Let's start with
field data. What should the following command produce as output?
echo hello | awk '{ printf("%s %s < 42\n", $1,
($1 < 42 ? "is" : "is not")) }'
Since 'hello' is alphabetic data, 'awk' can only do a string comparison.
Internally, it converts '42' into '"42"' and compares the two string
values '"hello"' and '"42"'. Here's the result:
$ echo hello | awk '{ printf("%s %s < 42\n", $1,
> ($1 < 42 ? "is" : "is not")) }'
-| hello is not < 42
However, what happens when data from a user _looks like_ a number?
On the one hand, in reality, the input data consists of characters, not
binary numeric values. But, on the other hand, the data looks numeric,
and 'awk' really ought to treat it as such. And indeed, it does:
$ echo 37 | awk '{ printf("%s %s < 42\n", $1,
> ($1 < 42 ? "is" : "is not")) }'
-| 37 is < 42
Here are the rules for when 'awk' treats data as a number, and for
when it treats data as a string.
The POSIX standard uses the term "numeric string" for input data that
looks numeric. The '37' in the previous example is a numeric string.
So what is the type of a numeric string? Answer: numeric.
The type of a variable is important because the types of two
variables determine how they are compared. Variable typing follows
these definitions and rules:
* A numeric constant or the result of a numeric operation has the
"numeric" attribute.
* A string constant or the result of a string operation has the
"string" attribute.
* Fields, 'getline' input, 'FILENAME', 'ARGV' elements, 'ENVIRON'
elements, and the elements of an array created by 'match()',
'split()', and 'patsplit()' that are numeric strings have the
"strnum" attribute.(2) Otherwise, they have the "string"
attribute. Uninitialized variables also have the "strnum"
attribute.
* Attributes propagate across assignments but are not changed by any
use.
The last rule is particularly important. In the following program,
'a' has numeric type, even though it is later used in a string
operation:
BEGIN {
a = 12.345
b = a " is a cute number"
print b
}
When two operands are compared, either string comparison or numeric
comparison may be used. This depends upon the attributes of the
operands, according to the following symmetric matrix:
+----------------------------------------------
| STRING NUMERIC STRNUM
--------+----------------------------------------------
|
STRING | string string string
|
NUMERIC | string numeric numeric
|
STRNUM | string numeric numeric
--------+----------------------------------------------
The basic idea is that user input that looks numeric--and _only_ user
input--should be treated as numeric, even though it is actually made of
characters and is therefore also a string. Thus, for example, the
string constant '" +3.14"', when it appears in program source code, is a
string--even though it looks numeric--and is _never_ treated as a number
for comparison purposes.
In short, when one operand is a "pure" string, such as a string
constant, then a string comparison is performed. Otherwise, a numeric
comparison is performed. (The primary difference between a number and a
strnum is that for strnums 'gawk' preserves the original string value
that the scalar had when it came in.)
This point bears additional emphasis: Input that looks numeric _is_
numeric. All other input is treated as strings.
Thus, the six-character input string ' +3.14' receives the strnum
attribute. In contrast, the eight characters '" +3.14"' appearing in
program text comprise a string constant. The following examples print
'1' when the comparison between the two different constants is true, and
'0' otherwise:
$ echo ' +3.14' | awk '{ print($0 == " +3.14") }' True
-| 1
$ echo ' +3.14' | awk '{ print($0 == "+3.14") }' False
-| 0
$ echo ' +3.14' | awk '{ print($0 == "3.14") }' False
-| 0
$ echo ' +3.14' | awk '{ print($0 == 3.14) }' True
-| 1
$ echo ' +3.14' | awk '{ print($1 == " +3.14") }' False
-| 0
$ echo ' +3.14' | awk '{ print($1 == "+3.14") }' True
-| 1
$ echo ' +3.14' | awk '{ print($1 == "3.14") }' False
-| 0
$ echo ' +3.14' | awk '{ print($1 == 3.14) }' True
-| 1
You can see the type of an input field (or other user input) using
'typeof()':
$ echo hello 37 | gawk '{ print typeof($1), typeof($2) }'
-| string strnum
---------- Footnotes ----------
(1) 'gawk' calls this "unassigned", as the following example shows.
(2) Thus, a POSIX numeric string and 'gawk''s strnum are the same
thing.
File: gawk.info, Node: Comparison Operators, Next: POSIX String Comparison, Prev: Variable Typing, Up: Typing and Comparison
6.3.2.2 Comparison Operators
............................
"Comparison expressions" compare strings or numbers for relationships
such as equality. They are written using "relational operators", which
are a superset of those in C. *note Table 6.3: table-relational-ops.
describes them.
Expression Result
--------------------------------------------------------------------------
X '<' Y True if X is less than Y
X '<=' Y True if X is less than or equal to Y
X '>' Y True if X is greater than Y
X '>=' Y True if X is greater than or equal to Y
X '==' Y True if X is equal to Y
X '!=' Y True if X is not equal to Y
X '~' Y True if the string X matches the regexp denoted by Y
X '!~' Y True if the string X does not match the regexp
denoted by Y
SUBSCRIPT 'in' True if the array ARRAY has an element with the
ARRAY subscript SUBSCRIPT
Table 6.3: Relational operators
Comparison expressions have the value one if true and zero if false.
When comparing operands of mixed types, numeric operands are converted
to strings using the value of 'CONVFMT' (*note Conversion::).
Strings are compared by comparing the first character of each, then
the second character of each, and so on. Thus, '"10"' is less than
'"9"'. If there are two strings where one is a prefix of the other, the
shorter string is less than the longer one. Thus, '"abc"' is less than
'"abcd"'.
It is very easy to accidentally mistype the '==' operator and leave
off one of the '=' characters. The result is still valid 'awk' code,
but the program does not do what is intended:
if (a = b) # oops! should be a == b
...
else
...
Unless 'b' happens to be zero or the null string, the 'if' part of the
test always succeeds. Because the operators are so similar, this kind
of error is very difficult to spot when scanning the source code.
The following list of expressions illustrates the kinds of
comparisons 'awk' performs, as well as what the result of each
comparison is:
'1.5 <= 2.0'
Numeric comparison (true)
'"abc" >= "xyz"'
String comparison (false)
'1.5 != " +2"'
String comparison (true)
'"1e2" < "3"'
String comparison (true)
'a = 2; b = "2"'
'a == b'
String comparison (true)
'a = 2; b = " +2"'
'a == b'
String comparison (false)
In this example:
$ echo 1e2 3 | awk '{ print ($1 < $2) ? "true" : "false" }'
-| false
the result is 'false' because both '$1' and '$2' are user input. They
are numeric strings--therefore both have the strnum attribute, dictating
a numeric comparison. The purpose of the comparison rules and the use
of numeric strings is to attempt to produce the behavior that is "least
surprising," while still "doing the right thing."
String comparisons and regular expression comparisons are very
different. For example:
x == "foo"
has the value one, or is true if the variable 'x' is precisely 'foo'.
By contrast:
x ~ /foo/
has the value one if 'x' contains 'foo', such as '"Oh, what a fool am
I!"'.
The righthand operand of the '~' and '!~' operators may be either a
regexp constant ('/'...'/') or an ordinary expression. In the latter
case, the value of the expression as a string is used as a dynamic
regexp (*note Regexp Usage::; also *note Computed Regexps::).
A constant regular expression in slashes by itself is also an
expression. '/REGEXP/' is an abbreviation for the following comparison
expression:
$0 ~ /REGEXP/
One special place where '/foo/' is _not_ an abbreviation for '$0 ~
/foo/' is when it is the righthand operand of '~' or '!~'. *Note Using
Constant Regexps::, where this is discussed in more detail.
File: gawk.info, Node: POSIX String Comparison, Prev: Comparison Operators, Up: Typing and Comparison
6.3.2.3 String Comparison Based on Locale Collating Order
.........................................................
The POSIX standard used to say that all string comparisons are performed
based on the locale's "collating order". This is the order in which
characters sort, as defined by the locale (for more discussion, *note
Locales::). This order is usually very different from the results
obtained when doing straight byte-by-byte comparison.(1)
Because this behavior differs considerably from existing practice,
'gawk' only implemented it when in POSIX mode (*note Options::). Here
is an example to illustrate the difference, in an 'en_US.UTF-8' locale:
$ gawk 'BEGIN { printf("ABC < abc = %s\n",
> ("ABC" < "abc" ? "TRUE" : "FALSE")) }'
-| ABC < abc = TRUE
$ gawk --posix 'BEGIN { printf("ABC < abc = %s\n",
> ("ABC" < "abc" ? "TRUE" : "FALSE")) }'
-| ABC < abc = FALSE
Fortunately, as of August 2016, comparison based on locale collating
order is no longer required for the '==' and '!=' operators.(2)
However, comparison based on locales is still required for '<', '<=',
'>', and '>='. POSIX thus recommends as follows:
Since the '==' operator checks whether strings are identical, not
whether they collate equally, applications needing to check whether
strings collate equally can use:
a <= b && a >= b
As of version 4.2, 'gawk' continues to use locale collating order for
'<', '<=', '>', and '>=' only in POSIX mode.
---------- Footnotes ----------
(1) Technically, string comparison is supposed to behave the same way
as if the strings were compared with the C 'strcoll()' function.
(2) See the Austin Group website
(http://austingroupbugs.net/view.php?id=1070).
File: gawk.info, Node: Boolean Ops, Next: Conditional Exp, Prev: Typing and Comparison, Up: Truth Values and Conditions
6.3.3 Boolean Expressions
-------------------------
A "Boolean expression" is a combination of comparison expressions or
matching expressions, using the Boolean operators "or" ('||'), "and"
('&&'), and "not" ('!'), along with parentheses to control nesting. The
truth value of the Boolean expression is computed by combining the truth
values of the component expressions. Boolean expressions are also
referred to as "logical expressions". The terms are equivalent.
Boolean expressions can be used wherever comparison and matching
expressions can be used. They can be used in 'if', 'while', 'do', and
'for' statements (*note Statements::). They have numeric values (one if
true, zero if false) that come into play if the result of the Boolean
expression is stored in a variable or used in arithmetic.
In addition, every Boolean expression is also a valid pattern, so you
can use one as a pattern to control the execution of rules. The Boolean
operators are:
'BOOLEAN1 && BOOLEAN2'
True if both BOOLEAN1 and BOOLEAN2 are true. For example, the
following statement prints the current input record if it contains
both 'edu' and 'li':
if ($0 ~ /edu/ && $0 ~ /li/) print
The subexpression BOOLEAN2 is evaluated only if BOOLEAN1 is true.
This can make a difference when BOOLEAN2 contains expressions that
have side effects. In the case of '$0 ~ /foo/ && ($2 == bar++)',
the variable 'bar' is not incremented if there is no substring
'foo' in the record.
'BOOLEAN1 || BOOLEAN2'
True if at least one of BOOLEAN1 or BOOLEAN2 is true. For example,
the following statement prints all records in the input that
contain _either_ 'edu' or 'li':
if ($0 ~ /edu/ || $0 ~ /li/) print
The subexpression BOOLEAN2 is evaluated only if BOOLEAN1 is false.
This can make a difference when BOOLEAN2 contains expressions that
have side effects. (Thus, this test never really distinguishes
records that contain both 'edu' and 'li'--as soon as 'edu' is
matched, the full test succeeds.)
'! BOOLEAN'
True if BOOLEAN is false. For example, the following program
prints 'no home!' in the unusual event that the 'HOME' environment
variable is not defined:
BEGIN { if (! ("HOME" in ENVIRON))
print "no home!" }
(The 'in' operator is described in *note Reference to Elements::.)
The '&&' and '||' operators are called "short-circuit" operators
because of the way they work. Evaluation of the full expression is
"short-circuited" if the result can be determined partway through its
evaluation.
Statements that end with '&&' or '||' can be continued simply by
putting a newline after them. But you cannot put a newline in front of
either of these operators without using backslash continuation (*note
Statements/Lines::).
The actual value of an expression using the '!' operator is either
one or zero, depending upon the truth value of the expression it is
applied to. The '!' operator is often useful for changing the sense of
a flag variable from false to true and back again. For example, the
following program is one way to print lines in between special
bracketing lines:
$1 == "START" { interested = ! interested; next }
interested { print }
$1 == "END" { interested = ! interested; next }
The variable 'interested', as with all 'awk' variables, starts out
initialized to zero, which is also false. When a line is seen whose
first field is 'START', the value of 'interested' is toggled to true,
using '!'. The next rule prints lines as long as 'interested' is true.
When a line is seen whose first field is 'END', 'interested' is toggled
back to false.(1)
Most commonly, the '!' operator is used in the conditions of 'if' and
'while' statements, where it often makes more sense to phrase the logic
in the negative:
if (! SOME CONDITION || SOME OTHER CONDITION) {
... DO WHATEVER PROCESSING ...
}
NOTE: The 'next' statement is discussed in *note Next Statement::.
'next' tells 'awk' to skip the rest of the rules, get the next
record, and start processing the rules over again at the top. The
reason it's there is to avoid printing the bracketing 'START' and
'END' lines.
---------- Footnotes ----------
(1) This program has a bug; it prints lines starting with 'END'. How
would you fix it?
File: gawk.info, Node: Conditional Exp, Prev: Boolean Ops, Up: Truth Values and Conditions
6.3.4 Conditional Expressions
-----------------------------
A "conditional expression" is a special kind of expression that has
three operands. It allows you to use one expression's value to select
one of two other expressions. The conditional expression in 'awk' is
the same as in the C language, as shown here:
SELECTOR ? IF-TRUE-EXP : IF-FALSE-EXP
There are three subexpressions. The first, SELECTOR, is always computed
first. If it is "true" (not zero or not null), then IF-TRUE-EXP is
computed next, and its value becomes the value of the whole expression.
Otherwise, IF-FALSE-EXP is computed next, and its value becomes the
value of the whole expression. For example, the following expression
produces the absolute value of 'x':
x >= 0 ? x : -x
Each time the conditional expression is computed, only one of
IF-TRUE-EXP and IF-FALSE-EXP is used; the other is ignored. This is
important when the expressions have side effects. For example, this
conditional expression examines element 'i' of either array 'a' or array
'b', and increments 'i':
x == y ? a[i++] : b[i++]
This is guaranteed to increment 'i' exactly once, because each time only
one of the two increment expressions is executed and the other is not.
*Note Arrays::, for more information about arrays.
As a minor 'gawk' extension, a statement that uses '?:' can be
continued simply by putting a newline after either character. However,
putting a newline in front of either character does not work without
using backslash continuation (*note Statements/Lines::). If '--posix'
is specified (*note Options::), this extension is disabled.
File: gawk.info, Node: Function Calls, Next: Precedence, Prev: Truth Values and Conditions, Up: Expressions
6.4 Function Calls
==================
A "function" is a name for a particular calculation. This enables you
to ask for it by name at any point in the program. For example, the
function 'sqrt()' computes the square root of a number.
A fixed set of functions are "built in", which means they are
available in every 'awk' program. The 'sqrt()' function is one of
these. *Note Built-in:: for a list of built-in functions and their
descriptions. In addition, you can define functions for use in your
program. *Note User-defined:: for instructions on how to do this.
Finally, 'gawk' lets you write functions in C or C++ that may be called
from your program (*note Dynamic Extensions::).
The way to use a function is with a "function call" expression, which
consists of the function name followed immediately by a list of
"arguments" in parentheses. The arguments are expressions that provide
the raw materials for the function's calculations. When there is more
than one argument, they are separated by commas. If there are no
arguments, just write '()' after the function name. The following
examples show function calls with and without arguments:
sqrt(x^2 + y^2) one argument
atan2(y, x) two arguments
rand() no arguments
CAUTION: Do not put any space between the function name and the
opening parenthesis! A user-defined function name looks just like
the name of a variable--a space would make the expression look like
concatenation of a variable with an expression inside parentheses.
With built-in functions, space before the parenthesis is harmless,
but it is best not to get into the habit of using space to avoid
mistakes with user-defined functions.
Each function expects a particular number of arguments. For example,
the 'sqrt()' function must be called with a single argument, the number
of which to take the square root:
sqrt(ARGUMENT)
Some of the built-in functions have one or more optional arguments.
If those arguments are not supplied, the functions use a reasonable
default value. *Note Built-in:: for full details. If arguments are
omitted in calls to user-defined functions, then those arguments are
treated as local variables. Such local variables act like the empty
string if referenced where a string value is required, and like zero if
referenced where a numeric value is required (*note User-defined::).
As an advanced feature, 'gawk' provides indirect function calls,
which is a way to choose the function to call at runtime, instead of
when you write the source code to your program. We defer discussion of
this feature until later; see *note Indirect Calls::.
Like every other expression, the function call has a value, often
called the "return value", which is computed by the function based on
the arguments you give it. In this example, the return value of
'sqrt(ARGUMENT)' is the square root of ARGUMENT. The following program
reads numbers, one number per line, and prints the square root of each
one:
$ awk '{ print "The square root of", $1, "is", sqrt($1) }'
1
-| The square root of 1 is 1
3
-| The square root of 3 is 1.73205
5
-| The square root of 5 is 2.23607
Ctrl-d
A function can also have side effects, such as assigning values to
certain variables or doing I/O. This program shows how the 'match()'
function (*note String Functions::) changes the variables 'RSTART' and
'RLENGTH':
{
if (match($1, $2))
print RSTART, RLENGTH
else
print "no match"
}
Here is a sample run:
$ awk -f matchit.awk
aaccdd c+
-| 3 2
foo bar
-| no match
abcdefg e
-| 5 1
File: gawk.info, Node: Precedence, Next: Locales, Prev: Function Calls, Up: Expressions
6.5 Operator Precedence (How Operators Nest)
============================================
"Operator precedence" determines how operators are grouped when
different operators appear close by in one expression. For example, '*'
has higher precedence than '+'; thus, 'a + b * c' means to multiply 'b'
and 'c', and then add 'a' to the product (i.e., 'a + (b * c)').
The normal precedence of the operators can be overruled by using
parentheses. Think of the precedence rules as saying where the
parentheses are assumed to be. In fact, it is wise to always use
parentheses whenever there is an unusual combination of operators,
because other people who read the program may not remember what the
precedence is in this case. Even experienced programmers occasionally
forget the exact rules, which leads to mistakes. Explicit parentheses
help prevent any such mistakes.
When operators of equal precedence are used together, the leftmost
operator groups first, except for the assignment, conditional, and
exponentiation operators, which group in the opposite order. Thus, 'a -
b + c' groups as '(a - b) + c' and 'a = b = c' groups as 'a = (b = c)'.
Normally the precedence of prefix unary operators does not matter,
because there is only one way to interpret them: innermost first. Thus,
'$++i' means '$(++i)' and '++$x' means '++($x)'. However, when another
operator follows the operand, then the precedence of the unary operators
can matter. '$x^2' means '($x)^2', but '-x^2' means '-(x^2)', because
'-' has lower precedence than '^', whereas '$' has higher precedence.
Also, operators cannot be combined in a way that violates the precedence
rules; for example, '$$0++--' is not a valid expression because the
first '$' has higher precedence than the '++'; to avoid the problem the
expression can be rewritten as '$($0++)--'.
This list presents 'awk''s operators, in order of highest to lowest
precedence:
'('...')'
Grouping.
'$'
Field reference.
'++ --'
Increment, decrement.
'^ **'
Exponentiation. These operators group right to left.
'+ - !'
Unary plus, minus, logical "not."
'* / %'
Multiplication, division, remainder.
'+ -'
Addition, subtraction.
String concatenation
There is no special symbol for concatenation. The operands are
simply written side by side (*note Concatenation::).
'< <= == != > >= >> | |&'
Relational and redirection. The relational operators and the
redirections have the same precedence level. Characters such as
'>' serve both as relationals and as redirections; the context
distinguishes between the two meanings.
Note that the I/O redirection operators in 'print' and 'printf'
statements belong to the statement level, not to expressions. The
redirection does not produce an expression that could be the
operand of another operator. As a result, it does not make sense
to use a redirection operator near another operator of lower
precedence without parentheses. Such combinations (e.g., 'print
foo > a ? b : c') result in syntax errors. The correct way to
write this statement is 'print foo > (a ? b : c)'.
'~ !~'
Matching, nonmatching.
'in'
Array membership.
'&&'
Logical "and."
'||'
Logical "or."
'?:'
Conditional. This operator groups right to left.
'= += -= *= /= %= ^= **='
Assignment. These operators group right to left.
NOTE: The '|&', '**', and '**=' operators are not specified by
POSIX. For maximum portability, do not use them.
File: gawk.info, Node: Locales, Next: Expressions Summary, Prev: Precedence, Up: Expressions
6.6 Where You Are Makes a Difference
====================================
Modern systems support the notion of "locales": a way to tell the system
about the local character set and language. The ISO C standard defines
a default '"C"' locale, which is an environment that is typical of what
many C programmers are used to.
Once upon a time, the locale setting used to affect regexp matching,
but this is no longer true (*note Ranges and Locales::).
Locales can affect record splitting. For the normal case of 'RS =
"\n"', the locale is largely irrelevant. For other single-character
record separators, setting 'LC_ALL=C' in the environment will give you
much better performance when reading records. Otherwise, 'gawk' has to
make several function calls, _per input character_, to find the record
terminator.
Locales can affect how dates and times are formatted (*note Time
Functions::). For example, a common way to abbreviate the date
September 4, 2015, in the United States is "9/4/15." In many countries
in Europe, however, it is abbreviated "4.9.15." Thus, the '%x'
specification in a '"US"' locale might produce '9/4/15', while in a
'"EUROPE"' locale, it might produce '4.9.15'.
According to POSIX, string comparison is also affected by locales
(similar to regular expressions). The details are presented in *note
POSIX String Comparison::.
Finally, the locale affects the value of the decimal point character
used when 'gawk' parses input data. This is discussed in detail in
*note Conversion::.
File: gawk.info, Node: Expressions Summary, Prev: Locales, Up: Expressions
6.7 Summary
===========
* Expressions are the basic elements of computation in programs.
They are built from constants, variables, function calls, and
combinations of the various kinds of values with operators.
* 'awk' supplies three kinds of constants: numeric, string, and
regexp. 'gawk' lets you specify numeric constants in octal and
hexadecimal (bases 8 and 16) as well as decimal (base 10). In
certain contexts, a standalone regexp constant such as '/foo/' has
the same meaning as '$0 ~ /foo/'.
* Variables hold values between uses in computations. A number of
built-in variables provide information to your 'awk' program, and a
number of others let you control how 'awk' behaves.
* Numbers are automatically converted to strings, and strings to
numbers, as needed by 'awk'. Numeric values are converted as if
they were formatted with 'sprintf()' using the format in 'CONVFMT'.
Locales can influence the conversions.
* 'awk' provides the usual arithmetic operators (addition,
subtraction, multiplication, division, modulus), and unary plus and
minus. It also provides comparison operators, Boolean operators,
an array membership testing operator, and regexp matching
operators. String concatenation is accomplished by placing two
expressions next to each other; there is no explicit operator. The
three-operand '?:' operator provides an "if-else" test within
expressions.
* Assignment operators provide convenient shorthands for common
arithmetic operations.
* In 'awk', a value is considered to be true if it is nonzero _or_
non-null. Otherwise, the value is false.
* A variable's type is set upon each assignment and may change over
its lifetime. The type determines how it behaves in comparisons
(string or numeric).
* Function calls return a value that may be used as part of a larger
expression. Expressions used to pass parameter values are fully
evaluated before the function is called. 'awk' provides built-in
and user-defined functions; this is described in *note Functions::.
* Operator precedence specifies the order in which operations are
performed, unless explicitly overridden by parentheses. 'awk''s
operator precedence is compatible with that of C.
* Locales can affect the format of data as output by an 'awk'
program, and occasionally the format for data read as input.
File: gawk.info, Node: Patterns and Actions, Next: Arrays, Prev: Expressions, Up: Top
7 Patterns, Actions, and Variables
**********************************
As you have already seen, each 'awk' statement consists of a pattern
with an associated action. This major node describes how you build
patterns and actions, what kinds of things you can do within actions,
and 'awk''s predefined variables.
The pattern-action rules and the statements available for use within
actions form the core of 'awk' programming. In a sense, everything
covered up to here has been the foundation that programs are built on
top of. Now it's time to start building something useful.
* Menu:
* Pattern Overview:: What goes into a pattern.
* Using Shell Variables:: How to use shell variables with 'awk'.
* Action Overview:: What goes into an action.
* Statements:: Describes the various control statements in
detail.
* Built-in Variables:: Summarizes the predefined variables.
* Pattern Action Summary:: Patterns and Actions summary.
File: gawk.info, Node: Pattern Overview, Next: Using Shell Variables, Up: Patterns and Actions
7.1 Pattern Elements
====================
* Menu:
* Regexp Patterns:: Using regexps as patterns.
* Expression Patterns:: Any expression can be used as a pattern.
* Ranges:: Pairs of patterns specify record ranges.
* BEGIN/END:: Specifying initialization and cleanup rules.
* BEGINFILE/ENDFILE:: Two special patterns for advanced control.
* Empty:: The empty pattern, which matches every record.
Patterns in 'awk' control the execution of rules--a rule is executed
when its pattern matches the current input record. The following is a
summary of the types of 'awk' patterns:
'/REGULAR EXPRESSION/'
A regular expression. It matches when the text of the input record
fits the regular expression. (*Note Regexp::.)
'EXPRESSION'
A single expression. It matches when its value is nonzero (if a
number) or non-null (if a string). (*Note Expression Patterns::.)
'BEGPAT, ENDPAT'
A pair of patterns separated by a comma, specifying a "range" of
records. The range includes both the initial record that matches
BEGPAT and the final record that matches ENDPAT. (*Note Ranges::.)
'BEGIN'
'END'
Special patterns for you to supply startup or cleanup actions for
your 'awk' program. (*Note BEGIN/END::.)
'BEGINFILE'
'ENDFILE'
Special patterns for you to supply startup or cleanup actions to be
done on a per-file basis. (*Note BEGINFILE/ENDFILE::.)
'EMPTY'
The empty pattern matches every input record. (*Note Empty::.)
File: gawk.info, Node: Regexp Patterns, Next: Expression Patterns, Up: Pattern Overview
7.1.1 Regular Expressions as Patterns
-------------------------------------
Regular expressions are one of the first kinds of patterns presented in
this book. This kind of pattern is simply a regexp constant in the
pattern part of a rule. Its meaning is '$0 ~ /PATTERN/'. The pattern
matches when the input record matches the regexp. For example:
/foo|bar|baz/ { buzzwords++ }
END { print buzzwords, "buzzwords seen" }
File: gawk.info, Node: Expression Patterns, Next: Ranges, Prev: Regexp Patterns, Up: Pattern Overview
7.1.2 Expressions as Patterns
-----------------------------
Any 'awk' expression is valid as an 'awk' pattern. The pattern matches
if the expression's value is nonzero (if a number) or non-null (if a
string). The expression is reevaluated each time the rule is tested
against a new input record. If the expression uses fields such as '$1',
the value depends directly on the new input record's text; otherwise, it
depends on only what has happened so far in the execution of the 'awk'
program.
Comparison expressions, using the comparison operators described in
*note Typing and Comparison::, are a very common kind of pattern.
Regexp matching and nonmatching are also very common expressions. The
left operand of the '~' and '!~' operators is a string. The right
operand is either a constant regular expression enclosed in slashes
('/REGEXP/'), or any expression whose string value is used as a dynamic
regular expression (*note Computed Regexps::). The following example
prints the second field of each input record whose first field is
precisely 'li':
$ awk '$1 == "li" { print $2 }' mail-list
(There is no output, because there is no person with the exact name
'li'.) Contrast this with the following regular expression match, which
accepts any record with a first field that contains 'li':
$ awk '$1 ~ /li/ { print $2 }' mail-list
-| 555-5553
-| 555-6699
A regexp constant as a pattern is also a special case of an
expression pattern. The expression '/li/' has the value one if 'li'
appears in the current input record. Thus, as a pattern, '/li/' matches
any record containing 'li'.
Boolean expressions are also commonly used as patterns. Whether the
pattern matches an input record depends on whether its subexpressions
match. For example, the following command prints all the records in
'mail-list' that contain both 'edu' and 'li':
$ awk '/edu/ && /li/' mail-list
-| Samuel 555-3430 samuel.lanceolis AT shu.edu A
The following command prints all records in 'mail-list' that contain
_either_ 'edu' or 'li' (or both, of course):
$ awk '/edu/ || /li/' mail-list
-| Amelia 555-5553 amelia.zodiacusque AT gmail.com F
-| Broderick 555-0542 broderick.aliquotiens AT yahoo.com R
-| Fabius 555-1234 fabius.undevicesimus AT ucb.edu F
-| Julie 555-6699 julie.perscrutabor AT skeeve.com F
-| Samuel 555-3430 samuel.lanceolis AT shu.edu A
-| Jean-Paul 555-2127 jeanpaul.campanorum AT nyu.edu R
The following command prints all records in 'mail-list' that do _not_
contain the string 'li':
$ awk '! /li/' mail-list
-| Anthony 555-3412 anthony.asserturo AT hotmail.com A
-| Becky 555-7685 becky.algebrarum AT gmail.com A
-| Bill 555-1675 bill.drowning AT hotmail.com A
-| Camilla 555-2912 camilla.infusarum AT skynet.be R
-| Fabius 555-1234 fabius.undevicesimus AT ucb.edu F
-| Martin 555-6480 martin.codicibus AT hotmail.com A
-| Jean-Paul 555-2127 jeanpaul.campanorum AT nyu.edu R
The subexpressions of a Boolean operator in a pattern can be constant
regular expressions, comparisons, or any other 'awk' expressions. Range
patterns are not expressions, so they cannot appear inside Boolean
patterns. Likewise, the special patterns 'BEGIN', 'END', 'BEGINFILE',
and 'ENDFILE', which never match any input record, are not expressions
and cannot appear inside Boolean patterns.
The precedence of the different operators that can appear in patterns
is described in *note Precedence::.
File: gawk.info, Node: Ranges, Next: BEGIN/END, Prev: Expression Patterns, Up: Pattern Overview
7.1.3 Specifying Record Ranges with Patterns
--------------------------------------------
A "range pattern" is made of two patterns separated by a comma, in the
form 'BEGPAT, ENDPAT'. It is used to match ranges of consecutive input
records. The first pattern, BEGPAT, controls where the range begins,
while ENDPAT controls where the pattern ends. For example, the
following:
awk '$1 == "on", $1 == "off"' myfile
prints every record in 'myfile' between 'on'/'off' pairs, inclusive.
A range pattern starts out by matching BEGPAT against every input
record. When a record matches BEGPAT, the range pattern is "turned on",
and the range pattern matches this record as well. As long as the range
pattern stays turned on, it automatically matches every input record
read. The range pattern also matches ENDPAT against every input record;
when this succeeds, the range pattern is "turned off" again for the
following record. Then the range pattern goes back to checking BEGPAT
against each record.
The record that turns on the range pattern and the one that turns it
off both match the range pattern. If you don't want to operate on these
records, you can write 'if' statements in the rule's action to
distinguish them from the records you are interested in.
It is possible for a pattern to be turned on and off by the same
record. If the record satisfies both conditions, then the action is
executed for just that record. For example, suppose there is text
between two identical markers (e.g., the '%' symbol), each on its own
line, that should be ignored. A first attempt would be to combine a
range pattern that describes the delimited text with the 'next'
statement (not discussed yet, *note Next Statement::). This causes
'awk' to skip any further processing of the current record and start
over again with the next input record. Such a program looks like this:
/^%$/,/^%$/ { next }
{ print }
This program fails because the range pattern is both turned on and
turned off by the first line, which just has a '%' on it. To accomplish
this task, write the program in the following manner, using a flag:
/^%$/ { skip = ! skip; next }
skip == 1 { next } # skip lines with `skip' set
In a range pattern, the comma (',') has the lowest precedence of all
the operators (i.e., it is evaluated last). Thus, the following program
attempts to combine a range pattern with another, simpler test:
echo Yes | awk '/1/,/2/ || /Yes/'
The intent of this program is '(/1/,/2/) || /Yes/'. However, 'awk'
interprets this as '/1/, (/2/ || /Yes/)'. This cannot be changed or
worked around; range patterns do not combine with other patterns:
$ echo Yes | gawk '(/1/,/2/) || /Yes/'
error-> gawk: cmd. line:1: (/1/,/2/) || /Yes/
error-> gawk: cmd. line:1: ^ syntax error
As a minor point of interest, although it is poor style, POSIX allows
you to put a newline after the comma in a range pattern. (d.c.)
File: gawk.info, Node: BEGIN/END, Next: BEGINFILE/ENDFILE, Prev: Ranges, Up: Pattern Overview
7.1.4 The 'BEGIN' and 'END' Special Patterns
--------------------------------------------
All the patterns described so far are for matching input records. The
'BEGIN' and 'END' special patterns are different. They supply startup
and cleanup actions for 'awk' programs. 'BEGIN' and 'END' rules must
have actions; there is no default action for these rules because there
is no current record when they run. 'BEGIN' and 'END' rules are often
referred to as "'BEGIN' and 'END' blocks" by longtime 'awk' programmers.
* Menu:
* Using BEGIN/END:: How and why to use BEGIN/END rules.
* I/O And BEGIN/END:: I/O issues in BEGIN/END rules.
File: gawk.info, Node: Using BEGIN/END, Next: I/O And BEGIN/END, Up: BEGIN/END
7.1.4.1 Startup and Cleanup Actions
...................................
A 'BEGIN' rule is executed once only, before the first input record is
read. Likewise, an 'END' rule is executed once only, after all the
input is read. For example:
$ awk '
> BEGIN { print "Analysis of \"li\"" }
> /li/ { ++n }
> END { print "\"li\" appears in", n, "records." }' mail-list
-| Analysis of "li"
-| "li" appears in 4 records.
This program finds the number of records in the input file
'mail-list' that contain the string 'li'. The 'BEGIN' rule prints a
title for the report. There is no need to use the 'BEGIN' rule to
initialize the counter 'n' to zero, as 'awk' does this automatically
(*note Variables::). The second rule increments the variable 'n' every
time a record containing the pattern 'li' is read. The 'END' rule
prints the value of 'n' at the end of the run.
The special patterns 'BEGIN' and 'END' cannot be used in ranges or
with Boolean operators (indeed, they cannot be used with any operators).
An 'awk' program may have multiple 'BEGIN' and/or 'END' rules. They are
executed in the order in which they appear: all the 'BEGIN' rules at
startup and all the 'END' rules at termination. 'BEGIN' and 'END' rules
may be intermixed with other rules. This feature was added in the 1987
version of 'awk' and is included in the POSIX standard. The original
(1978) version of 'awk' required the 'BEGIN' rule to be placed at the
beginning of the program, the 'END' rule to be placed at the end, and
only allowed one of each. This is no longer required, but it is a good
idea to follow this template in terms of program organization and
readability.
Multiple 'BEGIN' and 'END' rules are useful for writing library
functions, because each library file can have its own 'BEGIN' and/or
'END' rule to do its own initialization and/or cleanup. The order in
which library functions are named on the command line controls the order
in which their 'BEGIN' and 'END' rules are executed. Therefore, you
have to be careful when writing such rules in library files so that the
order in which they are executed doesn't matter. *Note Options:: for
more information on using library functions. *Note Library Functions::,
for a number of useful library functions.
If an 'awk' program has only 'BEGIN' rules and no other rules, then
the program exits after the 'BEGIN' rules are run.(1) However, if an
'END' rule exists, then the input is read, even if there are no other
rules in the program. This is necessary in case the 'END' rule checks
the 'FNR' and 'NR' variables.
---------- Footnotes ----------
(1) The original version of 'awk' kept reading and ignoring input
until the end of the file was seen.
File: gawk.info, Node: I/O And BEGIN/END, Prev: Using BEGIN/END, Up: BEGIN/END
7.1.4.2 Input/Output from 'BEGIN' and 'END' Rules
.................................................
There are several (sometimes subtle) points to be aware of when doing
I/O from a 'BEGIN' or 'END' rule. The first has to do with the value of
'$0' in a 'BEGIN' rule. Because 'BEGIN' rules are executed before any
input is read, there simply is no input record, and therefore no fields,
when executing 'BEGIN' rules. References to '$0' and the fields yield a
null string or zero, depending upon the context. One way to give '$0' a
real value is to execute a 'getline' command without a variable (*note
Getline::). Another way is simply to assign a value to '$0'.
The second point is similar to the first, but from the other
direction. Traditionally, due largely to implementation issues, '$0'
and 'NF' were _undefined_ inside an 'END' rule. The POSIX standard
specifies that 'NF' is available in an 'END' rule. It contains the
number of fields from the last input record. Most probably due to an
oversight, the standard does not say that '$0' is also preserved,
although logically one would think that it should be. In fact, all of
BWK 'awk', 'mawk', and 'gawk' preserve the value of '$0' for use in
'END' rules. Be aware, however, that some other implementations and
many older versions of Unix 'awk' do not.
The third point follows from the first two. The meaning of 'print'
inside a 'BEGIN' or 'END' rule is the same as always: 'print $0'. If
'$0' is the null string, then this prints an empty record. Many
longtime 'awk' programmers use an unadorned 'print' in 'BEGIN' and 'END'
rules, to mean 'print ""', relying on '$0' being null. Although one
might generally get away with this in 'BEGIN' rules, it is a very bad
idea in 'END' rules, at least in 'gawk'. It is also poor style, because
if an empty line is needed in the output, the program should print one
explicitly.
Finally, the 'next' and 'nextfile' statements are not allowed in a
'BEGIN' rule, because the implicit
read-a-record-and-match-against-the-rules loop has not started yet.
Similarly, those statements are not valid in an 'END' rule, because all
the input has been read. (*Note Next Statement:: and *note Nextfile
Statement::.)
File: gawk.info, Node: BEGINFILE/ENDFILE, Next: Empty, Prev: BEGIN/END, Up: Pattern Overview
7.1.5 The 'BEGINFILE' and 'ENDFILE' Special Patterns
----------------------------------------------------
This minor node describes a 'gawk'-specific feature.
Two special kinds of rule, 'BEGINFILE' and 'ENDFILE', give you
"hooks" into 'gawk''s command-line file processing loop. As with the
'BEGIN' and 'END' rules (*note BEGIN/END::), all 'BEGINFILE' rules in a
program are merged, in the order they are read by 'gawk', and all
'ENDFILE' rules are merged as well.
The body of the 'BEGINFILE' rules is executed just before 'gawk'
reads the first record from a file. 'FILENAME' is set to the name of
the current file, and 'FNR' is set to zero.
The 'BEGINFILE' rule provides you the opportunity to accomplish two
tasks that would otherwise be difficult or impossible to perform:
* You can test if the file is readable. Normally, it is a fatal
error if a file named on the command line cannot be opened for
reading. However, you can bypass the fatal error and move on to
the next file on the command line.
You do this by checking if the 'ERRNO' variable is not the empty
string; if so, then 'gawk' was not able to open the file. In this
case, your program can execute the 'nextfile' statement (*note
Nextfile Statement::). This causes 'gawk' to skip the file
entirely. Otherwise, 'gawk' exits with the usual fatal error.
* If you have written extensions that modify the record handling (by
inserting an "input parser"; *note Input Parsers::), you can invoke
them at this point, before 'gawk' has started processing the file.
(This is a _very_ advanced feature, currently used only by the
'gawkextlib' project
(https://sourceforge.net/projects/gawkextlib).)
The 'ENDFILE' rule is called when 'gawk' has finished processing the
last record in an input file. For the last input file, it will be
called before any 'END' rules. The 'ENDFILE' rule is executed even for
empty input files.
Normally, when an error occurs when reading input in the normal
input-processing loop, the error is fatal. However, if an 'ENDFILE'
rule is present, the error becomes non-fatal, and instead 'ERRNO' is
set. This makes it possible to catch and process I/O errors at the
level of the 'awk' program.
The 'next' statement (*note Next Statement::) is not allowed inside
either a 'BEGINFILE' or an 'ENDFILE' rule. The 'nextfile' statement is
allowed only inside a 'BEGINFILE' rule, not inside an 'ENDFILE' rule.
The 'getline' statement (*note Getline::) is restricted inside both
'BEGINFILE' and 'ENDFILE': only redirected forms of 'getline' are
allowed.
'BEGINFILE' and 'ENDFILE' are 'gawk' extensions. In most other 'awk'
implementations, or if 'gawk' is in compatibility mode (*note
Options::), they are not special.
File: gawk.info, Node: Empty, Prev: BEGINFILE/ENDFILE, Up: Pattern Overview
7.1.6 The Empty Pattern
-----------------------
An empty (i.e., nonexistent) pattern is considered to match _every_
input record. For example, the program:
awk '{ print $1 }' mail-list
prints the first field of every record.
File: gawk.info, Node: Using Shell Variables, Next: Action Overview, Prev: Pattern Overview, Up: Patterns and Actions
7.2 Using Shell Variables in Programs
=====================================
'awk' programs are often used as components in larger programs written
in shell. For example, it is very common to use a shell variable to
hold a pattern that the 'awk' program searches for. There are two ways
to get the value of the shell variable into the body of the 'awk'
program.
A common method is to use shell quoting to substitute the variable's
value into the program inside the script. For example, consider the
following program:
printf "Enter search pattern: "
read pattern
awk "/$pattern/ "'{ nmatches++ }
END { print nmatches, "found" }' /path/to/data
The 'awk' program consists of two pieces of quoted text that are
concatenated together to form the program. The first part is
double-quoted, which allows substitution of the 'pattern' shell variable
inside the quotes. The second part is single-quoted.
Variable substitution via quoting works, but can potentially be
messy. It requires a good understanding of the shell's quoting rules
(*note Quoting::), and it's often difficult to correctly match up the
quotes when reading the program.
A better method is to use 'awk''s variable assignment feature (*note
Assignment Options::) to assign the shell variable's value to an 'awk'
variable. Then use dynamic regexps to match the pattern (*note Computed
Regexps::). The following shows how to redo the previous example using
this technique:
printf "Enter search pattern: "
read pattern
awk -v pat="$pattern" '$0 ~ pat { nmatches++ }
END { print nmatches, "found" }' /path/to/data
Now, the 'awk' program is just one single-quoted string. The assignment
'-v pat="$pattern"' still requires double quotes, in case there is
whitespace in the value of '$pattern'. The 'awk' variable 'pat' could
be named 'pattern' too, but that would be more confusing. Using a
variable also provides more flexibility, as the variable can be used
anywhere inside the program--for printing, as an array subscript, or for
any other use--without requiring the quoting tricks at every point in
the program.
File: gawk.info, Node: Action Overview, Next: Statements, Prev: Using Shell Variables, Up: Patterns and Actions
7.3 Actions
===========
An 'awk' program or script consists of a series of rules and function
definitions interspersed. (Functions are described later. *Note
User-defined::.) A rule contains a pattern and an action, either of
which (but not both) may be omitted. The purpose of the "action" is to
tell 'awk' what to do once a match for the pattern is found. Thus, in
outline, an 'awk' program generally looks like this:
[PATTERN] '{ ACTION }'
PATTERN ['{ ACTION }']
...
'function NAME(ARGS) { ... }'
...
An action consists of one or more 'awk' "statements", enclosed in
braces ('{...}'). Each statement specifies one thing to do. The
statements are separated by newlines or semicolons. The braces around
an action must be used even if the action contains only one statement,
or if it contains no statements at all. However, if you omit the action
entirely, omit the braces as well. An omitted action is equivalent to
'{ print $0 }':
/foo/ { } match 'foo', do nothing -- empty action
/foo/ match 'foo', print the record -- omitted action
The following types of statements are supported in 'awk':
Expressions
Call functions or assign values to variables (*note Expressions::).
Executing this kind of statement simply computes the value of the
expression. This is useful when the expression has side effects
(*note Assignment Ops::).
Control statements
Specify the control flow of 'awk' programs. The 'awk' language
gives you C-like constructs ('if', 'for', 'while', and 'do') as
well as a few special ones (*note Statements::).
Compound statements
Enclose one or more statements in braces. A compound statement is
used in order to put several statements together in the body of an
'if', 'while', 'do', or 'for' statement.
Input statements
Use the 'getline' command (*note Getline::). Also supplied in
'awk' are the 'next' statement (*note Next Statement::) and the
'nextfile' statement (*note Nextfile Statement::).
Output statements
Such as 'print' and 'printf'. *Note Printing::.
Deletion statements
For deleting array elements. *Note Delete::.
File: gawk.info, Node: Statements, Next: Built-in Variables, Prev: Action Overview, Up: Patterns and Actions
7.4 Control Statements in Actions
=================================
"Control statements", such as 'if', 'while', and so on, control the flow
of execution in 'awk' programs. Most of 'awk''s control statements are
patterned after similar statements in C.
All the control statements start with special keywords, such as 'if'
and 'while', to distinguish them from simple expressions. Many control
statements contain other statements. For example, the 'if' statement
contains another statement that may or may not be executed. The
contained statement is called the "body". To include more than one
statement in the body, group them into a single "compound statement"
with braces, separating them with newlines or semicolons.
* Menu:
* If Statement:: Conditionally execute some 'awk'
statements.
* While Statement:: Loop until some condition is satisfied.
* Do Statement:: Do specified action while looping until some
condition is satisfied.
* For Statement:: Another looping statement, that provides
initialization and increment clauses.
* Switch Statement:: Switch/case evaluation for conditional
execution of statements based on a value.
* Break Statement:: Immediately exit the innermost enclosing loop.
* Continue Statement:: Skip to the end of the innermost enclosing
loop.
* Next Statement:: Stop processing the current input record.
* Nextfile Statement:: Stop processing the current file.
* Exit Statement:: Stop execution of 'awk'.
File: gawk.info, Node: If Statement, Next: While Statement, Up: Statements
7.4.1 The 'if'-'else' Statement
-------------------------------
The 'if'-'else' statement is 'awk''s decision-making statement. It
looks like this:
'if (CONDITION) THEN-BODY' ['else ELSE-BODY']
The CONDITION is an expression that controls what the rest of the
statement does. If the CONDITION is true, THEN-BODY is executed;
otherwise, ELSE-BODY is executed. The 'else' part of the statement is
optional. The condition is considered false if its value is zero or the
null string; otherwise, the condition is true. Refer to the following:
if (x % 2 == 0)
print "x is even"
else
print "x is odd"
In this example, if the expression 'x % 2 == 0' is true (i.e., if the
value of 'x' is evenly divisible by two), then the first 'print'
statement is executed; otherwise, the second 'print' statement is
executed. If the 'else' keyword appears on the same line as THEN-BODY
and THEN-BODY is not a compound statement (i.e., not surrounded by
braces), then a semicolon must separate THEN-BODY from the 'else'. To
illustrate this, the previous example can be rewritten as:
if (x % 2 == 0) print "x is even"; else
print "x is odd"
If the ';' is left out, 'awk' can't interpret the statement and it
produces a syntax error. Don't actually write programs this way,
because a human reader might fail to see the 'else' if it is not the
first thing on its line.
File: gawk.info, Node: While Statement, Next: Do Statement, Prev: If Statement, Up: Statements
7.4.2 The 'while' Statement
---------------------------
In programming, a "loop" is a part of a program that can be executed two
or more times in succession. The 'while' statement is the simplest
looping statement in 'awk'. It repeatedly executes a statement as long
as a condition is true. For example:
while (CONDITION)
BODY
BODY is a statement called the "body" of the loop, and CONDITION is an
expression that controls how long the loop keeps running. The first
thing the 'while' statement does is test the CONDITION. If the
CONDITION is true, it executes the statement BODY. (The CONDITION is
true when the value is not zero and not a null string.) After BODY has
been executed, CONDITION is tested again, and if it is still true, BODY
executes again. This process repeats until the CONDITION is no longer
true. If the CONDITION is initially false, the body of the loop never
executes and 'awk' continues with the statement following the loop.
This example prints the first three fields of each record, one per line:
awk '
{
i = 1
while (i <= 3) {
print $i
i++
}
}' inventory-shipped
The body of this loop is a compound statement enclosed in braces,
containing two statements. The loop works in the following manner:
first, the value of 'i' is set to one. Then, the 'while' statement
tests whether 'i' is less than or equal to three. This is true when 'i'
equals one, so the 'i'th field is printed. Then the 'i++' increments
the value of 'i' and the loop repeats. The loop terminates when 'i'
reaches four.
A newline is not required between the condition and the body;
however, using one makes the program clearer unless the body is a
compound statement or else is very simple. The newline after the open
brace that begins the compound statement is not required either, but the
program is harder to read without it.
File: gawk.info, Node: Do Statement, Next: For Statement, Prev: While Statement, Up: Statements
7.4.3 The 'do'-'while' Statement
--------------------------------
The 'do' loop is a variation of the 'while' looping statement. The 'do'
loop executes the BODY once and then repeats the BODY as long as the
CONDITION is true. It looks like this:
do
BODY
while (CONDITION)
Even if the CONDITION is false at the start, the BODY executes at
least once (and only once, unless executing BODY makes CONDITION true).
Contrast this with the corresponding 'while' statement:
while (CONDITION)
BODY
This statement does not execute the BODY even once if the CONDITION is
false to begin with. The following is an example of a 'do' statement:
{
i = 1
do {
print $0
i++
} while (i <= 10)
}
This program prints each input record 10 times. However, it isn't a
very realistic example, because in this case an ordinary 'while' would
do just as well. This situation reflects actual experience; only
occasionally is there a real use for a 'do' statement.
File: gawk.info, Node: For Statement, Next: Switch Statement, Prev: Do Statement, Up: Statements
7.4.4 The 'for' Statement
-------------------------
The 'for' statement makes it more convenient to count iterations of a
loop. The general form of the 'for' statement looks like this:
for (INITIALIZATION; CONDITION; INCREMENT)
BODY
The INITIALIZATION, CONDITION, and INCREMENT parts are arbitrary 'awk'
expressions, and BODY stands for any 'awk' statement.
The 'for' statement starts by executing INITIALIZATION. Then, as
long as the CONDITION is true, it repeatedly executes BODY and then
INCREMENT. Typically, INITIALIZATION sets a variable to either zero or
one, INCREMENT adds one to it, and CONDITION compares it against the
desired number of iterations. For example:
awk '
{
for (i = 1; i <= 3; i++)
print $i
}' inventory-shipped
This prints the first three fields of each input record, with one input
field per output line.
It isn't possible to set more than one variable in the INITIALIZATION
part without using a multiple assignment statement such as 'x = y = 0'.
This makes sense only if all the initial values are equal. (But it is
possible to initialize additional variables by writing their assignments
as separate statements preceding the 'for' loop.)
The same is true of the INCREMENT part. Incrementing additional
variables requires separate statements at the end of the loop. The C
compound expression, using C's comma operator, is useful in this
context, but it is not supported in 'awk'.
Most often, INCREMENT is an increment expression, as in the previous
example. But this is not required; it can be any expression whatsoever.
For example, the following statement prints all the powers of two
between 1 and 100:
for (i = 1; i <= 100; i *= 2)
print i
If there is nothing to be done, any of the three expressions in the
parentheses following the 'for' keyword may be omitted. Thus,
'for (; x > 0;)' is equivalent to 'while (x > 0)'. If the CONDITION is
omitted, it is treated as true, effectively yielding an "infinite loop"
(i.e., a loop that never terminates).
In most cases, a 'for' loop is an abbreviation for a 'while' loop, as
shown here:
INITIALIZATION
while (CONDITION) {
BODY
INCREMENT
}
The only exception is when the 'continue' statement (*note Continue
Statement::) is used inside the loop. Changing a 'for' statement to a
'while' statement in this way can change the effect of the 'continue'
statement inside the loop.
The 'awk' language has a 'for' statement in addition to a 'while'
statement because a 'for' loop is often both less work to type and more
natural to think of. Counting the number of iterations is very common
in loops. It can be easier to think of this counting as part of looping
rather than as something to do inside the loop.
There is an alternative version of the 'for' loop, for iterating over
all the indices of an array:
for (i in array)
DO SOMETHING WITH array[i]
*Note Scanning an Array:: for more information on this version of the
'for' loop.
File: gawk.info, Node: Switch Statement, Next: Break Statement, Prev: For Statement, Up: Statements
7.4.5 The 'switch' Statement
----------------------------
This minor node describes a 'gawk'-specific feature. If 'gawk' is in
compatibility mode (*note Options::), it is not available.
The 'switch' statement allows the evaluation of an expression and the
execution of statements based on a 'case' match. Case statements are
checked for a match in the order they are defined. If no suitable
'case' is found, the 'default' section is executed, if supplied.
Each 'case' contains a single constant, be it numeric, string, or
regexp. The 'switch' expression is evaluated, and then each 'case''s
constant is compared against the result in turn. The type of constant
determines the comparison: numeric or string do the usual comparisons.
A regexp constant does a regular expression match against the string
value of the original expression. The general form of the 'switch'
statement looks like this:
switch (EXPRESSION) {
case VALUE OR REGULAR EXPRESSION:
CASE-BODY
default:
DEFAULT-BODY
}
Control flow in the 'switch' statement works as it does in C. Once a
match to a given case is made, the case statement bodies execute until a
'break', 'continue', 'next', 'nextfile', or 'exit' is encountered, or
the end of the 'switch' statement itself. For example:
while ((c = getopt(ARGC, ARGV, "aksx")) != -1) {
switch (c) {
case "a":
# report size of all files
all_files = TRUE;
break
case "k":
BLOCK_SIZE = 1024 # 1K block size
break
case "s":
# do sums only
sum_only = TRUE
break
case "x":
# don't cross filesystems
fts_flags = or(fts_flags, FTS_XDEV)
break
case "?":
default:
usage()
break
}
}
Note that if none of the statements specified here halt execution of
a matched 'case' statement, execution falls through to the next 'case'
until execution halts. In this example, the 'case' for '"?"' falls
through to the 'default' case, which is to call a function named
'usage()'. (The 'getopt()' function being called here is described in
*note Getopt Function::.)
File: gawk.info, Node: Break Statement, Next: Continue Statement, Prev: Switch Statement, Up: Statements
7.4.6 The 'break' Statement
---------------------------
The 'break' statement jumps out of the innermost 'for', 'while', or 'do'
loop that encloses it. The following example finds the smallest divisor
of any integer, and also identifies prime numbers:
# find smallest divisor of num
{
num = $1
for (divisor = 2; divisor * divisor <= num; divisor++) {
if (num % divisor == 0)
break
}
if (num % divisor == 0)
printf "Smallest divisor of %d is %d\n", num, divisor
else
printf "%d is prime\n", num
}
When the remainder is zero in the first 'if' statement, 'awk'
immediately "breaks out" of the containing 'for' loop. This means that
'awk' proceeds immediately to the statement following the loop and
continues processing. (This is very different from the 'exit'
statement, which stops the entire 'awk' program. *Note Exit
Statement::.)
The following program illustrates how the CONDITION of a 'for' or
'while' statement could be replaced with a 'break' inside an 'if':
# find smallest divisor of num
{
num = $1
for (divisor = 2; ; divisor++) {
if (num % divisor == 0) {
printf "Smallest divisor of %d is %d\n", num, divisor
break
}
if (divisor * divisor > num) {
printf "%d is prime\n", num
break
}
}
}
The 'break' statement is also used to break out of the 'switch'
statement. This is discussed in *note Switch Statement::.
The 'break' statement has no meaning when used outside the body of a
loop or 'switch'. However, although it was never documented, historical
implementations of 'awk' treated the 'break' statement outside of a loop
as if it were a 'next' statement (*note Next Statement::). (d.c.)
Recent versions of BWK 'awk' no longer allow this usage, nor does
'gawk'.
File: gawk.info, Node: Continue Statement, Next: Next Statement, Prev: Break Statement, Up: Statements
7.4.7 The 'continue' Statement
------------------------------
Similar to 'break', the 'continue' statement is used only inside 'for',
'while', and 'do' loops. It skips over the rest of the loop body,
causing the next cycle around the loop to begin immediately. Contrast
this with 'break', which jumps out of the loop altogether.
The 'continue' statement in a 'for' loop directs 'awk' to skip the
rest of the body of the loop and resume execution with the
increment-expression of the 'for' statement. The following program
illustrates this fact:
BEGIN {
for (x = 0; x <= 20; x++) {
if (x == 5)
continue
printf "%d ", x
}
print ""
}
This program prints all the numbers from 0 to 20--except for 5, for
which the 'printf' is skipped. Because the increment 'x++' is not
skipped, 'x' does not remain stuck at 5. Contrast the 'for' loop from
the previous example with the following 'while' loop:
BEGIN {
x = 0
while (x <= 20) {
if (x == 5)
continue
printf "%d ", x
x++
}
print ""
}
This program loops forever once 'x' reaches 5, because the increment
('x++') is never reached.
The 'continue' statement has no special meaning with respect to the
'switch' statement, nor does it have any meaning when used outside the
body of a loop. Historical versions of 'awk' treated a 'continue'
statement outside a loop the same way they treated a 'break' statement
outside a loop: as if it were a 'next' statement (*note Next
Statement::). (d.c.) Recent versions of BWK 'awk' no longer work this
way, nor does 'gawk'.
File: gawk.info, Node: Next Statement, Next: Nextfile Statement, Prev: Continue Statement, Up: Statements
7.4.8 The 'next' Statement
--------------------------
The 'next' statement forces 'awk' to immediately stop processing the
current record and go on to the next record. This means that no further
rules are executed for the current record, and the rest of the current
rule's action isn't executed.
Contrast this with the effect of the 'getline' function (*note
Getline::). That also causes 'awk' to read the next record immediately,
but it does not alter the flow of control in any way (i.e., the rest of
the current action executes with a new input record).
At the highest level, 'awk' program execution is a loop that reads an
input record and then tests each rule's pattern against it. If you
think of this loop as a 'for' statement whose body contains the rules,
then the 'next' statement is analogous to a 'continue' statement. It
skips to the end of the body of this implicit loop and executes the
increment (which reads another record).
For example, suppose an 'awk' program works only on records with four
fields, and it shouldn't fail when given bad input. To avoid
complicating the rest of the program, write a "weed out" rule near the
beginning, in the following manner:
NF != 4 {
printf("%s:%d: skipped: NF != 4\n", FILENAME, FNR) > "/dev/stderr"
next
}
Because of the 'next' statement, the program's subsequent rules won't
see the bad record. The error message is redirected to the standard
error output stream, as error messages should be. For more detail, see
*note Special Files::.
If the 'next' statement causes the end of the input to be reached,
then the code in any 'END' rules is executed. *Note BEGIN/END::.
The 'next' statement is not allowed inside 'BEGINFILE' and 'ENDFILE'
rules. *Note BEGINFILE/ENDFILE::.
According to the POSIX standard, the behavior is undefined if the
'next' statement is used in a 'BEGIN' or 'END' rule. 'gawk' treats it
as a syntax error. Although POSIX does not disallow it, most other
'awk' implementations don't allow the 'next' statement inside function
bodies (*note User-defined::). Just as with any other 'next' statement,
a 'next' statement inside a function body reads the next record and
starts processing it with the first rule in the program.
File: gawk.info, Node: Nextfile Statement, Next: Exit Statement, Prev: Next Statement, Up: Statements
7.4.9 The 'nextfile' Statement
------------------------------
The 'nextfile' statement is similar to the 'next' statement. However,
instead of abandoning processing of the current record, the 'nextfile'
statement instructs 'awk' to stop processing the current data file.
Upon execution of the 'nextfile' statement, 'FILENAME' is updated to
the name of the next data file listed on the command line, 'FNR' is
reset to one, and processing starts over with the first rule in the
program. If the 'nextfile' statement causes the end of the input to be
reached, then the code in any 'END' rules is executed. An exception to
this is when 'nextfile' is invoked during execution of any statement in
an 'END' rule; in this case, it causes the program to stop immediately.
*Note BEGIN/END::.
The 'nextfile' statement is useful when there are many data files to
process but it isn't necessary to process every record in every file.
Without 'nextfile', in order to move on to the next data file, a program
would have to continue scanning the unwanted records. The 'nextfile'
statement accomplishes this much more efficiently.
In 'gawk', execution of 'nextfile' causes additional things to
happen: any 'ENDFILE' rules are executed if 'gawk' is not currently in
an 'END' or 'BEGINFILE' rule, 'ARGIND' is incremented, and any
'BEGINFILE' rules are executed. ('ARGIND' hasn't been introduced yet.
*Note Built-in Variables::.)
With 'gawk', 'nextfile' is useful inside a 'BEGINFILE' rule to skip
over a file that would otherwise cause 'gawk' to exit with a fatal
error. In this case, 'ENDFILE' rules are not executed. *Note
BEGINFILE/ENDFILE::.
Although it might seem that 'close(FILENAME)' would accomplish the
same as 'nextfile', this isn't true. 'close()' is reserved for closing
files, pipes, and coprocesses that are opened with redirections. It is
not related to the main processing that 'awk' does with the files listed
in 'ARGV'.
NOTE: For many years, 'nextfile' was a common extension. In
September 2012, it was accepted for inclusion into the POSIX
standard. See the Austin Group website
(http://austingroupbugs.net/view.php?id=607).
The current version of BWK 'awk' and 'mawk' also support 'nextfile'.
However, they don't allow the 'nextfile' statement inside function
bodies (*note User-defined::). 'gawk' does; a 'nextfile' inside a
function body reads the first record from the next file and starts
processing it with the first rule in the program, just as any other
'nextfile' statement.
File: gawk.info, Node: Exit Statement, Prev: Nextfile Statement, Up: Statements
7.4.10 The 'exit' Statement
---------------------------
The 'exit' statement causes 'awk' to immediately stop executing the
current rule and to stop processing input; any remaining input is
ignored. The 'exit' statement is written as follows:
'exit' [RETURN CODE]
When an 'exit' statement is executed from a 'BEGIN' rule, the program
stops processing everything immediately. No input records are read.
However, if an 'END' rule is present, as part of executing the 'exit'
statement, the 'END' rule is executed (*note BEGIN/END::). If 'exit' is
used in the body of an 'END' rule, it causes the program to stop
immediately.
An 'exit' statement that is not part of a 'BEGIN' or 'END' rule stops
the execution of any further automatic rules for the current record,
skips reading any remaining input records, and executes the 'END' rule
if there is one. 'gawk' also skips any 'ENDFILE' rules; they do not
execute.
In such a case, if you don't want the 'END' rule to do its job, set a
variable to a nonzero value before the 'exit' statement and check that
variable in the 'END' rule. *Note Assert Function:: for an example that
does this.
If an argument is supplied to 'exit', its value is used as the exit
status code for the 'awk' process. If no argument is supplied, 'exit'
causes 'awk' to return a "success" status. In the case where an
argument is supplied to a first 'exit' statement, and then 'exit' is
called a second time from an 'END' rule with no argument, 'awk' uses the
previously supplied exit value. (d.c.) *Note Exit Status:: for more
information.
For example, suppose an error condition occurs that is difficult or
impossible to handle. Conventionally, programs report this by exiting
with a nonzero status. An 'awk' program can do this using an 'exit'
statement with a nonzero argument, as shown in the following example:
BEGIN {
if (("date" | getline date_now) <= 0) {
print "Can't get system date" > "/dev/stderr"
exit 1
}
print "current date is", date_now
close("date")
}
NOTE: For full portability, exit values should be between zero and
126, inclusive. Negative values, and values of 127 or greater, may
not produce consistent results across different operating systems.
File: gawk.info, Node: Built-in Variables, Next: Pattern Action Summary, Prev: Statements, Up: Patterns and Actions
7.5 Predefined Variables
========================
Most 'awk' variables are available to use for your own purposes; they
never change unless your program assigns values to them, and they never
affect anything unless your program examines them. However, a few
variables in 'awk' have special built-in meanings. 'awk' examines some
of these automatically, so that they enable you to tell 'awk' how to do
certain things. Others are set automatically by 'awk', so that they
carry information from the internal workings of 'awk' to your program.
This minor node documents all of 'gawk''s predefined variables, most
of which are also documented in the major nodes describing their areas
of activity.
* Menu:
* User-modified:: Built-in variables that you change to control
'awk'.
* Auto-set:: Built-in variables where 'awk' gives
you information.
* ARGC and ARGV:: Ways to use 'ARGC' and 'ARGV'.
File: gawk.info, Node: User-modified, Next: Auto-set, Up: Built-in Variables
7.5.1 Built-in Variables That Control 'awk'
-------------------------------------------
The following is an alphabetical list of variables that you can change
to control how 'awk' does certain things.
The variables that are specific to 'gawk' are marked with a pound
sign ('#'). These variables are 'gawk' extensions. In other 'awk'
implementations or if 'gawk' is in compatibility mode (*note Options::),
they are not special. (Any exceptions are noted in the description of
each variable.)
'BINMODE #'
On non-POSIX systems, this variable specifies use of binary mode
for all I/O. Numeric values of one, two, or three specify that
input files, output files, or all files, respectively, should use
binary I/O. A numeric value less than zero is treated as zero, and
a numeric value greater than three is treated as three.
Alternatively, string values of '"r"' or '"w"' specify that input
files and output files, respectively, should use binary I/O. A
string value of '"rw"' or '"wr"' indicates that all files should
use binary I/O. Any other string value is treated the same as
'"rw"', but causes 'gawk' to generate a warning message. 'BINMODE'
is described in more detail in *note PC Using::. 'mawk' (*note
Other Versions::) also supports this variable, but only using
numeric values.
'CONVFMT'
A string that controls the conversion of numbers to strings (*note
Conversion::). It works by being passed, in effect, as the first
argument to the 'sprintf()' function (*note String Functions::).
Its default value is '"%.6g"'. 'CONVFMT' was introduced by the
POSIX standard.
'FIELDWIDTHS #'
A space-separated list of columns that tells 'gawk' how to split
input with fixed columnar boundaries. Starting in version 4.2,
each field width may optionally be preceded by a colon-separated
value specifying the number of characters to skip before the field
starts. Assigning a value to 'FIELDWIDTHS' overrides the use of
'FS' and 'FPAT' for field splitting. *Note Constant Size:: for
more information.
'FPAT #'
A regular expression (as a string) that tells 'gawk' to create the
fields based on text that matches the regular expression.
Assigning a value to 'FPAT' overrides the use of 'FS' and
'FIELDWIDTHS' for field splitting. *Note Splitting By Content::
for more information.
'FS'
The input field separator (*note Field Separators::). The value is
a single-character string or a multicharacter regular expression
that matches the separations between fields in an input record. If
the value is the null string ('""'), then each character in the
record becomes a separate field. (This behavior is a 'gawk'
extension. POSIX 'awk' does not specify the behavior when 'FS' is
the null string. Nonetheless, some other versions of 'awk' also
treat '""' specially.)
The default value is '" "', a string consisting of a single space.
As a special exception, this value means that any sequence of
spaces, TABs, and/or newlines is a single separator. It also
causes spaces, TABs, and newlines at the beginning and end of a
record to be ignored.
You can set the value of 'FS' on the command line using the '-F'
option:
awk -F, 'PROGRAM' INPUT-FILES
If 'gawk' is using 'FIELDWIDTHS' or 'FPAT' for field splitting,
assigning a value to 'FS' causes 'gawk' to return to the normal,
'FS'-based field splitting. An easy way to do this is to simply
say 'FS = FS', perhaps with an explanatory comment.
'IGNORECASE #'
If 'IGNORECASE' is nonzero or non-null, then all string comparisons
and all regular expression matching are case-independent. This
applies to regexp matching with '~' and '!~', the 'gensub()',
'gsub()', 'index()', 'match()', 'patsplit()', 'split()', and
'sub()' functions, record termination with 'RS', and field
splitting with 'FS' and 'FPAT'. However, the value of 'IGNORECASE'
does _not_ affect array subscripting and it does not affect field
splitting when using a single-character field separator. *Note
Case-sensitivity::.
'LINT #'
When this variable is true (nonzero or non-null), 'gawk' behaves as
if the '--lint' command-line option is in effect (*note Options::).
With a value of '"fatal"', lint warnings become fatal errors. With
a value of '"invalid"', only warnings about things that are
actually invalid are issued. (This is not fully implemented yet.)
Any other true value prints nonfatal warnings. Assigning a false
value to 'LINT' turns off the lint warnings.
This variable is a 'gawk' extension. It is not special in other
'awk' implementations. Unlike with the other special variables,
changing 'LINT' does affect the production of lint warnings, even
if 'gawk' is in compatibility mode. Much as the '--lint' and
'--traditional' options independently control different aspects of
'gawk''s behavior, the control of lint warnings during program
execution is independent of the flavor of 'awk' being executed.
'OFMT'
A string that controls conversion of numbers to strings (*note
Conversion::) for printing with the 'print' statement. It works by
being passed as the first argument to the 'sprintf()' function
(*note String Functions::). Its default value is '"%.6g"'.
Earlier versions of 'awk' used 'OFMT' to specify the format for
converting numbers to strings in general expressions; this is now
done by 'CONVFMT'.
'OFS'
The output field separator (*note Output Separators::). It is
output between the fields printed by a 'print' statement. Its
default value is '" "', a string consisting of a single space.
'ORS'
The output record separator. It is output at the end of every
'print' statement. Its default value is '"\n"', the newline
character. (*Note Output Separators::.)
'PREC #'
The working precision of arbitrary-precision floating-point
numbers, 53 bits by default (*note Setting precision::).
'ROUNDMODE #'
The rounding mode to use for arbitrary-precision arithmetic on
numbers, by default '"N"' ('roundTiesToEven' in the IEEE 754
standard; *note Setting the rounding mode::).
'RS'
The input record separator. Its default value is a string
containing a single newline character, which means that an input
record consists of a single line of text. It can also be the null
string, in which case records are separated by runs of blank lines.
If it is a regexp, records are separated by matches of the regexp
in the input text. (*Note Records::.)
The ability for 'RS' to be a regular expression is a 'gawk'
extension. In most other 'awk' implementations, or if 'gawk' is in
compatibility mode (*note Options::), just the first character of
'RS''s value is used.
'SUBSEP'
The subscript separator. It has the default value of '"\034"' and
is used to separate the parts of the indices of a multidimensional
array. Thus, the expression 'foo["A", "B"]' really accesses
'foo["A\034B"]' (*note Multidimensional::).
'TEXTDOMAIN #'
Used for internationalization of programs at the 'awk' level. It
sets the default text domain for specially marked string constants
in the source text, as well as for the 'dcgettext()',
'dcngettext()', and 'bindtextdomain()' functions (*note
Internationalization::). The default value of 'TEXTDOMAIN' is
'"messages"'.
File: gawk.info, Node: Auto-set, Next: ARGC and ARGV, Prev: User-modified, Up: Built-in Variables
7.5.2 Built-in Variables That Convey Information
------------------------------------------------
The following is an alphabetical list of variables that 'awk' sets
automatically on certain occasions in order to provide information to
your program.
The variables that are specific to 'gawk' are marked with a pound
sign ('#'). These variables are 'gawk' extensions. In other 'awk'
implementations or if 'gawk' is in compatibility mode (*note Options::),
they are not special:
'ARGC', 'ARGV'
The command-line arguments available to 'awk' programs are stored
in an array called 'ARGV'. 'ARGC' is the number of command-line
arguments present. *Note Other Arguments::. Unlike most 'awk'
arrays, 'ARGV' is indexed from 0 to 'ARGC' - 1. In the following
example:
$ awk 'BEGIN {
> for (i = 0; i < ARGC; i++)
> print ARGV[i]
> }' inventory-shipped mail-list
-| awk
-| inventory-shipped
-| mail-list
'ARGV[0]' contains 'awk', 'ARGV[1]' contains 'inventory-shipped',
and 'ARGV[2]' contains 'mail-list'. The value of 'ARGC' is three,
one more than the index of the last element in 'ARGV', because the
elements are numbered from zero.
The names 'ARGC' and 'ARGV', as well as the convention of indexing
the array from 0 to 'ARGC' - 1, are derived from the C language's
method of accessing command-line arguments.
The value of 'ARGV[0]' can vary from system to system. Also, you
should note that the program text is _not_ included in 'ARGV', nor
are any of 'awk''s command-line options. *Note ARGC and ARGV:: for
information about how 'awk' uses these variables. (d.c.)
'ARGIND #'
The index in 'ARGV' of the current file being processed. Every
time 'gawk' opens a new data file for processing, it sets 'ARGIND'
to the index in 'ARGV' of the file name. When 'gawk' is processing
the input files, 'FILENAME == ARGV[ARGIND]' is always true.
This variable is useful in file processing; it allows you to tell
how far along you are in the list of data files as well as to
distinguish between successive instances of the same file name on
the command line.
While you can change the value of 'ARGIND' within your 'awk'
program, 'gawk' automatically sets it to a new value when it opens
the next file.
'ENVIRON'
An associative array containing the values of the environment. The
array indices are the environment variable names; the elements are
the values of the particular environment variables. For example,
'ENVIRON["HOME"]' might be '/home/arnold'.
For POSIX 'awk', changing this array does not affect the
environment passed on to any programs that 'awk' may spawn via
redirection or the 'system()' function.
However, beginning with version 4.2, if not in POSIX compatibility
mode, 'gawk' does update its own environment when 'ENVIRON' is
changed, thus changing the environment seen by programs that it
creates. You should therefore be especially careful if you modify
'ENVIRON["PATH"]', which is the search path for finding executable
programs.
This can also affect the running 'gawk' program, since some of the
built-in functions may pay attention to certain environment
variables. The most notable instance of this is 'mktime()' (*note
Time Functions::), which pays attention the value of the 'TZ'
environment variable on many systems.
Some operating systems may not have environment variables. On such
systems, the 'ENVIRON' array is empty (except for
'ENVIRON["AWKPATH"]' and 'ENVIRON["AWKLIBPATH"]'; *note AWKPATH
Variable:: and *note AWKLIBPATH Variable::).
'ERRNO #'
If a system error occurs during a redirection for 'getline', during
a read for 'getline', or during a 'close()' operation, then 'ERRNO'
contains a string describing the error.
In addition, 'gawk' clears 'ERRNO' before opening each command-line
input file. This enables checking if the file is readable inside a
'BEGINFILE' pattern (*note BEGINFILE/ENDFILE::).
Otherwise, 'ERRNO' works similarly to the C variable 'errno'.
Except for the case just mentioned, 'gawk' _never_ clears it (sets
it to zero or '""'). Thus, you should only expect its value to be
meaningful when an I/O operation returns a failure value, such as
'getline' returning -1. You are, of course, free to clear it
yourself before doing an I/O operation.
If the value of 'ERRNO' corresponds to a system error in the C
'errno' variable, then 'PROCINFO["errno"]' will be set to the value
of 'errno'. For non-system errors, 'PROCINFO["errno"]' will be
zero.
'FILENAME'
The name of the current input file. When no data files are listed
on the command line, 'awk' reads from the standard input and
'FILENAME' is set to '"-"'. 'FILENAME' changes each time a new
file is read (*note Reading Files::). Inside a 'BEGIN' rule, the
value of 'FILENAME' is '""', because there are no input files being
processed yet.(1) (d.c.) Note, though, that using 'getline'
(*note Getline::) inside a 'BEGIN' rule can give 'FILENAME' a
value.
'FNR'
The current record number in the current file. 'awk' increments
'FNR' each time it reads a new record (*note Records::). 'awk'
resets 'FNR' to zero each time it starts a new input file.
'NF'
The number of fields in the current input record. 'NF' is set each
time a new record is read, when a new field is created, or when
'$0' changes (*note Fields::).
Unlike most of the variables described in this node, assigning a
value to 'NF' has the potential to affect 'awk''s internal
workings. In particular, assignments to 'NF' can be used to create
fields in or remove fields from the current record. *Note Changing
Fields::.
'FUNCTAB #'
An array whose indices and corresponding values are the names of
all the built-in, user-defined, and extension functions in the
program.
NOTE: Attempting to use the 'delete' statement with the
'FUNCTAB' array causes a fatal error. Any attempt to assign
to an element of 'FUNCTAB' also causes a fatal error.
'NR'
The number of input records 'awk' has processed since the beginning
of the program's execution (*note Records::). 'awk' increments
'NR' each time it reads a new record.
'PROCINFO #'
The elements of this array provide access to information about the
running 'awk' program. The following elements (listed
alphabetically) are guaranteed to be available:
'PROCINFO["argv"]'
The 'PROCINFO["argv"]' array contains all of the command-line
arguments (after glob expansion and redirection processing on
platforms where that must be done manually by the program)
with subscripts ranging from 0 through 'argc' - 1. For
example, 'PROCINFO["argv"][0]' will contain the name by which
'gawk' was invoked. Here is an example of how this feature
may be used:
gawk '
BEGIN {
for (i = 0; i < length(PROCINFO["argv"]); i++)
print i, PROCINFO["argv"][i]
}'
Please note that this differs from the standard 'ARGV' array
which does not include command-line arguments that have
already been processed by 'gawk' (*note ARGC and ARGV::).
'PROCINFO["egid"]'
The value of the 'getegid()' system call.
'PROCINFO["errno"]'
The value of the C 'errno' variable when 'ERRNO' is set to the
associated error message.
'PROCINFO["euid"]'
The value of the 'geteuid()' system call.
'PROCINFO["FS"]'
This is '"FS"' if field splitting with 'FS' is in effect,
'"FIELDWIDTHS"' if field splitting with 'FIELDWIDTHS' is in
effect, '"FPAT"' if field matching with 'FPAT' is in effect,
or '"API"' if field splitting is controlled by an API input
parser.
'PROCINFO["gid"]'
The value of the 'getgid()' system call.
'PROCINFO["identifiers"]'
A subarray, indexed by the names of all identifiers used in
the text of the 'awk' program. An "identifier" is simply the
name of a variable (be it scalar or array), built-in function,
user-defined function, or extension function. For each
identifier, the value of the element is one of the following:
'"array"'
The identifier is an array.
'"builtin"'
The identifier is a built-in function.
'"extension"'
The identifier is an extension function loaded via
'@load' or '-l'.
'"scalar"'
The identifier is a scalar.
'"untyped"'
The identifier is untyped (could be used as a scalar or
an array; 'gawk' doesn't know yet).
'"user"'
The identifier is a user-defined function.
The values indicate what 'gawk' knows about the identifiers
after it has finished parsing the program; they are _not_
updated while the program runs.
'PROCINFO["pgrpid"]'
The process group ID of the current process.
'PROCINFO["pid"]'
The process ID of the current process.
'PROCINFO["ppid"]'
The parent process ID of the current process.
'PROCINFO["strftime"]'
The default time format string for 'strftime()'. Assigning a
new value to this element changes the default. *Note Time
Functions::.
'PROCINFO["uid"]'
The value of the 'getuid()' system call.
'PROCINFO["version"]'
The version of 'gawk'.
The following additional elements in the array are available to
provide information about the MPFR and GMP libraries if your
version of 'gawk' supports arbitrary-precision arithmetic (*note
Arbitrary Precision Arithmetic::):
'PROCINFO["gmp_version"]'
The version of the GNU MP library.
'PROCINFO["mpfr_version"]'
The version of the GNU MPFR library.
'PROCINFO["prec_max"]'
The maximum precision supported by MPFR.
'PROCINFO["prec_min"]'
The minimum precision required by MPFR.
The following additional elements in the array are available to
provide information about the version of the extension API, if your
version of 'gawk' supports dynamic loading of extension functions
(*note Dynamic Extensions::):
'PROCINFO["api_major"]'
The major version of the extension API.
'PROCINFO["api_minor"]'
The minor version of the extension API.
On some systems, there may be elements in the array, '"group1"'
through '"groupN"' for some N. N is the number of supplementary
groups that the process has. Use the 'in' operator to test for
these elements (*note Reference to Elements::).
The following elements allow you to change 'gawk''s behavior:
'PROCINFO["NONFATAL"]'
If this element exists, then I/O errors for all redirections
become nonfatal. *Note Nonfatal::.
'PROCINFO["NAME", "NONFATAL"]'
Make I/O errors for NAME be nonfatal. *Note Nonfatal::.
'PROCINFO["COMMAND", "pty"]'
For two-way communication to COMMAND, use a pseudo-tty instead
of setting up a two-way pipe. *Note Two-way I/O:: for more
information.
'PROCINFO["INPUT_NAME", "READ_TIMEOUT"]'
Set a timeout for reading from input redirection INPUT_NAME.
*Note Read Timeout:: for more information.
'PROCINFO["INPUT_NAME", "RETRY"]'
If an I/O error that may be retried occurs when reading data
from INPUT_NAME, and this array entry exists, then 'getline'
returns -2 instead of following the default behavior of
returning -1 and configuring INPUT_NAME to return no further
data. An I/O error that may be retried is one where 'errno'
has the value 'EAGAIN', 'EWOULDBLOCK', 'EINTR', or
'ETIMEDOUT'. This may be useful in conjunction with
'PROCINFO["INPUT_NAME", "READ_TIMEOUT"]' or situations where a
file descriptor has been configured to behave in a
non-blocking fashion. *Note Retrying Input:: for more
information.
'PROCINFO["sorted_in"]'
If this element exists in 'PROCINFO', its value controls the
order in which array indices will be processed by 'for (INDX
in ARRAY)' loops. This is an advanced feature, so we defer
the full description until later; see *note Scanning an
Array::.
'RLENGTH'
The length of the substring matched by the 'match()' function
(*note String Functions::). 'RLENGTH' is set by invoking the
'match()' function. Its value is the length of the matched string,
or -1 if no match is found.
'RSTART'
The start index in characters of the substring that is matched by
the 'match()' function (*note String Functions::). 'RSTART' is set
by invoking the 'match()' function. Its value is the position of
the string where the matched substring starts, or zero if no match
was found.
'RT #'
The input text that matched the text denoted by 'RS', the record
separator. It is set every time a record is read.
'SYMTAB #'
An array whose indices are the names of all defined global
variables and arrays in the program. 'SYMTAB' makes 'gawk''s
symbol table visible to the 'awk' programmer. It is built as
'gawk' parses the program and is complete before the program starts
to run.
The array may be used for indirect access to read or write the
value of a variable:
foo = 5
SYMTAB["foo"] = 4
print foo # prints 4
The 'isarray()' function (*note Type Functions::) may be used to
test if an element in 'SYMTAB' is an array. Also, you may not use
the 'delete' statement with the 'SYMTAB' array.
You may use an index for 'SYMTAB' that is not a predefined
identifier:
SYMTAB["xxx"] = 5
print SYMTAB["xxx"]
This works as expected: in this case 'SYMTAB' acts just like a
regular array. The only difference is that you can't then delete
'SYMTAB["xxx"]'.
The 'SYMTAB' array is more interesting than it looks. Andrew
Schorr points out that it effectively gives 'awk' data pointers.
Consider his example:
# Indirect multiply of any variable by amount, return result
function multiply(variable, amount)
{
return SYMTAB[variable] *= amount
}
You would use it like this:
BEGIN {
answer = 10.5
multiply("answer", 4)
print "The answer is", answer
}
When run, this produces:
$ gawk -f answer.awk
-| The answer is 42
NOTE: In order to avoid severe time-travel paradoxes,(2)
neither 'FUNCTAB' nor 'SYMTAB' is available as an element
within the 'SYMTAB' array.
Changing 'NR' and 'FNR'
'awk' increments 'NR' and 'FNR' each time it reads a record, instead
of setting them to the absolute value of the number of records read.
This means that a program can change these variables and their new
values are incremented for each record. (d.c.) The following example
shows this:
$ echo '1
> 2
> 3
> 4' | awk 'NR == 2 { NR = 17 }
> { print NR }'
-| 1
-| 17
-| 18
-| 19
Before 'FNR' was added to the 'awk' language (*note V7/SVR3.1::), many
'awk' programs used this feature to track the number of records in a
file by resetting 'NR' to zero when 'FILENAME' changed.
---------- Footnotes ----------
(1) Some early implementations of Unix 'awk' initialized 'FILENAME'
to '"-"', even if there were data files to be processed. This behavior
was incorrect and should not be relied upon in your programs.
(2) Not to mention difficult implementation issues.
File: gawk.info, Node: ARGC and ARGV, Prev: Auto-set, Up: Built-in Variables
7.5.3 Using 'ARGC' and 'ARGV'
-----------------------------
*note Auto-set:: presented the following program describing the
information contained in 'ARGC' and 'ARGV':
$ awk 'BEGIN {
> for (i = 0; i < ARGC; i++)
> print ARGV[i]
> }' inventory-shipped mail-list
-| awk
-| inventory-shipped
-| mail-list
In this example, 'ARGV[0]' contains 'awk', 'ARGV[1]' contains
'inventory-shipped', and 'ARGV[2]' contains 'mail-list'. Notice that
the 'awk' program is not entered in 'ARGV'. The other command-line
options, with their arguments, are also not entered. This includes
variable assignments done with the '-v' option (*note Options::).
Normal variable assignments on the command line _are_ treated as
arguments and do show up in the 'ARGV' array. Given the following
program in a file named 'showargs.awk':
BEGIN {
printf "A=%d, B=%d\n", A, B
for (i = 0; i < ARGC; i++)
printf "\tARGV[%d] = %s\n", i, ARGV[i]
}
END { printf "A=%d, B=%d\n", A, B }
Running it produces the following:
$ awk -v A=1 -f showargs.awk B=2 /dev/null
-| A=1, B=0
-| ARGV[0] = awk
-| ARGV[1] = B=2
-| ARGV[2] = /dev/null
-| A=1, B=2
A program can alter 'ARGC' and the elements of 'ARGV'. Each time
'awk' reaches the end of an input file, it uses the next element of
'ARGV' as the name of the next input file. By storing a different
string there, a program can change which files are read. Use '"-"' to
represent the standard input. Storing additional elements and
incrementing 'ARGC' causes additional files to be read.
If the value of 'ARGC' is decreased, that eliminates input files from
the end of the list. By recording the old value of 'ARGC' elsewhere, a
program can treat the eliminated arguments as something other than file
names.
To eliminate a file from the middle of the list, store the null
string ('""') into 'ARGV' in place of the file's name. As a special
feature, 'awk' ignores file names that have been replaced with the null
string. Another option is to use the 'delete' statement to remove
elements from 'ARGV' (*note Delete::).
All of these actions are typically done in the 'BEGIN' rule, before
actual processing of the input begins. *Note Split Program:: and *note
Tee Program:: for examples of each way of removing elements from 'ARGV'.
To actually get options into an 'awk' program, end the 'awk' options
with '--' and then supply the 'awk' program's options, in the following
manner:
awk -f myprog.awk -- -v -q file1 file2 ...
The following fragment processes 'ARGV' in order to examine, and then
remove, the previously mentioned command-line options:
BEGIN {
for (i = 1; i < ARGC; i++) {
if (ARGV[i] == "-v")
verbose = 1
else if (ARGV[i] == "-q")
debug = 1
else if (ARGV[i] ~ /^-./) {
e = sprintf("%s: unrecognized option -- %c",
ARGV[0], substr(ARGV[i], 2, 1))
print e > "/dev/stderr"
} else
break
delete ARGV[i]
}
}
Ending the 'awk' options with '--' isn't necessary in 'gawk'. Unless
'--posix' has been specified, 'gawk' silently puts any unrecognized
options into 'ARGV' for the 'awk' program to deal with. As soon as it
sees an unknown option, 'gawk' stops looking for other options that it
might otherwise recognize. The previous command line with 'gawk' would
be:
gawk -f myprog.awk -q -v file1 file2 ...
Because '-q' is not a valid 'gawk' option, it and the following '-v' are
passed on to the 'awk' program. (*Note Getopt Function:: for an 'awk'
library function that parses command-line options.)
When designing your program, you should choose options that don't
conflict with 'gawk''s, because it will process any options that it
accepts before passing the rest of the command line on to your program.
Using '#!' with the '-E' option may help (*note Executable Scripts:: and
*note Options::).
File: gawk.info, Node: Pattern Action Summary, Prev: Built-in Variables, Up: Patterns and Actions
7.6 Summary
===========
* Pattern-action pairs make up the basic elements of an 'awk'
program. Patterns are either normal expressions, range
expressions, or regexp constants; one of the special keywords
'BEGIN', 'END', 'BEGINFILE', or 'ENDFILE'; or empty. The action
executes if the current record matches the pattern. Empty
(missing) patterns match all records.
* I/O from 'BEGIN' and 'END' rules has certain constraints. This is
also true, only more so, for 'BEGINFILE' and 'ENDFILE' rules. The
latter two give you "hooks" into 'gawk''s file processing, allowing
you to recover from a file that otherwise would cause a fatal error
(such as a file that cannot be opened).
* Shell variables can be used in 'awk' programs by careful use of
shell quoting. It is easier to pass a shell variable into 'awk' by
using the '-v' option and an 'awk' variable.
* Actions consist of statements enclosed in curly braces. Statements
are built up from expressions, control statements, compound
statements, input and output statements, and deletion statements.
* The control statements in 'awk' are 'if'-'else', 'while', 'for',
and 'do'-'while'. 'gawk' adds the 'switch' statement. There are
two flavors of 'for' statement: one for performing general looping,
and the other for iterating through an array.
* 'break' and 'continue' let you exit early or start the next
iteration of a loop (or get out of a 'switch').
* 'next' and 'nextfile' let you read the next record and start over
at the top of your program or skip to the next input file and start
over, respectively.
* The 'exit' statement terminates your program. When executed from
an action (or function body), it transfers control to the 'END'
statements. From an 'END' statement body, it exits immediately.
You may pass an optional numeric value to be used as 'awk''s exit
status.
* Some predefined variables provide control over 'awk', mainly for
I/O. Other variables convey information from 'awk' to your program.
* 'ARGC' and 'ARGV' make the command-line arguments available to your
program. Manipulating them from a 'BEGIN' rule lets you control
how 'awk' will process the provided data files.
File: gawk.info, Node: Arrays, Next: Functions, Prev: Patterns and Actions, Up: Top
8 Arrays in 'awk'
*****************
An "array" is a table of values called "elements". The elements of an
array are distinguished by their "indices". Indices may be either
numbers or strings.
This major node describes how arrays work in 'awk', how to use array
elements, how to scan through every element in an array, and how to
remove array elements. It also describes how 'awk' simulates
multidimensional arrays, as well as some of the less obvious points
about array usage. The major node moves on to discuss 'gawk''s facility
for sorting arrays, and ends with a brief description of 'gawk''s
ability to support true arrays of arrays.
* Menu:
* Array Basics:: The basics of arrays.
* Numeric Array Subscripts:: How to use numbers as subscripts in
'awk'.
* Uninitialized Subscripts:: Using Uninitialized variables as subscripts.
* Delete:: The 'delete' statement removes an element
from an array.
* Multidimensional:: Emulating multidimensional arrays in
'awk'.
* Arrays of Arrays:: True multidimensional arrays.
* Arrays Summary:: Summary of arrays.
File: gawk.info, Node: Array Basics, Next: Numeric Array Subscripts, Up: Arrays
8.1 The Basics of Arrays
========================
This minor node presents the basics: working with elements in arrays one
at a time, and traversing all of the elements in an array.
* Menu:
* Array Intro:: Introduction to Arrays
* Reference to Elements:: How to examine one element of an array.
* Assigning Elements:: How to change an element of an array.
* Array Example:: Basic Example of an Array
* Scanning an Array:: A variation of the 'for' statement. It
loops through the indices of an array's
existing elements.
* Controlling Scanning:: Controlling the order in which arrays are
scanned.
File: gawk.info, Node: Array Intro, Next: Reference to Elements, Up: Array Basics
8.1.1 Introduction to Arrays
----------------------------
Doing linear scans over an associative array is like trying to club
someone to death with a loaded Uzi.
-- _Larry Wall_
The 'awk' language provides one-dimensional arrays for storing groups
of related strings or numbers. Every 'awk' array must have a name.
Array names have the same syntax as variable names; any valid variable
name would also be a valid array name. But one name cannot be used in
both ways (as an array and as a variable) in the same 'awk' program.
Arrays in 'awk' superficially resemble arrays in other programming
languages, but there are fundamental differences. In 'awk', it isn't
necessary to specify the size of an array before starting to use it.
Additionally, any number or string, not just consecutive integers, may
be used as an array index.
In most other languages, arrays must be "declared" before use,
including a specification of how many elements or components they
contain. In such languages, the declaration causes a contiguous block
of memory to be allocated for that many elements. Usually, an index in
the array must be a nonnegative integer. For example, the index zero
specifies the first element in the array, which is actually stored at
the beginning of the block of memory. Index one specifies the second
element, which is stored in memory right after the first element, and so
on. It is impossible to add more elements to the array, because it has
room only for as many elements as given in the declaration. (Some
languages allow arbitrary starting and ending indices--e.g., '15 ..
27'--but the size of the array is still fixed when the array is
declared.)
A contiguous array of four elements might look like *note Figure 8.1:
figure-array-elements, conceptually, if the element values are eight,
'"foo"', '""', and 30.
+---------+---------+--------+---------+
| 8 | "foo" | "" | 30 | Value
+---------+---------+--------+---------+
0 1 2 3 Index
Figure 8.1: A contiguous array
Only the values are stored; the indices are implicit from the order of
the values. Here, eight is the value at index zero, because eight
appears in the position with zero elements before it.
Arrays in 'awk' are different--they are "associative". This means
that each array is a collection of pairs--an index and its corresponding
array element value:
Index Value
------------------------
'3' '30'
'1' '"foo"'
'0' '8'
'2' '""'
The pairs are shown in jumbled order because their order is
irrelevant.(1)
One advantage of associative arrays is that new pairs can be added at
any time. For example, suppose a tenth element is added to the array
whose value is '"number ten"'. The result is:
Index Value
-------------------------------
'10' '"number
ten"'
'3' '30'
'1' '"foo"'
'0' '8'
'2' '""'
Now the array is "sparse", which just means some indices are missing.
It has elements 0-3 and 10, but doesn't have elements 4, 5, 6, 7, 8, or
9.
Another consequence of associative arrays is that the indices don't
have to be nonnegative integers. Any number, or even a string, can be
an index. For example, the following is an array that translates words
from English to French:
Index Value
------------------------
'"dog"' '"chien"'
'"cat"' '"chat"'
'"one"' '"un"'
'1' '"un"'
Here we decided to translate the number one in both spelled-out and
numeric form--thus illustrating that a single array can have both
numbers and strings as indices. (In fact, array subscripts are always
strings. There are some subtleties to how numbers work when used as
array subscripts; this is discussed in more detail in *note Numeric
Array Subscripts::.) Here, the number '1' isn't double-quoted, because
'awk' automatically converts it to a string.
The value of 'IGNORECASE' has no effect upon array subscripting. The
identical string value used to store an array element must be used to
retrieve it. When 'awk' creates an array (e.g., with the 'split()'
built-in function), that array's indices are consecutive integers
starting at one. (*Note String Functions::.)
'awk''s arrays are efficient--the time to access an element is
independent of the number of elements in the array.
---------- Footnotes ----------
(1) The ordering will vary among 'awk' implementations, which
typically use hash tables to store array elements and values.
File: gawk.info, Node: Reference to Elements, Next: Assigning Elements, Prev: Array Intro, Up: Array Basics
8.1.2 Referring to an Array Element
-----------------------------------
The principal way to use an array is to refer to one of its elements.
An "array reference" is an expression as follows:
ARRAY[INDEX-EXPRESSION]
Here, ARRAY is the name of an array. The expression INDEX-EXPRESSION is
the index of the desired element of the array.
The value of the array reference is the current value of that array
element. For example, 'foo[4.3]' is an expression referencing the
element of array 'foo' at index '4.3'.
A reference to an array element that has no recorded value yields a
value of '""', the null string. This includes elements that have not
been assigned any value as well as elements that have been deleted
(*note Delete::).
NOTE: A reference to an element that does not exist _automatically_
creates that array element, with the null string as its value. (In
some cases, this is unfortunate, because it might waste memory
inside 'awk'.)
Novice 'awk' programmers often make the mistake of checking if an
element exists by checking if the value is empty:
# Check if "foo" exists in a: Incorrect!
if (a["foo"] != "") ...
This is incorrect for two reasons. First, it _creates_ 'a["foo"]'
if it didn't exist before! Second, it is valid (if a bit unusual)
to set an array element equal to the empty string.
To determine whether an element exists in an array at a certain
index, use the following expression:
INDX in ARRAY
This expression tests whether the particular index INDX exists, without
the side effect of creating that element if it is not present. The
expression has the value one (true) if 'ARRAY[INDX]' exists and zero
(false) if it does not exist. (We use INDX here, because 'index' is the
name of a built-in function.) For example, this statement tests whether
the array 'frequencies' contains the index '2':
if (2 in frequencies)
print "Subscript 2 is present."
Note that this is _not_ a test of whether the array 'frequencies'
contains an element whose _value_ is two. There is no way to do that
except to scan all the elements. Also, this _does not_ create
'frequencies[2]', while the following (incorrect) alternative does:
if (frequencies[2] != "")
print "Subscript 2 is present."
File: gawk.info, Node: Assigning Elements, Next: Array Example, Prev: Reference to Elements, Up: Array Basics
8.1.3 Assigning Array Elements
------------------------------
Array elements can be assigned values just like 'awk' variables:
ARRAY[INDEX-EXPRESSION] = VALUE
ARRAY is the name of an array. The expression INDEX-EXPRESSION is the
index of the element of the array that is assigned a value. The
expression VALUE is the value to assign to that element of the array.
File: gawk.info, Node: Array Example, Next: Scanning an Array, Prev: Assigning Elements, Up: Array Basics
8.1.4 Basic Array Example
-------------------------
The following program takes a list of lines, each beginning with a line
number, and prints them out in order of line number. The line numbers
are not in order when they are first read--instead, they are scrambled.
This program sorts the lines by making an array using the line numbers
as subscripts. The program then prints out the lines in sorted order of
their numbers. It is a very simple program and gets confused upon
encountering repeated numbers, gaps, or lines that don't begin with a
number:
{
if ($1 > max)
max = $1
arr[$1] = $0
}
END {
for (x = 1; x <= max; x++)
print arr[x]
}
The first rule keeps track of the largest line number seen so far; it
also stores each line into the array 'arr', at an index that is the
line's number. The second rule runs after all the input has been read,
to print out all the lines. When this program is run with the following
input:
5 I am the Five man
2 Who are you? The new number two!
4 . . . And four on the floor
1 Who is number one?
3 I three you.
Its output is:
1 Who is number one?
2 Who are you? The new number two!
3 I three you.
4 . . . And four on the floor
5 I am the Five man
If a line number is repeated, the last line with a given number
overrides the others. Gaps in the line numbers can be handled with an
easy improvement to the program's 'END' rule, as follows:
END {
for (x = 1; x <= max; x++)
if (x in arr)
print arr[x]
}
File: gawk.info, Node: Scanning an Array, Next: Controlling Scanning, Prev: Array Example, Up: Array Basics
8.1.5 Scanning All Elements of an Array
---------------------------------------
In programs that use arrays, it is often necessary to use a loop that
executes once for each element of an array. In other languages, where
arrays are contiguous and indices are limited to nonnegative integers,
this is easy: all the valid indices can be found by counting from the
lowest index up to the highest. This technique won't do the job in
'awk', because any number or string can be an array index. So 'awk' has
a special kind of 'for' statement for scanning an array:
for (VAR in ARRAY)
BODY
This loop executes BODY once for each index in ARRAY that the program
has previously used, with the variable VAR set to that index.
The following program uses this form of the 'for' statement. The
first rule scans the input records and notes which words appear (at
least once) in the input, by storing a one into the array 'used' with
the word as the index. The second rule scans the elements of 'used' to
find all the distinct words that appear in the input. It prints each
word that is more than 10 characters long and also prints the number of
such words. *Note String Functions:: for more information on the
built-in function 'length()'.
# Record a 1 for each word that is used at least once
{
for (i = 1; i <= NF; i++)
used[$i] = 1
}
# Find number of distinct words more than 10 characters long
END {
for (x in used) {
if (length(x) > 10) {
++num_long_words
print x
}
}
print num_long_words, "words longer than 10 characters"
}
*Note Word Sorting:: for a more detailed example of this type.
The order in which elements of the array are accessed by this
statement is determined by the internal arrangement of the array
elements within 'awk' and in standard 'awk' cannot be controlled or
changed. This can lead to problems if new elements are added to ARRAY
by statements in the loop body; it is not predictable whether the 'for'
loop will reach them. Similarly, changing VAR inside the loop may
produce strange results. It is best to avoid such things.
As a point of information, 'gawk' sets up the list of elements to be
iterated over before the loop starts, and does not change it. But not
all 'awk' versions do so. Consider this program, named 'loopcheck.awk':
BEGIN {
a["here"] = "here"
a["is"] = "is"
a["a"] = "a"
a["loop"] = "loop"
for (i in a) {
j++
a[j] = j
print i
}
}
Here is what happens when run with 'gawk' (and 'mawk'):
$ gawk -f loopcheck.awk
-| here
-| loop
-| a
-| is
Contrast this to BWK 'awk':
$ nawk -f loopcheck.awk
-| loop
-| here
-| is
-| a
-| 1
File: gawk.info, Node: Controlling Scanning, Prev: Scanning an Array, Up: Array Basics
8.1.6 Using Predefined Array Scanning Orders with 'gawk'
--------------------------------------------------------
This node describes a feature that is specific to 'gawk'.
By default, when a 'for' loop traverses an array, the order is
undefined, meaning that the 'awk' implementation determines the order in
which the array is traversed. This order is usually based on the
internal implementation of arrays and will vary from one version of
'awk' to the next.
Often, though, you may wish to do something simple, such as "traverse
the array by comparing the indices in ascending order," or "traverse the
array by comparing the values in descending order." 'gawk' provides two
mechanisms that give you this control:
* Set 'PROCINFO["sorted_in"]' to one of a set of predefined values.
We describe this now.
* Set 'PROCINFO["sorted_in"]' to the name of a user-defined function
to use for comparison of array elements. This advanced feature is
described later in *note Array Sorting::.
The following special values for 'PROCINFO["sorted_in"]' are
available:
'"@unsorted"'
Array elements are processed in arbitrary order, which is the
default 'awk' behavior.
'"@ind_str_asc"'
Order by indices in ascending order compared as strings; this is
the most basic sort. (Internally, array indices are always
strings, so with 'a[2*5] = 1' the index is '"10"' rather than
numeric 10.)
'"@ind_num_asc"'
Order by indices in ascending order but force them to be treated as
numbers in the process. Any index with a non-numeric value will
end up positioned as if it were zero.
'"@val_type_asc"'
Order by element values in ascending order (rather than by
indices). Ordering is by the type assigned to the element (*note
Typing and Comparison::). All numeric values come before all
string values, which in turn come before all subarrays. (Subarrays
have not been described yet; *note Arrays of Arrays::.)
'"@val_str_asc"'
Order by element values in ascending order (rather than by
indices). Scalar values are compared as strings. Subarrays, if
present, come out last.
'"@val_num_asc"'
Order by element values in ascending order (rather than by
indices). Scalar values are compared as numbers. Subarrays, if
present, come out last. When numeric values are equal, the string
values are used to provide an ordering: this guarantees consistent
results across different versions of the C 'qsort()' function,(1)
which 'gawk' uses internally to perform the sorting.
'"@ind_str_desc"'
Like '"@ind_str_asc"', but the string indices are ordered from high
to low.
'"@ind_num_desc"'
Like '"@ind_num_asc"', but the numeric indices are ordered from
high to low.
'"@val_type_desc"'
Like '"@val_type_asc"', but the element values, based on type, are
ordered from high to low. Subarrays, if present, come out first.
'"@val_str_desc"'
Like '"@val_str_asc"', but the element values, treated as strings,
are ordered from high to low. Subarrays, if present, come out
first.
'"@val_num_desc"'
Like '"@val_num_asc"', but the element values, treated as numbers,
are ordered from high to low. Subarrays, if present, come out
first.
The array traversal order is determined before the 'for' loop starts
to run. Changing 'PROCINFO["sorted_in"]' in the loop body does not
affect the loop. For example:
$ gawk '
> BEGIN {
> a[4] = 4
> a[3] = 3
> for (i in a)
> print i, a[i]
> }'
-| 4 4
-| 3 3
$ gawk '
> BEGIN {
> PROCINFO["sorted_in"] = "@ind_str_asc"
> a[4] = 4
> a[3] = 3
> for (i in a)
> print i, a[i]
> }'
-| 3 3
-| 4 4
When sorting an array by element values, if a value happens to be a
subarray then it is considered to be greater than any string or numeric
value, regardless of what the subarray itself contains, and all
subarrays are treated as being equal to each other. Their order
relative to each other is determined by their index strings.
Here are some additional things to bear in mind about sorted array
traversal:
* The value of 'PROCINFO["sorted_in"]' is global. That is, it
affects all array traversal 'for' loops. If you need to change it
within your own code, you should see if it's defined and save and
restore the value:
...
if ("sorted_in" in PROCINFO) {
save_sorted = PROCINFO["sorted_in"]
PROCINFO["sorted_in"] = "@val_str_desc" # or whatever
}
...
if (save_sorted)
PROCINFO["sorted_in"] = save_sorted
* As already mentioned, the default array traversal order is
represented by '"@unsorted"'. You can also get the default
behavior by assigning the null string to 'PROCINFO["sorted_in"]' or
by just deleting the '"sorted_in"' element from the 'PROCINFO'
array with the 'delete' statement. (The 'delete' statement hasn't
been described yet; *note Delete::.)
In addition, 'gawk' provides built-in functions for sorting arrays;
see *note Array Sorting Functions::.
---------- Footnotes ----------
(1) When two elements compare as equal, the C 'qsort()' function does
not guarantee that they will maintain their original relative order
after sorting. Using the string value to provide a unique ordering when
the numeric values are equal ensures that 'gawk' behaves consistently
across different environments.
File: gawk.info, Node: Numeric Array Subscripts, Next: Uninitialized Subscripts, Prev: Array Basics, Up: Arrays
8.2 Using Numbers to Subscript Arrays
=====================================
An important aspect to remember about arrays is that _array subscripts
are always strings_. When a numeric value is used as a subscript, it is
converted to a string value before being used for subscripting (*note
Conversion::). This means that the value of the predefined variable
'CONVFMT' can affect how your program accesses elements of an array.
For example:
xyz = 12.153
data[xyz] = 1
CONVFMT = "%2.2f"
if (xyz in data)
printf "%s is in data\n", xyz
else
printf "%s is not in data\n", xyz
This prints '12.15 is not in data'. The first statement gives 'xyz' a
numeric value. Assigning to 'data[xyz]' subscripts 'data' with the
string value '"12.153"' (using the default conversion value of
'CONVFMT', '"%.6g"'). Thus, the array element 'data["12.153"]' is
assigned the value one. The program then changes the value of
'CONVFMT'. The test '(xyz in data)' generates a new string value from
'xyz'--this time '"12.15"'--because the value of 'CONVFMT' only allows
two significant digits. This test fails, because '"12.15"' is different
from '"12.153"'.
According to the rules for conversions (*note Conversion::), integer
values always convert to strings as integers, no matter what the value
of 'CONVFMT' may happen to be. So the usual case of the following
works:
for (i = 1; i <= maxsub; i++)
do something with array[i]
The "integer values always convert to strings as integers" rule has
an additional consequence for array indexing. Octal and hexadecimal
constants (*note Nondecimal-numbers::) are converted internally into
numbers, and their original form is forgotten. This means, for example,
that 'array[17]', 'array[021]', and 'array[0x11]' all refer to the same
element!
As with many things in 'awk', the majority of the time things work as
you would expect them to. But it is useful to have a precise knowledge
of the actual rules, as they can sometimes have a subtle effect on your
programs.
File: gawk.info, Node: Uninitialized Subscripts, Next: Delete, Prev: Numeric Array Subscripts, Up: Arrays
8.3 Using Uninitialized Variables as Subscripts
===============================================
Suppose it's necessary to write a program to print the input data in
reverse order. A reasonable attempt to do so (with some test data)
might look like this:
$ echo 'line 1
> line 2
> line 3' | awk '{ l[lines] = $0; ++lines }
> END {
> for (i = lines - 1; i >= 0; i--)
> print l[i]
> }'
-| line 3
-| line 2
Unfortunately, the very first line of input data did not appear in
the output!
Upon first glance, we would think that this program should have
worked. The variable 'lines' is uninitialized, and uninitialized
variables have the numeric value zero. So, 'awk' should have printed
the value of 'l[0]'.
The issue here is that subscripts for 'awk' arrays are _always_
strings. Uninitialized variables, when used as strings, have the value
'""', not zero. Thus, 'line 1' ends up stored in 'l[""]'. The
following version of the program works correctly:
{ l[lines++] = $0 }
END {
for (i = lines - 1; i >= 0; i--)
print l[i]
}
Here, the '++' forces 'lines' to be numeric, thus making the "old
value" numeric zero. This is then converted to '"0"' as the array
subscript.
Even though it is somewhat unusual, the null string ('""') is a valid
array subscript. (d.c.) 'gawk' warns about the use of the null string
as a subscript if '--lint' is provided on the command line (*note
Options::).
File: gawk.info, Node: Delete, Next: Multidimensional, Prev: Uninitialized Subscripts, Up: Arrays
8.4 The 'delete' Statement
==========================
To remove an individual element of an array, use the 'delete' statement:
delete ARRAY[INDEX-EXPRESSION]
Once an array element has been deleted, any value the element once
had is no longer available. It is as if the element had never been
referred to or been given a value. The following is an example of
deleting elements in an array:
for (i in frequencies)
delete frequencies[i]
This example removes all the elements from the array 'frequencies'.
Once an element is deleted, a subsequent 'for' statement to scan the
array does not report that element and using the 'in' operator to check
for the presence of that element returns zero (i.e., false):
delete foo[4]
if (4 in foo)
print "This will never be printed"
It is important to note that deleting an element is _not_ the same as
assigning it a null value (the empty string, '""'). For example:
foo[4] = ""
if (4 in foo)
print "This is printed, even though foo[4] is empty"
It is not an error to delete an element that does not exist.
However, if '--lint' is provided on the command line (*note Options::),
'gawk' issues a warning message when an element that is not in the array
is deleted.
All the elements of an array may be deleted with a single statement
by leaving off the subscript in the 'delete' statement, as follows:
delete ARRAY
Using this version of the 'delete' statement is about three times
more efficient than the equivalent loop that deletes each element one at
a time.
This form of the 'delete' statement is also supported by BWK 'awk'
and 'mawk', as well as by a number of other implementations.
NOTE: For many years, using 'delete' without a subscript was a
common extension. In September 2012, it was accepted for inclusion
into the POSIX standard. See the Austin Group website
(http://austingroupbugs.net/view.php?id=544).
The following statement provides a portable but nonobvious way to
clear out an array:(1)
split("", array)
The 'split()' function (*note String Functions::) clears out the
target array first. This call asks it to split apart the null string.
Because there is no data to split out, the function simply clears the
array and then returns.
CAUTION: Deleting all the elements from an array does not change
its type; you cannot clear an array and then use the array's name
as a scalar (i.e., a regular variable). For example, the following
does not work:
a[1] = 3
delete a
a = 3
---------- Footnotes ----------
(1) Thanks to Michael Brennan for pointing this out.
File: gawk.info, Node: Multidimensional, Next: Arrays of Arrays, Prev: Delete, Up: Arrays
8.5 Multidimensional Arrays
===========================
* Menu:
* Multiscanning:: Scanning multidimensional arrays.
A "multidimensional array" is an array in which an element is identified
by a sequence of indices instead of a single index. For example, a
two-dimensional array requires two indices. The usual way (in many
languages, including 'awk') to refer to an element of a two-dimensional
array named 'grid' is with 'grid[X,Y]'.
Multidimensional arrays are supported in 'awk' through concatenation
of indices into one string. 'awk' converts the indices into strings
(*note Conversion::) and concatenates them together, with a separator
between them. This creates a single string that describes the values of
the separate indices. The combined string is used as a single index
into an ordinary, one-dimensional array. The separator used is the
value of the built-in variable 'SUBSEP'.
For example, suppose we evaluate the expression 'foo[5,12] = "value"'
when the value of 'SUBSEP' is '"@"'. The numbers 5 and 12 are converted
to strings and concatenated with an '@' between them, yielding '"5@12"';
thus, the array element 'foo["5@12"]' is set to '"value"'.
Once the element's value is stored, 'awk' has no record of whether it
was stored with a single index or a sequence of indices. The two
expressions 'foo[5,12]' and 'foo[5 SUBSEP 12]' are always equivalent.
The default value of 'SUBSEP' is the string '"\034"', which contains
a nonprinting character that is unlikely to appear in an 'awk' program
or in most input data. The usefulness of choosing an unlikely character
comes from the fact that index values that contain a string matching
'SUBSEP' can lead to combined strings that are ambiguous. Suppose that
'SUBSEP' is '"@"'; then 'foo["a@b", "c"]' and 'foo["a", "b@c"]' are
indistinguishable because both are actually stored as 'foo["a@b@c"]'.
To test whether a particular index sequence exists in a
multidimensional array, use the same operator ('in') that is used for
single-dimensional arrays. Write the whole sequence of indices in
parentheses, separated by commas, as the left operand:
if ((SUBSCRIPT1, SUBSCRIPT2, ...) in ARRAY)
...
Here is an example that treats its input as a two-dimensional array
of fields; it rotates this array 90 degrees clockwise and prints the
result. It assumes that all lines have the same number of elements:
{
if (max_nf < NF)
max_nf = NF
max_nr = NR
for (x = 1; x <= NF; x++)
vector[x, NR] = $x
}
END {
for (x = 1; x <= max_nf; x++) {
for (y = max_nr; y >= 1; --y)
printf("%s ", vector[x, y])
printf("\n")
}
}
When given the input:
1 2 3 4 5 6
2 3 4 5 6 1
3 4 5 6 1 2
4 5 6 1 2 3
the program produces the following output:
4 3 2 1
5 4 3 2
6 5 4 3
1 6 5 4
2 1 6 5
3 2 1 6
File: gawk.info, Node: Multiscanning, Up: Multidimensional
8.5.1 Scanning Multidimensional Arrays
--------------------------------------
There is no special 'for' statement for scanning a "multidimensional"
array. There cannot be one, because, in truth, 'awk' does not have
multidimensional arrays or elements--there is only a multidimensional
_way of accessing_ an array.
However, if your program has an array that is always accessed as
multidimensional, you can get the effect of scanning it by combining the
scanning 'for' statement (*note Scanning an Array::) with the built-in
'split()' function (*note String Functions::). It works in the
following manner:
for (combined in array) {
split(combined, separate, SUBSEP)
...
}
This sets the variable 'combined' to each concatenated combined index in
the array, and splits it into the individual indices by breaking it
apart where the value of 'SUBSEP' appears. The individual indices then
become the elements of the array 'separate'.
Thus, if a value is previously stored in 'array[1, "foo"]', then an
element with index '"1\034foo"' exists in 'array'. (Recall that the
default value of 'SUBSEP' is the character with code 034.) Sooner or
later, the 'for' statement finds that index and does an iteration with
the variable 'combined' set to '"1\034foo"'. Then the 'split()'
function is called as follows:
split("1\034foo", separate, "\034")
The result is to set 'separate[1]' to '"1"' and 'separate[2]' to
'"foo"'. Presto! The original sequence of separate indices is
recovered.
File: gawk.info, Node: Arrays of Arrays, Next: Arrays Summary, Prev: Multidimensional, Up: Arrays
8.6 Arrays of Arrays
====================
'gawk' goes beyond standard 'awk''s multidimensional array access and
provides true arrays of arrays. Elements of a subarray are referred to
by their own indices enclosed in square brackets, just like the elements
of the main array. For example, the following creates a two-element
subarray at index '1' of the main array 'a':
a[1][1] = 1
a[1][2] = 2
This simulates a true two-dimensional array. Each subarray element
can contain another subarray as a value, which in turn can hold other
arrays as well. In this way, you can create arrays of three or more
dimensions. The indices can be any 'awk' expressions, including scalars
separated by commas (i.e., a regular 'awk' simulated multidimensional
subscript). So the following is valid in 'gawk':
a[1][3][1, "name"] = "barney"
Each subarray and the main array can be of different length. In
fact, the elements of an array or its subarray do not all have to have
the same type. This means that the main array and any of its subarrays
can be nonrectangular, or jagged in structure. You can assign a scalar
value to the index '4' of the main array 'a', even though 'a[1]' is
itself an array and not a scalar:
a[4] = "An element in a jagged array"
The terms "dimension", "row", and "column" are meaningless when
applied to such an array, but we will use "dimension" henceforth to
imply the maximum number of indices needed to refer to an existing
element. The type of any element that has already been assigned cannot
be changed by assigning a value of a different type. You have to first
delete the current element, which effectively makes 'gawk' forget about
the element at that index:
delete a[4]
a[4][5][6][7] = "An element in a four-dimensional array"
This removes the scalar value from index '4' and then inserts a
three-level nested subarray containing a scalar. You can also delete an
entire subarray or subarray of subarrays:
delete a[4][5]
a[4][5] = "An element in subarray a[4]"
But recall that you can not delete the main array 'a' and then use it
as a scalar.
The built-in functions that take array arguments can also be used
with subarrays. For example, the following code fragment uses
'length()' (*note String Functions::) to determine the number of
elements in the main array 'a' and its subarrays:
print length(a), length(a[1]), length(a[1][3])
This results in the following output for our main array 'a':
2, 3, 1
The 'SUBSCRIPT in ARRAY' expression (*note Reference to Elements::)
works similarly for both regular 'awk'-style arrays and arrays of
arrays. For example, the tests '1 in a', '3 in a[1]', and '(1, "name")
in a[1][3]' all evaluate to one (true) for our array 'a'.
The 'for (item in array)' statement (*note Scanning an Array::) can
be nested to scan all the elements of an array of arrays if it is
rectangular in structure. In order to print the contents (scalar
values) of a two-dimensional array of arrays (i.e., in which each
first-level element is itself an array, not necessarily of the same
length), you could use the following code:
for (i in array)
for (j in array[i])
print array[i][j]
The 'isarray()' function (*note Type Functions::) lets you test if an
array element is itself an array:
for (i in array) {
if (isarray(array[i]) {
for (j in array[i]) {
print array[i][j]
}
}
else
print array[i]
}
If the structure of a jagged array of arrays is known in advance, you
can often devise workarounds using control statements. For example, the
following code prints the elements of our main array 'a':
for (i in a) {
for (j in a[i]) {
if (j == 3) {
for (k in a[i][j])
print a[i][j][k]
} else
print a[i][j]
}
}
*Note Walking Arrays:: for a user-defined function that "walks" an
arbitrarily dimensioned array of arrays.
Recall that a reference to an uninitialized array element yields a
value of '""', the null string. This has one important implication when
you intend to use a subarray as an argument to a function, as
illustrated by the following example:
$ gawk 'BEGIN { split("a b c d", b[1]); print b[1][1] }'
error-> gawk: cmd. line:1: fatal: split: second argument is not an array
The way to work around this is to first force 'b[1]' to be an array
by creating an arbitrary index:
$ gawk 'BEGIN { b[1][1] = ""; split("a b c d", b[1]); print b[1][1] }'
-| a
File: gawk.info, Node: Arrays Summary, Prev: Arrays of Arrays, Up: Arrays
8.7 Summary
===========
* Standard 'awk' provides one-dimensional associative arrays (arrays
indexed by string values). All arrays are associative; numeric
indices are converted automatically to strings.
* Array elements are referenced as 'ARRAY[INDX]'. Referencing an
element creates it if it did not exist previously.
* The proper way to see if an array has an element with a given index
is to use the 'in' operator: 'INDX in ARRAY'.
* Use 'for (INDX in ARRAY) ...' to scan through all the individual
elements of an array. In the body of the loop, INDX takes on the
value of each element's index in turn.
* The order in which a 'for (INDX in ARRAY)' loop traverses an array
is undefined in POSIX 'awk' and varies among implementations.
'gawk' lets you control the order by assigning special predefined
values to 'PROCINFO["sorted_in"]'.
* Use 'delete ARRAY[INDX]' to delete an individual element. To
delete all of the elements in an array, use 'delete ARRAY'. This
latter feature has been a common extension for many years and is
now standard, but may not be supported by all commercial versions
of 'awk'.
* Standard 'awk' simulates multidimensional arrays by separating
subscript values with commas. The values are concatenated into a
single string, separated by the value of 'SUBSEP'. The fact that
such a subscript was created in this way is not retained; thus,
changing 'SUBSEP' may have unexpected consequences. You can use
'(SUB1, SUB2, ...) in ARRAY' to see if such a multidimensional
subscript exists in ARRAY.
* 'gawk' provides true arrays of arrays. You use a separate set of
square brackets for each dimension in such an array:
'data[row][col]', for example. Array elements may thus be either
scalar values (number or string) or other arrays.
* Use the 'isarray()' built-in function to determine if an array
element is itself a subarray.
File: gawk.info, Node: Functions, Next: Library Functions, Prev: Arrays, Up: Top
9 Functions
***********
This major node describes 'awk''s built-in functions, which fall into
three categories: numeric, string, and I/O. 'gawk' provides additional
groups of functions to work with values that represent time, do bit
manipulation, sort arrays, provide type information, and
internationalize and localize programs.
Besides the built-in functions, 'awk' has provisions for writing new
functions that the rest of a program can use. The second half of this
major node describes these "user-defined" functions. Finally, we
explore indirect function calls, a 'gawk'-specific extension that lets
you determine at runtime what function is to be called.
* Menu:
* Built-in:: Summarizes the built-in functions.
* User-defined:: Describes User-defined functions in detail.
* Indirect Calls:: Choosing the function to call at runtime.
* Functions Summary:: Summary of functions.
File: gawk.info, Node: Built-in, Next: User-defined, Up: Functions
9.1 Built-in Functions
======================
"Built-in" functions are always available for your 'awk' program to
call. This minor node defines all the built-in functions in 'awk'; some
of these are mentioned in other minor nodes but are summarized here for
your convenience.
* Menu:
* Calling Built-in:: How to call built-in functions.
* Numeric Functions:: Functions that work with numbers, including
'int()', 'sin()' and 'rand()'.
* String Functions:: Functions for string manipulation, such as
'split()', 'match()' and
'sprintf()'.
* I/O Functions:: Functions for files and shell commands.
* Time Functions:: Functions for dealing with timestamps.
* Bitwise Functions:: Functions for bitwise operations.
* Type Functions:: Functions for type information.
* I18N Functions:: Functions for string translation.
File: gawk.info, Node: Calling Built-in, Next: Numeric Functions, Up: Built-in
9.1.1 Calling Built-in Functions
--------------------------------
To call one of 'awk''s built-in functions, write the name of the
function followed by arguments in parentheses. For example, 'atan2(y +
z, 1)' is a call to the function 'atan2()' and has two arguments.
Whitespace is ignored between the built-in function name and the
opening parenthesis, but nonetheless it is good practice to avoid using
whitespace there. User-defined functions do not permit whitespace in
this way, and it is easier to avoid mistakes by following a simple
convention that always works--no whitespace after a function name.
Each built-in function accepts a certain number of arguments. In
some cases, arguments can be omitted. The defaults for omitted
arguments vary from function to function and are described under the
individual functions. In some 'awk' implementations, extra arguments
given to built-in functions are ignored. However, in 'gawk', it is a
fatal error to give extra arguments to a built-in function.
When a function is called, expressions that create the function's
actual parameters are evaluated completely before the call is performed.
For example, in the following code fragment:
i = 4
j = sqrt(i++)
the variable 'i' is incremented to the value five before 'sqrt()' is
called with a value of four for its actual parameter. The order of
evaluation of the expressions used for the function's parameters is
undefined. Thus, avoid writing programs that assume that parameters are
evaluated from left to right or from right to left. For example:
i = 5
j = atan2(++i, i *= 2)
If the order of evaluation is left to right, then 'i' first becomes
six, and then 12, and 'atan2()' is called with the two arguments six and
12. But if the order of evaluation is right to left, 'i' first becomes
10, then 11, and 'atan2()' is called with the two arguments 11 and 10.
File: gawk.info, Node: Numeric Functions, Next: String Functions, Prev: Calling Built-in, Up: Built-in
9.1.2 Numeric Functions
-----------------------
The following list describes all of the built-in functions that work
with numbers. Optional parameters are enclosed in square
brackets ([ ]):
'atan2(Y, X)'
Return the arctangent of 'Y / X' in radians. You can use 'pi =
atan2(0, -1)' to retrieve the value of pi.
'cos(X)'
Return the cosine of X, with X in radians.
'exp(X)'
Return the exponential of X ('e ^ X') or report an error if X is
out of range. The range of values X can have depends on your
machine's floating-point representation.
'int(X)'
Return the nearest integer to X, located between X and zero and
truncated toward zero. For example, 'int(3)' is 3, 'int(3.9)' is
3, 'int(-3.9)' is -3, and 'int(-3)' is -3 as well.
'log(X)'
Return the natural logarithm of X, if X is positive; otherwise,
return 'NaN' ("not a number") on IEEE 754 systems. Additionally,
'gawk' prints a warning message when 'x' is negative.
'rand()'
Return a random number. The values of 'rand()' are uniformly
distributed between zero and one. The value could be zero but is
never one.(1)
Often random integers are needed instead. Following is a
user-defined function that can be used to obtain a random
nonnegative integer less than N:
function randint(n)
{
return int(n * rand())
}
The multiplication produces a random number greater than or equal
to zero and less than 'n'. Using 'int()', this result is made into
an integer between zero and 'n' - 1, inclusive.
The following example uses a similar function to produce random
integers between one and N. This program prints a new random
number for each input record:
# Function to roll a simulated die.
function roll(n) { return 1 + int(rand() * n) }
# Roll 3 six-sided dice and
# print total number of points.
{
printf("%d points\n", roll(6) + roll(6) + roll(6))
}
CAUTION: In most 'awk' implementations, including 'gawk',
'rand()' starts generating numbers from the same starting
number, or "seed", each time you run 'awk'.(2) Thus, a
program generates the same results each time you run it. The
numbers are random within one 'awk' run but predictable from
run to run. This is convenient for debugging, but if you want
a program to do different things each time it is used, you
must change the seed to a value that is different in each run.
To do this, use 'srand()'.
'sin(X)'
Return the sine of X, with X in radians.
'sqrt(X)'
Return the positive square root of X. 'gawk' prints a warning
message if X is negative. Thus, 'sqrt(4)' is 2.
'srand('[X]')'
Set the starting point, or seed, for generating random numbers to
the value X.
Each seed value leads to a particular sequence of random
numbers.(3) Thus, if the seed is set to the same value a second
time, the same sequence of random numbers is produced again.
CAUTION: Different 'awk' implementations use different
random-number generators internally. Don't expect the same
'awk' program to produce the same series of random numbers
when executed by different versions of 'awk'.
If the argument X is omitted, as in 'srand()', then the current
date and time of day are used for a seed. This is the way to get
random numbers that are truly unpredictable.
The return value of 'srand()' is the previous seed. This makes it
easy to keep track of the seeds in case you need to consistently
reproduce sequences of random numbers.
POSIX does not specify the initial seed; it differs among 'awk'
implementations.
---------- Footnotes ----------
(1) The C version of 'rand()' on many Unix systems is known to
produce fairly poor sequences of random numbers. However, nothing
requires that an 'awk' implementation use the C 'rand()' to implement
the 'awk' version of 'rand()'. In fact, 'gawk' uses the BSD 'random()'
function, which is considerably better than 'rand()', to produce random
numbers.
(2) 'mawk' uses a different seed each time.
(3) Computer-generated random numbers really are not truly random.
They are technically known as "pseudorandom". This means that although
the numbers in a sequence appear to be random, you can in fact generate
the same sequence of random numbers over and over again.
File: gawk.info, Node: String Functions, Next: I/O Functions, Prev: Numeric Functions, Up: Built-in
9.1.3 String-Manipulation Functions
-----------------------------------
The functions in this minor node look at or change the text of one or
more strings.
'gawk' understands locales (*note Locales::) and does all string
processing in terms of _characters_, not _bytes_. This distinction is
particularly important to understand for locales where one character may
be represented by multiple bytes. Thus, for example, 'length()' returns
the number of characters in a string, and not the number of bytes used
to represent those characters. Similarly, 'index()' works with
character indices, and not byte indices.
CAUTION: A number of functions deal with indices into strings. For
these functions, the first character of a string is at position
(index) one. This is different from C and the languages descended
from it, where the first character is at position zero. You need
to remember this when doing index calculations, particularly if you
are used to C.
In the following list, optional parameters are enclosed in square
brackets ([ ]). Several functions perform string substitution; the full
discussion is provided in the description of the 'sub()' function, which
comes toward the end, because the list is presented alphabetically.
Those functions that are specific to 'gawk' are marked with a pound
sign ('#'). They are not available in compatibility mode (*note
Options::):
* Menu:
* Gory Details:: More than you want to know about '\' and
'&' with 'sub()', 'gsub()', and
'gensub()'.
'asort('SOURCE [',' DEST [',' HOW ] ]') #'
'asorti('SOURCE [',' DEST [',' HOW ] ]') #'
These two functions are similar in behavior, so they are described
together.
NOTE: The following description ignores the third argument,
HOW, as it requires understanding features that we have not
discussed yet. Thus, the discussion here is a deliberate
simplification. (We do provide all the details later on; see
*note Array Sorting Functions:: for the full story.)
Both functions return the number of elements in the array SOURCE.
For 'asort()', 'gawk' sorts the values of SOURCE and replaces the
indices of the sorted values of SOURCE with sequential integers
starting with one. If the optional array DEST is specified, then
SOURCE is duplicated into DEST. DEST is then sorted, leaving the
indices of SOURCE unchanged.
When comparing strings, 'IGNORECASE' affects the sorting (*note
Array Sorting Functions::). If the SOURCE array contains subarrays
as values (*note Arrays of Arrays::), they will come last, after
all scalar values. Subarrays are _not_ recursively sorted.
For example, if the contents of 'a' are as follows:
a["last"] = "de"
a["first"] = "sac"
a["middle"] = "cul"
A call to 'asort()':
asort(a)
results in the following contents of 'a':
a[1] = "cul"
a[2] = "de"
a[3] = "sac"
The 'asorti()' function works similarly to 'asort()'; however, the
_indices_ are sorted, instead of the values. Thus, in the previous
example, starting with the same initial set of indices and values
in 'a', calling 'asorti(a)' would yield:
a[1] = "first"
a[2] = "last"
a[3] = "middle"
'gensub(REGEXP, REPLACEMENT, HOW' [', TARGET']') #'
Search the target string TARGET for matches of the regular
expression REGEXP. If HOW is a string beginning with 'g' or 'G'
(short for "global"), then replace all matches of REGEXP with
REPLACEMENT. Otherwise, HOW is treated as a number indicating
which match of REGEXP to replace. If no TARGET is supplied, use
'$0'. It returns the modified string as the result of the function
and the original target string is _not_ changed.
'gensub()' is a general substitution function. Its purpose is to
provide more features than the standard 'sub()' and 'gsub()'
functions.
'gensub()' provides an additional feature that is not available in
'sub()' or 'gsub()': the ability to specify components of a regexp
in the replacement text. This is done by using parentheses in the
regexp to mark the components and then specifying '\N' in the
replacement text, where N is a digit from 1 to 9. For example:
$ gawk '
> BEGIN {
> a = "abc def"
> b = gensub(/(.+) (.+)/, "\\2 \\1", "g", a)
> print b
> }'
-| def abc
As with 'sub()', you must type two backslashes in order to get one
into the string. In the replacement text, the sequence '\0'
represents the entire matched text, as does the character '&'.
The following example shows how you can use the third argument to
control which match of the regexp should be changed:
$ echo a b c a b c |
> gawk '{ print gensub(/a/, "AA", 2) }'
-| a b c AA b c
In this case, '$0' is the default target string. 'gensub()'
returns the new string as its result, which is passed directly to
'print' for printing.
If the HOW argument is a string that does not begin with 'g' or
'G', or if it is a number that is less than or equal to zero, only
one substitution is performed. If HOW is zero, 'gawk' issues a
warning message.
If REGEXP does not match TARGET, 'gensub()''s return value is the
original unchanged value of TARGET.
'gsub(REGEXP, REPLACEMENT' [', TARGET']')'
Search TARGET for _all_ of the longest, leftmost, _nonoverlapping_
matching substrings it can find and replace them with REPLACEMENT.
The 'g' in 'gsub()' stands for "global," which means replace
everywhere. For example:
{ gsub(/Britain/, "United Kingdom"); print }
replaces all occurrences of the string 'Britain' with 'United
Kingdom' for all input records.
The 'gsub()' function returns the number of substitutions made. If
the variable to search and alter (TARGET) is omitted, then the
entire input record ('$0') is used. As in 'sub()', the characters
'&' and '\' are special, and the third argument must be assignable.
'index(IN, FIND)'
Search the string IN for the first occurrence of the string FIND,
and return the position in characters where that occurrence begins
in the string IN. Consider the following example:
$ awk 'BEGIN { print index("peanut", "an") }'
-| 3
If FIND is not found, 'index()' returns zero.
With BWK 'awk' and 'gawk', it is a fatal error to use a regexp
constant for FIND. Other implementations allow it, simply treating
the regexp constant as an expression meaning '$0 ~ /regexp/'.
(d.c.)
'length('[STRING]')'
Return the number of characters in STRING. If STRING is a number,
the length of the digit string representing that number is
returned. For example, 'length("abcde")' is five. By contrast,
'length(15 * 35)' works out to three. In this example, 15 * 35 =
525, and 525 is then converted to the string '"525"', which has
three characters.
If no argument is supplied, 'length()' returns the length of '$0'.
NOTE: In older versions of 'awk', the 'length()' function
could be called without any parentheses. Doing so is
considered poor practice, although the 2008 POSIX standard
explicitly allows it, to support historical practice. For
programs to be maximally portable, always supply the
parentheses.
If 'length()' is called with a variable that has not been used,
'gawk' forces the variable to be a scalar. Other implementations
of 'awk' leave the variable without a type. (d.c.) Consider:
$ gawk 'BEGIN { print length(x) ; x[1] = 1 }'
-| 0
error-> gawk: fatal: attempt to use scalar `x' as array
$ nawk 'BEGIN { print length(x) ; x[1] = 1 }'
-| 0
If '--lint' has been specified on the command line, 'gawk' issues a
warning about this.
With 'gawk' and several other 'awk' implementations, when given an
array argument, the 'length()' function returns the number of
elements in the array. (c.e.) This is less useful than it might
seem at first, as the array is not guaranteed to be indexed from
one to the number of elements in it. If '--lint' is provided on
the command line (*note Options::), 'gawk' warns that passing an
array argument is not portable. If '--posix' is supplied, using an
array argument is a fatal error (*note Arrays::).
'match(STRING, REGEXP' [', ARRAY']')'
Search STRING for the longest, leftmost substring matched by the
regular expression REGEXP and return the character position (index)
at which that substring begins (one, if it starts at the beginning
of STRING). If no match is found, return zero.
The REGEXP argument may be either a regexp constant ('/'...'/') or
a string constant ('"'...'"'). In the latter case, the string is
treated as a regexp to be matched. *Note Computed Regexps:: for a
discussion of the difference between the two forms, and the
implications for writing your program correctly.
The order of the first two arguments is the opposite of most other
string functions that work with regular expressions, such as
'sub()' and 'gsub()'. It might help to remember that for
'match()', the order is the same as for the '~' operator: 'STRING ~
REGEXP'.
The 'match()' function sets the predefined variable 'RSTART' to the
index. It also sets the predefined variable 'RLENGTH' to the
length in characters of the matched substring. If no match is
found, 'RSTART' is set to zero, and 'RLENGTH' to -1.
For example:
{
if ($1 == "FIND")
regex = $2
else {
where = match($0, regex)
if (where != 0)
print "Match of", regex, "found at", where, "in", $0
}
}
This program looks for lines that match the regular expression
stored in the variable 'regex'. This regular expression can be
changed. If the first word on a line is 'FIND', 'regex' is changed
to be the second word on that line. Therefore, if given:
FIND ru+n
My program runs
but not very quickly
FIND Melvin
JF+KM
This line is property of Reality Engineering Co.
Melvin was here.
'awk' prints:
Match of ru+n found at 12 in My program runs
Match of Melvin found at 1 in Melvin was here.
If ARRAY is present, it is cleared, and then the zeroth element of
ARRAY is set to the entire portion of STRING matched by REGEXP. If
REGEXP contains parentheses, the integer-indexed elements of ARRAY
are set to contain the portion of STRING matching the corresponding
parenthesized subexpression. For example:
$ echo foooobazbarrrrr |
> gawk '{ match($0, /(fo+).+(bar*)/, arr)
> print arr[1], arr[2] }'
-| foooo barrrrr
In addition, multidimensional subscripts are available providing
the start index and length of each matched subexpression:
$ echo foooobazbarrrrr |
> gawk '{ match($0, /(fo+).+(bar*)/, arr)
> print arr[1], arr[2]
> print arr[1, "start"], arr[1, "length"]
> print arr[2, "start"], arr[2, "length"]
> }'
-| foooo barrrrr
-| 1 5
-| 9 7
There may not be subscripts for the start and index for every
parenthesized subexpression, because they may not all have matched
text; thus, they should be tested for with the 'in' operator (*note
Reference to Elements::).
The ARRAY argument to 'match()' is a 'gawk' extension. In
compatibility mode (*note Options::), using a third argument is a
fatal error.
'patsplit(STRING, ARRAY' [', FIELDPAT' [', SEPS' ] ]') #'
Divide STRING into pieces (or "fields") defined by FIELDPAT and
store the pieces in ARRAY and the separator strings in the SEPS
array. The first piece is stored in 'ARRAY[1]', the second piece
in 'ARRAY[2]', and so forth. The third argument, FIELDPAT, is a
regexp describing the fields in STRING (just as 'FPAT' is a regexp
describing the fields in input records). It may be either a regexp
constant or a string. If FIELDPAT is omitted, the value of 'FPAT'
is used. 'patsplit()' returns the number of elements created.
'SEPS[I]' is the possibly null separator string after 'ARRAY[I]'.
The possibly null leading separator will be in 'SEPS[0]'. So a
non-null STRING with N fields will have N+1 separators. A null
STRING will not have neither fields nor separators.
The 'patsplit()' function splits strings into pieces in a manner
similar to the way input lines are split into fields using 'FPAT'
(*note Splitting By Content::).
Before splitting the string, 'patsplit()' deletes any previously
existing elements in the arrays ARRAY and SEPS.
'split(STRING, ARRAY' [', FIELDSEP' [', SEPS' ] ]')'
Divide STRING into pieces separated by FIELDSEP and store the
pieces in ARRAY and the separator strings in the SEPS array. The
first piece is stored in 'ARRAY[1]', the second piece in
'ARRAY[2]', and so forth. The string value of the third argument,
FIELDSEP, is a regexp describing where to split STRING (much as
'FS' can be a regexp describing where to split input records). If
FIELDSEP is omitted, the value of 'FS' is used. 'split()' returns
the number of elements created. SEPS is a 'gawk' extension, with
'SEPS[I]' being the separator string between 'ARRAY[I]' and
'ARRAY[I+1]'. If FIELDSEP is a single space, then any leading
whitespace goes into 'SEPS[0]' and any trailing whitespace goes
into 'SEPS[N]', where N is the return value of 'split()' (i.e., the
number of elements in ARRAY).
The 'split()' function splits strings into pieces in a manner
similar to the way input lines are split into fields. For example:
split("cul-de-sac", a, "-", seps)
splits the string '"cul-de-sac"' into three fields using '-' as the
separator. It sets the contents of the array 'a' as follows:
a[1] = "cul"
a[2] = "de"
a[3] = "sac"
and sets the contents of the array 'seps' as follows:
seps[1] = "-"
seps[2] = "-"
The value returned by this call to 'split()' is three.
As with input field-splitting, when the value of FIELDSEP is '" "',
leading and trailing whitespace is ignored in values assigned to
the elements of ARRAY but not in SEPS, and the elements are
separated by runs of whitespace. Also, as with input field
splitting, if FIELDSEP is the null string, each individual
character in the string is split into its own array element.
(c.e.)
Note, however, that 'RS' has no effect on the way 'split()' works.
Even though 'RS = ""' causes the newline character to also be an
input field separator, this does not affect how 'split()' splits
strings.
Modern implementations of 'awk', including 'gawk', allow the third
argument to be a regexp constant ('/'...'/') as well as a string.
(d.c.) The POSIX standard allows this as well. *Note Computed
Regexps:: for a discussion of the difference between using a string
constant or a regexp constant, and the implications for writing
your program correctly.
Before splitting the string, 'split()' deletes any previously
existing elements in the arrays ARRAY and SEPS.
If STRING is null, the array has no elements. (So this is a
portable way to delete an entire array with one statement. *Note
Delete::.)
If STRING does not match FIELDSEP at all (but is not null), ARRAY
has one element only. The value of that element is the original
STRING.
In POSIX mode (*note Options::), the fourth argument is not
allowed.
'sprintf(FORMAT, EXPRESSION1, ...)'
Return (without printing) the string that 'printf' would have
printed out with the same arguments (*note Printf::). For example:
pival = sprintf("pi = %.2f (approx.)", 22/7)
assigns the string 'pi = 3.14 (approx.)' to the variable 'pival'.
'strtonum(STR) #'
Examine STR and return its numeric value. If STR begins with a
leading '0', 'strtonum()' assumes that STR is an octal number. If
STR begins with a leading '0x' or '0X', 'strtonum()' assumes that
STR is a hexadecimal number. For example:
$ echo 0x11 |
> gawk '{ printf "%d\n", strtonum($1) }'
-| 17
Using the 'strtonum()' function is _not_ the same as adding zero to
a string value; the automatic coercion of strings to numbers works
only for decimal data, not for octal or hexadecimal.(1)
Note also that 'strtonum()' uses the current locale's decimal point
for recognizing numbers (*note Locales::).
'sub(REGEXP, REPLACEMENT' [', TARGET']')'
Search TARGET, which is treated as a string, for the leftmost,
longest substring matched by the regular expression REGEXP. Modify
the entire string by replacing the matched text with REPLACEMENT.
The modified string becomes the new value of TARGET. Return the
number of substitutions made (zero or one).
The REGEXP argument may be either a regexp constant ('/'...'/') or
a string constant ('"'...'"'). In the latter case, the string is
treated as a regexp to be matched. *Note Computed Regexps:: for a
discussion of the difference between the two forms, and the
implications for writing your program correctly.
This function is peculiar because TARGET is not simply used to
compute a value, and not just any expression will do--it must be a
variable, field, or array element so that 'sub()' can store a
modified value there. If this argument is omitted, then the
default is to use and alter '$0'.(2) For example:
str = "water, water, everywhere"
sub(/at/, "ith", str)
sets 'str' to 'wither, water, everywhere', by replacing the
leftmost longest occurrence of 'at' with 'ith'.
If the special character '&' appears in REPLACEMENT, it stands for
the precise substring that was matched by REGEXP. (If the regexp
can match more than one string, then this precise substring may
vary.) For example:
{ sub(/candidate/, "& and his wife"); print }
changes the first occurrence of 'candidate' to 'candidate and his
wife' on each input line. Here is another example:
$ awk 'BEGIN {
> str = "daabaaa"
> sub(/a+/, "C&C", str)
> print str
> }'
-| dCaaCbaaa
This shows how '&' can represent a nonconstant string and also
illustrates the "leftmost, longest" rule in regexp matching (*note
Leftmost Longest::).
The effect of this special character ('&') can be turned off by
putting a backslash before it in the string. As usual, to insert
one backslash in the string, you must write two backslashes.
Therefore, write '\\&' in a string constant to include a literal
'&' in the replacement. For example, the following shows how to
replace the first '|' on each line with an '&':
{ sub(/\|/, "\\&"); print }
As mentioned, the third argument to 'sub()' must be a variable,
field, or array element. Some versions of 'awk' allow the third
argument to be an expression that is not an lvalue. In such a
case, 'sub()' still searches for the pattern and returns zero or
one, but the result of the substitution (if any) is thrown away
because there is no place to put it. Such versions of 'awk' accept
expressions like the following:
sub(/USA/, "United States", "the USA and Canada")
For historical compatibility, 'gawk' accepts such erroneous code.
However, using any other nonchangeable object as the third
parameter causes a fatal error and your program will not run.
Finally, if the REGEXP is not a regexp constant, it is converted
into a string, and then the value of that string is treated as the
regexp to match.
'substr(STRING, START' [', LENGTH' ]')'
Return a LENGTH-character-long substring of STRING, starting at
character number START. The first character of a string is
character number one.(3) For example, 'substr("washington", 5, 3)'
returns '"ing"'.
If LENGTH is not present, 'substr()' returns the whole suffix of
STRING that begins at character number START. For example,
'substr("washington", 5)' returns '"ington"'. The whole suffix is
also returned if LENGTH is greater than the number of characters
remaining in the string, counting from character START.
If START is less than one, 'substr()' treats it as if it was one.
(POSIX doesn't specify what to do in this case: BWK 'awk' acts this
way, and therefore 'gawk' does too.) If START is greater than the
number of characters in the string, 'substr()' returns the null
string. Similarly, if LENGTH is present but less than or equal to
zero, the null string is returned.
The string returned by 'substr()' _cannot_ be assigned. Thus, it
is a mistake to attempt to change a portion of a string, as shown
in the following example:
string = "abcdef"
# try to get "abCDEf", won't work
substr(string, 3, 3) = "CDE"
It is also a mistake to use 'substr()' as the third argument of
'sub()' or 'gsub()':
gsub(/xyz/, "pdq", substr($0, 5, 20)) # WRONG
(Some commercial versions of 'awk' treat 'substr()' as assignable,
but doing so is not portable.)
If you need to replace bits and pieces of a string, combine
'substr()' with string concatenation, in the following manner:
string = "abcdef"
...
string = substr(string, 1, 2) "CDE" substr(string, 6)
'tolower(STRING)'
Return a copy of STRING, with each uppercase character in the
string replaced with its corresponding lowercase character.
Nonalphabetic characters are left unchanged. For example,
'tolower("MiXeD cAsE 123")' returns '"mixed case 123"'.
'toupper(STRING)'
Return a copy of STRING, with each lowercase character in the
string replaced with its corresponding uppercase character.
Nonalphabetic characters are left unchanged. For example,
'toupper("MiXeD cAsE 123")' returns '"MIXED CASE 123"'.
Matching the Null String
In 'awk', the '*' operator can match the null string. This is
particularly important for the 'sub()', 'gsub()', and 'gensub()'
functions. For example:
$ echo abc | awk '{ gsub(/m*/, "X"); print }'
-| XaXbXcX
Although this makes a certain amount of sense, it can be surprising.
---------- Footnotes ----------
(1) Unless you use the '--non-decimal-data' option, which isn't
recommended. *Note Nondecimal Data:: for more information.
(2) Note that this means that the record will first be regenerated
using the value of 'OFS' if any fields have been changed, and that the
fields will be updated after the substitution, even if the operation is
a "no-op" such as 'sub(/^/, "")'.
(3) This is different from C and C++, in which the first character is
number zero.
File: gawk.info, Node: Gory Details, Up: String Functions
9.1.3.1 More about '\' and '&' with 'sub()', 'gsub()', and 'gensub()'
.....................................................................
CAUTION: This subsubsection has been reported to cause headaches.
You might want to skip it upon first reading.
When using 'sub()', 'gsub()', or 'gensub()', and trying to get
literal backslashes and ampersands into the replacement text, you need
to remember that there are several levels of "escape processing" going
on.
First, there is the "lexical" level, which is when 'awk' reads your
program and builds an internal copy of it to execute. Then there is the
runtime level, which is when 'awk' actually scans the replacement string
to determine what to generate.
At both levels, 'awk' looks for a defined set of characters that can
come after a backslash. At the lexical level, it looks for the escape
sequences listed in *note Escape Sequences::. Thus, for every '\' that
'awk' processes at the runtime level, you must type two backslashes at
the lexical level. When a character that is not valid for an escape
sequence follows the '\', BWK 'awk' and 'gawk' both simply remove the
initial '\' and put the next character into the string. Thus, for
example, '"a\qb"' is treated as '"aqb"'.
At the runtime level, the various functions handle sequences of '\'
and '&' differently. The situation is (sadly) somewhat complex.
Historically, the 'sub()' and 'gsub()' functions treated the
two-character sequence '\&' specially; this sequence was replaced in the
generated text with a single '&'. Any other '\' within the REPLACEMENT
string that did not precede an '&' was passed through unchanged. This
is illustrated in *note Table 9.1: table-sub-escapes.
You type 'sub()' sees 'sub()' generates
----- ------- ----------
'\&' '&' The matched text
'\\&' '\&' A literal '&'
'\\\&' '\&' A literal '&'
'\\\\&' '\\&' A literal '\&'
'\\\\\&' '\\&' A literal '\&'
'\\\\\\&' '\\\&' A literal '\\&'
'\\q' '\q' A literal '\q'
Table 9.1: Historical escape sequence processing for 'sub()' and
'gsub()'
This table shows the lexical-level processing, where an odd number of
backslashes becomes an even number at the runtime level, as well as the
runtime processing done by 'sub()'. (For the sake of simplicity, the
rest of the following tables only show the case of even numbers of
backslashes entered at the lexical level.)
The problem with the historical approach is that there is no way to
get a literal '\' followed by the matched text.
Several editions of the POSIX standard attempted to fix this problem
but weren't successful. The details are irrelevant at this point in
time.
At one point, the 'gawk' maintainer submitted proposed text for a
revised standard that reverts to rules that correspond more closely to
the original existing practice. The proposed rules have special cases
that make it possible to produce a '\' preceding the matched text. This
is shown in *note Table 9.2: table-sub-proposed.
You type 'sub()' sees 'sub()' generates
----- ------- ----------
'\\\\\\&' '\\\&' A literal '\&'
'\\\\&' '\\&' A literal '\', followed by the matched text
'\\&' '\&' A literal '&'
'\\q' '\q' A literal '\q'
'\\\\' '\\' '\\'
Table 9.2: 'gawk' rules for 'sub()' and backslash
In a nutshell, at the runtime level, there are now three special
sequences of characters ('\\\&', '\\&', and '\&') whereas historically
there was only one. However, as in the historical case, any '\' that is
not part of one of these three sequences is not special and appears in
the output literally.
'gawk' 3.0 and 3.1 follow these rules for 'sub()' and 'gsub()'. The
POSIX standard took much longer to be revised than was expected. In
addition, the 'gawk' maintainer's proposal was lost during the
standardization process. The final rules are somewhat simpler. The
results are similar except for one case.
The POSIX rules state that '\&' in the replacement string produces a
literal '&', '\\' produces a literal '\', and '\' followed by anything
else is not special; the '\' is placed straight into the output. These
rules are presented in *note Table 9.3: table-posix-sub.
You type 'sub()' sees 'sub()' generates
----- ------- ----------
'\\\\\\&' '\\\&' A literal '\&'
'\\\\&' '\\&' A literal '\', followed by the matched text
'\\&' '\&' A literal '&'
'\\q' '\q' A literal '\q'
'\\\\' '\\' '\'
Table 9.3: POSIX rules for 'sub()' and 'gsub()'
The only case where the difference is noticeable is the last one:
'\\\\' is seen as '\\' and produces '\' instead of '\\'.
Starting with version 3.1.4, 'gawk' followed the POSIX rules when
'--posix' was specified (*note Options::). Otherwise, it continued to
follow the proposed rules, as that had been its behavior for many years.
When version 4.0.0 was released, the 'gawk' maintainer made the POSIX
rules the default, breaking well over a decade's worth of backward
compatibility.(1) Needless to say, this was a bad idea, and as of
version 4.0.1, 'gawk' resumed its historical behavior, and only follows
the POSIX rules when '--posix' is given.
The rules for 'gensub()' are considerably simpler. At the runtime
level, whenever 'gawk' sees a '\', if the following character is a
digit, then the text that matched the corresponding parenthesized
subexpression is placed in the generated output. Otherwise, no matter
what character follows the '\', it appears in the generated text and the
'\' does not, as shown in *note Table 9.4: table-gensub-escapes.
You type 'gensub()' sees 'gensub()' generates
----- --------- ------------
'&' '&' The matched text
'\\&' '\&' A literal '&'
'\\\\' '\\' A literal '\'
'\\\\&' '\\&' A literal '\', then the matched text
'\\\\\\&' '\\\&' A literal '\&'
'\\q' '\q' A literal 'q'
Table 9.4: Escape sequence processing for 'gensub()'
Because of the complexity of the lexical- and runtime-level
processing and the special cases for 'sub()' and 'gsub()', we recommend
the use of 'gawk' and 'gensub()' when you have to do substitutions.
---------- Footnotes ----------
(1) This was rather naive of him, despite there being a note in this
minor node indicating that the next major version would move to the
POSIX rules.
File: gawk.info, Node: I/O Functions, Next: Time Functions, Prev: String Functions, Up: Built-in
9.1.4 Input/Output Functions
----------------------------
The following functions relate to input/output (I/O). Optional
parameters are enclosed in square brackets ([ ]):
'close('FILENAME [',' HOW]')'
Close the file FILENAME for input or output. Alternatively, the
argument may be a shell command that was used for creating a
coprocess, or for redirecting to or from a pipe; then the coprocess
or pipe is closed. *Note Close Files And Pipes:: for more
information.
When closing a coprocess, it is occasionally useful to first close
one end of the two-way pipe and then to close the other. This is
done by providing a second argument to 'close()'. This second
argument (HOW) should be one of the two string values '"to"' or
'"from"', indicating which end of the pipe to close. Case in the
string does not matter. *Note Two-way I/O::, which discusses this
feature in more detail and gives an example.
Note that the second argument to 'close()' is a 'gawk' extension;
it is not available in compatibility mode (*note Options::).
'fflush('[FILENAME]')'
Flush any buffered output associated with FILENAME, which is either
a file opened for writing or a shell command for redirecting output
to a pipe or coprocess.
Many utility programs "buffer" their output (i.e., they save
information to write to a disk file or the screen in memory until
there is enough for it to be worthwhile to send the data to the
output device). This is often more efficient than writing every
little bit of information as soon as it is ready. However,
sometimes it is necessary to force a program to "flush" its buffers
(i.e., write the information to its destination, even if a buffer
is not full). This is the purpose of the 'fflush()'
function--'gawk' also buffers its output, and the 'fflush()'
function forces 'gawk' to flush its buffers.
Brian Kernighan added 'fflush()' to his 'awk' in April 1992. For
two decades, it was a common extension. In December 2012, it was
accepted for inclusion into the POSIX standard. See the Austin
Group website (http://austingroupbugs.net/view.php?id=634).
POSIX standardizes 'fflush()' as follows: if there is no argument,
or if the argument is the null string ('""'), then 'awk' flushes
the buffers for _all_ open output files and pipes.
NOTE: Prior to version 4.0.2, 'gawk' would flush only the
standard output if there was no argument, and flush all output
files and pipes if the argument was the null string. This was
changed in order to be compatible with Brian Kernighan's
'awk', in the hope that standardizing this feature in POSIX
would then be easier (which indeed proved to be the case).
With 'gawk', you can use 'fflush("/dev/stdout")' if you wish
to flush only the standard output.
'fflush()' returns zero if the buffer is successfully flushed;
otherwise, it returns a nonzero value. ('gawk' returns -1.) In
the case where all buffers are flushed, the return value is zero
only if all buffers were flushed successfully. Otherwise, it is
-1, and 'gawk' warns about the problem FILENAME.
'gawk' also issues a warning message if you attempt to flush a file
or pipe that was opened for reading (such as with 'getline'), or if
FILENAME is not an open file, pipe, or coprocess. In such a case,
'fflush()' returns -1, as well.
Interactive Versus Noninteractive Buffering
As a side point, buffering issues can be even more confusing if your
program is "interactive" (i.e., communicating with a user sitting at a
keyboard).(1)
Interactive programs generally "line buffer" their output (i.e., they
write out every line). Noninteractive programs wait until they have a
full buffer, which may be many lines of output. Here is an example of
the difference:
$ awk '{ print $1 + $2 }'
1 1
-| 2
2 3
-| 5
Ctrl-d
Each line of output is printed immediately. Compare that behavior with
this example:
$ awk '{ print $1 + $2 }' | cat
1 1
2 3
Ctrl-d
-| 2
-| 5
Here, no output is printed until after the 'Ctrl-d' is typed, because it
is all buffered and sent down the pipe to 'cat' in one shot.
'system(COMMAND)'
Execute the operating system command COMMAND and then return to the
'awk' program. Return COMMAND's exit status (see further on).
For example, if the following fragment of code is put in your 'awk'
program:
END {
system("date | mail -s 'awk run done' root")
}
the system administrator is sent mail when the 'awk' program
finishes processing input and begins its end-of-input processing.
Note that redirecting 'print' or 'printf' into a pipe is often
enough to accomplish your task. If you need to run many commands,
it is more efficient to simply print them down a pipeline to the
shell:
while (MORE STUFF TO DO)
print COMMAND | "/bin/sh"
close("/bin/sh")
However, if your 'awk' program is interactive, 'system()' is useful
for running large self-contained programs, such as a shell or an
editor. Some operating systems cannot implement the 'system()'
function. 'system()' causes a fatal error if it is not supported.
NOTE: When '--sandbox' is specified, the 'system()' function
is disabled (*note Options::).
On POSIX systems, a command's exit status is a 16-bit number. The
exit value passed to the C 'exit()' function is held in the
high-order eight bits. The low-order bits indicate if the process
was killed by a signal (bit 7) and if so, the guilty signal number
(bits 0-6).
Traditionally, 'awk''s 'system()' function has simply returned the
exit status value divided by 256. In the normal case this gives
the exit status but in the case of death-by-signal it yields a
fractional floating-point value.(2) POSIX states that 'awk''s
'system()' should return the full 16-bit value.
'gawk' steers a middle ground. The return values are summarized in
*note Table 9.5: table-system-return-values.
Situation Return value from 'system()'
--------------------------------------------------------------------------
'--traditional' C 'system()''s value divided by 256
'--posix' C 'system()''s value
Normal exit of command Command's exit status
Death by signal of command 256 + number of murderous signal
Death by signal of command 512 + number of murderous signal
with core dump
Some kind of error -1
Table 9.5: Return values from 'system()'
Controlling Output Buffering with 'system()'
The 'fflush()' function provides explicit control over output
buffering for individual files and pipes. However, its use is not
portable to many older 'awk' implementations. An alternative method to
flush output buffers is to call 'system()' with a null string as its
argument:
system("") # flush output
'gawk' treats this use of the 'system()' function as a special case and
is smart enough not to run a shell (or other command interpreter) with
the empty command. Therefore, with 'gawk', this idiom is not only
useful, it is also efficient. Although this method should work with
other 'awk' implementations, it does not necessarily avoid starting an
unnecessary shell. (Other implementations may only flush the buffer
associated with the standard output and not necessarily all buffered
output.)
If you think about what a programmer expects, it makes sense that
'system()' should flush any pending output. The following program:
BEGIN {
print "first print"
system("echo system echo")
print "second print"
}
must print:
first print
system echo
second print
and not:
system echo
first print
second print
If 'awk' did not flush its buffers before calling 'system()', you
would see the latter (undesirable) output.
---------- Footnotes ----------
(1) A program is interactive if the standard output is connected to a
terminal device. On modern systems, this means your keyboard and
screen.
(2) In private correspondence, Dr. Kernighan has indicated to me that
the way this was done was probably a mistake.
File: gawk.info, Node: Time Functions, Next: Bitwise Functions, Prev: I/O Functions, Up: Built-in
9.1.5 Time Functions
--------------------
'awk' programs are commonly used to process log files containing
timestamp information, indicating when a particular log record was
written. Many programs log their timestamps in the form returned by the
'time()' system call, which is the number of seconds since a particular
epoch. On POSIX-compliant systems, it is the number of seconds since
1970-01-01 00:00:00 UTC, not counting leap seconds.(1) All known
POSIX-compliant systems support timestamps from 0 through 2^31 - 1,
which is sufficient to represent times through 2038-01-19 03:14:07 UTC.
Many systems support a wider range of timestamps, including negative
timestamps that represent times before the epoch.
In order to make it easier to process such log files and to produce
useful reports, 'gawk' provides the following functions for working with
timestamps. They are 'gawk' extensions; they are not specified in the
POSIX standard.(2) However, recent versions of 'mawk' (*note Other
Versions::) also support these functions. Optional parameters are
enclosed in square brackets ([ ]):
'mktime(DATESPEC' [', UTC-FLAG' ]')'
Turn DATESPEC into a timestamp in the same form as is returned by
'systime()'. It is similar to the function of the same name in ISO
C. The argument, DATESPEC, is a string of the form
'"YYYY MM DD HH MM SS [DST]"'. The string consists of six or seven
numbers representing, respectively, the full year including
century, the month from 1 to 12, the day of the month from 1 to 31,
the hour of the day from 0 to 23, the minute from 0 to 59, the
second from 0 to 60,(3) and an optional daylight-savings flag.
The values of these numbers need not be within the ranges
specified; for example, an hour of -1 means 1 hour before midnight.
The origin-zero Gregorian calendar is assumed, with year 0
preceding year 1 and year -1 preceding year 0. If UTC-FLAG is
present and is either nonzero or non-null, the time is assumed to
be in the UTC time zone; otherwise, the time is assumed to be in
the local time zone. If the DST daylight-savings flag is positive,
the time is assumed to be daylight savings time; if zero, the time
is assumed to be standard time; and if negative (the default),
'mktime()' attempts to determine whether daylight savings time is
in effect for the specified time.
If DATESPEC does not contain enough elements or if the resulting
time is out of range, 'mktime()' returns -1.
'strftime('[FORMAT [',' TIMESTAMP [',' UTC-FLAG] ] ]')'
Format the time specified by TIMESTAMP based on the contents of the
FORMAT string and return the result. It is similar to the function
of the same name in ISO C. If UTC-FLAG is present and is either
nonzero or non-null, the value is formatted as UTC (Coordinated
Universal Time, formerly GMT or Greenwich Mean Time). Otherwise,
the value is formatted for the local time zone. The TIMESTAMP is
in the same format as the value returned by the 'systime()'
function. If no TIMESTAMP argument is supplied, 'gawk' uses the
current time of day as the timestamp. Without a FORMAT argument,
'strftime()' uses the value of 'PROCINFO["strftime"]' as the format
string (*note Built-in Variables::). The default string value is
'"%a %b %e %H:%M:%S %Z %Y"'. This format string produces output
that is equivalent to that of the 'date' utility. You can assign a
new value to 'PROCINFO["strftime"]' to change the default format;
see the following list for the various format directives.
'systime()'
Return the current time as the number of seconds since the system
epoch. On POSIX systems, this is the number of seconds since
1970-01-01 00:00:00 UTC, not counting leap seconds. It may be a
different number on other systems.
The 'systime()' function allows you to compare a timestamp from a log
file with the current time of day. In particular, it is easy to
determine how long ago a particular record was logged. It also allows
you to produce log records using the "seconds since the epoch" format.
The 'mktime()' function allows you to convert a textual
representation of a date and time into a timestamp. This makes it easy
to do before/after comparisons of dates and times, particularly when
dealing with date and time data coming from an external source, such as
a log file.
The 'strftime()' function allows you to easily turn a timestamp into
human-readable information. It is similar in nature to the 'sprintf()'
function (*note String Functions::), in that it copies nonformat
specification characters verbatim to the returned string, while
substituting date and time values for format specifications in the
FORMAT string.
'strftime()' is guaranteed by the 1999 ISO C standard(4) to support
the following date format specifications:
'%a'
The locale's abbreviated weekday name.
'%A'
The locale's full weekday name.
'%b'
The locale's abbreviated month name.
'%B'
The locale's full month name.
'%c'
The locale's "appropriate" date and time representation. (This is
'%A %B %d %T %Y' in the '"C"' locale.)
'%C'
The century part of the current year. This is the year divided by
100 and truncated to the next lower integer.
'%d'
The day of the month as a decimal number (01-31).
'%D'
Equivalent to specifying '%m/%d/%y'.
'%e'
The day of the month, padded with a space if it is only one digit.
'%F'
Equivalent to specifying '%Y-%m-%d'. This is the ISO 8601 date
format.
'%g'
The year modulo 100 of the ISO 8601 week number, as a decimal
number (00-99). For example, January 1, 2012, is in week 53 of
2011. Thus, the year of its ISO 8601 week number is 2011, even
though its year is 2012. Similarly, December 31, 2012, is in week
1 of 2013. Thus, the year of its ISO week number is 2013, even
though its year is 2012.
'%G'
The full year of the ISO week number, as a decimal number.
'%h'
Equivalent to '%b'.
'%H'
The hour (24-hour clock) as a decimal number (00-23).
'%I'
The hour (12-hour clock) as a decimal number (01-12).
'%j'
The day of the year as a decimal number (001-366).
'%m'
The month as a decimal number (01-12).
'%M'
The minute as a decimal number (00-59).
'%n'
A newline character (ASCII LF).
'%p'
The locale's equivalent of the AM/PM designations associated with a
12-hour clock.
'%r'
The locale's 12-hour clock time. (This is '%I:%M:%S %p' in the
'"C"' locale.)
'%R'
Equivalent to specifying '%H:%M'.
'%S'
The second as a decimal number (00-60).
'%t'
A TAB character.
'%T'
Equivalent to specifying '%H:%M:%S'.
'%u'
The weekday as a decimal number (1-7). Monday is day one.
'%U'
The week number of the year (with the first Sunday as the first day
of week one) as a decimal number (00-53).
'%V'
The week number of the year (with the first Monday as the first day
of week one) as a decimal number (01-53). The method for
determining the week number is as specified by ISO 8601. (To wit:
if the week containing January 1 has four or more days in the new
year, then it is week one; otherwise it is the last week [52 or 53]
of the previous year and the next week is week one.)
'%w'
The weekday as a decimal number (0-6). Sunday is day zero.
'%W'
The week number of the year (with the first Monday as the first day
of week one) as a decimal number (00-53).
'%x'
The locale's "appropriate" date representation. (This is '%A %B %d
%Y' in the '"C"' locale.)
'%X'
The locale's "appropriate" time representation. (This is '%T' in
the '"C"' locale.)
'%y'
The year modulo 100 as a decimal number (00-99).
'%Y'
The full year as a decimal number (e.g., 2015).
'%z'
The time zone offset in a '+HHMM' format (e.g., the format
necessary to produce RFC 822/RFC 1036 date headers).
'%Z'
The time zone name or abbreviation; no characters if no time zone
is determinable.
'%Ec %EC %Ex %EX %Ey %EY %Od %Oe %OH'
'%OI %Om %OM %OS %Ou %OU %OV %Ow %OW %Oy'
"Alternative representations" for the specifications that use only
the second letter ('%c', '%C', and so on).(5) (These facilitate
compliance with the POSIX 'date' utility.)
'%%'
A literal '%'.
If a conversion specifier is not one of those just listed, the
behavior is undefined.(6)
For systems that are not yet fully standards-compliant, 'gawk'
supplies a copy of 'strftime()' from the GNU C Library. It supports all
of the just-listed format specifications. If that version is used to
compile 'gawk' (*note Installation::), then the following additional
format specifications are available:
'%k'
The hour (24-hour clock) as a decimal number (0-23). Single-digit
numbers are padded with a space.
'%l'
The hour (12-hour clock) as a decimal number (1-12). Single-digit
numbers are padded with a space.
'%s'
The time as a decimal timestamp in seconds since the epoch.
Additionally, the alternative representations are recognized but
their normal representations are used.
The following example is an 'awk' implementation of the POSIX 'date'
utility. Normally, the 'date' utility prints the current date and time
of day in a well-known format. However, if you provide an argument to
it that begins with a '+', 'date' copies nonformat specifier characters
to the standard output and interprets the current time according to the
format specifiers in the string. For example:
$ date '+Today is %A, %B %d, %Y.'
-| Today is Monday, September 22, 2014.
Here is the 'gawk' version of the 'date' utility. It has a shell
"wrapper" to handle the '-u' option, which requires that 'date' run as
if the time zone is set to UTC:
#! /bin/sh
#
# date --- approximate the POSIX 'date' command
case $1 in
-u) TZ=UTC0 # use UTC
export TZ
shift ;;
esac
gawk 'BEGIN {
format = PROCINFO["strftime"]
exitval = 0
if (ARGC > 2)
exitval = 1
else if (ARGC == 2) {
format = ARGV[1]
if (format ~ /^\+/)
format = substr(format, 2) # remove leading +
}
print strftime(format)
exit exitval
}' "$@"
---------- Footnotes ----------
(1) *Note Glossary::, especially the entries "Epoch" and "UTC."
(2) The GNU 'date' utility can also do many of the things described
here. Its use may be preferable for simple time-related operations in
shell scripts.
(3) Occasionally there are minutes in a year with a leap second,
which is why the seconds can go up to 60.
(4) Unfortunately, not every system's 'strftime()' necessarily
supports all of the conversions listed here.
(5) If you don't understand any of this, don't worry about it; these
facilities are meant to make it easier to "internationalize" programs.
Other internationalization features are described in *note
Internationalization::.
(6) This is because ISO C leaves the behavior of the C version of
'strftime()' undefined and 'gawk' uses the system's version of
'strftime()' if it's there. Typically, the conversion specifier either
does not appear in the returned string or appears literally.
File: gawk.info, Node: Bitwise Functions, Next: Type Functions, Prev: Time Functions, Up: Built-in
9.1.6 Bit-Manipulation Functions
--------------------------------
I can explain it for you, but I can't understand it for you.
-- _Anonymous_
Many languages provide the ability to perform "bitwise" operations on
two integer numbers. In other words, the operation is performed on each
successive pair of bits in the operands. Three common operations are
bitwise AND, OR, and XOR. The operations are described in *note Table
9.6: table-bitwise-ops.
Bit operator
| AND | OR | XOR
|---+---+---+---+---+---
Operands | 0 | 1 | 0 | 1 | 0 | 1
----------+---+---+---+---+---+---
0 | 0 0 | 0 1 | 0 1
1 | 0 1 | 1 1 | 1 0
Table 9.6: Bitwise operations
As you can see, the result of an AND operation is 1 only when _both_
bits are 1. The result of an OR operation is 1 if _either_ bit is 1.
The result of an XOR operation is 1 if either bit is 1, but not both.
The next operation is the "complement"; the complement of 1 is 0 and the
complement of 0 is 1. Thus, this operation "flips" all the bits of a
given value.
Finally, two other common operations are to shift the bits left or
right. For example, if you have a bit string '10111001' and you shift
it right by three bits, you end up with '00010111'.(1) If you start
over again with '10111001' and shift it left by three bits, you end up
with '11001000'. The following list describes 'gawk''s built-in
functions that implement the bitwise operations. Optional parameters
are enclosed in square brackets ([ ]):
'and(V1, V2 [, ...])'
Return the bitwise AND of the arguments. There must be at least
two.
'compl(VAL)'
Return the bitwise complement of VAL.
'lshift(VAL, COUNT)'
Return the value of VAL, shifted left by COUNT bits.
'or(V1, V2 [, ...])'
Return the bitwise OR of the arguments. There must be at least
two.
'rshift(VAL, COUNT)'
Return the value of VAL, shifted right by COUNT bits.
'xor(V1, V2 [, ...])'
Return the bitwise XOR of the arguments. There must be at least
two.
CAUTION: Beginning with 'gawk' version 4.2, negative operands are
not allowed for any of these functions. A negative operand
produces a fatal error. See the sidebar "Beware The Smoke and
Mirrors!" for more information as to why.
Here is a user-defined function (*note User-defined::) that
illustrates the use of these functions:
# bits2str --- turn an integer into readable ones and zeros
function bits2str(bits, data, mask)
{
if (bits == 0)
return "0"
mask = 1
for (; bits != 0; bits = rshift(bits, 1))
data = (and(bits, mask) ? "1" : "0") data
while ((length(data) % 8) != 0)
data = "0" data
return data
}
BEGIN {
printf "123 = %s\n", bits2str(123)
printf "0123 = %s\n", bits2str(0123)
printf "0x99 = %s\n", bits2str(0x99)
comp = compl(0x99)
printf "compl(0x99) = %#x = %s\n", comp, bits2str(comp)
shift = lshift(0x99, 2)
printf "lshift(0x99, 2) = %#x = %s\n", shift, bits2str(shift)
shift = rshift(0x99, 2)
printf "rshift(0x99, 2) = %#x = %s\n", shift, bits2str(shift)
}
This program produces the following output when run:
$ gawk -f testbits.awk
-| 123 = 01111011
-| 0123 = 01010011
-| 0x99 = 10011001
-| compl(0x99) = 0x3fffffffffff66 =
-| 00111111111111111111111111111111111111111111111101100110
-| lshift(0x99, 2) = 0x264 = 0000001001100100
-| rshift(0x99, 2) = 0x26 = 00100110
The 'bits2str()' function turns a binary number into a string.
Initializing 'mask' to one creates a binary value where the rightmost
bit is set to one. Using this mask, the function repeatedly checks the
rightmost bit. ANDing the mask with the value indicates whether the
rightmost bit is one or not. If so, a '"1"' is concatenated onto the
front of the string. Otherwise, a '"0"' is added. The value is then
shifted right by one bit and the loop continues until there are no more
one bits.
If the initial value is zero, it returns a simple '"0"'. Otherwise,
at the end, it pads the value with zeros to represent multiples of 8-bit
quantities. This is typical in modern computers.
The main code in the 'BEGIN' rule shows the difference between the
decimal and octal values for the same numbers (*note
Nondecimal-numbers::), and then demonstrates the results of the
'compl()', 'lshift()', and 'rshift()' functions.
Beware The Smoke and Mirrors!
It other languages, bitwise operations are performed on integer
values, not floating-point values. As a general statement, such
operations work best when performed on unsigned integers.
'gawk' attempts to treat the arguments to the bitwise functions as
unsigned integers. For this reason, negative arguments produce a fatal
error.
In normal operation, for all of these functions, first the
double-precision floating-point value is converted to the widest C
unsigned integer type, then the bitwise operation is performed. If the
result cannot be represented exactly as a C 'double', leading nonzero
bits are removed one by one until it can be represented exactly. The
result is then converted back into a C 'double'.(2)
However, when using arbitrary precision arithmetic with the '-M'
option (*note Arbitrary Precision Arithmetic::), the results may differ.
This is particularly noticeable with the 'compl()' function:
$ gawk 'BEGIN { print compl(42) }'
-| 9007199254740949
$ gawk -M 'BEGIN { print compl(42) }'
-| -43
What's going on becomes clear when printing the results in
hexadecimal:
$ gawk 'BEGIN { printf "%#x\n", compl(42) }'
-| 0x1fffffffffffd5
$ gawk -M 'BEGIN { printf "%#x\n", compl(42) }'
-| 0xffffffffffffffd5
When using the '-M' option, under the hood, 'gawk' uses GNU MP
arbitrary precision integers which have at least 64 bits of precision.
When not using '-M', 'gawk' stores integral values in regular
double-precision floating point, which only maintain 53 bits of
precision. Furthermore, the GNU MP library treats (or at least seems to
treat) the leading bit as a sign bit; thus the result with '-M' in this
case is a negative number.
In short, using 'gawk' for any but the simplest kind of bitwise
operations is probably a bad idea; caveat emptor!
---------- Footnotes ----------
(1) This example shows that zeros come in on the left side. For
'gawk', this is always true, but in some languages, it's possible to
have the left side fill with ones.
(2) If you don't understand this paragraph, the upshot is that 'gawk'
can only store a particular range of integer values; numbers outside
that range are reduced to fit within the range.
File: gawk.info, Node: Type Functions, Next: I18N Functions, Prev: Bitwise Functions, Up: Built-in
9.1.7 Getting Type Information
------------------------------
'gawk' provides two functions that let you distinguish the type of a
variable. This is necessary for writing code that traverses every
element of an array of arrays (*note Arrays of Arrays::), and in other
contexts.
'isarray(X)'
Return a true value if X is an array. Otherwise, return false.
'typeof(X)'
Return one of the following strings, depending upon the type of X:
'"array"'
X is an array.
'"regexp"'
X is a strongly typed regexp (*note Strong Regexp
Constants::).
'"number"'
X is a number.
'"string"'
X is a string.
'"strnum"'
X is a number that started life as user input, such as a field
or the result of calling 'split()'. (I.e., X has the strnum
attribute; *note Variable Typing::.)
'"unassigned"'
X is a scalar variable that has not been assigned a value yet.
For example:
BEGIN {
# creates a[1] but it has no assigned value
a[1]
print typeof(a[1]) # unassigned
}
'"untyped"'
X has not yet been used yet at all; it can become a scalar or
an array. The typing could even conceivably differ from run
to run of the same program! For example:
BEGIN {
print "initially, typeof(v) = ", typeof(v)
if ("FOO" in ENVIRON)
make_scalar(v)
else
make_array(v)
print "typeof(v) =", typeof(v)
}
function make_scalar(p, l) { l = p }
function make_array(p) { p[1] = 1 }
'isarray()' is meant for use in two circumstances. The first is when
traversing a multidimensional array: you can test if an element is
itself an array or not. The second is inside the body of a user-defined
function (not discussed yet; *note User-defined::), to test if a
parameter is an array or not.
NOTE: Using 'isarray()' at the global level to test variables makes
no sense. Because you are the one writing the program, you are
supposed to know if your variables are arrays or not. And in fact,
due to the way 'gawk' works, if you pass the name of a variable
that has not been previously used to 'isarray()', 'gawk' ends up
turning it into a scalar.
The 'typeof()' function is general; it allows you to determine if a
variable or function parameter is a scalar, an array, or a strongly
typed regexp.
File: gawk.info, Node: I18N Functions, Prev: Type Functions, Up: Built-in
9.1.8 String-Translation Functions
----------------------------------
'gawk' provides facilities for internationalizing 'awk' programs. These
include the functions described in the following list. The descriptions
here are purposely brief. *Note Internationalization::, for the full
story. Optional parameters are enclosed in square brackets ([ ]):
'bindtextdomain(DIRECTORY' [',' DOMAIN]')'
Set the directory in which 'gawk' will look for message translation
files, in case they will not or cannot be placed in the "standard"
locations (e.g., during testing). It returns the directory in
which DOMAIN is "bound."
The default DOMAIN is the value of 'TEXTDOMAIN'. If DIRECTORY is
the null string ('""'), then 'bindtextdomain()' returns the current
binding for the given DOMAIN.
'dcgettext(STRING' [',' DOMAIN [',' CATEGORY] ]')'
Return the translation of STRING in text domain DOMAIN for locale
category CATEGORY. The default value for DOMAIN is the current
value of 'TEXTDOMAIN'. The default value for CATEGORY is
'"LC_MESSAGES"'.
'dcngettext(STRING1, STRING2, NUMBER' [',' DOMAIN [',' CATEGORY] ]')'
Return the plural form used for NUMBER of the translation of
STRING1 and STRING2 in text domain DOMAIN for locale category
CATEGORY. STRING1 is the English singular variant of a message,
and STRING2 is the English plural variant of the same message. The
default value for DOMAIN is the current value of 'TEXTDOMAIN'. The
default value for CATEGORY is '"LC_MESSAGES"'.
File: gawk.info, Node: User-defined, Next: Indirect Calls, Prev: Built-in, Up: Functions
9.2 User-Defined Functions
==========================
Complicated 'awk' programs can often be simplified by defining your own
functions. User-defined functions can be called just like built-in ones
(*note Function Calls::), but it is up to you to define them (i.e., to
tell 'awk' what they should do).
* Menu:
* Definition Syntax:: How to write definitions and what they mean.
* Function Example:: An example function definition and what it
does.
* Function Caveats:: Things to watch out for.
* Return Statement:: Specifying the value a function returns.
* Dynamic Typing:: How variable types can change at runtime.
File: gawk.info, Node: Definition Syntax, Next: Function Example, Up: User-defined
9.2.1 Function Definition Syntax
--------------------------------
It's entirely fair to say that the awk syntax for local variable
definitions is appallingly awful.
-- _Brian Kernighan_
Definitions of functions can appear anywhere between the rules of an
'awk' program. Thus, the general form of an 'awk' program is extended
to include sequences of rules _and_ user-defined function definitions.
There is no need to put the definition of a function before all uses of
the function. This is because 'awk' reads the entire program before
starting to execute any of it.
The definition of a function named NAME looks like this:
'function' NAME'('[PARAMETER-LIST]')'
'{'
BODY-OF-FUNCTION
'}'
Here, NAME is the name of the function to define. A valid function name
is like a valid variable name: a sequence of letters, digits, and
underscores that doesn't start with a digit. Here too, only the 52
upper- and lowercase English letters may be used in a function name.
Within a single 'awk' program, any particular name can only be used as a
variable, array, or function.
PARAMETER-LIST is an optional list of the function's arguments and
local variable names, separated by commas. When the function is called,
the argument names are used to hold the argument values given in the
call.
A function cannot have two parameters with the same name, nor may it
have a parameter with the same name as the function itself.
CAUTION: According to the POSIX standard, function parameters
cannot have the same name as one of the special predefined
variables (*note Built-in Variables::), nor may a function
parameter have the same name as another function.
Not all versions of 'awk' enforce these restrictions. 'gawk'
always enforces the first restriction. With '--posix' (*note
Options::), it also enforces the second restriction.
Local variables act like the empty string if referenced where a
string value is required, and like zero if referenced where a numeric
value is required. This is the same as the behavior of regular
variables that have never been assigned a value. (There is more to
understand about local variables; *note Dynamic Typing::.)
The BODY-OF-FUNCTION consists of 'awk' statements. It is the most
important part of the definition, because it says what the function
should actually _do_. The argument names exist to give the body a way
to talk about the arguments; local variables exist to give the body
places to keep temporary values.
Argument names are not distinguished syntactically from local
variable names. Instead, the number of arguments supplied when the
function is called determines how many argument variables there are.
Thus, if three argument values are given, the first three names in
PARAMETER-LIST are arguments and the rest are local variables.
It follows that if the number of arguments is not the same in all
calls to the function, some of the names in PARAMETER-LIST may be
arguments on some occasions and local variables on others. Another way
to think of this is that omitted arguments default to the null string.
Usually when you write a function, you know how many names you intend
to use for arguments and how many you intend to use as local variables.
It is conventional to place some extra space between the arguments and
the local variables, in order to document how your function is supposed
to be used.
During execution of the function body, the arguments and local
variable values hide, or "shadow", any variables of the same names used
in the rest of the program. The shadowed variables are not accessible
in the function definition, because there is no way to name them while
their names have been taken away for the arguments and local variables.
All other variables used in the 'awk' program can be referenced or set
normally in the function's body.
The arguments and local variables last only as long as the function
body is executing. Once the body finishes, you can once again access
the variables that were shadowed while the function was running.
The function body can contain expressions that call functions. They
can even call this function, either directly or by way of another
function. When this happens, we say the function is "recursive". The
act of a function calling itself is called "recursion".
All the built-in functions return a value to their caller.
User-defined functions can do so also, using the 'return' statement,
which is described in detail in *note Return Statement::. Many of the
subsequent examples in this minor node use the 'return' statement.
In many 'awk' implementations, including 'gawk', the keyword
'function' may be abbreviated 'func'. (c.e.) However, POSIX only
specifies the use of the keyword 'function'. This actually has some
practical implications. If 'gawk' is in POSIX-compatibility mode (*note
Options::), then the following statement does _not_ define a function:
func foo() { a = sqrt($1) ; print a }
Instead, it defines a rule that, for each record, concatenates the value
of the variable 'func' with the return value of the function 'foo'. If
the resulting string is non-null, the action is executed. This is
probably not what is desired. ('awk' accepts this input as
syntactically valid, because functions may be used before they are
defined in 'awk' programs.(1))
To ensure that your 'awk' programs are portable, always use the
keyword 'function' when defining a function.
---------- Footnotes ----------
(1) This program won't actually run, because 'foo()' is undefined.
File: gawk.info, Node: Function Example, Next: Function Caveats, Prev: Definition Syntax, Up: User-defined
9.2.2 Function Definition Examples
----------------------------------
Here is an example of a user-defined function, called 'myprint()', that
takes a number and prints it in a specific format:
function myprint(num)
{
printf "%6.3g\n", num
}
To illustrate, here is an 'awk' rule that uses our 'myprint()' function:
$3 > 0 { myprint($3) }
This program prints, in our special format, all the third fields that
contain a positive number in our input. Therefore, when given the
following input:
1.2 3.4 5.6 7.8
9.10 11.12 -13.14 15.16
17.18 19.20 21.22 23.24
this program, using our function to format the results, prints:
5.6
21.2
This function deletes all the elements in an array (recall that the
extra whitespace signifies the start of the local variable list):
function delarray(a, i)
{
for (i in a)
delete a[i]
}
When working with arrays, it is often necessary to delete all the
elements in an array and start over with a new list of elements (*note
Delete::). Instead of having to repeat this loop everywhere that you
need to clear out an array, your program can just call 'delarray()'.
(This guarantees portability. The use of 'delete ARRAY' to delete the
contents of an entire array is a relatively recent(1) addition to the
POSIX standard.)
The following is an example of a recursive function. It takes a
string as an input parameter and returns the string in reverse order.
Recursive functions must always have a test that stops the recursion.
In this case, the recursion terminates when the input string is already
empty:
function rev(str)
{
if (str == "")
return ""
return (rev(substr(str, 2)) substr(str, 1, 1))
}
If this function is in a file named 'rev.awk', it can be tested this
way:
$ echo "Don't Panic!" |
> gawk -e '{ print rev($0) }' -f rev.awk
-| !cinaP t'noD
The C 'ctime()' function takes a timestamp and returns it as a
string, formatted in a well-known fashion. The following example uses
the built-in 'strftime()' function (*note Time Functions::) to create an
'awk' version of 'ctime()':
# ctime.awk
#
# awk version of C ctime(3) function
function ctime(ts, format)
{
format = "%a %b %e %H:%M:%S %Z %Y"
if (ts == 0)
ts = systime() # use current time as default
return strftime(format, ts)
}
You might think that 'ctime()' could use 'PROCINFO["strftime"]' for
its format string. That would be a mistake, because 'ctime()' is
supposed to return the time formatted in a standard fashion, and
user-level code could have changed 'PROCINFO["strftime"]'.
---------- Footnotes ----------
(1) Late in 2012.
File: gawk.info, Node: Function Caveats, Next: Return Statement, Prev: Function Example, Up: User-defined
9.2.3 Calling User-Defined Functions
------------------------------------
"Calling a function" means causing the function to run and do its job.
A function call is an expression and its value is the value returned by
the function.
* Menu:
* Calling A Function:: Don't use spaces.
* Variable Scope:: Controlling variable scope.
* Pass By Value/Reference:: Passing parameters.
File: gawk.info, Node: Calling A Function, Next: Variable Scope, Up: Function Caveats
9.2.3.1 Writing a Function Call
...............................
A function call consists of the function name followed by the arguments
in parentheses. 'awk' expressions are what you write in the call for
the arguments. Each time the call is executed, these expressions are
evaluated, and the values become the actual arguments. For example,
here is a call to 'foo()' with three arguments (the first being a string
concatenation):
foo(x y, "lose", 4 * z)
CAUTION: Whitespace characters (spaces and TABs) are not allowed
between the function name and the opening parenthesis of the
argument list. If you write whitespace by mistake, 'awk' might
think that you mean to concatenate a variable with an expression in
parentheses. However, it notices that you used a function name and
not a variable name, and reports an error.
File: gawk.info, Node: Variable Scope, Next: Pass By Value/Reference, Prev: Calling A Function, Up: Function Caveats
9.2.3.2 Controlling Variable Scope
..................................
Unlike in many languages, there is no way to make a variable local to a
'{' ... '}' block in 'awk', but you can make a variable local to a
function. It is good practice to do so whenever a variable is needed
only in that function.
To make a variable local to a function, simply declare the variable
as an argument after the actual function arguments (*note Definition
Syntax::). Look at the following example, where variable 'i' is a
global variable used by both functions 'foo()' and 'bar()':
function bar()
{
for (i = 0; i < 3; i++)
print "bar's i=" i
}
function foo(j)
{
i = j + 1
print "foo's i=" i
bar()
print "foo's i=" i
}
BEGIN {
i = 10
print "top's i=" i
foo(0)
print "top's i=" i
}
Running this script produces the following, because the 'i' in
functions 'foo()' and 'bar()' and at the top level refer to the same
variable instance:
top's i=10
foo's i=1
bar's i=0
bar's i=1
bar's i=2
foo's i=3
top's i=3
If you want 'i' to be local to both 'foo()' and 'bar()', do as
follows (the extra space before 'i' is a coding convention to indicate
that 'i' is a local variable, not an argument):
function bar( i)
{
for (i = 0; i < 3; i++)
print "bar's i=" i
}
function foo(j, i)
{
i = j + 1
print "foo's i=" i
bar()
print "foo's i=" i
}
BEGIN {
i = 10
print "top's i=" i
foo(0)
print "top's i=" i
}
Running the corrected script produces the following:
top's i=10
foo's i=1
bar's i=0
bar's i=1
bar's i=2
foo's i=1
top's i=10
Besides scalar values (strings and numbers), you may also have local
arrays. By using a parameter name as an array, 'awk' treats it as an
array, and it is local to the function. In addition, recursive calls
create new arrays. Consider this example:
function some_func(p1, a)
{
if (p1++ > 3)
return
a[p1] = p1
some_func(p1)
printf("At level %d, index %d %s found in a\n",
p1, (p1 - 1), (p1 - 1) in a ? "is" : "is not")
printf("At level %d, index %d %s found in a\n",
p1, p1, p1 in a ? "is" : "is not")
print ""
}
BEGIN {
some_func(1)
}
When run, this program produces the following output:
At level 4, index 3 is not found in a
At level 4, index 4 is found in a
At level 3, index 2 is not found in a
At level 3, index 3 is found in a
At level 2, index 1 is not found in a
At level 2, index 2 is found in a
File: gawk.info, Node: Pass By Value/Reference, Prev: Variable Scope, Up: Function Caveats
9.2.3.3 Passing Function Arguments by Value Or by Reference
...........................................................
In 'awk', when you declare a function, there is no way to declare
explicitly whether the arguments are passed "by value" or "by
reference".
Instead, the passing convention is determined at runtime when the
function is called, according to the following rule: if the argument is
an array variable, then it is passed by reference. Otherwise, the
argument is passed by value.
Passing an argument by value means that when a function is called, it
is given a _copy_ of the value of this argument. The caller may use a
variable as the expression for the argument, but the called function
does not know this--it only knows what value the argument had. For
example, if you write the following code:
foo = "bar"
z = myfunc(foo)
then you should not think of the argument to 'myfunc()' as being "the
variable 'foo'." Instead, think of the argument as the string value
'"bar"'. If the function 'myfunc()' alters the values of its local
variables, this has no effect on any other variables. Thus, if
'myfunc()' does this:
function myfunc(str)
{
print str
str = "zzz"
print str
}
to change its first argument variable 'str', it does _not_ change the
value of 'foo' in the caller. The role of 'foo' in calling 'myfunc()'
ended when its value ('"bar"') was computed. If 'str' also exists
outside of 'myfunc()', the function body cannot alter this outer value,
because it is shadowed during the execution of 'myfunc()' and cannot be
seen or changed from there.
However, when arrays are the parameters to functions, they are _not_
copied. Instead, the array itself is made available for direct
manipulation by the function. This is usually termed "call by
reference". Changes made to an array parameter inside the body of a
function _are_ visible outside that function.
NOTE: Changing an array parameter inside a function can be very
dangerous if you do not watch what you are doing. For example:
function changeit(array, ind, nvalue)
{
array[ind] = nvalue
}
BEGIN {
a[1] = 1; a[2] = 2; a[3] = 3
changeit(a, 2, "two")
printf "a[1] = %s, a[2] = %s, a[3] = %s\n",
a[1], a[2], a[3]
}
prints 'a[1] = 1, a[2] = two, a[3] = 3', because 'changeit()'
stores '"two"' in the second element of 'a'.
Some 'awk' implementations allow you to call a function that has not
been defined. They only report a problem at runtime, when the program
actually tries to call the function. For example:
BEGIN {
if (0)
foo()
else
bar()
}
function bar() { ... }
# note that `foo' is not defined
Because the 'if' statement will never be true, it is not really a
problem that 'foo()' has not been defined. Usually, though, it is a
problem if a program calls an undefined function.
If '--lint' is specified (*note Options::), 'gawk' reports calls to
undefined functions.
Some 'awk' implementations generate a runtime error if you use either
the 'next' statement or the 'nextfile' statement (*note Next
Statement::, and *note Nextfile Statement::) inside a user-defined
function. 'gawk' does not have this limitation.
File: gawk.info, Node: Return Statement, Next: Dynamic Typing, Prev: Function Caveats, Up: User-defined
9.2.4 The 'return' Statement
----------------------------
As seen in several earlier examples, the body of a user-defined function
can contain a 'return' statement. This statement returns control to the
calling part of the 'awk' program. It can also be used to return a
value for use in the rest of the 'awk' program. It looks like this:
'return' [EXPRESSION]
The EXPRESSION part is optional. Due most likely to an oversight,
POSIX does not define what the return value is if you omit the
EXPRESSION. Technically speaking, this makes the returned value
undefined, and therefore, unpredictable. In practice, though, all
versions of 'awk' simply return the null string, which acts like zero if
used in a numeric context.
A 'return' statement without an EXPRESSION is assumed at the end of
every function definition. So, if control reaches the end of the
function body, then technically the function returns an unpredictable
value. In practice, it returns the empty string. 'awk' does _not_ warn
you if you use the return value of such a function.
Sometimes, you want to write a function for what it does, not for
what it returns. Such a function corresponds to a 'void' function in C,
C++, or Java, or to a 'procedure' in Ada. Thus, it may be appropriate
to not return any value; simply bear in mind that you should not be
using the return value of such a function.
The following is an example of a user-defined function that returns a
value for the largest number among the elements of an array:
function maxelt(vec, i, ret)
{
for (i in vec) {
if (ret == "" || vec[i] > ret)
ret = vec[i]
}
return ret
}
You call 'maxelt()' with one argument, which is an array name. The
local variables 'i' and 'ret' are not intended to be arguments; there is
nothing to stop you from passing more than one argument to 'maxelt()'
but the results would be strange. The extra space before 'i' in the
function parameter list indicates that 'i' and 'ret' are local
variables. You should follow this convention when defining functions.
The following program uses the 'maxelt()' function. It loads an
array, calls 'maxelt()', and then reports the maximum number in that
array:
function maxelt(vec, i, ret)
{
for (i in vec) {
if (ret == "" || vec[i] > ret)
ret = vec[i]
}
return ret
}
# Load all fields of each record into nums.
{
for(i = 1; i <= NF; i++)
nums[NR, i] = $i
}
END {
print maxelt(nums)
}
Given the following input:
1 5 23 8 16
44 3 5 2 8 26
256 291 1396 2962 100
-6 467 998 1101
99385 11 0 225
the program reports (predictably) that 99,385 is the largest value in
the array.
File: gawk.info, Node: Dynamic Typing, Prev: Return Statement, Up: User-defined
9.2.5 Functions and Their Effects on Variable Typing
----------------------------------------------------
'awk' is a very fluid language. It is possible that 'awk' can't tell if
an identifier represents a scalar variable or an array until runtime.
Here is an annotated sample program:
function foo(a)
{
a[1] = 1 # parameter is an array
}
BEGIN {
b = 1
foo(b) # invalid: fatal type mismatch
foo(x) # x uninitialized, becomes an array dynamically
x = 1 # now not allowed, runtime error
}
In this example, the first call to 'foo()' generates a fatal error,
so 'awk' will not report the second error. If you comment out that
call, though, then 'awk' does report the second error.
Usually, such things aren't a big issue, but it's worth being aware
of them.
File: gawk.info, Node: Indirect Calls, Next: Functions Summary, Prev: User-defined, Up: Functions
9.3 Indirect Function Calls
===========================
This section describes an advanced, 'gawk'-specific extension.
Often, you may wish to defer the choice of function to call until
runtime. For example, you may have different kinds of records, each of
which should be processed differently.
Normally, you would have to use a series of 'if'-'else' statements to
decide which function to call. By using "indirect" function calls, you
can specify the name of the function to call as a string variable, and
then call the function. Let's look at an example.
Suppose you have a file with your test scores for the classes you are
taking, and you wish to get the sum and the average of your test scores.
The first field is the class name. The following fields are the
functions to call to process the data, up to a "marker" field 'data:'.
Following the marker, to the end of the record, are the various numeric
test scores.
Here is the initial file:
Biology_101 sum average data: 87.0 92.4 78.5 94.9
Chemistry_305 sum average data: 75.2 98.3 94.7 88.2
English_401 sum average data: 100.0 95.6 87.1 93.4
To process the data, you might write initially:
{
class = $1
for (i = 2; $i != "data:"; i++) {
if ($i == "sum")
sum() # processes the whole record
else if ($i == "average")
average()
... # and so on
}
}
This style of programming works, but can be awkward. With "indirect"
function calls, you tell 'gawk' to use the _value_ of a variable as the
_name_ of the function to call.
The syntax is similar to that of a regular function call: an
identifier immediately followed by an opening parenthesis, any
arguments, and then a closing parenthesis, with the addition of a
leading '@' character:
the_func = "sum"
result = @the_func() # calls the sum() function
Here is a full program that processes the previously shown data,
using indirect function calls:
# indirectcall.awk --- Demonstrate indirect function calls
# average --- return the average of the values in fields $first - $last
function average(first, last, sum, i)
{
sum = 0;
for (i = first; i <= last; i++)
sum += $i
return sum / (last - first + 1)
}
# sum --- return the sum of the values in fields $first - $last
function sum(first, last, ret, i)
{
ret = 0;
for (i = first; i <= last; i++)
ret += $i
return ret
}
These two functions expect to work on fields; thus, the parameters
'first' and 'last' indicate where in the fields to start and end.
Otherwise, they perform the expected computations and are not unusual:
# For each record, print the class name and the requested statistics
{
class_name = $1
gsub(/_/, " ", class_name) # Replace _ with spaces
# find start
for (i = 1; i <= NF; i++) {
if ($i == "data:") {
start = i + 1
break
}
}
printf("%s:\n", class_name)
for (i = 2; $i != "data:"; i++) {
the_function = $i
printf("\t%s: <%s>\n", $i, @the_function(start, NF) "")
}
print ""
}
This is the main processing for each record. It prints the class
name (with underscores replaced with spaces). It then finds the start
of the actual data, saving it in 'start'. The last part of the code
loops through each function name (from '$2' up to the marker, 'data:'),
calling the function named by the field. The indirect function call
itself occurs as a parameter in the call to 'printf'. (The 'printf'
format string uses '%s' as the format specifier so that we can use
functions that return strings, as well as numbers. Note that the result
from the indirect call is concatenated with the empty string, in order
to force it to be a string value.)
Here is the result of running the program:
$ gawk -f indirectcall.awk class_data1
-| Biology 101:
-| sum: <352.8>
-| average: <88.2>
-|
-| Chemistry 305:
-| sum: <356.4>
-| average: <89.1>
-|
-| English 401:
-| sum: <376.1>
-| average: <94.025>
The ability to use indirect function calls is more powerful than you
may think at first. The C and C++ languages provide "function
pointers," which are a mechanism for calling a function chosen at
runtime. One of the most well-known uses of this ability is the C
'qsort()' function, which sorts an array using the famous "quicksort"
algorithm (see the Wikipedia article
(https://en.wikipedia.org/wiki/Quicksort) for more information). To use
this function, you supply a pointer to a comparison function. This
mechanism allows you to sort arbitrary data in an arbitrary fashion.
We can do something similar using 'gawk', like this:
# quicksort.awk --- Quicksort algorithm, with user-supplied
# comparison function
# quicksort --- C.A.R. Hoare's quicksort algorithm. See Wikipedia
# or almost any algorithms or computer science text.
function quicksort(data, left, right, less_than, i, last)
{
if (left >= right) # do nothing if array contains fewer
return # than two elements
quicksort_swap(data, left, int((left + right) / 2))
last = left
for (i = left + 1; i <= right; i++)
if (@less_than(data[i], data[left]))
quicksort_swap(data, ++last, i)
quicksort_swap(data, left, last)
quicksort(data, left, last - 1, less_than)
quicksort(data, last + 1, right, less_than)
}
# quicksort_swap --- helper function for quicksort, should really be inline
function quicksort_swap(data, i, j, temp)
{
temp = data[i]
data[i] = data[j]
data[j] = temp
}
The 'quicksort()' function receives the 'data' array, the starting
and ending indices to sort ('left' and 'right'), and the name of a
function that performs a "less than" comparison. It then implements the
quicksort algorithm.
To make use of the sorting function, we return to our previous
example. The first thing to do is write some comparison functions:
# num_lt --- do a numeric less than comparison
function num_lt(left, right)
{
return ((left + 0) < (right + 0))
}
# num_ge --- do a numeric greater than or equal to comparison
function num_ge(left, right)
{
return ((left + 0) >= (right + 0))
}
The 'num_ge()' function is needed to perform a descending sort; when
used to perform a "less than" test, it actually does the opposite
(greater than or equal to), which yields data sorted in descending
order.
Next comes a sorting function. It is parameterized with the starting
and ending field numbers and the comparison function. It builds an
array with the data and calls 'quicksort()' appropriately, and then
formats the results as a single string:
# do_sort --- sort the data according to `compare'
# and return it as a string
function do_sort(first, last, compare, data, i, retval)
{
delete data
for (i = 1; first <= last; first++) {
data[i] = $first
i++
}
quicksort(data, 1, i-1, compare)
retval = data[1]
for (i = 2; i in data; i++)
retval = retval " " data[i]
return retval
}
Finally, the two sorting functions call 'do_sort()', passing in the
names of the two comparison functions:
# sort --- sort the data in ascending order and return it as a string
function sort(first, last)
{
return do_sort(first, last, "num_lt")
}
# rsort --- sort the data in descending order and return it as a string
function rsort(first, last)
{
return do_sort(first, last, "num_ge")
}
Here is an extended version of the data file:
Biology_101 sum average sort rsort data: 87.0 92.4 78.5 94.9
Chemistry_305 sum average sort rsort data: 75.2 98.3 94.7 88.2
English_401 sum average sort rsort data: 100.0 95.6 87.1 93.4
Finally, here are the results when the enhanced program is run:
$ gawk -f quicksort.awk -f indirectcall.awk class_data2
-| Biology 101:
-| sum: <352.8>
-| average: <88.2>
-| sort: <78.5 87.0 92.4 94.9>
-| rsort: <94.9 92.4 87.0 78.5>
-|
-| Chemistry 305:
-| sum: <356.4>
-| average: <89.1>
-| sort: <75.2 88.2 94.7 98.3>
-| rsort: <98.3 94.7 88.2 75.2>
-|
-| English 401:
-| sum: <376.1>
-| average: <94.025>
-| sort: <87.1 93.4 95.6 100.0>
-| rsort: <100.0 95.6 93.4 87.1>
Another example where indirect functions calls are useful can be
found in processing arrays. This is described in *note Walking
Arrays::.
Remember that you must supply a leading '@' in front of an indirect
function call.
Starting with version 4.1.2 of 'gawk', indirect function calls may
also be used with built-in functions and with extension functions (*note
Dynamic Extensions::). There are some limitations when calling built-in
functions indirectly, as follows.
* You cannot pass a regular expression constant to a built-in
function through an indirect function call.(1) This applies to the
'sub()', 'gsub()', 'gensub()', 'match()', 'split()' and
'patsplit()' functions.
* If calling 'sub()' or 'gsub()', you may only pass two arguments,
since those functions are unusual in that they update their third
argument. This means that '$0' will be updated.
'gawk' does its best to make indirect function calls efficient. For
example, in the following case:
for (i = 1; i <= n; i++)
@the_func()
'gawk' looks up the actual function to call only once.
---------- Footnotes ----------
(1) This may change in a future version; recheck the documentation
that comes with your version of 'gawk' to see if it has.
File: gawk.info, Node: Functions Summary, Prev: Indirect Calls, Up: Functions
9.4 Summary
===========
* 'awk' provides built-in functions and lets you define your own
functions.
* POSIX 'awk' provides three kinds of built-in functions: numeric,
string, and I/O. 'gawk' provides functions that sort arrays, work
with values representing time, do bit manipulation, determine
variable type (array versus scalar), and internationalize and
localize programs. 'gawk' also provides several extensions to some
of standard functions, typically in the form of additional
arguments.
* Functions accept zero or more arguments and return a value. The
expressions that provide the argument values are completely
evaluated before the function is called. Order of evaluation is
not defined. The return value can be ignored.
* The handling of backslash in 'sub()' and 'gsub()' is not simple.
It is more straightforward in 'gawk''s 'gensub()' function, but
that function still requires care in its use.
* User-defined functions provide important capabilities but come with
some syntactic inelegancies. In a function call, there cannot be
any space between the function name and the opening left
parenthesis of the argument list. Also, there is no provision for
local variables, so the convention is to add extra parameters, and
to separate them visually from the real parameters by extra
whitespace.
* User-defined functions may call other user-defined (and built-in)
functions and may call themselves recursively. Function parameters
"hide" any global variables of the same names. You cannot use the
name of a reserved variable (such as 'ARGC') as the name of a
parameter in user-defined functions.
* Scalar values are passed to user-defined functions by value. Array
parameters are passed by reference; any changes made by the
function to array parameters are thus visible after the function
has returned.
* Use the 'return' statement to return from a user-defined function.
An optional expression becomes the function's return value. Only
scalar values may be returned by a function.
* If a variable that has never been used is passed to a user-defined
function, how that function treats the variable can set its nature:
either scalar or array.
* 'gawk' provides indirect function calls using a special syntax. By
setting a variable to the name of a function, you can determine at
runtime what function will be called at that point in the program.
This is equivalent to function pointers in C and C++.
File: gawk.info, Node: Library Functions, Next: Sample Programs, Prev: Functions, Up: Top
10 A Library of 'awk' Functions
*******************************
*note User-defined:: describes how to write your own 'awk' functions.
Writing functions is important, because it allows you to encapsulate
algorithms and program tasks in a single place. It simplifies
programming, making program development more manageable and making
programs more readable.
In their seminal 1976 book, 'Software Tools',(1) Brian Kernighan and
P.J. Plauger wrote:
Good Programming is not learned from generalities, but by seeing
how significant programs can be made clean, easy to read, easy to
maintain and modify, human-engineered, efficient and reliable, by
the application of common sense and good programming practices.
Careful study and imitation of good programs leads to better
writing.
In fact, they felt this idea was so important that they placed this
statement on the cover of their book. Because we believe strongly that
their statement is correct, this major node and *note Sample Programs::,
provide a good-sized body of code for you to read and, we hope, to learn
from.
This major node presents a library of useful 'awk' functions. Many
of the sample programs presented later in this Info file use these
functions. The functions are presented here in a progression from
simple to complex.
*note Extract Program:: presents a program that you can use to
extract the source code for these example library functions and programs
from the Texinfo source for this Info file. (This has already been done
as part of the 'gawk' distribution.)
If you have written one or more useful, general-purpose 'awk'
functions and would like to contribute them to the 'awk' user community,
see *note How To Contribute::, for more information.
The programs in this major node and in *note Sample Programs::,
freely use 'gawk'-specific features. Rewriting these programs for
different implementations of 'awk' is pretty straightforward:
* Diagnostic error messages are sent to '/dev/stderr'. Use '| "cat
1>&2"' instead of '> "/dev/stderr"' if your system does not have a
'/dev/stderr', or if you cannot use 'gawk'.
* A number of programs use 'nextfile' (*note Nextfile Statement::) to
skip any remaining input in the input file.
* Finally, some of the programs choose to ignore upper- and lowercase
distinctions in their input. They do so by assigning one to
'IGNORECASE'. You can achieve almost the same effect(2) by adding
the following rule to the beginning of the program:
# ignore case
{ $0 = tolower($0) }
Also, verify that all regexp and string constants used in
comparisons use only lowercase letters.
* Menu:
* Library Names:: How to best name private global variables in
library functions.
* General Functions:: Functions that are of general use.
* Data File Management:: Functions for managing command-line data
files.
* Getopt Function:: A function for processing command-line
arguments.
* Passwd Functions:: Functions for getting user information.
* Group Functions:: Functions for getting group information.
* Walking Arrays:: A function to walk arrays of arrays.
* Library Functions Summary:: Summary of library functions.
* Library Exercises:: Exercises.
---------- Footnotes ----------
(1) Sadly, over 35 years later, many of the lessons taught by this
book have yet to be learned by a vast number of practicing programmers.
(2) The effects are not identical. Output of the transformed record
will be in all lowercase, while 'IGNORECASE' preserves the original
contents of the input record.
File: gawk.info, Node: Library Names, Next: General Functions, Up: Library Functions
10.1 Naming Library Function Global Variables
=============================================
Due to the way the 'awk' language evolved, variables are either "global"
(usable by the entire program) or "local" (usable just by a specific
function). There is no intermediate state analogous to 'static'
variables in C.
Library functions often need to have global variables that they can
use to preserve state information between calls to the function--for
example, 'getopt()''s variable '_opti' (*note Getopt Function::). Such
variables are called "private", as the only functions that need to use
them are the ones in the library.
When writing a library function, you should try to choose names for
your private variables that will not conflict with any variables used by
either another library function or a user's main program. For example,
a name like 'i' or 'j' is not a good choice, because user programs often
use variable names like these for their own purposes.
The example programs shown in this major node all start the names of
their private variables with an underscore ('_'). Users generally don't
use leading underscores in their variable names, so this convention
immediately decreases the chances that the variable names will be
accidentally shared with the user's program.
In addition, several of the library functions use a prefix that helps
indicate what function or set of functions use the variables--for
example, '_pw_byname()' in the user database routines (*note Passwd
Functions::). This convention is recommended, as it even further
decreases the chance of inadvertent conflict among variable names. Note
that this convention is used equally well for variable names and for
private function names.(1)
As a final note on variable naming, if a function makes global
variables available for use by a main program, it is a good convention
to start those variables' names with a capital letter--for example,
'getopt()''s 'Opterr' and 'Optind' variables (*note Getopt Function::).
The leading capital letter indicates that it is global, while the fact
that the variable name is not all capital letters indicates that the
variable is not one of 'awk''s predefined variables, such as 'FS'.
It is also important that _all_ variables in library functions that
do not need to save state are, in fact, declared local.(2) If this is
not done, the variables could accidentally be used in the user's
program, leading to bugs that are very difficult to track down:
function lib_func(x, y, l1, l2)
{
...
# some_var should be local but by oversight is not
USE VARIABLE some_var
...
}
A different convention, common in the Tcl community, is to use a
single associative array to hold the values needed by the library
function(s), or "package." This significantly decreases the number of
actual global names in use. For example, the functions described in
*note Passwd Functions:: might have used array elements
'PW_data["inited"]', 'PW_data["total"]', 'PW_data["count"]', and
'PW_data["awklib"]', instead of '_pw_inited', '_pw_awklib', '_pw_total',
and '_pw_count'.
The conventions presented in this minor node are exactly that:
conventions. You are not required to write your programs this way--we
merely recommend that you do so.
---------- Footnotes ----------
(1) Although all the library routines could have been rewritten to
use this convention, this was not done, in order to show how our own
'awk' programming style has evolved and to provide some basis for this
discussion.
(2) 'gawk''s '--dump-variables' command-line option is useful for
verifying this.
File: gawk.info, Node: General Functions, Next: Data File Management, Prev: Library Names, Up: Library Functions
10.2 General Programming
========================
This minor node presents a number of functions that are of general
programming use.
* Menu:
* Strtonum Function:: A replacement for the built-in
'strtonum()' function.
* Assert Function:: A function for assertions in 'awk'
programs.
* Round Function:: A function for rounding if 'sprintf()'
does not do it correctly.
* Cliff Random Function:: The Cliff Random Number Generator.
* Ordinal Functions:: Functions for using characters as numbers and
vice versa.
* Join Function:: A function to join an array into a string.
* Getlocaltime Function:: A function to get formatted times.
* Readfile Function:: A function to read an entire file at once.
* Shell Quoting:: A function to quote strings for the shell.
File: gawk.info, Node: Strtonum Function, Next: Assert Function, Up: General Functions
10.2.1 Converting Strings to Numbers
------------------------------------
The 'strtonum()' function (*note String Functions::) is a 'gawk'
extension. The following function provides an implementation for other
versions of 'awk':
# mystrtonum --- convert string to number
function mystrtonum(str, ret, n, i, k, c)
{
if (str ~ /^0[0-7]*$/) {
# octal
n = length(str)
ret = 0
for (i = 1; i <= n; i++) {
c = substr(str, i, 1)
# index() returns 0 if c not in string,
# includes c == "0"
k = index("1234567", c)
ret = ret * 8 + k
}
} else if (str ~ /^0[xX][[:xdigit:]]+$/) {
# hexadecimal
str = substr(str, 3) # lop off leading 0x
n = length(str)
ret = 0
for (i = 1; i <= n; i++) {
c = substr(str, i, 1)
c = tolower(c)
# index() returns 0 if c not in string,
# includes c == "0"
k = index("123456789abcdef", c)
ret = ret * 16 + k
}
} else if (str ~ \
/^[-+]?([0-9]+([.][0-9]*([Ee][0-9]+)?)?|([.][0-9]+([Ee][-+]?[0-9]+)?))$/) {
# decimal number, possibly floating point
ret = str + 0
} else
ret = "NOT-A-NUMBER"
return ret
}
# BEGIN { # gawk test harness
# a[1] = "25"
# a[2] = ".31"
# a[3] = "0123"
# a[4] = "0xdeadBEEF"
# a[5] = "123.45"
# a[6] = "1.e3"
# a[7] = "1.32"
# a[8] = "1.32E2"
#
# for (i = 1; i in a; i++)
# print a[i], strtonum(a[i]), mystrtonum(a[i])
# }
The function first looks for C-style octal numbers (base 8). If the
input string matches a regular expression describing octal numbers, then
'mystrtonum()' loops through each character in the string. It sets 'k'
to the index in '"1234567"' of the current octal digit. The return
value will either be the same number as the digit, or zero if the
character is not there, which will be true for a '0'. This is safe,
because the regexp test in the 'if' ensures that only octal values are
converted.
Similar logic applies to the code that checks for and converts a
hexadecimal value, which starts with '0x' or '0X'. The use of
'tolower()' simplifies the computation for finding the correct numeric
value for each hexadecimal digit.
Finally, if the string matches the (rather complicated) regexp for a
regular decimal integer or floating-point number, the computation 'ret =
str + 0' lets 'awk' convert the value to a number.
A commented-out test program is included, so that the function can be
tested with 'gawk' and the results compared to the built-in 'strtonum()'
function.
File: gawk.info, Node: Assert Function, Next: Round Function, Prev: Strtonum Function, Up: General Functions
10.2.2 Assertions
-----------------
When writing large programs, it is often useful to know that a condition
or set of conditions is true. Before proceeding with a particular
computation, you make a statement about what you believe to be the case.
Such a statement is known as an "assertion". The C language provides an
'<assert.h>' header file and corresponding 'assert()' macro that a
programmer can use to make assertions. If an assertion fails, the
'assert()' macro arranges to print a diagnostic message describing the
condition that should have been true but was not, and then it kills the
program. In C, using 'assert()' looks this:
#include <assert.h>
int myfunc(int a, double b)
{
assert(a <= 5 && b >= 17.1);
...
}
If the assertion fails, the program prints a message similar to this:
prog.c:5: assertion failed: a <= 5 && b >= 17.1
The C language makes it possible to turn the condition into a string
for use in printing the diagnostic message. This is not possible in
'awk', so this 'assert()' function also requires a string version of the
condition that is being tested. Following is the function:
# assert --- assert that a condition is true. Otherwise, exit.
function assert(condition, string)
{
if (! condition) {
printf("%s:%d: assertion failed: %s\n",
FILENAME, FNR, string) > "/dev/stderr"
_assert_exit = 1
exit 1
}
}
END {
if (_assert_exit)
exit 1
}
The 'assert()' function tests the 'condition' parameter. If it is
false, it prints a message to standard error, using the 'string'
parameter to describe the failed condition. It then sets the variable
'_assert_exit' to one and executes the 'exit' statement. The 'exit'
statement jumps to the 'END' rule. If the 'END' rule finds
'_assert_exit' to be true, it exits immediately.
The purpose of the test in the 'END' rule is to keep any other 'END'
rules from running. When an assertion fails, the program should exit
immediately. If no assertions fail, then '_assert_exit' is still false
when the 'END' rule is run normally, and the rest of the program's 'END'
rules execute. For all of this to work correctly, 'assert.awk' must be
the first source file read by 'awk'. The function can be used in a
program in the following way:
function myfunc(a, b)
{
assert(a <= 5 && b >= 17.1, "a <= 5 && b >= 17.1")
...
}
If the assertion fails, you see a message similar to the following:
mydata:1357: assertion failed: a <= 5 && b >= 17.1
There is a small problem with this version of 'assert()'. An 'END'
rule is automatically added to the program calling 'assert()'.
Normally, if a program consists of just a 'BEGIN' rule, the input files
and/or standard input are not read. However, now that the program has
an 'END' rule, 'awk' attempts to read the input data files or standard
input (*note Using BEGIN/END::), most likely causing the program to hang
as it waits for input.
There is a simple workaround to this: make sure that such a 'BEGIN'
rule always ends with an 'exit' statement.
File: gawk.info, Node: Round Function, Next: Cliff Random Function, Prev: Assert Function, Up: General Functions
10.2.3 Rounding Numbers
-----------------------
The way 'printf' and 'sprintf()' (*note Printf::) perform rounding often
depends upon the system's C 'sprintf()' subroutine. On many machines,
'sprintf()' rounding is "unbiased", which means it doesn't always round
a trailing .5 up, contrary to naive expectations. In unbiased rounding,
.5 rounds to even, rather than always up, so 1.5 rounds to 2 but 4.5
rounds to 4. This means that if you are using a format that does
rounding (e.g., '"%.0f"'), you should check what your system does. The
following function does traditional rounding; it might be useful if your
'awk''s 'printf' does unbiased rounding:
# round.awk --- do normal rounding
function round(x, ival, aval, fraction)
{
ival = int(x) # integer part, int() truncates
# see if fractional part
if (ival == x) # no fraction
return ival # ensure no decimals
if (x < 0) {
aval = -x # absolute value
ival = int(aval)
fraction = aval - ival
if (fraction >= .5)
return int(x) - 1 # -2.5 --> -3
else
return int(x) # -2.3 --> -2
} else {
fraction = x - ival
if (fraction >= .5)
return ival + 1
else
return ival
}
}
# test harness
# { print $0, round($0) }
File: gawk.info, Node: Cliff Random Function, Next: Ordinal Functions, Prev: Round Function, Up: General Functions
10.2.4 The Cliff Random Number Generator
----------------------------------------
The Cliff random number generator
(http://mathworld.wolfram.com/CliffRandomNumberGenerator.html) is a very
simple random number generator that "passes the noise sphere test for
randomness by showing no structure." It is easily programmed, in less
than 10 lines of 'awk' code:
# cliff_rand.awk --- generate Cliff random numbers
BEGIN { _cliff_seed = 0.1 }
function cliff_rand()
{
_cliff_seed = (100 * log(_cliff_seed)) % 1
if (_cliff_seed < 0)
_cliff_seed = - _cliff_seed
return _cliff_seed
}
This algorithm requires an initial "seed" of 0.1. Each new value
uses the current seed as input for the calculation. If the built-in
'rand()' function (*note Numeric Functions::) isn't random enough, you
might try using this function instead.
File: gawk.info, Node: Ordinal Functions, Next: Join Function, Prev: Cliff Random Function, Up: General Functions
10.2.5 Translating Between Characters and Numbers
-------------------------------------------------
One commercial implementation of 'awk' supplies a built-in function,
'ord()', which takes a character and returns the numeric value for that
character in the machine's character set. If the string passed to
'ord()' has more than one character, only the first one is used.
The inverse of this function is 'chr()' (from the function of the
same name in Pascal), which takes a number and returns the corresponding
character. Both functions are written very nicely in 'awk'; there is no
real reason to build them into the 'awk' interpreter:
# ord.awk --- do ord and chr
# Global identifiers:
# _ord_: numerical values indexed by characters
# _ord_init: function to initialize _ord_
BEGIN { _ord_init() }
function _ord_init( low, high, i, t)
{
low = sprintf("%c", 7) # BEL is ascii 7
if (low == "\a") { # regular ascii
low = 0
high = 127
} else if (sprintf("%c", 128 + 7) == "\a") {
# ascii, mark parity
low = 128
high = 255
} else { # ebcdic(!)
low = 0
high = 255
}
for (i = low; i <= high; i++) {
t = sprintf("%c", i)
_ord_[t] = i
}
}
Some explanation of the numbers used by '_ord_init()' is worthwhile.
The most prominent character set in use today is ASCII.(1) Although an
8-bit byte can hold 256 distinct values (from 0 to 255), ASCII only
defines characters that use the values from 0 to 127.(2) In the now
distant past, at least one minicomputer manufacturer used ASCII, but
with mark parity, meaning that the leftmost bit in the byte is always 1.
This means that on those systems, characters have numeric values from
128 to 255. Finally, large mainframe systems use the EBCDIC character
set, which uses all 256 values. There are other character sets in use
on some older systems, but they are not really worth worrying about:
function ord(str, c)
{
# only first character is of interest
c = substr(str, 1, 1)
return _ord_[c]
}
function chr(c)
{
# force c to be numeric by adding 0
return sprintf("%c", c + 0)
}
#### test code ####
# BEGIN {
# for (;;) {
# printf("enter a character: ")
# if (getline var <= 0)
# break
# printf("ord(%s) = %d\n", var, ord(var))
# }
# }
An obvious improvement to these functions is to move the code for the
'_ord_init' function into the body of the 'BEGIN' rule. It was written
this way initially for ease of development. There is a "test program"
in a 'BEGIN' rule, to test the function. It is commented out for
production use.
---------- Footnotes ----------
(1) This is changing; many systems use Unicode, a very large
character set that includes ASCII as a subset. On systems with full
Unicode support, a character can occupy up to 32 bits, making simple
tests such as used here prohibitively expensive.
(2) ASCII has been extended in many countries to use the values from
128 to 255 for country-specific characters. If your system uses these
extensions, you can simplify '_ord_init()' to loop from 0 to 255.
File: gawk.info, Node: Join Function, Next: Getlocaltime Function, Prev: Ordinal Functions, Up: General Functions
10.2.6 Merging an Array into a String
-------------------------------------
When doing string processing, it is often useful to be able to join all
the strings in an array into one long string. The following function,
'join()', accomplishes this task. It is used later in several of the
application programs (*note Sample Programs::).
Good function design is important; this function needs to be general,
but it should also have a reasonable default behavior. It is called
with an array as well as the beginning and ending indices of the
elements in the array to be merged. This assumes that the array indices
are numeric--a reasonable assumption, as the array was likely created
with 'split()' (*note String Functions::):
# join.awk --- join an array into a string
function join(array, start, end, sep, result, i)
{
if (sep == "")
sep = " "
else if (sep == SUBSEP) # magic value
sep = ""
result = array[start]
for (i = start + 1; i <= end; i++)
result = result sep array[i]
return result
}
An optional additional argument is the separator to use when joining
the strings back together. If the caller supplies a nonempty value,
'join()' uses it; if it is not supplied, it has a null value. In this
case, 'join()' uses a single space as a default separator for the
strings. If the value is equal to 'SUBSEP', then 'join()' joins the
strings with no separator between them. 'SUBSEP' serves as a "magic"
value to indicate that there should be no separation between the
component strings.(1)
---------- Footnotes ----------
(1) It would be nice if 'awk' had an assignment operator for
concatenation. The lack of an explicit operator for concatenation makes
string operations more difficult than they really need to be.
File: gawk.info, Node: Getlocaltime Function, Next: Readfile Function, Prev: Join Function, Up: General Functions
10.2.7 Managing the Time of Day
-------------------------------
The 'systime()' and 'strftime()' functions described in *note Time
Functions:: provide the minimum functionality necessary for dealing with
the time of day in human-readable form. Although 'strftime()' is
extensive, the control formats are not necessarily easy to remember or
intuitively obvious when reading a program.
The following function, 'getlocaltime()', populates a user-supplied
array with preformatted time information. It returns a string with the
current time formatted in the same way as the 'date' utility:
# getlocaltime.awk --- get the time of day in a usable format
# Returns a string in the format of output of date(1)
# Populates the array argument time with individual values:
# time["second"] -- seconds (0 - 59)
# time["minute"] -- minutes (0 - 59)
# time["hour"] -- hours (0 - 23)
# time["althour"] -- hours (0 - 12)
# time["monthday"] -- day of month (1 - 31)
# time["month"] -- month of year (1 - 12)
# time["monthname"] -- name of the month
# time["shortmonth"] -- short name of the month
# time["year"] -- year modulo 100 (0 - 99)
# time["fullyear"] -- full year
# time["weekday"] -- day of week (Sunday = 0)
# time["altweekday"] -- day of week (Monday = 0)
# time["dayname"] -- name of weekday
# time["shortdayname"] -- short name of weekday
# time["yearday"] -- day of year (0 - 365)
# time["timezone"] -- abbreviation of timezone name
# time["ampm"] -- AM or PM designation
# time["weeknum"] -- week number, Sunday first day
# time["altweeknum"] -- week number, Monday first day
function getlocaltime(time, ret, now, i)
{
# get time once, avoids unnecessary system calls
now = systime()
# return date(1)-style output
ret = strftime("%a %b %e %H:%M:%S %Z %Y", now)
# clear out target array
delete time
# fill in values, force numeric values to be
# numeric by adding 0
time["second"] = strftime("%S", now) + 0
time["minute"] = strftime("%M", now) + 0
time["hour"] = strftime("%H", now) + 0
time["althour"] = strftime("%I", now) + 0
time["monthday"] = strftime("%d", now) + 0
time["month"] = strftime("%m", now) + 0
time["monthname"] = strftime("%B", now)
time["shortmonth"] = strftime("%b", now)
time["year"] = strftime("%y", now) + 0
time["fullyear"] = strftime("%Y", now) + 0
time["weekday"] = strftime("%w", now) + 0
time["altweekday"] = strftime("%u", now) + 0
time["dayname"] = strftime("%A", now)
time["shortdayname"] = strftime("%a", now)
time["yearday"] = strftime("%j", now) + 0
time["timezone"] = strftime("%Z", now)
time["ampm"] = strftime("%p", now)
time["weeknum"] = strftime("%U", now) + 0
time["altweeknum"] = strftime("%W", now) + 0
return ret
}
The string indices are easier to use and read than the various
formats required by 'strftime()'. The 'alarm' program presented in
*note Alarm Program:: uses this function. A more general design for the
'getlocaltime()' function would have allowed the user to supply an
optional timestamp value to use instead of the current time.
File: gawk.info, Node: Readfile Function, Next: Shell Quoting, Prev: Getlocaltime Function, Up: General Functions
10.2.8 Reading a Whole File at Once
-----------------------------------
Often, it is convenient to have the entire contents of a file available
in memory as a single string. A straightforward but naive way to do
that might be as follows:
function readfile1(file, tmp, contents)
{
if ((getline tmp < file) < 0)
return
contents = tmp RT
while ((getline tmp < file) > 0)
contents = contents tmp RT
close(file)
return contents
}
This function reads from 'file' one record at a time, building up the
full contents of the file in the local variable 'contents'. It works,
but is not necessarily efficient.
The following function, based on a suggestion by Denis Shirokov,
reads the entire contents of the named file in one shot:
# readfile.awk --- read an entire file at once
function readfile(file, tmp, save_rs)
{
save_rs = RS
RS = "^$"
getline tmp < file
close(file)
RS = save_rs
return tmp
}
It works by setting 'RS' to '^$', a regular expression that will
never match if the file has contents. 'gawk' reads data from the file
into 'tmp', attempting to match 'RS'. The match fails after each read,
but fails quickly, such that 'gawk' fills 'tmp' with the entire contents
of the file. (*Note Records:: for information on 'RT' and 'RS'.)
In the case that 'file' is empty, the return value is the null
string. Thus, calling code may use something like:
contents = readfile("/some/path")
if (length(contents) == 0)
# file was empty ...
This tests the result to see if it is empty or not. An equivalent
test would be 'contents == ""'.
*Note Extension Sample Readfile:: for an extension function that also
reads an entire file into memory.
File: gawk.info, Node: Shell Quoting, Prev: Readfile Function, Up: General Functions
10.2.9 Quoting Strings to Pass to the Shell
-------------------------------------------
Michael Brennan offers the following programming pattern, which he uses
frequently:
#! /bin/sh
awkp='
...
'
INPUT_PROGRAM | awk "$awkp" | /bin/sh
For example, a program of his named 'flac-edit' has this form:
$ flac-edit -song="Whoope! That's Great" file.flac
It generates the following output, which is to be piped to the shell
('/bin/sh'):
chmod +w file.flac
metaflac --remove-tag=TITLE file.flac
LANG=en_US.88591 metaflac --set-tag=TITLE='Whoope! That'"'"'s Great' file.flac
chmod -w file.flac
Note the need for shell quoting. The function 'shell_quote()' does
it. 'SINGLE' is the one-character string '"'"' and 'QSINGLE' is the
three-character string '"\"'\""':
# shell_quote --- quote an argument for passing to the shell
function shell_quote(s, # parameter
SINGLE, QSINGLE, i, X, n, ret) # locals
{
if (s == "")
return "\"\""
SINGLE = "\x27" # single quote
QSINGLE = "\"\x27\""
n = split(s, X, SINGLE)
ret = SINGLE X[1] SINGLE
for (i = 2; i <= n; i++)
ret = ret QSINGLE SINGLE X[i] SINGLE
return ret
}
File: gawk.info, Node: Data File Management, Next: Getopt Function, Prev: General Functions, Up: Library Functions
10.3 Data file Management
=========================
This minor node presents functions that are useful for managing
command-line data files.
* Menu:
* Filetrans Function:: A function for handling data file transitions.
* Rewind Function:: A function for rereading the current file.
* File Checking:: Checking that data files are readable.
* Empty Files:: Checking for zero-length files.
* Ignoring Assigns:: Treating assignments as file names.
File: gawk.info, Node: Filetrans Function, Next: Rewind Function, Up: Data File Management
10.3.1 Noting Data file Boundaries
----------------------------------
The 'BEGIN' and 'END' rules are each executed exactly once, at the
beginning and end of your 'awk' program, respectively (*note
BEGIN/END::). We (the 'gawk' authors) once had a user who mistakenly
thought that the 'BEGIN' rules were executed at the beginning of each
data file and the 'END' rules were executed at the end of each data
file.
When informed that this was not the case, the user requested that we
add new special patterns to 'gawk', named 'BEGIN_FILE' and 'END_FILE',
that would have the desired behavior. He even supplied us the code to
do so.
Adding these special patterns to 'gawk' wasn't necessary; the job can
be done cleanly in 'awk' itself, as illustrated by the following library
program. It arranges to call two user-supplied functions, 'beginfile()'
and 'endfile()', at the beginning and end of each data file. Besides
solving the problem in only nine(!) lines of code, it does so
_portably_; this works with any implementation of 'awk':
# transfile.awk
#
# Give the user a hook for filename transitions
#
# The user must supply functions beginfile() and endfile()
# that each take the name of the file being started or
# finished, respectively.
FILENAME != _oldfilename {
if (_oldfilename != "")
endfile(_oldfilename)
_oldfilename = FILENAME
beginfile(FILENAME)
}
END { endfile(FILENAME) }
This file must be loaded before the user's "main" program, so that
the rule it supplies is executed first.
This rule relies on 'awk''s 'FILENAME' variable, which automatically
changes for each new data file. The current file name is saved in a
private variable, '_oldfilename'. If 'FILENAME' does not equal
'_oldfilename', then a new data file is being processed and it is
necessary to call 'endfile()' for the old file. Because 'endfile()'
should only be called if a file has been processed, the program first
checks to make sure that '_oldfilename' is not the null string. The
program then assigns the current file name to '_oldfilename' and calls
'beginfile()' for the file. Because, like all 'awk' variables,
'_oldfilename' is initialized to the null string, this rule executes
correctly even for the first data file.
The program also supplies an 'END' rule to do the final processing
for the last file. Because this 'END' rule comes before any 'END' rules
supplied in the "main" program, 'endfile()' is called first. Once
again, the value of multiple 'BEGIN' and 'END' rules should be clear.
If the same data file occurs twice in a row on the command line, then
'endfile()' and 'beginfile()' are not executed at the end of the first
pass and at the beginning of the second pass. The following version
solves the problem:
# ftrans.awk --- handle datafile transitions
#
# user supplies beginfile() and endfile() functions
FNR == 1 {
if (_filename_ != "")
endfile(_filename_)
_filename_ = FILENAME
beginfile(FILENAME)
}
END { endfile(_filename_) }
*note Wc Program:: shows how this library function can be used and
how it simplifies writing the main program.
So Why Does 'gawk' Have 'BEGINFILE' and 'ENDFILE'?
You are probably wondering, if 'beginfile()' and 'endfile()'
functions can do the job, why does 'gawk' have 'BEGINFILE' and 'ENDFILE'
patterns?
Good question. Normally, if 'awk' cannot open a file, this causes an
immediate fatal error. In this case, there is no way for a user-defined
function to deal with the problem, as the mechanism for calling it
relies on the file being open and at the first record. Thus, the main
reason for 'BEGINFILE' is to give you a "hook" to catch files that
cannot be processed. 'ENDFILE' exists for symmetry, and because it
provides an easy way to do per-file cleanup processing. For more
information, refer to *note BEGINFILE/ENDFILE::.
File: gawk.info, Node: Rewind Function, Next: File Checking, Prev: Filetrans Function, Up: Data File Management
10.3.2 Rereading the Current File
---------------------------------
Another request for a new built-in function was for a function that
would make it possible to reread the current file. The requesting user
didn't want to have to use 'getline' (*note Getline::) inside a loop.
However, as long as you are not in the 'END' rule, it is quite easy
to arrange to immediately close the current input file and then start
over with it from the top. For lack of a better name, we'll call the
function 'rewind()':
# rewind.awk --- rewind the current file and start over
function rewind( i)
{
# shift remaining arguments up
for (i = ARGC; i > ARGIND; i--)
ARGV[i] = ARGV[i-1]
# make sure gawk knows to keep going
ARGC++
# make current file next to get done
ARGV[ARGIND+1] = FILENAME
# do it
nextfile
}
The 'rewind()' function relies on the 'ARGIND' variable (*note
Auto-set::), which is specific to 'gawk'. It also relies on the
'nextfile' keyword (*note Nextfile Statement::). Because of this, you
should not call it from an 'ENDFILE' rule. (This isn't necessary
anyway, because 'gawk' goes to the next file as soon as an 'ENDFILE'
rule finishes!)
You need to be careful calling 'rewind()'. You can end up causing
infinite recursion if you don't pay attention. Here is an example use:
$ cat data
-| a
-| b
-| c
-| d
-| e
$ cat test.awk
-| FNR == 3 && ! rewound {
-| rewound = 1
-| rewind()
-| }
-|
-| { print FILENAME, FNR, $0 }
$ gawk -f rewind.awk -f test.awk data
-| data 1 a
-| data 2 b
-| data 1 a
-| data 2 b
-| data 3 c
-| data 4 d
-| data 5 e
File: gawk.info, Node: File Checking, Next: Empty Files, Prev: Rewind Function, Up: Data File Management
10.3.3 Checking for Readable Data files
---------------------------------------
Normally, if you give 'awk' a data file that isn't readable, it stops
with a fatal error. There are times when you might want to just ignore
such files and keep going.(1) You can do this by prepending the
following program to your 'awk' program:
# readable.awk --- library file to skip over unreadable files
BEGIN {
for (i = 1; i < ARGC; i++) {
if (ARGV[i] ~ /^[a-zA-Z_][a-zA-Z0-9_]*=.*/ \
|| ARGV[i] == "-" || ARGV[i] == "/dev/stdin")
continue # assignment or standard input
else if ((getline junk < ARGV[i]) < 0) # unreadable
delete ARGV[i]
else
close(ARGV[i])
}
}
This works, because the 'getline' won't be fatal. Removing the
element from 'ARGV' with 'delete' skips the file (because it's no longer
in the list). See also *note ARGC and ARGV::.
Because 'awk' variable names only allow the English letters, the
regular expression check purposely does not use character classes such
as '[:alpha:]' and '[:alnum:]' (*note Bracket Expressions::).
---------- Footnotes ----------
(1) The 'BEGINFILE' special pattern (*note BEGINFILE/ENDFILE::)
provides an alternative mechanism for dealing with files that can't be
opened. However, the code here provides a portable solution.
File: gawk.info, Node: Empty Files, Next: Ignoring Assigns, Prev: File Checking, Up: Data File Management
10.3.4 Checking for Zero-Length Files
-------------------------------------
All known 'awk' implementations silently skip over zero-length files.
This is a by-product of 'awk''s implicit
read-a-record-and-match-against-the-rules loop: when 'awk' tries to read
a record from an empty file, it immediately receives an end-of-file
indication, closes the file, and proceeds on to the next command-line
data file, _without_ executing any user-level 'awk' program code.
Using 'gawk''s 'ARGIND' variable (*note Built-in Variables::), it is
possible to detect when an empty data file has been skipped. Similar to
the library file presented in *note Filetrans Function::, the following
library file calls a function named 'zerofile()' that the user must
provide. The arguments passed are the file name and the position in
'ARGV' where it was found:
# zerofile.awk --- library file to process empty input files
BEGIN { Argind = 0 }
ARGIND > Argind + 1 {
for (Argind++; Argind < ARGIND; Argind++)
zerofile(ARGV[Argind], Argind)
}
ARGIND != Argind { Argind = ARGIND }
END {
if (ARGIND > Argind)
for (Argind++; Argind <= ARGIND; Argind++)
zerofile(ARGV[Argind], Argind)
}
The user-level variable 'Argind' allows the 'awk' program to track
its progress through 'ARGV'. Whenever the program detects that 'ARGIND'
is greater than 'Argind + 1', it means that one or more empty files were
skipped. The action then calls 'zerofile()' for each such file,
incrementing 'Argind' along the way.
The 'Argind != ARGIND' rule simply keeps 'Argind' up to date in the
normal case.
Finally, the 'END' rule catches the case of any empty files at the
end of the command-line arguments. Note that the test in the condition
of the 'for' loop uses the '<=' operator, not '<'.
File: gawk.info, Node: Ignoring Assigns, Prev: Empty Files, Up: Data File Management
10.3.5 Treating Assignments as File names
-----------------------------------------
Occasionally, you might not want 'awk' to process command-line variable
assignments (*note Assignment Options::). In particular, if you have a
file name that contains an '=' character, 'awk' treats the file name as
an assignment and does not process it.
Some users have suggested an additional command-line option for
'gawk' to disable command-line assignments. However, some simple
programming with a library file does the trick:
# noassign.awk --- library file to avoid the need for a
# special option that disables command-line assignments
function disable_assigns(argc, argv, i)
{
for (i = 1; i < argc; i++)
if (argv[i] ~ /^[a-zA-Z_][a-zA-Z0-9_]*=.*/)
argv[i] = ("./" argv[i])
}
BEGIN {
if (No_command_assign)
disable_assigns(ARGC, ARGV)
}
You then run your program this way:
awk -v No_command_assign=1 -f noassign.awk -f yourprog.awk *
The function works by looping through the arguments. It prepends
'./' to any argument that matches the form of a variable assignment,
turning that argument into a file name.
The use of 'No_command_assign' allows you to disable command-line
assignments at invocation time, by giving the variable a true value.
When not set, it is initially zero (i.e., false), so the command-line
arguments are left alone.
File: gawk.info, Node: Getopt Function, Next: Passwd Functions, Prev: Data File Management, Up: Library Functions
10.4 Processing Command-Line Options
====================================
Most utilities on POSIX-compatible systems take options on the command
line that can be used to change the way a program behaves. 'awk' is an
example of such a program (*note Options::). Often, options take
"arguments" (i.e., data that the program needs to correctly obey the
command-line option). For example, 'awk''s '-F' option requires a
string to use as the field separator. The first occurrence on the
command line of either '--' or a string that does not begin with '-'
ends the options.
Modern Unix systems provide a C function named 'getopt()' for
processing command-line arguments. The programmer provides a string
describing the one-letter options. If an option requires an argument,
it is followed in the string with a colon. 'getopt()' is also passed
the count and values of the command-line arguments and is called in a
loop. 'getopt()' processes the command-line arguments for option
letters. Each time around the loop, it returns a single character
representing the next option letter that it finds, or '?' if it finds an
invalid option. When it returns -1, there are no options left on the
command line.
When using 'getopt()', options that do not take arguments can be
grouped together. Furthermore, options that take arguments require that
the argument be present. The argument can immediately follow the option
letter, or it can be a separate command-line argument.
Given a hypothetical program that takes three command-line options,
'-a', '-b', and '-c', where '-b' requires an argument, all of the
following are valid ways of invoking the program:
prog -a -b foo -c data1 data2 data3
prog -ac -bfoo -- data1 data2 data3
prog -acbfoo data1 data2 data3
Notice that when the argument is grouped with its option, the rest of
the argument is considered to be the option's argument. In this
example, '-acbfoo' indicates that all of the '-a', '-b', and '-c'
options were supplied, and that 'foo' is the argument to the '-b'
option.
'getopt()' provides four external variables that the programmer can
use:
'optind'
The index in the argument value array ('argv') where the first
nonoption command-line argument can be found.
'optarg'
The string value of the argument to an option.
'opterr'
Usually 'getopt()' prints an error message when it finds an invalid
option. Setting 'opterr' to zero disables this feature. (An
application might want to print its own error message.)
'optopt'
The letter representing the command-line option.
The following C fragment shows how 'getopt()' might process
command-line arguments for 'awk':
int
main(int argc, char *argv[])
{
...
/* print our own message */
opterr = 0;
while ((c = getopt(argc, argv, "v:f:F:W:")) != -1) {
switch (c) {
case 'f': /* file */
...
break;
case 'F': /* field separator */
...
break;
case 'v': /* variable assignment */
...
break;
case 'W': /* extension */
...
break;
case '?':
default:
usage();
break;
}
}
...
}
As a side point, 'gawk' actually uses the GNU 'getopt_long()'
function to process both normal and GNU-style long options (*note
Options::).
The abstraction provided by 'getopt()' is very useful and is quite
handy in 'awk' programs as well. Following is an 'awk' version of
'getopt()'. This function highlights one of the greatest weaknesses in
'awk', which is that it is very poor at manipulating single characters.
Repeated calls to 'substr()' are necessary for accessing individual
characters (*note String Functions::).(1)
The discussion that follows walks through the code a bit at a time:
# getopt.awk --- Do C library getopt(3) function in awk
# External variables:
# Optind -- index in ARGV of first nonoption argument
# Optarg -- string value of argument to current option
# Opterr -- if nonzero, print our own diagnostic
# Optopt -- current option letter
# Returns:
# -1 at end of options
# "?" for unrecognized option
# <c> a character representing the current option
# Private Data:
# _opti -- index in multiflag option, e.g., -abc
The function starts out with comments presenting a list of the global
variables it uses, what the return values are, what they mean, and any
global variables that are "private" to this library function. Such
documentation is essential for any program, and particularly for library
functions.
The 'getopt()' function first checks that it was indeed called with a
string of options (the 'options' parameter). If 'options' has a zero
length, 'getopt()' immediately returns -1:
function getopt(argc, argv, options, thisopt, i)
{
if (length(options) == 0) # no options given
return -1
if (argv[Optind] == "--") { # all done
Optind++
_opti = 0
return -1
} else if (argv[Optind] !~ /^-[^:[:space:]]/) {
_opti = 0
return -1
}
The next thing to check for is the end of the options. A '--' ends
the command-line options, as does any command-line argument that does
not begin with a '-'. 'Optind' is used to step through the array of
command-line arguments; it retains its value across calls to 'getopt()',
because it is a global variable.
The regular expression that is used, '/^-[^:[:space:]/', checks for a
'-' followed by anything that is not whitespace and not a colon. If the
current command-line argument does not match this pattern, it is not an
option, and it ends option processing. Continuing on:
if (_opti == 0)
_opti = 2
thisopt = substr(argv[Optind], _opti, 1)
Optopt = thisopt
i = index(options, thisopt)
if (i == 0) {
if (Opterr)
printf("%c -- invalid option\n", thisopt) > "/dev/stderr"
if (_opti >= length(argv[Optind])) {
Optind++
_opti = 0
} else
_opti++
return "?"
}
The '_opti' variable tracks the position in the current command-line
argument ('argv[Optind]'). If multiple options are grouped together
with one '-' (e.g., '-abx'), it is necessary to return them to the user
one at a time.
If '_opti' is equal to zero, it is set to two, which is the index in
the string of the next character to look at (we skip the '-', which is
at position one). The variable 'thisopt' holds the character, obtained
with 'substr()'. It is saved in 'Optopt' for the main program to use.
If 'thisopt' is not in the 'options' string, then it is an invalid
option. If 'Opterr' is nonzero, 'getopt()' prints an error message on
the standard error that is similar to the message from the C version of
'getopt()'.
Because the option is invalid, it is necessary to skip it and move on
to the next option character. If '_opti' is greater than or equal to
the length of the current command-line argument, it is necessary to move
on to the next argument, so 'Optind' is incremented and '_opti' is reset
to zero. Otherwise, 'Optind' is left alone and '_opti' is merely
incremented.
In any case, because the option is invalid, 'getopt()' returns '"?"'.
The main program can examine 'Optopt' if it needs to know what the
invalid option letter actually is. Continuing on:
if (substr(options, i + 1, 1) == ":") {
# get option argument
if (length(substr(argv[Optind], _opti + 1)) > 0)
Optarg = substr(argv[Optind], _opti + 1)
else
Optarg = argv[++Optind]
_opti = 0
} else
Optarg = ""
If the option requires an argument, the option letter is followed by
a colon in the 'options' string. If there are remaining characters in
the current command-line argument ('argv[Optind]'), then the rest of
that string is assigned to 'Optarg'. Otherwise, the next command-line
argument is used ('-xFOO' versus '-x FOO'). In either case, '_opti' is
reset to zero, because there are no more characters left to examine in
the current command-line argument. Continuing:
if (_opti == 0 || _opti >= length(argv[Optind])) {
Optind++
_opti = 0
} else
_opti++
return thisopt
}
Finally, if '_opti' is either zero or greater than the length of the
current command-line argument, it means this element in 'argv' is
through being processed, so 'Optind' is incremented to point to the next
element in 'argv'. If neither condition is true, then only '_opti' is
incremented, so that the next option letter can be processed on the next
call to 'getopt()'.
The 'BEGIN' rule initializes both 'Opterr' and 'Optind' to one.
'Opterr' is set to one, because the default behavior is for 'getopt()'
to print a diagnostic message upon seeing an invalid option. 'Optind'
is set to one, because there's no reason to look at the program name,
which is in 'ARGV[0]':
BEGIN {
Opterr = 1 # default is to diagnose
Optind = 1 # skip ARGV[0]
# test program
if (_getopt_test) {
while ((_go_c = getopt(ARGC, ARGV, "ab:cd")) != -1)
printf("c = <%c>, Optarg = <%s>\n",
_go_c, Optarg)
printf("non-option arguments:\n")
for (; Optind < ARGC; Optind++)
printf("\tARGV[%d] = <%s>\n",
Optind, ARGV[Optind])
}
}
The rest of the 'BEGIN' rule is a simple test program. Here are the
results of two sample runs of the test program:
$ awk -f getopt.awk -v _getopt_test=1 -- -a -cbARG bax -x
-| c = <a>, Optarg = <>
-| c = <c>, Optarg = <>
-| c = <b>, Optarg = <ARG>
-| non-option arguments:
-| ARGV[3] = <bax>
-| ARGV[4] = <-x>
$ awk -f getopt.awk -v _getopt_test=1 -- -a -x -- xyz abc
-| c = <a>, Optarg = <>
error-> x -- invalid option
-| c = <?>, Optarg = <>
-| non-option arguments:
-| ARGV[4] = <xyz>
-| ARGV[5] = <abc>
In both runs, the first '--' terminates the arguments to 'awk', so
that it does not try to interpret the '-a', etc., as its own options.
NOTE: After 'getopt()' is through, user-level code must clear out
all the elements of 'ARGV' from 1 to 'Optind', so that 'awk' does
not try to process the command-line options as file names.
Using '#!' with the '-E' option may help avoid conflicts between your
program's options and 'gawk''s options, as '-E' causes 'gawk' to abandon
processing of further options (*note Executable Scripts:: and *note
Options::).
Several of the sample programs presented in *note Sample Programs::,
use 'getopt()' to process their arguments.
---------- Footnotes ----------
(1) This function was written before 'gawk' acquired the ability to
split strings into single characters using '""' as the separator. We
have left it alone, as using 'substr()' is more portable.
File: gawk.info, Node: Passwd Functions, Next: Group Functions, Prev: Getopt Function, Up: Library Functions
10.5 Reading the User Database
==============================
The 'PROCINFO' array (*note Built-in Variables::) provides access to the
current user's real and effective user and group ID numbers, and, if
available, the user's supplementary group set. However, because these
are numbers, they do not provide very useful information to the average
user. There needs to be some way to find the user information
associated with the user and group ID numbers. This minor node presents
a suite of functions for retrieving information from the user database.
*Note Group Functions:: for a similar suite that retrieves information
from the group database.
The POSIX standard does not define the file where user information is
kept. Instead, it provides the '<pwd.h>' header file and several C
language subroutines for obtaining user information. The primary
function is 'getpwent()', for "get password entry." The "password"
comes from the original user database file, '/etc/passwd', which stores
user information along with the encrypted passwords (hence the name).
Although an 'awk' program could simply read '/etc/passwd' directly,
this file may not contain complete information about the system's set of
users.(1) To be sure you are able to produce a readable and complete
version of the user database, it is necessary to write a small C program
that calls 'getpwent()'. 'getpwent()' is defined as returning a pointer
to a 'struct passwd'. Each time it is called, it returns the next entry
in the database. When there are no more entries, it returns 'NULL', the
null pointer. When this happens, the C program should call 'endpwent()'
to close the database. Following is 'pwcat', a C program that "cats"
the password database:
/*
* pwcat.c
*
* Generate a printable version of the password database.
*/
#include <stdio.h>
#include <pwd.h>
int
main(int argc, char **argv)
{
struct passwd *p;
while ((p = getpwent()) != NULL)
printf("%s:%s:%ld:%ld:%s:%s:%s\n",
p->pw_name, p->pw_passwd, (long) p->pw_uid,
(long) p->pw_gid, p->pw_gecos, p->pw_dir, p->pw_shell);
endpwent();
return 0;
}
If you don't understand C, don't worry about it. The output from
'pwcat' is the user database, in the traditional '/etc/passwd' format of
colon-separated fields. The fields are:
Login name
The user's login name.
Encrypted password
The user's encrypted password. This may not be available on some
systems.
User-ID
The user's numeric user ID number. (On some systems, it's a C
'long', and not an 'int'. Thus, we cast it to 'long' for all
cases.)
Group-ID
The user's numeric group ID number. (Similar comments about 'long'
versus 'int' apply here.)
Full name
The user's full name, and perhaps other information associated with
the user.
Home directory
The user's login (or "home") directory (familiar to shell
programmers as '$HOME').
Login shell
The program that is run when the user logs in. This is usually a
shell, such as Bash.
A few lines representative of 'pwcat''s output are as follows:
$ pwcat
-| root:x:0:1:Operator:/:/bin/sh
-| nobody:*:65534:65534::/:
-| daemon:*:1:1::/:
-| sys:*:2:2::/:/bin/csh
-| bin:*:3:3::/bin:
-| arnold:xyzzy:2076:10:Arnold Robbins:/home/arnold:/bin/sh
-| miriam:yxaay:112:10:Miriam Robbins:/home/miriam:/bin/sh
-| andy:abcca2:113:10:Andy Jacobs:/home/andy:/bin/sh
...
With that introduction, following is a group of functions for getting
user information. There are several functions here, corresponding to
the C functions of the same names:
# passwd.awk --- access password file information
BEGIN {
# tailor this to suit your system
_pw_awklib = "/usr/local/libexec/awk/"
}
function _pw_init( oldfs, oldrs, olddol0, pwcat, using_fw, using_fpat)
{
if (_pw_inited)
return
oldfs = FS
oldrs = RS
olddol0 = $0
using_fw = (PROCINFO["FS"] == "FIELDWIDTHS")
using_fpat = (PROCINFO["FS"] == "FPAT")
FS = ":"
RS = "\n"
pwcat = _pw_awklib "pwcat"
while ((pwcat | getline) > 0) {
_pw_byname[$1] = $0
_pw_byuid[$3] = $0
_pw_bycount[++_pw_total] = $0
}
close(pwcat)
_pw_count = 0
_pw_inited = 1
FS = oldfs
if (using_fw)
FIELDWIDTHS = FIELDWIDTHS
else if (using_fpat)
FPAT = FPAT
RS = oldrs
$0 = olddol0
}
The 'BEGIN' rule sets a private variable to the directory where
'pwcat' is stored. Because it is used to help out an 'awk' library
routine, we have chosen to put it in '/usr/local/libexec/awk'; however,
you might want it to be in a different directory on your system.
The function '_pw_init()' fills three copies of the user information
into three associative arrays. The arrays are indexed by username
('_pw_byname'), by user ID number ('_pw_byuid'), and by order of
occurrence ('_pw_bycount'). The variable '_pw_inited' is used for
efficiency, as '_pw_init()' needs to be called only once.
Because this function uses 'getline' to read information from
'pwcat', it first saves the values of 'FS', 'RS', and '$0'. It notes in
the variable 'using_fw' whether field splitting with 'FIELDWIDTHS' is in
effect or not. Doing so is necessary, as these functions could be
called from anywhere within a user's program, and the user may have his
or her own way of splitting records and fields. This makes it possible
to restore the correct field-splitting mechanism later. The test can
only be true for 'gawk'. It is false if using 'FS' or 'FPAT', or on
some other 'awk' implementation.
The code that checks for using 'FPAT', using 'using_fpat' and
'PROCINFO["FS"]', is similar.
The main part of the function uses a loop to read database lines,
split the lines into fields, and then store the lines into each array as
necessary. When the loop is done, '_pw_init()' cleans up by closing the
pipeline, setting '_pw_inited' to one, and restoring 'FS' (and
'FIELDWIDTHS' or 'FPAT' if necessary), 'RS', and '$0'. The use of
'_pw_count' is explained shortly.
The 'getpwnam()' function takes a username as a string argument. If
that user is in the database, it returns the appropriate line.
Otherwise, it relies on the array reference to a nonexistent element to
create the element with the null string as its value:
function getpwnam(name)
{
_pw_init()
return _pw_byname[name]
}
Similarly, the 'getpwuid()' function takes a user ID number argument.
If that user number is in the database, it returns the appropriate line.
Otherwise, it returns the null string:
function getpwuid(uid)
{
_pw_init()
return _pw_byuid[uid]
}
The 'getpwent()' function simply steps through the database, one
entry at a time. It uses '_pw_count' to track its current position in
the '_pw_bycount' array:
function getpwent()
{
_pw_init()
if (_pw_count < _pw_total)
return _pw_bycount[++_pw_count]
return ""
}
The 'endpwent()' function resets '_pw_count' to zero, so that
subsequent calls to 'getpwent()' start over again:
function endpwent()
{
_pw_count = 0
}
A conscious design decision in this suite is that each subroutine
calls '_pw_init()' to initialize the database arrays. The overhead of
running a separate process to generate the user database, and the I/O to
scan it, are only incurred if the user's main program actually calls one
of these functions. If this library file is loaded along with a user's
program, but none of the routines are ever called, then there is no
extra runtime overhead. (The alternative is move the body of
'_pw_init()' into a 'BEGIN' rule, which always runs 'pwcat'. This
simplifies the code but runs an extra process that may never be needed.)
In turn, calling '_pw_init()' is not too expensive, because the
'_pw_inited' variable keeps the program from reading the data more than
once. If you are worried about squeezing every last cycle out of your
'awk' program, the check of '_pw_inited' could be moved out of
'_pw_init()' and duplicated in all the other functions. In practice,
this is not necessary, as most 'awk' programs are I/O-bound, and such a
change would clutter up the code.
The 'id' program in *note Id Program:: uses these functions.
---------- Footnotes ----------
(1) It is often the case that password information is stored in a
network database.
File: gawk.info, Node: Group Functions, Next: Walking Arrays, Prev: Passwd Functions, Up: Library Functions
10.6 Reading the Group Database
===============================
Much of the discussion presented in *note Passwd Functions:: applies to
the group database as well. Although there has traditionally been a
well-known file ('/etc/group') in a well-known format, the POSIX
standard only provides a set of C library routines ('<grp.h>' and
'getgrent()') for accessing the information. Even though this file may
exist, it may not have complete information. Therefore, as with the
user database, it is necessary to have a small C program that generates
the group database as its output. 'grcat', a C program that "cats" the
group database, is as follows:
/*
* grcat.c
*
* Generate a printable version of the group database.
*/
#include <stdio.h>
#include <grp.h>
int
main(int argc, char **argv)
{
struct group *g;
int i;
while ((g = getgrent()) != NULL) {
printf("%s:%s:%ld:", g->gr_name, g->gr_passwd,
(long) g->gr_gid);
for (i = 0; g->gr_mem[i] != NULL; i++) {
printf("%s", g->gr_mem[i]);
if (g->gr_mem[i+1] != NULL)
putchar(',');
}
putchar('\n');
}
endgrent();
return 0;
}
Each line in the group database represents one group. The fields are
separated with colons and represent the following information:
Group Name
The group's name.
Group Password
The group's encrypted password. In practice, this field is never
used; it is usually empty or set to '*'.
Group ID Number
The group's numeric group ID number; the association of name to
number must be unique within the file. (On some systems it's a C
'long', and not an 'int'. Thus, we cast it to 'long' for all
cases.)
Group Member List
A comma-separated list of usernames. These users are members of
the group. Modern Unix systems allow users to be members of
several groups simultaneously. If your system does, then there are
elements '"group1"' through '"groupN"' in 'PROCINFO' for those
group ID numbers. (Note that 'PROCINFO' is a 'gawk' extension;
*note Built-in Variables::.)
Here is what running 'grcat' might produce:
$ grcat
-| wheel:*:0:arnold
-| nogroup:*:65534:
-| daemon:*:1:
-| kmem:*:2:
-| staff:*:10:arnold,miriam,andy
-| other:*:20:
...
Here are the functions for obtaining information from the group
database. There are several, modeled after the C library functions of
the same names:
# group.awk --- functions for dealing with the group file
BEGIN {
# Change to suit your system
_gr_awklib = "/usr/local/libexec/awk/"
}
function _gr_init( oldfs, oldrs, olddol0, grcat,
using_fw, using_fpat, n, a, i)
{
if (_gr_inited)
return
oldfs = FS
oldrs = RS
olddol0 = $0
using_fw = (PROCINFO["FS"] == "FIELDWIDTHS")
using_fpat = (PROCINFO["FS"] == "FPAT")
FS = ":"
RS = "\n"
grcat = _gr_awklib "grcat"
while ((grcat | getline) > 0) {
if ($1 in _gr_byname)
_gr_byname[$1] = _gr_byname[$1] "," $4
else
_gr_byname[$1] = $0
if ($3 in _gr_bygid)
_gr_bygid[$3] = _gr_bygid[$3] "," $4
else
_gr_bygid[$3] = $0
n = split($4, a, "[ \t]*,[ \t]*")
for (i = 1; i <= n; i++)
if (a[i] in _gr_groupsbyuser)
_gr_groupsbyuser[a[i]] = _gr_groupsbyuser[a[i]] " " $1
else
_gr_groupsbyuser[a[i]] = $1
_gr_bycount[++_gr_count] = $0
}
close(grcat)
_gr_count = 0
_gr_inited++
FS = oldfs
if (using_fw)
FIELDWIDTHS = FIELDWIDTHS
else if (using_fpat)
FPAT = FPAT
RS = oldrs
$0 = olddol0
}
The 'BEGIN' rule sets a private variable to the directory where
'grcat' is stored. Because it is used to help out an 'awk' library
routine, we have chosen to put it in '/usr/local/libexec/awk'. You
might want it to be in a different directory on your system.
These routines follow the same general outline as the user database
routines (*note Passwd Functions::). The '_gr_inited' variable is used
to ensure that the database is scanned no more than once. The
'_gr_init()' function first saves 'FS', 'RS', and '$0', and then sets
'FS' and 'RS' to the correct values for scanning the group information.
It also takes care to note whether 'FIELDWIDTHS' or 'FPAT' is being
used, and to restore the appropriate field-splitting mechanism.
The group information is stored in several associative arrays. The
arrays are indexed by group name ('_gr_byname'), by group ID number
('_gr_bygid'), and by position in the database ('_gr_bycount'). There
is an additional array indexed by username ('_gr_groupsbyuser'), which
is a space-separated list of groups to which each user belongs.
Unlike in the user database, it is possible to have multiple records
in the database for the same group. This is common when a group has a
large number of members. A pair of such entries might look like the
following:
tvpeople:*:101:johnny,jay,arsenio
tvpeople:*:101:david,conan,tom,joan
For this reason, '_gr_init()' looks to see if a group name or group
ID number is already seen. If so, the usernames are simply concatenated
onto the previous list of users.(1)
Finally, '_gr_init()' closes the pipeline to 'grcat', restores 'FS'
(and 'FIELDWIDTHS' or 'FPAT', if necessary), 'RS', and '$0', initializes
'_gr_count' to zero (it is used later), and makes '_gr_inited' nonzero.
The 'getgrnam()' function takes a group name as its argument, and if
that group exists, it is returned. Otherwise, it relies on the array
reference to a nonexistent element to create the element with the null
string as its value:
function getgrnam(group)
{
_gr_init()
return _gr_byname[group]
}
The 'getgrgid()' function is similar; it takes a numeric group ID and
looks up the information associated with that group ID:
function getgrgid(gid)
{
_gr_init()
return _gr_bygid[gid]
}
The 'getgruser()' function does not have a C counterpart. It takes a
username and returns the list of groups that have the user as a member:
function getgruser(user)
{
_gr_init()
return _gr_groupsbyuser[user]
}
The 'getgrent()' function steps through the database one entry at a
time. It uses '_gr_count' to track its position in the list:
function getgrent()
{
_gr_init()
if (++_gr_count in _gr_bycount)
return _gr_bycount[_gr_count]
return ""
}
The 'endgrent()' function resets '_gr_count' to zero so that
'getgrent()' can start over again:
function endgrent()
{
_gr_count = 0
}
As with the user database routines, each function calls '_gr_init()'
to initialize the arrays. Doing so only incurs the extra overhead of
running 'grcat' if these functions are used (as opposed to moving the
body of '_gr_init()' into a 'BEGIN' rule).
Most of the work is in scanning the database and building the various
associative arrays. The functions that the user calls are themselves
very simple, relying on 'awk''s associative arrays to do work.
The 'id' program in *note Id Program:: uses these functions.
---------- Footnotes ----------
(1) There is a subtle problem with the code just presented. Suppose
that the first time there were no names. This code adds the names with
a leading comma. It also doesn't check that there is a '$4'.
File: gawk.info, Node: Walking Arrays, Next: Library Functions Summary, Prev: Group Functions, Up: Library Functions
10.7 Traversing Arrays of Arrays
================================
*note Arrays of Arrays:: described how 'gawk' provides arrays of arrays.
In particular, any element of an array may be either a scalar or another
array. The 'isarray()' function (*note Type Functions::) lets you
distinguish an array from a scalar. The following function,
'walk_array()', recursively traverses an array, printing the element
indices and values. You call it with the array and a string
representing the name of the array:
function walk_array(arr, name, i)
{
for (i in arr) {
if (isarray(arr[i]))
walk_array(arr[i], (name "[" i "]"))
else
printf("%s[%s] = %s\n", name, i, arr[i])
}
}
It works by looping over each element of the array. If any given
element is itself an array, the function calls itself recursively,
passing the subarray and a new string representing the current index.
Otherwise, the function simply prints the element's name, index, and
value. Here is a main program to demonstrate:
BEGIN {
a[1] = 1
a[2][1] = 21
a[2][2] = 22
a[3] = 3
a[4][1][1] = 411
a[4][2] = 42
walk_array(a, "a")
}
When run, the program produces the following output:
$ gawk -f walk_array.awk
-| a[1] = 1
-| a[2][1] = 21
-| a[2][2] = 22
-| a[3] = 3
-| a[4][1][1] = 411
-| a[4][2] = 42
The function just presented simply prints the name and value of each
scalar array element. However, it is easy to generalize it, by passing
in the name of a function to call when walking an array. The modified
function looks like this:
function process_array(arr, name, process, do_arrays, i, new_name)
{
for (i in arr) {
new_name = (name "[" i "]")
if (isarray(arr[i])) {
if (do_arrays)
@process(new_name, arr[i])
process_array(arr[i], new_name, process, do_arrays)
} else
@process(new_name, arr[i])
}
}
The arguments are as follows:
'arr'
The array.
'name'
The name of the array (a string).
'process'
The name of the function to call.
'do_arrays'
If this is true, the function can handle elements that are
subarrays.
If subarrays are to be processed, that is done before walking them
further.
When run with the following scaffolding, the function produces the
same results as does the earlier version of 'walk_array()':
BEGIN {
a[1] = 1
a[2][1] = 21
a[2][2] = 22
a[3] = 3
a[4][1][1] = 411
a[4][2] = 42
process_array(a, "a", "do_print", 0)
}
function do_print(name, element)
{
printf "%s = %s\n", name, element
}
File: gawk.info, Node: Library Functions Summary, Next: Library Exercises, Prev: Walking Arrays, Up: Library Functions
10.8 Summary
============
* Reading programs is an excellent way to learn Good Programming.
The functions and programs provided in this major node and the next
are intended to serve that purpose.
* When writing general-purpose library functions, put some thought
into how to name any global variables so that they won't conflict
with variables from a user's program.
* The functions presented here fit into the following categories:
General problems
Number-to-string conversion, testing assertions, rounding,
random number generation, converting characters to numbers,
joining strings, getting easily usable time-of-day
information, and reading a whole file in one shot
Managing data files
Noting data file boundaries, rereading the current file,
checking for readable files, checking for zero-length files,
and treating assignments as file names
Processing command-line options
An 'awk' version of the standard C 'getopt()' function
Reading the user and group databases
Two sets of routines that parallel the C library versions
Traversing arrays of arrays
Two functions that traverse an array of arrays to any depth
File: gawk.info, Node: Library Exercises, Prev: Library Functions Summary, Up: Library Functions
10.9 Exercises
==============
1. In *note Empty Files::, we presented the 'zerofile.awk' program,
which made use of 'gawk''s 'ARGIND' variable. Can this problem be
solved without relying on 'ARGIND'? If so, how?
2. As a related challenge, revise that code to handle the case where
an intervening value in 'ARGV' is a variable assignment.
File: gawk.info, Node: Sample Programs, Next: Advanced Features, Prev: Library Functions, Up: Top
11 Practical 'awk' Programs
***************************
*note Library Functions::, presents the idea that reading programs in a
language contributes to learning that language. This major node
continues that theme, presenting a potpourri of 'awk' programs for your
reading enjoyment.
Many of these programs use library functions presented in *note
Library Functions::.
* Menu:
* Running Examples:: How to run these examples.
* Clones:: Clones of common utilities.
* Miscellaneous Programs:: Some interesting 'awk' programs.
* Programs Summary:: Summary of programs.
* Programs Exercises:: Exercises.
File: gawk.info, Node: Running Examples, Next: Clones, Up: Sample Programs
11.1 Running the Example Programs
=================================
To run a given program, you would typically do something like this:
awk -f PROGRAM -- OPTIONS FILES
Here, PROGRAM is the name of the 'awk' program (such as 'cut.awk'),
OPTIONS are any command-line options for the program that start with a
'-', and FILES are the actual data files.
If your system supports the '#!' executable interpreter mechanism
(*note Executable Scripts::), you can instead run your program directly:
cut.awk -c1-8 myfiles > results
If your 'awk' is not 'gawk', you may instead need to use this:
cut.awk -- -c1-8 myfiles > results
File: gawk.info, Node: Clones, Next: Miscellaneous Programs, Prev: Running Examples, Up: Sample Programs
11.2 Reinventing Wheels for Fun and Profit
==========================================
This minor node presents a number of POSIX utilities implemented in
'awk'. Reinventing these programs in 'awk' is often enjoyable, because
the algorithms can be very clearly expressed, and the code is usually
very concise and simple. This is true because 'awk' does so much for
you.
It should be noted that these programs are not necessarily intended
to replace the installed versions on your system. Nor may all of these
programs be fully compliant with the most recent POSIX standard. This
is not a problem; their purpose is to illustrate 'awk' language
programming for "real-world" tasks.
The programs are presented in alphabetical order.
* Menu:
* Cut Program:: The 'cut' utility.
* Egrep Program:: The 'egrep' utility.
* Id Program:: The 'id' utility.
* Split Program:: The 'split' utility.
* Tee Program:: The 'tee' utility.
* Uniq Program:: The 'uniq' utility.
* Wc Program:: The 'wc' utility.
File: gawk.info, Node: Cut Program, Next: Egrep Program, Up: Clones
11.2.1 Cutting Out Fields and Columns
-------------------------------------
The 'cut' utility selects, or "cuts," characters or fields from its
standard input and sends them to its standard output. Fields are
separated by TABs by default, but you may supply a command-line option
to change the field "delimiter" (i.e., the field-separator character).
'cut''s definition of fields is less general than 'awk''s.
A common use of 'cut' might be to pull out just the login names of
logged-on users from the output of 'who'. For example, the following
pipeline generates a sorted, unique list of the logged-on users:
who | cut -c1-8 | sort | uniq
The options for 'cut' are:
'-c LIST'
Use LIST as the list of characters to cut out. Items within the
list may be separated by commas, and ranges of characters can be
separated with dashes. The list '1-8,15,22-35' specifies
characters 1 through 8, 15, and 22 through 35.
'-f LIST'
Use LIST as the list of fields to cut out.
'-d DELIM'
Use DELIM as the field-separator character instead of the TAB
character.
'-s'
Suppress printing of lines that do not contain the field delimiter.
The 'awk' implementation of 'cut' uses the 'getopt()' library
function (*note Getopt Function::) and the 'join()' library function
(*note Join Function::).
The program begins with a comment describing the options, the library
functions needed, and a 'usage()' function that prints out a usage
message and exits. 'usage()' is called if invalid arguments are
supplied:
# cut.awk --- implement cut in awk
# Options:
# -f list Cut fields
# -d c Field delimiter character
# -c list Cut characters
#
# -s Suppress lines without the delimiter
#
# Requires getopt() and join() library functions
function usage()
{
print("usage: cut [-f list] [-d c] [-s] [files...]") > "/dev/stderr"
print("usage: cut [-c list] [files...]") > "/dev/stderr"
exit 1
}
Next comes a 'BEGIN' rule that parses the command-line options. It
sets 'FS' to a single TAB character, because that is 'cut''s default
field separator. The rule then sets the output field separator to be
the same as the input field separator. A loop using 'getopt()' steps
through the command-line options. Exactly one of the variables
'by_fields' or 'by_chars' is set to true, to indicate that processing
should be done by fields or by characters, respectively. When cutting
by characters, the output field separator is set to the null string:
BEGIN {
FS = "\t" # default
OFS = FS
while ((c = getopt(ARGC, ARGV, "sf:c:d:")) != -1) {
if (c == "f") {
by_fields = 1
fieldlist = Optarg
} else if (c == "c") {
by_chars = 1
fieldlist = Optarg
OFS = ""
} else if (c == "d") {
if (length(Optarg) > 1) {
printf("cut: using first character of %s" \
" for delimiter\n", Optarg) > "/dev/stderr"
Optarg = substr(Optarg, 1, 1)
}
fs = FS = Optarg
OFS = FS
if (FS == " ") # defeat awk semantics
FS = "[ ]"
} else if (c == "s")
suppress = 1
else
usage()
}
# Clear out options
for (i = 1; i < Optind; i++)
ARGV[i] = ""
The code must take special care when the field delimiter is a space.
Using a single space ('" "') for the value of 'FS' is incorrect--'awk'
would separate fields with runs of spaces, TABs, and/or newlines, and we
want them to be separated with individual spaces. To this end, we save
the original space character in the variable 'fs' for later use; after
setting 'FS' to '"[ ]"' we can't use it directly to see if the field
delimiter character is in the string.
Also remember that after 'getopt()' is through (as described in *note
Getopt Function::), we have to clear out all the elements of 'ARGV' from
1 to 'Optind', so that 'awk' does not try to process the command-line
options as file names.
After dealing with the command-line options, the program verifies
that the options make sense. Only one or the other of '-c' and '-f'
should be used, and both require a field list. Then the program calls
either 'set_fieldlist()' or 'set_charlist()' to pull apart the list of
fields or characters:
if (by_fields && by_chars)
usage()
if (by_fields == 0 && by_chars == 0)
by_fields = 1 # default
if (fieldlist == "") {
print "cut: needs list for -c or -f" > "/dev/stderr"
exit 1
}
if (by_fields)
set_fieldlist()
else
set_charlist()
}
'set_fieldlist()' splits the field list apart at the commas into an
array. Then, for each element of the array, it looks to see if the
element is actually a range, and if so, splits it apart. The function
checks the range to make sure that the first number is smaller than the
second. Each number in the list is added to the 'flist' array, which
simply lists the fields that will be printed. Normal field splitting is
used. The program lets 'awk' handle the job of doing the field
splitting:
function set_fieldlist( n, m, i, j, k, f, g)
{
n = split(fieldlist, f, ",")
j = 1 # index in flist
for (i = 1; i <= n; i++) {
if (index(f[i], "-") != 0) { # a range
m = split(f[i], g, "-")
if (m != 2 || g[1] >= g[2]) {
printf("cut: bad field list: %s\n",
f[i]) > "/dev/stderr"
exit 1
}
for (k = g[1]; k <= g[2]; k++)
flist[j++] = k
} else
flist[j++] = f[i]
}
nfields = j - 1
}
The 'set_charlist()' function is more complicated than
'set_fieldlist()'. The idea here is to use 'gawk''s 'FIELDWIDTHS'
variable (*note Constant Size::), which describes constant-width input.
When using a character list, that is exactly what we have.
Setting up 'FIELDWIDTHS' is more complicated than simply listing the
fields that need to be printed. We have to keep track of the fields to
print and also the intervening characters that have to be skipped. For
example, suppose you wanted characters 1 through 8, 15, and 22 through
35. You would use '-c 1-8,15,22-35'. The necessary value for
'FIELDWIDTHS' is '"8 6 1 6 14"'. This yields five fields, and the
fields to print are '$1', '$3', and '$5'. The intermediate fields are
"filler", which is stuff in between the desired data. 'flist' lists the
fields to print, and 't' tracks the complete field list, including
filler fields:
function set_charlist( field, i, j, f, g, n, m, t,
filler, last, len)
{
field = 1 # count total fields
n = split(fieldlist, f, ",")
j = 1 # index in flist
for (i = 1; i <= n; i++) {
if (index(f[i], "-") != 0) { # range
m = split(f[i], g, "-")
if (m != 2 || g[1] >= g[2]) {
printf("cut: bad character list: %s\n",
f[i]) > "/dev/stderr"
exit 1
}
len = g[2] - g[1] + 1
if (g[1] > 1) # compute length of filler
filler = g[1] - last - 1
else
filler = 0
if (filler)
t[field++] = filler
t[field++] = len # length of field
last = g[2]
flist[j++] = field - 1
} else {
if (f[i] > 1)
filler = f[i] - last - 1
else
filler = 0
if (filler)
t[field++] = filler
t[field++] = 1
last = f[i]
flist[j++] = field - 1
}
}
FIELDWIDTHS = join(t, 1, field - 1)
nfields = j - 1
}
Next is the rule that processes the data. If the '-s' option is
given, then 'suppress' is true. The first 'if' statement makes sure
that the input record does have the field separator. If 'cut' is
processing fields, 'suppress' is true, and the field separator character
is not in the record, then the record is skipped.
If the record is valid, then 'gawk' has split the data into fields,
either using the character in 'FS' or using fixed-length fields and
'FIELDWIDTHS'. The loop goes through the list of fields that should be
printed. The corresponding field is printed if it contains data. If
the next field also has data, then the separator character is written
out between the fields:
{
if (by_fields && suppress && index($0, fs) == 0)
next
for (i = 1; i <= nfields; i++) {
if ($flist[i] != "") {
printf "%s", $flist[i]
if (i < nfields && $flist[i+1] != "")
printf "%s", OFS
}
}
print ""
}
This version of 'cut' relies on 'gawk''s 'FIELDWIDTHS' variable to do
the character-based cutting. It is possible in other 'awk'
implementations to use 'substr()' (*note String Functions::), but it is
also extremely painful. The 'FIELDWIDTHS' variable supplies an elegant
solution to the problem of picking the input line apart by characters.
File: gawk.info, Node: Egrep Program, Next: Id Program, Prev: Cut Program, Up: Clones
11.2.2 Searching for Regular Expressions in Files
-------------------------------------------------
The 'egrep' utility searches files for patterns. It uses regular
expressions that are almost identical to those available in 'awk' (*note
Regexp::). You invoke it as follows:
'egrep' [OPTIONS] ''PATTERN'' FILES ...
The PATTERN is a regular expression. In typical usage, the regular
expression is quoted to prevent the shell from expanding any of the
special characters as file name wildcards. Normally, 'egrep' prints the
lines that matched. If multiple file names are provided on the command
line, each output line is preceded by the name of the file and a colon.
The options to 'egrep' are as follows:
'-c'
Print out a count of the lines that matched the pattern, instead of
the lines themselves.
'-s'
Be silent. No output is produced and the exit value indicates
whether the pattern was matched.
'-v'
Invert the sense of the test. 'egrep' prints the lines that do
_not_ match the pattern and exits successfully if the pattern is
not matched.
'-i'
Ignore case distinctions in both the pattern and the input data.
'-l'
Only print (list) the names of the files that matched, not the
lines that matched.
'-e PATTERN'
Use PATTERN as the regexp to match. The purpose of the '-e' option
is to allow patterns that start with a '-'.
This version uses the 'getopt()' library function (*note Getopt
Function::) and the file transition library program (*note Filetrans
Function::).
The program begins with a descriptive comment and then a 'BEGIN' rule
that processes the command-line arguments with 'getopt()'. The '-i'
(ignore case) option is particularly easy with 'gawk'; we just use the
'IGNORECASE' predefined variable (*note Built-in Variables::):
# egrep.awk --- simulate egrep in awk
#
# Options:
# -c count of lines
# -s silent - use exit value
# -v invert test, success if no match
# -i ignore case
# -l print filenames only
# -e argument is pattern
#
# Requires getopt and file transition library functions
BEGIN {
while ((c = getopt(ARGC, ARGV, "ce:svil")) != -1) {
if (c == "c")
count_only++
else if (c == "s")
no_print++
else if (c == "v")
invert++
else if (c == "i")
IGNORECASE = 1
else if (c == "l")
filenames_only++
else if (c == "e")
pattern = Optarg
else
usage()
}
Next comes the code that handles the 'egrep'-specific behavior. If
no pattern is supplied with '-e', the first nonoption on the command
line is used. The 'awk' command-line arguments up to 'ARGV[Optind]' are
cleared, so that 'awk' won't try to process them as files. If no files
are specified, the standard input is used, and if multiple files are
specified, we make sure to note this so that the file names can precede
the matched lines in the output:
if (pattern == "")
pattern = ARGV[Optind++]
for (i = 1; i < Optind; i++)
ARGV[i] = ""
if (Optind >= ARGC) {
ARGV[1] = "-"
ARGC = 2
} else if (ARGC - Optind > 1)
do_filenames++
# if (IGNORECASE)
# pattern = tolower(pattern)
}
The last two lines are commented out, as they are not needed in
'gawk'. They should be uncommented if you have to use another version
of 'awk'.
The next set of lines should be uncommented if you are not using
'gawk'. This rule translates all the characters in the input line into
lowercase if the '-i' option is specified.(1) The rule is commented out
as it is not necessary with 'gawk':
#{
# if (IGNORECASE)
# $0 = tolower($0)
#}
The 'beginfile()' function is called by the rule in 'ftrans.awk' when
each new file is processed. In this case, it is very simple; all it
does is initialize a variable 'fcount' to zero. 'fcount' tracks how
many lines in the current file matched the pattern. Naming the
parameter 'junk' shows we know that 'beginfile()' is called with a
parameter, but that we're not interested in its value:
function beginfile(junk)
{
fcount = 0
}
The 'endfile()' function is called after each file has been
processed. It affects the output only when the user wants a count of
the number of lines that matched. 'no_print' is true only if the exit
status is desired. 'count_only' is true if line counts are desired.
'egrep' therefore only prints line counts if printing and counting are
enabled. The output format must be adjusted depending upon the number
of files to process. Finally, 'fcount' is added to 'total', so that we
know the total number of lines that matched the pattern:
function endfile(file)
{
if (! no_print && count_only) {
if (do_filenames)
print file ":" fcount
else
print fcount
}
total += fcount
}
The 'BEGINFILE' and 'ENDFILE' special patterns (*note
BEGINFILE/ENDFILE::) could be used, but then the program would be
'gawk'-specific. Additionally, this example was written before 'gawk'
acquired 'BEGINFILE' and 'ENDFILE'.
The following rule does most of the work of matching lines. The
variable 'matches' is true if the line matched the pattern. If the user
wants lines that did not match, the sense of 'matches' is inverted using
the '!' operator. 'fcount' is incremented with the value of 'matches',
which is either one or zero, depending upon a successful or unsuccessful
match. If the line does not match, the 'next' statement just moves on
to the next record.
A number of additional tests are made, but they are only done if we
are not counting lines. First, if the user only wants the exit status
('no_print' is true), then it is enough to know that _one_ line in this
file matched, and we can skip on to the next file with 'nextfile'.
Similarly, if we are only printing file names, we can print the file
name, and then skip to the next file with 'nextfile'. Finally, each
line is printed, with a leading file name and colon if necessary:
{
matches = ($0 ~ pattern)
if (invert)
matches = ! matches
fcount += matches # 1 or 0
if (! matches)
next
if (! count_only) {
if (no_print)
nextfile
if (filenames_only) {
print FILENAME
nextfile
}
if (do_filenames)
print FILENAME ":" $0
else
print
}
}
The 'END' rule takes care of producing the correct exit status. If
there are no matches, the exit status is one; otherwise, it is zero:
END {
exit (total == 0)
}
The 'usage()' function prints a usage message in case of invalid
options, and then exits:
function usage()
{
print("Usage: egrep [-csvil] [-e pat] [files ...]") > "/dev/stderr"
print("\n\tegrep [-csvil] pat [files ...]") > "/dev/stderr"
exit 1
}
---------- Footnotes ----------
(1) It also introduces a subtle bug; if a match happens, we output
the translated line, not the original.
File: gawk.info, Node: Id Program, Next: Split Program, Prev: Egrep Program, Up: Clones
11.2.3 Printing Out User Information
------------------------------------
The 'id' utility lists a user's real and effective user ID numbers, real
and effective group ID numbers, and the user's group set, if any. 'id'
only prints the effective user ID and group ID if they are different
from the real ones. If possible, 'id' also supplies the corresponding
user and group names. The output might look like this:
$ id
-| uid=1000(arnold) gid=1000(arnold) groups=1000(arnold),4(adm),7(lp),27(sudo)
This information is part of what is provided by 'gawk''s 'PROCINFO'
array (*note Built-in Variables::). However, the 'id' utility provides
a more palatable output than just individual numbers.
Here is a simple version of 'id' written in 'awk'. It uses the user
database library functions (*note Passwd Functions::) and the group
database library functions (*note Group Functions::) from *note Library
Functions::.
The program is fairly straightforward. All the work is done in the
'BEGIN' rule. The user and group ID numbers are obtained from
'PROCINFO'. The code is repetitive. The entry in the user database for
the real user ID number is split into parts at the ':'. The name is the
first field. Similar code is used for the effective user ID number and
the group numbers:
# id.awk --- implement id in awk
#
# Requires user and group library functions
# output is:
# uid=12(foo) euid=34(bar) gid=3(baz) \
# egid=5(blat) groups=9(nine),2(two),1(one)
BEGIN {
uid = PROCINFO["uid"]
euid = PROCINFO["euid"]
gid = PROCINFO["gid"]
egid = PROCINFO["egid"]
printf("uid=%d", uid)
pw = getpwuid(uid)
pr_first_field(pw)
if (euid != uid) {
printf(" euid=%d", euid)
pw = getpwuid(euid)
pr_first_field(pw)
}
printf(" gid=%d", gid)
pw = getgrgid(gid)
pr_first_field(pw)
if (egid != gid) {
printf(" egid=%d", egid)
pw = getgrgid(egid)
pr_first_field(pw)
}
for (i = 1; ("group" i) in PROCINFO; i++) {
if (i == 1)
printf(" groups=")
group = PROCINFO["group" i]
printf("%d", group)
pw = getgrgid(group)
pr_first_field(pw)
if (("group" (i+1)) in PROCINFO)
printf(",")
}
print ""
}
function pr_first_field(str, a)
{
if (str != "") {
split(str, a, ":")
printf("(%s)", a[1])
}
}
The test in the 'for' loop is worth noting. Any supplementary groups
in the 'PROCINFO' array have the indices '"group1"' through '"groupN"'
for some N (i.e., the total number of supplementary groups). However,
we don't know in advance how many of these groups there are.
This loop works by starting at one, concatenating the value with
'"group"', and then using 'in' to see if that value is in the array
(*note Reference to Elements::). Eventually, 'i' is incremented past
the last group in the array and the loop exits.
The loop is also correct if there are _no_ supplementary groups; then
the condition is false the first time it's tested, and the loop body
never executes.
The 'pr_first_field()' function simply isolates out some code that is
used repeatedly, making the whole program shorter and cleaner. In
particular, moving the check for the empty string into this function
saves several lines of code.
File: gawk.info, Node: Split Program, Next: Tee Program, Prev: Id Program, Up: Clones
11.2.4 Splitting a Large File into Pieces
-----------------------------------------
The 'split' program splits large text files into smaller pieces. Usage
is as follows:(1)
'split' ['-COUNT'] [FILE] [PREFIX]
By default, the output files are named 'xaa', 'xab', and so on. Each
file has 1,000 lines in it, with the likely exception of the last file.
To change the number of lines in each file, supply a number on the
command line preceded with a minus sign (e.g., '-500' for files with 500
lines in them instead of 1,000). To change the names of the output
files to something like 'myfileaa', 'myfileab', and so on, supply an
additional argument that specifies the file name prefix.
Here is a version of 'split' in 'awk'. It uses the 'ord()' and
'chr()' functions presented in *note Ordinal Functions::.
The program first sets its defaults, and then tests to make sure
there are not too many arguments. It then looks at each argument in
turn. The first argument could be a minus sign followed by a number.
If it is, this happens to look like a negative number, so it is made
positive, and that is the count of lines. The data file name is skipped
over and the final argument is used as the prefix for the output file
names:
# split.awk --- do split in awk
#
# Requires ord() and chr() library functions
# usage: split [-count] [file] [outname]
BEGIN {
outfile = "x" # default
count = 1000
if (ARGC > 4)
usage()
i = 1
if (i in ARGV && ARGV[i] ~ /^-[[:digit:]]+$/) {
count = -ARGV[i]
ARGV[i] = ""
i++
}
# test argv in case reading from stdin instead of file
if (i in ARGV)
i++ # skip datafile name
if (i in ARGV) {
outfile = ARGV[i]
ARGV[i] = ""
}
s1 = s2 = "a"
out = (outfile s1 s2)
}
The next rule does most of the work. 'tcount' (temporary count)
tracks how many lines have been printed to the output file so far. If
it is greater than 'count', it is time to close the current file and
start a new one. 's1' and 's2' track the current suffixes for the file
name. If they are both 'z', the file is just too big. Otherwise, 's1'
moves to the next letter in the alphabet and 's2' starts over again at
'a':
{
if (++tcount > count) {
close(out)
if (s2 == "z") {
if (s1 == "z") {
printf("split: %s is too large to split\n",
FILENAME) > "/dev/stderr"
exit 1
}
s1 = chr(ord(s1) + 1)
s2 = "a"
}
else
s2 = chr(ord(s2) + 1)
out = (outfile s1 s2)
tcount = 1
}
print > out
}
The 'usage()' function simply prints an error message and exits:
function usage()
{
print("usage: split [-num] [file] [outname]") > "/dev/stderr"
exit 1
}
This program is a bit sloppy; it relies on 'awk' to automatically
close the last file instead of doing it in an 'END' rule. It also
assumes that letters are contiguous in the character set, which isn't
true for EBCDIC systems.
---------- Footnotes ----------
(1) This is the traditional usage. The POSIX usage is different, but
not relevant for what the program aims to demonstrate.
File: gawk.info, Node: Tee Program, Next: Uniq Program, Prev: Split Program, Up: Clones
11.2.5 Duplicating Output into Multiple Files
---------------------------------------------
The 'tee' program is known as a "pipe fitting." 'tee' copies its
standard input to its standard output and also duplicates it to the
files named on the command line. Its usage is as follows:
'tee' ['-a'] FILE ...
The '-a' option tells 'tee' to append to the named files, instead of
truncating them and starting over.
The 'BEGIN' rule first makes a copy of all the command-line arguments
into an array named 'copy'. 'ARGV[0]' is not needed, so it is not
copied. 'tee' cannot use 'ARGV' directly, because 'awk' attempts to
process each file name in 'ARGV' as input data.
If the first argument is '-a', then the flag variable 'append' is set
to true, and both 'ARGV[1]' and 'copy[1]' are deleted. If 'ARGC' is
less than two, then no file names were supplied and 'tee' prints a usage
message and exits. Finally, 'awk' is forced to read the standard input
by setting 'ARGV[1]' to '"-"' and 'ARGC' to two:
# tee.awk --- tee in awk
#
# Copy standard input to all named output files.
# Append content if -a option is supplied.
#
BEGIN {
for (i = 1; i < ARGC; i++)
copy[i] = ARGV[i]
if (ARGV[1] == "-a") {
append = 1
delete ARGV[1]
delete copy[1]
ARGC--
}
if (ARGC < 2) {
print "usage: tee [-a] file ..." > "/dev/stderr"
exit 1
}
ARGV[1] = "-"
ARGC = 2
}
The following single rule does all the work. Because there is no
pattern, it is executed for each line of input. The body of the rule
simply prints the line into each file on the command line, and then to
the standard output:
{
# moving the if outside the loop makes it run faster
if (append)
for (i in copy)
print >> copy[i]
else
for (i in copy)
print > copy[i]
print
}
It is also possible to write the loop this way:
for (i in copy)
if (append)
print >> copy[i]
else
print > copy[i]
This is more concise, but it is also less efficient. The 'if' is tested
for each record and for each output file. By duplicating the loop body,
the 'if' is only tested once for each input record. If there are N
input records and M output files, the first method only executes N 'if'
statements, while the second executes N'*'M 'if' statements.
Finally, the 'END' rule cleans up by closing all the output files:
END {
for (i in copy)
close(copy[i])
}
File: gawk.info, Node: Uniq Program, Next: Wc Program, Prev: Tee Program, Up: Clones
11.2.6 Printing Nonduplicated Lines of Text
-------------------------------------------
The 'uniq' utility reads sorted lines of data on its standard input, and
by default removes duplicate lines. In other words, it only prints
unique lines--hence the name. 'uniq' has a number of options. The
usage is as follows:
'uniq' ['-udc' ['-N']] ['+N'] [INPUTFILE [OUTPUTFILE]]
The options for 'uniq' are:
'-d'
Print only repeated (duplicated) lines.
'-u'
Print only nonrepeated (unique) lines.
'-c'
Count lines. This option overrides '-d' and '-u'. Both repeated
and nonrepeated lines are counted.
'-N'
Skip N fields before comparing lines. The definition of fields is
similar to 'awk''s default: nonwhitespace characters separated by
runs of spaces and/or TABs.
'+N'
Skip N characters before comparing lines. Any fields specified
with '-N' are skipped first.
'INPUTFILE'
Data is read from the input file named on the command line, instead
of from the standard input.
'OUTPUTFILE'
The generated output is sent to the named output file, instead of
to the standard output.
Normally 'uniq' behaves as if both the '-d' and '-u' options are
provided.
'uniq' uses the 'getopt()' library function (*note Getopt Function::)
and the 'join()' library function (*note Join Function::).
The program begins with a 'usage()' function and then a brief outline
of the options and their meanings in comments. The 'BEGIN' rule deals
with the command-line arguments and options. It uses a trick to get
'getopt()' to handle options of the form '-25', treating such an option
as the option letter '2' with an argument of '5'. If indeed two or more
digits are supplied ('Optarg' looks like a number), 'Optarg' is
concatenated with the option digit and then the result is added to zero
to make it into a number. If there is only one digit in the option,
then 'Optarg' is not needed. In this case, 'Optind' must be decremented
so that 'getopt()' processes it next time. This code is admittedly a
bit tricky.
If no options are supplied, then the default is taken, to print both
repeated and nonrepeated lines. The output file, if provided, is
assigned to 'outputfile'. Early on, 'outputfile' is initialized to the
standard output, '/dev/stdout':
# uniq.awk --- do uniq in awk
#
# Requires getopt() and join() library functions
function usage()
{
print("Usage: uniq [-udc [-n]] [+n] [ in [ out ]]") > "/dev/stderr"
exit 1
}
# -c count lines. overrides -d and -u
# -d only repeated lines
# -u only nonrepeated lines
# -n skip n fields
# +n skip n characters, skip fields first
BEGIN {
count = 1
outputfile = "/dev/stdout"
opts = "udc0:1:2:3:4:5:6:7:8:9:"
while ((c = getopt(ARGC, ARGV, opts)) != -1) {
if (c == "u")
non_repeated_only++
else if (c == "d")
repeated_only++
else if (c == "c")
do_count++
else if (index("0123456789", c) != 0) {
# getopt() requires args to options
# this messes us up for things like -5
if (Optarg ~ /^[[:digit:]]+$/)
fcount = (c Optarg) + 0
else {
fcount = c + 0
Optind--
}
} else
usage()
}
if (ARGV[Optind] ~ /^\+[[:digit:]]+$/) {
charcount = substr(ARGV[Optind], 2) + 0
Optind++
}
for (i = 1; i < Optind; i++)
ARGV[i] = ""
if (repeated_only == 0 && non_repeated_only == 0)
repeated_only = non_repeated_only = 1
if (ARGC - Optind == 2) {
outputfile = ARGV[ARGC - 1]
ARGV[ARGC - 1] = ""
}
}
The following function, 'are_equal()', compares the current line,
'$0', to the previous line, 'last'. It handles skipping fields and
characters. If no field count and no character count are specified,
'are_equal()' returns one or zero depending upon the result of a simple
string comparison of 'last' and '$0'.
Otherwise, things get more complicated. If fields have to be
skipped, each line is broken into an array using 'split()' (*note String
Functions::); the desired fields are then joined back into a line using
'join()'. The joined lines are stored in 'clast' and 'cline'. If no
fields are skipped, 'clast' and 'cline' are set to 'last' and '$0',
respectively. Finally, if characters are skipped, 'substr()' is used to
strip off the leading 'charcount' characters in 'clast' and 'cline'.
The two strings are then compared and 'are_equal()' returns the result:
function are_equal( n, m, clast, cline, alast, aline)
{
if (fcount == 0 && charcount == 0)
return (last == $0)
if (fcount > 0) {
n = split(last, alast)
m = split($0, aline)
clast = join(alast, fcount+1, n)
cline = join(aline, fcount+1, m)
} else {
clast = last
cline = $0
}
if (charcount) {
clast = substr(clast, charcount + 1)
cline = substr(cline, charcount + 1)
}
return (clast == cline)
}
The following two rules are the body of the program. The first one
is executed only for the very first line of data. It sets 'last' equal
to '$0', so that subsequent lines of text have something to be compared
to.
The second rule does the work. The variable 'equal' is one or zero,
depending upon the results of 'are_equal()''s comparison. If 'uniq' is
counting repeated lines, and the lines are equal, then it increments the
'count' variable. Otherwise, it prints the line and resets 'count',
because the two lines are not equal.
If 'uniq' is not counting, and if the lines are equal, 'count' is
incremented. Nothing is printed, as the point is to remove duplicates.
Otherwise, if 'uniq' is counting repeated lines and more than one line
is seen, or if 'uniq' is counting nonrepeated lines and only one line is
seen, then the line is printed, and 'count' is reset.
Finally, similar logic is used in the 'END' rule to print the final
line of input data:
NR == 1 {
last = $0
next
}
{
equal = are_equal()
if (do_count) { # overrides -d and -u
if (equal)
count++
else {
printf("%4d %s\n", count, last) > outputfile
last = $0
count = 1 # reset
}
next
}
if (equal)
count++
else {
if ((repeated_only && count > 1) ||
(non_repeated_only && count == 1))
print last > outputfile
last = $0
count = 1
}
}
END {
if (do_count)
printf("%4d %s\n", count, last) > outputfile
else if ((repeated_only && count > 1) ||
(non_repeated_only && count == 1))
print last > outputfile
close(outputfile)
}
File: gawk.info, Node: Wc Program, Prev: Uniq Program, Up: Clones
11.2.7 Counting Things
----------------------
The 'wc' (word count) utility counts lines, words, and characters in one
or more input files. Its usage is as follows:
'wc' ['-lwc'] [FILES ...]
If no files are specified on the command line, 'wc' reads its
standard input. If there are multiple files, it also prints total
counts for all the files. The options and their meanings are as
follows:
'-l'
Count only lines.
'-w'
Count only words. A "word" is a contiguous sequence of
nonwhitespace characters, separated by spaces and/or TABs.
Luckily, this is the normal way 'awk' separates fields in its input
data.
'-c'
Count only characters.
Implementing 'wc' in 'awk' is particularly elegant, because 'awk'
does a lot of the work for us; it splits lines into words (i.e., fields)
and counts them, it counts lines (i.e., records), and it can easily tell
us how long a line is.
This program uses the 'getopt()' library function (*note Getopt
Function::) and the file-transition functions (*note Filetrans
Function::).
This version has one notable difference from traditional versions of
'wc': it always prints the counts in the order lines, words, and
characters. Traditional versions note the order of the '-l', '-w', and
'-c' options on the command line, and print the counts in that order.
The 'BEGIN' rule does the argument processing. The variable
'print_total' is true if more than one file is named on the command
line:
# wc.awk --- count lines, words, characters
# Options:
# -l only count lines
# -w only count words
# -c only count characters
#
# Default is to count lines, words, characters
#
# Requires getopt() and file transition library functions
BEGIN {
# let getopt() print a message about
# invalid options. we ignore them
while ((c = getopt(ARGC, ARGV, "lwc")) != -1) {
if (c == "l")
do_lines = 1
else if (c == "w")
do_words = 1
else if (c == "c")
do_chars = 1
}
for (i = 1; i < Optind; i++)
ARGV[i] = ""
# if no options, do all
if (! do_lines && ! do_words && ! do_chars)
do_lines = do_words = do_chars = 1
print_total = (ARGC - i > 1)
}
The 'beginfile()' function is simple; it just resets the counts of
lines, words, and characters to zero, and saves the current file name in
'fname':
function beginfile(file)
{
lines = words = chars = 0
fname = FILENAME
}
The 'endfile()' function adds the current file's numbers to the
running totals of lines, words, and characters. It then prints out
those numbers for the file that was just read. It relies on
'beginfile()' to reset the numbers for the following data file:
function endfile(file)
{
tlines += lines
twords += words
tchars += chars
if (do_lines)
printf "\t%d", lines
if (do_words)
printf "\t%d", words
if (do_chars)
printf "\t%d", chars
printf "\t%s\n", fname
}
There is one rule that is executed for each line. It adds the length
of the record, plus one, to 'chars'.(1) Adding one plus the record
length is needed because the newline character separating records (the
value of 'RS') is not part of the record itself, and thus not included
in its length. Next, 'lines' is incremented for each line read, and
'words' is incremented by the value of 'NF', which is the number of
"words" on this line:
# do per line
{
chars += length($0) + 1 # get newline
lines++
words += NF
}
Finally, the 'END' rule simply prints the totals for all the files:
END {
if (print_total) {
if (do_lines)
printf "\t%d", tlines
if (do_words)
printf "\t%d", twords
if (do_chars)
printf "\t%d", tchars
print "\ttotal"
}
}
---------- Footnotes ----------
(1) Because 'gawk' understands multibyte locales, this code counts
characters, not bytes.
File: gawk.info, Node: Miscellaneous Programs, Next: Programs Summary, Prev: Clones, Up: Sample Programs
11.3 A Grab Bag of 'awk' Programs
=================================
This minor node is a large "grab bag" of miscellaneous programs. We
hope you find them both interesting and enjoyable.
* Menu:
* Dupword Program:: Finding duplicated words in a document.
* Alarm Program:: An alarm clock.
* Translate Program:: A program similar to the 'tr' utility.
* Labels Program:: Printing mailing labels.
* Word Sorting:: A program to produce a word usage count.
* History Sorting:: Eliminating duplicate entries from a history
file.
* Extract Program:: Pulling out programs from Texinfo source
files.
* Simple Sed:: A Simple Stream Editor.
* Igawk Program:: A wrapper for 'awk' that includes
files.
* Anagram Program:: Finding anagrams from a dictionary.
* Signature Program:: People do amazing things with too much time on
their hands.
File: gawk.info, Node: Dupword Program, Next: Alarm Program, Up: Miscellaneous Programs
11.3.1 Finding Duplicated Words in a Document
---------------------------------------------
A common error when writing large amounts of prose is to accidentally
duplicate words. Typically you will see this in text as something like
"the the program does the following..." When the text is online, often
the duplicated words occur at the end of one line and the beginning of
another, making them very difficult to spot.
This program, 'dupword.awk', scans through a file one line at a time
and looks for adjacent occurrences of the same word. It also saves the
last word on a line (in the variable 'prev') for comparison with the
first word on the next line.
The first two statements make sure that the line is all lowercase, so
that, for example, "The" and "the" compare equal to each other. The
next statement replaces nonalphanumeric and nonwhitespace characters
with spaces, so that punctuation does not affect the comparison either.
The characters are replaced with spaces so that formatting controls
don't create nonsense words (e.g., the Texinfo '@code{NF}' becomes
'codeNF' if punctuation is simply deleted). The record is then resplit
into fields, yielding just the actual words on the line, and ensuring
that there are no empty fields.
If there are no fields left after removing all the punctuation, the
current record is skipped. Otherwise, the program loops through each
word, comparing it to the previous one:
# dupword.awk --- find duplicate words in text
{
$0 = tolower($0)
gsub(/[^[:alnum:][:blank:]]/, " ");
$0 = $0 # re-split
if (NF == 0)
next
if ($1 == prev)
printf("%s:%d: duplicate %s\n",
FILENAME, FNR, $1)
for (i = 2; i <= NF; i++)
if ($i == $(i-1))
printf("%s:%d: duplicate %s\n",
FILENAME, FNR, $i)
prev = $NF
}
File: gawk.info, Node: Alarm Program, Next: Translate Program, Prev: Dupword Program, Up: Miscellaneous Programs
11.3.2 An Alarm Clock Program
-----------------------------
Nothing cures insomnia like a ringing alarm clock.
-- _Arnold Robbins_
Sleep is for web developers.
-- _Erik Quanstrom_
The following program is a simple "alarm clock" program. You give it
a time of day and an optional message. At the specified time, it prints
the message on the standard output. In addition, you can give it the
number of times to repeat the message as well as a delay between
repetitions.
This program uses the 'getlocaltime()' function from *note
Getlocaltime Function::.
All the work is done in the 'BEGIN' rule. The first part is argument
checking and setting of defaults: the delay, the count, and the message
to print. If the user supplied a message without the ASCII BEL
character (known as the "alert" character, '"\a"'), then it is added to
the message. (On many systems, printing the ASCII BEL generates an
audible alert. Thus, when the alarm goes off, the system calls
attention to itself in case the user is not looking at the computer.)
Just for a change, this program uses a 'switch' statement (*note Switch
Statement::), but the processing could be done with a series of
'if'-'else' statements instead. Here is the program:
# alarm.awk --- set an alarm
#
# Requires getlocaltime() library function
# usage: alarm time [ "message" [ count [ delay ] ] ]
BEGIN {
# Initial argument sanity checking
usage1 = "usage: alarm time ['message' [count [delay]]]"
usage2 = sprintf("\t(%s) time ::= hh:mm", ARGV[1])
if (ARGC < 2) {
print usage1 > "/dev/stderr"
print usage2 > "/dev/stderr"
exit 1
}
switch (ARGC) {
case 5:
delay = ARGV[4] + 0
# fall through
case 4:
count = ARGV[3] + 0
# fall through
case 3:
message = ARGV[2]
break
default:
if (ARGV[1] !~ /[[:digit:]]?[[:digit:]]:[[:digit:]]{2}/) {
print usage1 > "/dev/stderr"
print usage2 > "/dev/stderr"
exit 1
}
break
}
# set defaults for once we reach the desired time
if (delay == 0)
delay = 180 # 3 minutes
if (count == 0)
count = 5
if (message == "")
message = sprintf("\aIt is now %s!\a", ARGV[1])
else if (index(message, "\a") == 0)
message = "\a" message "\a"
The next minor node of code turns the alarm time into hours and
minutes, converts it (if necessary) to a 24-hour clock, and then turns
that time into a count of the seconds since midnight. Next it turns the
current time into a count of seconds since midnight. The difference
between the two is how long to wait before setting off the alarm:
# split up alarm time
split(ARGV[1], atime, ":")
hour = atime[1] + 0 # force numeric
minute = atime[2] + 0 # force numeric
# get current broken down time
getlocaltime(now)
# if time given is 12-hour hours and it's after that
# hour, e.g., `alarm 5:30' at 9 a.m. means 5:30 p.m.,
# then add 12 to real hour
if (hour < 12 && now["hour"] > hour)
hour += 12
# set target time in seconds since midnight
target = (hour * 60 * 60) + (minute * 60)
# get current time in seconds since midnight
current = (now["hour"] * 60 * 60) + \
(now["minute"] * 60) + now["second"]
# how long to sleep for
naptime = target - current
if (naptime <= 0) {
print "alarm: time is in the past!" > "/dev/stderr"
exit 1
}
Finally, the program uses the 'system()' function (*note I/O
Functions::) to call the 'sleep' utility. The 'sleep' utility simply
pauses for the given number of seconds. If the exit status is not zero,
the program assumes that 'sleep' was interrupted and exits. If 'sleep'
exited with an OK status (zero), then the program prints the message in
a loop, again using 'sleep' to delay for however many seconds are
necessary:
# zzzzzz..... go away if interrupted
if (system(sprintf("sleep %d", naptime)) != 0)
exit 1
# time to notify!
command = sprintf("sleep %d", delay)
for (i = 1; i <= count; i++) {
print message
# if sleep command interrupted, go away
if (system(command) != 0)
break
}
exit 0
}
File: gawk.info, Node: Translate Program, Next: Labels Program, Prev: Alarm Program, Up: Miscellaneous Programs
11.3.3 Transliterating Characters
---------------------------------
The system 'tr' utility transliterates characters. For example, it is
often used to map uppercase letters into lowercase for further
processing:
GENERATE DATA | tr 'A-Z' 'a-z' | PROCESS DATA ...
'tr' requires two lists of characters.(1) When processing the input,
the first character in the first list is replaced with the first
character in the second list, the second character in the first list is
replaced with the second character in the second list, and so on. If
there are more characters in the "from" list than in the "to" list, the
last character of the "to" list is used for the remaining characters in
the "from" list.
Once upon a time, a user proposed adding a transliteration function
to 'gawk'. The following program was written to prove that character
transliteration could be done with a user-level function. This program
is not as complete as the system 'tr' utility, but it does most of the
job.
The 'translate' program was written long before 'gawk' acquired the
ability to split each character in a string into separate array
elements. Thus, it makes repeated use of the 'substr()', 'index()', and
'gsub()' built-in functions (*note String Functions::). There are two
functions. The first, 'stranslate()', takes three arguments:
'from'
A list of characters from which to translate
'to'
A list of characters to which to translate
'target'
The string on which to do the translation
Associative arrays make the translation part fairly easy. 't_ar'
holds the "to" characters, indexed by the "from" characters. Then a
simple loop goes through 'from', one character at a time. For each
character in 'from', if the character appears in 'target', it is
replaced with the corresponding 'to' character.
The 'translate()' function calls 'stranslate()', using '$0' as the
target. The main program sets two global variables, 'FROM' and 'TO',
from the command line, and then changes 'ARGV' so that 'awk' reads from
the standard input.
Finally, the processing rule simply calls 'translate()' for each
record:
# translate.awk --- do tr-like stuff
# Bugs: does not handle things like tr A-Z a-z; it has
# to be spelled out. However, if `to' is shorter than `from',
# the last character in `to' is used for the rest of `from'.
function stranslate(from, to, target, lf, lt, ltarget, t_ar, i, c,
result)
{
lf = length(from)
lt = length(to)
ltarget = length(target)
for (i = 1; i <= lt; i++)
t_ar[substr(from, i, 1)] = substr(to, i, 1)
if (lt < lf)
for (; i <= lf; i++)
t_ar[substr(from, i, 1)] = substr(to, lt, 1)
for (i = 1; i <= ltarget; i++) {
c = substr(target, i, 1)
if (c in t_ar)
c = t_ar[c]
result = result c
}
return result
}
function translate(from, to)
{
return $0 = stranslate(from, to, $0)
}
# main program
BEGIN {
if (ARGC < 3) {
print "usage: translate from to" > "/dev/stderr"
exit
}
FROM = ARGV[1]
TO = ARGV[2]
ARGC = 2
ARGV[1] = "-"
}
{
translate(FROM, TO)
print
}
It is possible to do character transliteration in a user-level
function, but it is not necessarily efficient, and we (the 'gawk'
developers) started to consider adding a built-in function. However,
shortly after writing this program, we learned that Brian Kernighan had
added the 'toupper()' and 'tolower()' functions to his 'awk' (*note
String Functions::). These functions handle the vast majority of the
cases where character transliteration is necessary, and so we chose to
simply add those functions to 'gawk' as well and then leave well enough
alone.
An obvious improvement to this program would be to set up the 't_ar'
array only once, in a 'BEGIN' rule. However, this assumes that the
"from" and "to" lists will never change throughout the lifetime of the
program.
Another obvious improvement is to enable the use of ranges, such as
'a-z', as allowed by the 'tr' utility. Look at the code for 'cut.awk'
(*note Cut Program::) for inspiration.
---------- Footnotes ----------
(1) On some older systems, including Solaris, the system version of
'tr' may require that the lists be written as range expressions enclosed
in square brackets ('[a-z]') and quoted, to prevent the shell from
attempting a file name expansion. This is not a feature.
File: gawk.info, Node: Labels Program, Next: Word Sorting, Prev: Translate Program, Up: Miscellaneous Programs
11.3.4 Printing Mailing Labels
------------------------------
Here is a "real-world"(1) program. This script reads lists of names and
addresses and generates mailing labels. Each page of labels has 20
labels on it, two across and 10 down. The addresses are guaranteed to
be no more than five lines of data. Each address is separated from the
next by a blank line.
The basic idea is to read 20 labels' worth of data. Each line of
each label is stored in the 'line' array. The single rule takes care of
filling the 'line' array and printing the page when 20 labels have been
read.
The 'BEGIN' rule simply sets 'RS' to the empty string, so that 'awk'
splits records at blank lines (*note Records::). It sets 'MAXLINES' to
100, because 100 is the maximum number of lines on the page (20 * 5 =
100).
Most of the work is done in the 'printpage()' function. The label
lines are stored sequentially in the 'line' array. But they have to
print horizontally: 'line[1]' next to 'line[6]', 'line[2]' next to
'line[7]', and so on. Two loops accomplish this. The outer loop,
controlled by 'i', steps through every 10 lines of data; this is each
row of labels. The inner loop, controlled by 'j', goes through the
lines within the row. As 'j' goes from 0 to 4, 'i+j' is the 'j'th line
in the row, and 'i+j+5' is the entry next to it. The output ends up
looking something like this:
line 1 line 6
line 2 line 7
line 3 line 8
line 4 line 9
line 5 line 10
...
The 'printf' format string '%-41s' left-aligns the data and prints it
within a fixed-width field.
As a final note, an extra blank line is printed at lines 21 and 61,
to keep the output lined up on the labels. This is dependent on the
particular brand of labels in use when the program was written. You
will also note that there are two blank lines at the top and two blank
lines at the bottom.
The 'END' rule arranges to flush the final page of labels; there may
not have been an even multiple of 20 labels in the data:
# labels.awk --- print mailing labels
# Each label is 5 lines of data that may have blank lines.
# The label sheets have 2 blank lines at the top and 2 at
# the bottom.
BEGIN { RS = "" ; MAXLINES = 100 }
function printpage( i, j)
{
if (Nlines <= 0)
return
printf "\n\n" # header
for (i = 1; i <= Nlines; i += 10) {
if (i == 21 || i == 61)
print ""
for (j = 0; j < 5; j++) {
if (i + j > MAXLINES)
break
printf " %-41s %s\n", line[i+j], line[i+j+5]
}
print ""
}
printf "\n\n" # footer
delete line
}
# main rule
{
if (Count >= 20) {
printpage()
Count = 0
Nlines = 0
}
n = split($0, a, "\n")
for (i = 1; i <= n; i++)
line[++Nlines] = a[i]
for (; i <= 5; i++)
line[++Nlines] = ""
Count++
}
END {
printpage()
}
---------- Footnotes ----------
(1) "Real world" is defined as "a program actually used to get
something done."
File: gawk.info, Node: Word Sorting, Next: History Sorting, Prev: Labels Program, Up: Miscellaneous Programs
11.3.5 Generating Word-Usage Counts
-----------------------------------
When working with large amounts of text, it can be interesting to know
how often different words appear. For example, an author may overuse
certain words, in which case he or she might wish to find synonyms to
substitute for words that appear too often. This node develops a
program for counting words and presenting the frequency information in a
useful format.
At first glance, a program like this would seem to do the job:
# wordfreq-first-try.awk --- print list of word frequencies
{
for (i = 1; i <= NF; i++)
freq[$i]++
}
END {
for (word in freq)
printf "%s\t%d\n", word, freq[word]
}
The program relies on 'awk''s default field-splitting mechanism to
break each line up into "words" and uses an associative array named
'freq', indexed by each word, to count the number of times the word
occurs. In the 'END' rule, it prints the counts.
This program has several problems that prevent it from being useful
on real text files:
* The 'awk' language considers upper- and lowercase characters to be
distinct. Therefore, "bartender" and "Bartender" are not treated
as the same word. This is undesirable, because words are
capitalized if they begin sentences in normal text, and a frequency
analyzer should not be sensitive to capitalization.
* Words are detected using the 'awk' convention that fields are
separated just by whitespace. Other characters in the input
(except newlines) don't have any special meaning to 'awk'. This
means that punctuation characters count as part of words.
* The output does not come out in any useful order. You're more
likely to be interested in which words occur most frequently or in
having an alphabetized table of how frequently each word occurs.
The first problem can be solved by using 'tolower()' to remove case
distinctions. The second problem can be solved by using 'gsub()' to
remove punctuation characters. Finally, we solve the third problem by
using the system 'sort' utility to process the output of the 'awk'
script. Here is the new version of the program:
# wordfreq.awk --- print list of word frequencies
{
$0 = tolower($0) # remove case distinctions
# remove punctuation
gsub(/[^[:alnum:]_[:blank:]]/, "", $0)
for (i = 1; i <= NF; i++)
freq[$i]++
}
END {
for (word in freq)
printf "%s\t%d\n", word, freq[word]
}
The regexp '/[^[:alnum:]_[:blank:]]/' might have been written
'/[[:punct:]]/', but then underscores would also be removed, and we want
to keep them.
Assuming we have saved this program in a file named 'wordfreq.awk',
and that the data is in 'file1', the following pipeline:
awk -f wordfreq.awk file1 | sort -k 2nr
produces a table of the words appearing in 'file1' in order of
decreasing frequency.
The 'awk' program suitably massages the data and produces a word
frequency table, which is not ordered. The 'awk' script's output is
then sorted by the 'sort' utility and printed on the screen.
The options given to 'sort' specify a sort that uses the second field
of each input line (skipping one field), that the sort keys should be
treated as numeric quantities (otherwise '15' would come before '5'),
and that the sorting should be done in descending (reverse) order.
The 'sort' could even be done from within the program, by changing
the 'END' action to:
END {
sort = "sort -k 2nr"
for (word in freq)
printf "%s\t%d\n", word, freq[word] | sort
close(sort)
}
This way of sorting must be used on systems that do not have true
pipes at the command-line (or batch-file) level. See the general
operating system documentation for more information on how to use the
'sort' program.
File: gawk.info, Node: History Sorting, Next: Extract Program, Prev: Word Sorting, Up: Miscellaneous Programs
11.3.6 Removing Duplicates from Unsorted Text
---------------------------------------------
The 'uniq' program (*note Uniq Program::) removes duplicate lines from
_sorted_ data.
Suppose, however, you need to remove duplicate lines from a data file
but that you want to preserve the order the lines are in. A good
example of this might be a shell history file. The history file keeps a
copy of all the commands you have entered, and it is not unusual to
repeat a command several times in a row. Occasionally you might want to
compact the history by removing duplicate entries. Yet it is desirable
to maintain the order of the original commands.
This simple program does the job. It uses two arrays. The 'data'
array is indexed by the text of each line. For each line, 'data[$0]' is
incremented. If a particular line has not been seen before, then
'data[$0]' is zero. In this case, the text of the line is stored in
'lines[count]'. Each element of 'lines' is a unique command, and the
indices of 'lines' indicate the order in which those lines are
encountered. The 'END' rule simply prints out the lines, in order:
# histsort.awk --- compact a shell history file
# Thanks to Byron Rakitzis for the general idea
{
if (data[$0]++ == 0)
lines[++count] = $0
}
END {
for (i = 1; i <= count; i++)
print lines[i]
}
This program also provides a foundation for generating other useful
information. For example, using the following 'print' statement in the
'END' rule indicates how often a particular command is used:
print data[lines[i]], lines[i]
This works because 'data[$0]' is incremented each time a line is seen.
File: gawk.info, Node: Extract Program, Next: Simple Sed, Prev: History Sorting, Up: Miscellaneous Programs
11.3.7 Extracting Programs from Texinfo Source Files
----------------------------------------------------
The nodes *note Library Functions::, and *note Sample Programs::, are
the top level nodes for a large number of 'awk' programs. If you want
to experiment with these programs, it is tedious to type them in by
hand. Here we present a program that can extract parts of a Texinfo
input file into separate files.
This Info file is written in Texinfo
(https://www.gnu.org/software/texinfo/), the GNU Project's document
formatting language. A single Texinfo source file can be used to
produce both printed documentation, with TeX, and online documentation.
(The Texinfo language is described fully, starting with *note (Texinfo,
texinfo,Texinfo---The GNU Documentation Format)Top::.)
For our purposes, it is enough to know three things about Texinfo
input files:
* The "at" symbol ('@') is special in Texinfo, much as the backslash
('\') is in C or 'awk'. Literal '@' symbols are represented in
Texinfo source files as '@@'.
* Comments start with either '@c' or '@comment'. The file-extraction
program works by using special comments that start at the beginning
of a line.
* Lines containing '@group' and '@end group' commands bracket example
text that should not be split across a page boundary.
(Unfortunately, TeX isn't always smart enough to do things exactly
right, so we have to give it some help.)
The following program, 'extract.awk', reads through a Texinfo source
file and does two things, based on the special comments. Upon seeing
'@c system ...', it runs a command, by extracting the command text from
the control line and passing it on to the 'system()' function (*note I/O
Functions::). Upon seeing '@c file FILENAME', each subsequent line is
sent to the file FILENAME, until '@c endfile' is encountered. The rules
in 'extract.awk' match either '@c' or '@comment' by letting the 'omment'
part be optional. Lines containing '@group' and '@end group' are simply
removed. 'extract.awk' uses the 'join()' library function (*note Join
Function::).
The example programs in the online Texinfo source for 'GAWK:
Effective AWK Programming' ('gawktexi.in') have all been bracketed
inside 'file' and 'endfile' lines. The 'gawk' distribution uses a copy
of 'extract.awk' to extract the sample programs and install many of them
in a standard directory where 'gawk' can find them. The Texinfo file
looks something like this:
...
This program has a @code{BEGIN} rule
that prints a nice message:
@example
@c file examples/messages.awk
BEGIN @{ print "Don't panic!" @}
@c endfile
@end example
It also prints some final advice:
@example
@c file examples/messages.awk
END @{ print "Always avoid bored archaeologists!" @}
@c endfile
@end example
...
'extract.awk' begins by setting 'IGNORECASE' to one, so that mixed
upper- and lowercase letters in the directives won't matter.
The first rule handles calling 'system()', checking that a command is
given ('NF' is at least three) and also checking that the command exits
with a zero exit status, signifying OK:
# extract.awk --- extract files and run programs from Texinfo files
BEGIN { IGNORECASE = 1 }
/^@c(omment)?[ \t]+system/ {
if (NF < 3) {
e = ("extract: " FILENAME ":" FNR)
e = (e ": badly formed `system' line")
print e > "/dev/stderr"
next
}
$1 = ""
$2 = ""
stat = system($0)
if (stat != 0) {
e = ("extract: " FILENAME ":" FNR)
e = (e ": warning: system returned " stat)
print e > "/dev/stderr"
}
}
The variable 'e' is used so that the rule fits nicely on the screen.
The second rule handles moving data into files. It verifies that a
file name is given in the directive. If the file named is not the
current file, then the current file is closed. Keeping the current file
open until a new file is encountered allows the use of the '>'
redirection for printing the contents, keeping open-file management
simple.
The 'for' loop does the work. It reads lines using 'getline' (*note
Getline::). For an unexpected end-of-file, it calls the
'unexpected_eof()' function. If the line is an "endfile" line, then it
breaks out of the loop. If the line is an '@group' or '@end group'
line, then it ignores it and goes on to the next line. Similarly,
comments within examples are also ignored.
Most of the work is in the following few lines. If the line has no
'@' symbols, the program can print it directly. Otherwise, each leading
'@' must be stripped off. To remove the '@' symbols, the line is split
into separate elements of the array 'a', using the 'split()' function
(*note String Functions::). The '@' symbol is used as the separator
character. Each element of 'a' that is empty indicates two successive
'@' symbols in the original line. For each two empty elements ('@@' in
the original file), we have to add a single '@' symbol back in.
When the processing of the array is finished, 'join()' is called with
the value of 'SUBSEP' (*note Multidimensional::), to rejoin the pieces
back into a single line. That line is then printed to the output file:
/^@c(omment)?[ \t]+file/ {
if (NF != 3) {
e = ("extract: " FILENAME ":" FNR ": badly formed `file' line")
print e > "/dev/stderr"
next
}
if ($3 != curfile) {
if (curfile != "")
close(curfile)
curfile = $3
}
for (;;) {
if ((getline line) <= 0)
unexpected_eof()
if (line ~ /^@c(omment)?[ \t]+endfile/)
break
else if (line ~ /^@(end[ \t]+)?group/)
continue
else if (line ~ /^@c(omment+)?[ \t]+/)
continue
if (index(line, "@") == 0) {
print line > curfile
continue
}
n = split(line, a, "@")
# if a[1] == "", means leading @,
# don't add one back in.
for (i = 2; i <= n; i++) {
if (a[i] == "") { # was an @@
a[i] = "@"
if (a[i+1] == "")
i++
}
}
print join(a, 1, n, SUBSEP) > curfile
}
}
An important thing to note is the use of the '>' redirection. Output
done with '>' only opens the file once; it stays open and subsequent
output is appended to the file (*note Redirection::). This makes it
easy to mix program text and explanatory prose for the same sample
source file (as has been done here!) without any hassle. The file is
only closed when a new data file name is encountered or at the end of
the input file.
Finally, the function 'unexpected_eof()' prints an appropriate error
message and then exits. The 'END' rule handles the final cleanup,
closing the open file:
function unexpected_eof()
{
printf("extract: %s:%d: unexpected EOF or error\n",
FILENAME, FNR) > "/dev/stderr"
exit 1
}
END {
if (curfile)
close(curfile)
}
File: gawk.info, Node: Simple Sed, Next: Igawk Program, Prev: Extract Program, Up: Miscellaneous Programs
11.3.8 A Simple Stream Editor
-----------------------------
The 'sed' utility is a "stream editor", a program that reads a stream of
data, makes changes to it, and passes it on. It is often used to make
global changes to a large file or to a stream of data generated by a
pipeline of commands. Although 'sed' is a complicated program in its
own right, its most common use is to perform global substitutions in the
middle of a pipeline:
COMMAND1 < orig.data | sed 's/old/new/g' | COMMAND2 > result
Here, 's/old/new/g' tells 'sed' to look for the regexp 'old' on each
input line and globally replace it with the text 'new' (i.e., all the
occurrences on a line). This is similar to 'awk''s 'gsub()' function
(*note String Functions::).
The following program, 'awksed.awk', accepts at least two
command-line arguments: the pattern to look for and the text to replace
it with. Any additional arguments are treated as data file names to
process. If none are provided, the standard input is used:
# awksed.awk --- do s/foo/bar/g using just print
# Thanks to Michael Brennan for the idea
function usage()
{
print "usage: awksed pat repl [files...]" > "/dev/stderr"
exit 1
}
BEGIN {
# validate arguments
if (ARGC < 3)
usage()
RS = ARGV[1]
ORS = ARGV[2]
# don't use arguments as files
ARGV[1] = ARGV[2] = ""
}
# look ma, no hands!
{
if (RT == "")
printf "%s", $0
else
print
}
The program relies on 'gawk''s ability to have 'RS' be a regexp, as
well as on the setting of 'RT' to the actual text that terminates the
record (*note Records::).
The idea is to have 'RS' be the pattern to look for. 'gawk'
automatically sets '$0' to the text between matches of the pattern.
This is text that we want to keep, unmodified. Then, by setting 'ORS'
to the replacement text, a simple 'print' statement outputs the text we
want to keep, followed by the replacement text.
There is one wrinkle to this scheme, which is what to do if the last
record doesn't end with text that matches 'RS'. Using a 'print'
statement unconditionally prints the replacement text, which is not
correct. However, if the file did not end in text that matches 'RS',
'RT' is set to the null string. In this case, we can print '$0' using
'printf' (*note Printf::).
The 'BEGIN' rule handles the setup, checking for the right number of
arguments and calling 'usage()' if there is a problem. Then it sets
'RS' and 'ORS' from the command-line arguments and sets 'ARGV[1]' and
'ARGV[2]' to the null string, so that they are not treated as file names
(*note ARGC and ARGV::).
The 'usage()' function prints an error message and exits. Finally,
the single rule handles the printing scheme outlined earlier, using
'print' or 'printf' as appropriate, depending upon the value of 'RT'.
File: gawk.info, Node: Igawk Program, Next: Anagram Program, Prev: Simple Sed, Up: Miscellaneous Programs
11.3.9 An Easy Way to Use Library Functions
-------------------------------------------
In *note Include Files::, we saw how 'gawk' provides a built-in
file-inclusion capability. However, this is a 'gawk' extension. This
minor node provides the motivation for making file inclusion available
for standard 'awk', and shows how to do it using a combination of shell
and 'awk' programming.
Using library functions in 'awk' can be very beneficial. It
encourages code reuse and the writing of general functions. Programs
are smaller and therefore clearer. However, using library functions is
only easy when writing 'awk' programs; it is painful when running them,
requiring multiple '-f' options. If 'gawk' is unavailable, then so too
is the 'AWKPATH' environment variable and the ability to put 'awk'
functions into a library directory (*note Options::). It would be nice
to be able to write programs in the following manner:
# library functions
@include getopt.awk
@include join.awk
...
# main program
BEGIN {
while ((c = getopt(ARGC, ARGV, "a:b:cde")) != -1)
...
...
}
The following program, 'igawk.sh', provides this service. It
simulates 'gawk''s searching of the 'AWKPATH' variable and also allows
"nested" includes (i.e., a file that is included with '@include' can
contain further '@include' statements). 'igawk' makes an effort to only
include files once, so that nested includes don't accidentally include a
library function twice.
'igawk' should behave just like 'gawk' externally. This means it
should accept all of 'gawk''s command-line arguments, including the
ability to have multiple source files specified via '-f' and the ability
to mix command-line and library source files.
The program is written using the POSIX Shell ('sh') command
language.(1) It works as follows:
1. Loop through the arguments, saving anything that doesn't represent
'awk' source code for later, when the expanded program is run.
2. For any arguments that do represent 'awk' text, put the arguments
into a shell variable that will be expanded. There are two cases:
a. Literal text, provided with '-e' or '--source'. This text is
just appended directly.
b. Source file names, provided with '-f'. We use a neat trick
and append '@include FILENAME' to the shell variable's
contents. Because the file-inclusion program works the way
'gawk' does, this gets the text of the file included in the
program at the correct point.
3. Run an 'awk' program (naturally) over the shell variable's contents
to expand '@include' statements. The expanded program is placed in
a second shell variable.
4. Run the expanded program with 'gawk' and any other original
command-line arguments that the user supplied (such as the data
file names).
This program uses shell variables extensively: for storing
command-line arguments and the text of the 'awk' program that will
expand the user's program, for the user's original program, and for the
expanded program. Doing so removes some potential problems that might
arise were we to use temporary files instead, at the cost of making the
script somewhat more complicated.
The initial part of the program turns on shell tracing if the first
argument is 'debug'.
The next part loops through all the command-line arguments. There
are several cases of interest:
'--'
This ends the arguments to 'igawk'. Anything else should be passed
on to the user's 'awk' program without being evaluated.
'-W'
This indicates that the next option is specific to 'gawk'. To make
argument processing easier, the '-W' is appended to the front of
the remaining arguments and the loop continues. (This is an 'sh'
programming trick. Don't worry about it if you are not familiar
with 'sh'.)
'-v', '-F'
These are saved and passed on to 'gawk'.
'-f', '--file', '--file=', '-Wfile='
The file name is appended to the shell variable 'program' with an
'@include' statement. The 'expr' utility is used to remove the
leading option part of the argument (e.g., '--file='). (Typical
'sh' usage would be to use the 'echo' and 'sed' utilities to do
this work. Unfortunately, some versions of 'echo' evaluate escape
sequences in their arguments, possibly mangling the program text.
Using 'expr' avoids this problem.)
'--source', '--source=', '-Wsource='
The source text is appended to 'program'.
'--version', '-Wversion'
'igawk' prints its version number, runs 'gawk --version' to get the
'gawk' version information, and then exits.
If none of the '-f', '--file', '-Wfile', '--source', or '-Wsource'
arguments are supplied, then the first nonoption argument should be the
'awk' program. If there are no command-line arguments left, 'igawk'
prints an error message and exits. Otherwise, the first argument is
appended to 'program'. In any case, after the arguments have been
processed, the shell variable 'program' contains the complete text of
the original 'awk' program.
The program is as follows:
#! /bin/sh
# igawk --- like gawk but do @include processing
if [ "$1" = debug ]
then
set -x
shift
fi
# A literal newline, so that program text is formatted correctly
n='
'
# Initialize variables to empty
program=
opts=
while [ $# -ne 0 ] # loop over arguments
do
case $1 in
--) shift
break ;;
-W) shift
# The ${x?'message here'} construct prints a
# diagnostic if $x is the null string
set -- -W"${@?'missing operand'}"
continue ;;
-[vF]) opts="$opts $1 '${2?'missing operand'}'"
shift ;;
-[vF]*) opts="$opts '$1'" ;;
-f) program="$program$n@include ${2?'missing operand'}"
shift ;;
-f*) f=$(expr "$1" : '-f\(.*\)')
program="$program$n@include $f" ;;
-[W-]file=*)
f=$(expr "$1" : '-.file=\(.*\)')
program="$program$n@include $f" ;;
-[W-]file)
program="$program$n@include ${2?'missing operand'}"
shift ;;
-[W-]source=*)
t=$(expr "$1" : '-.source=\(.*\)')
program="$program$n$t" ;;
-[W-]source)
program="$program$n${2?'missing operand'}"
shift ;;
-[W-]version)
echo igawk: version 3.0 1>&2
gawk --version
exit 0 ;;
-[W-]*) opts="$opts '$1'" ;;
*) break ;;
esac
shift
done
if [ -z "$program" ]
then
program=${1?'missing program'}
shift
fi
# At this point, `program' has the program.
The 'awk' program to process '@include' directives is stored in the
shell variable 'expand_prog'. Doing this keeps the shell script
readable. The 'awk' program reads through the user's program, one line
at a time, using 'getline' (*note Getline::). The input file names and
'@include' statements are managed using a stack. As each '@include' is
encountered, the current file name is "pushed" onto the stack and the
file named in the '@include' directive becomes the current file name.
As each file is finished, the stack is "popped," and the previous input
file becomes the current input file again. The process is started by
making the original file the first one on the stack.
The 'pathto()' function does the work of finding the full path to a
file. It simulates 'gawk''s behavior when searching the 'AWKPATH'
environment variable (*note AWKPATH Variable::). If a file name has a
'/' in it, no path search is done. Similarly, if the file name is
'"-"', then that string is used as-is. Otherwise, the file name is
concatenated with the name of each directory in the path, and an attempt
is made to open the generated file name. The only way to test if a file
can be read in 'awk' is to go ahead and try to read it with 'getline';
this is what 'pathto()' does.(2) If the file can be read, it is closed
and the file name is returned:
expand_prog='
function pathto(file, i, t, junk)
{
if (index(file, "/") != 0)
return file
if (file == "-")
return file
for (i = 1; i <= ndirs; i++) {
t = (pathlist[i] "/" file)
if ((getline junk < t) > 0) {
# found it
close(t)
return t
}
}
return ""
}
The main program is contained inside one 'BEGIN' rule. The first
thing it does is set up the 'pathlist' array that 'pathto()' uses.
After splitting the path on ':', null elements are replaced with '"."',
which represents the current directory:
BEGIN {
path = ENVIRON["AWKPATH"]
ndirs = split(path, pathlist, ":")
for (i = 1; i <= ndirs; i++) {
if (pathlist[i] == "")
pathlist[i] = "."
}
The stack is initialized with 'ARGV[1]', which will be
'"/dev/stdin"'. The main loop comes next. Input lines are read in
succession. Lines that do not start with '@include' are printed
verbatim. If the line does start with '@include', the file name is in
'$2'. 'pathto()' is called to generate the full path. If it cannot,
then the program prints an error message and continues.
The next thing to check is if the file is included already. The
'processed' array is indexed by the full file name of each included file
and it tracks this information for us. If the file is seen again, a
warning message is printed. Otherwise, the new file name is pushed onto
the stack and processing continues.
Finally, when 'getline' encounters the end of the input file, the
file is closed and the stack is popped. When 'stackptr' is less than
zero, the program is done:
stackptr = 0
input[stackptr] = ARGV[1] # ARGV[1] is first file
for (; stackptr >= 0; stackptr--) {
while ((getline < input[stackptr]) > 0) {
if (tolower($1) != "@include") {
print
continue
}
fpath = pathto($2)
if (fpath == "") {
printf("igawk: %s:%d: cannot find %s\n",
input[stackptr], FNR, $2) > "/dev/stderr"
continue
}
if (! (fpath in processed)) {
processed[fpath] = input[stackptr]
input[++stackptr] = fpath # push onto stack
} else
print $2, "included in", input[stackptr],
"already included in",
processed[fpath] > "/dev/stderr"
}
close(input[stackptr])
}
}' # close quote ends `expand_prog' variable
processed_program=$(gawk -- "$expand_prog" /dev/stdin << EOF
$program
EOF
)
The shell construct 'COMMAND << MARKER' is called a "here document".
Everything in the shell script up to the MARKER is fed to COMMAND as
input. The shell processes the contents of the here document for
variable and command substitution (and possibly other things as well,
depending upon the shell).
The shell construct '$(...)' is called "command substitution". The
output of the command inside the parentheses is substituted into the
command line. Because the result is used in a variable assignment, it
is saved as a single string, even if the results contain whitespace.
The expanded program is saved in the variable 'processed_program'.
It's done in these steps:
1. Run 'gawk' with the '@include'-processing program (the value of the
'expand_prog' shell variable) reading standard input.
2. Standard input is the contents of the user's program, from the
shell variable 'program'. Feed its contents to 'gawk' via a here
document.
3. Save the results of this processing in the shell variable
'processed_program' by using command substitution.
The last step is to call 'gawk' with the expanded program, along with
the original options and command-line arguments that the user supplied:
eval gawk $opts -- '"$processed_program"' '"$@"'
The 'eval' command is a shell construct that reruns the shell's
parsing process. This keeps things properly quoted.
This version of 'igawk' represents the fifth version of this program.
There are four key simplifications that make the program work better:
* Using '@include' even for the files named with '-f' makes building
the initial collected 'awk' program much simpler; all the
'@include' processing can be done once.
* Not trying to save the line read with 'getline' in the 'pathto()'
function when testing for the file's accessibility for use with the
main program simplifies things considerably.
* Using a 'getline' loop in the 'BEGIN' rule does it all in one
place. It is not necessary to call out to a separate loop for
processing nested '@include' statements.
* Instead of saving the expanded program in a temporary file, putting
it in a shell variable avoids some potential security problems.
This has the disadvantage that the script relies upon more features
of the 'sh' language, making it harder to follow for those who
aren't familiar with 'sh'.
Also, this program illustrates that it is often worthwhile to combine
'sh' and 'awk' programming together. You can usually accomplish quite a
lot, without having to resort to low-level programming in C or C++, and
it is frequently easier to do certain kinds of string and argument
manipulation using the shell than it is in 'awk'.
Finally, 'igawk' shows that it is not always necessary to add new
features to a program; they can often be layered on top.(3)
---------- Footnotes ----------
(1) Fully explaining the 'sh' language is beyond the scope of this
book. We provide some minimal explanations, but see a good shell
programming book if you wish to understand things in more depth.
(2) On some very old versions of 'awk', the test 'getline junk < t'
can loop forever if the file exists but is empty.
(3) 'gawk' does '@include' processing itself in order to support the
use of 'awk' programs as Web CGI scripts.
File: gawk.info, Node: Anagram Program, Next: Signature Program, Prev: Igawk Program, Up: Miscellaneous Programs
11.3.10 Finding Anagrams from a Dictionary
------------------------------------------
An interesting programming challenge is to search for "anagrams" in a
word list (such as '/usr/share/dict/words' on many GNU/Linux systems).
One word is an anagram of another if both words contain the same letters
(e.g., "babbling" and "blabbing").
Column 2, Problem C, of Jon Bentley's 'Programming Pearls', Second
Edition, presents an elegant algorithm. The idea is to give words that
are anagrams a common signature, sort all the words together by their
signatures, and then print them. Dr. Bentley observes that taking the
letters in each word and sorting them produces those common signatures.
The following program uses arrays of arrays to bring together words
with the same signature and array sorting to print the words in sorted
order:
# anagram.awk --- An implementation of the anagram-finding algorithm
# from Jon Bentley's "Programming Pearls," 2nd edition.
# Addison Wesley, 2000, ISBN 0-201-65788-0.
# Column 2, Problem C, section 2.8, pp 18-20.
/'s$/ { next } # Skip possessives
The program starts with a header, and then a rule to skip possessives
in the dictionary file. The next rule builds up the data structure.
The first dimension of the array is indexed by the signature; the second
dimension is the word itself:
{
key = word2key($1) # Build signature
data[key][$1] = $1 # Store word with signature
}
The 'word2key()' function creates the signature. It splits the word
apart into individual letters, sorts the letters, and then joins them
back together:
# word2key --- split word apart into letters, sort, and join back together
function word2key(word, a, i, n, result)
{
n = split(word, a, "")
asort(a)
for (i = 1; i <= n; i++)
result = result a[i]
return result
}
Finally, the 'END' rule traverses the array and prints out the
anagram lists. It sends the output to the system 'sort' command because
otherwise the anagrams would appear in arbitrary order:
END {
sort = "sort"
for (key in data) {
# Sort words with same key
nwords = asorti(data[key], words)
if (nwords == 1)
continue
# And print. Minor glitch: trailing space at end of each line
for (j = 1; j <= nwords; j++)
printf("%s ", words[j]) | sort
print "" | sort
}
close(sort)
}
Here is some partial output when the program is run:
$ gawk -f anagram.awk /usr/share/dict/words | grep '^b'
...
babbled blabbed
babbler blabber brabble
babblers blabbers brabbles
babbling blabbing
babbly blabby
babel bable
babels beslab
babery yabber
...
File: gawk.info, Node: Signature Program, Prev: Anagram Program, Up: Miscellaneous Programs
11.3.11 And Now for Something Completely Different
--------------------------------------------------
The following program was written by Davide Brini and is published on
his website (http://backreference.org/2011/02/03/obfuscated-awk/). It
serves as his signature in the Usenet group 'comp.lang.awk'. He
supplies the following copyright terms:
Copyright (C) 2008 Davide Brini
Copying and distribution of the code published in this page, with
or without modification, are permitted in any medium without
royalty provided the copyright notice and this notice are
preserved.
Here is the program:
awk 'BEGIN{O="~"~"~";o="=="=="==";o+=+o;x=O""O;while(X++<=x+o+o)c=c"%c";
printf c,(x-O)*(x-O),x*(x-o)-o,x*(x-O)+x-O-o,+x*(x-O)-x+o,X*(o*o+O)+x-O,
X*(X-x)-o*o,(x+X)*o*o+o,x*(X-x)-O-O,x-O+(O+o+X+x)*(o+O),X*X-X*(x-O)-x+O,
O+X*(o*(o+O)+O),+x+O+X*o,x*(x-o),(o+X+x)*o*o-(x-O-O),O+(X-x)*(X+O),x-O}'
We leave it to you to determine what the program does. (If you are
truly desperate to understand it, see Chris Johansen's explanation,
which is embedded in the Texinfo source file for this Info file.)
File: gawk.info, Node: Programs Summary, Next: Programs Exercises, Prev: Miscellaneous Programs, Up: Sample Programs
11.4 Summary
============
* The programs provided in this major node continue on the theme that
reading programs is an excellent way to learn Good Programming.
* Using '#!' to make 'awk' programs directly runnable makes them
easier to use. Otherwise, invoke the program using 'awk -f ...'.
* Reimplementing standard POSIX programs in 'awk' is a pleasant
exercise; 'awk''s expressive power lets you write such programs in
relatively few lines of code, yet they are functionally complete
and usable.
* One of standard 'awk''s weaknesses is working with individual
characters. The ability to use 'split()' with the empty string as
the separator can considerably simplify such tasks.
* The examples here demonstrate the usefulness of the library
functions from *note Library Functions:: for a number of real (if
small) programs.
* Besides reinventing POSIX wheels, other programs solved a selection
of interesting problems, such as finding duplicate words in text,
printing mailing labels, and finding anagrams.
File: gawk.info, Node: Programs Exercises, Prev: Programs Summary, Up: Sample Programs
11.5 Exercises
==============
1. Rewrite 'cut.awk' (*note Cut Program::) using 'split()' with '""'
as the separator.
2. In *note Egrep Program::, we mentioned that 'egrep -i' could be
simulated in versions of 'awk' without 'IGNORECASE' by using
'tolower()' on the line and the pattern. In a footnote there, we
also mentioned that this solution has a bug: the translated line is
output, and not the original one. Fix this problem.
3. The POSIX version of 'id' takes options that control which
information is printed. Modify the 'awk' version (*note Id
Program::) to accept the same arguments and perform in the same
way.
4. The 'split.awk' program (*note Split Program::) assumes that
letters are contiguous in the character set, which isn't true for
EBCDIC systems. Fix this problem. (Hint: Consider a different way
to work through the alphabet, without relying on 'ord()' and
'chr()'.)
5. In 'uniq.awk' (*note Uniq Program::, the logic for choosing which
lines to print represents a "state machine", which is "a device
that can be in one of a set number of stable conditions depending
on its previous condition and on the present values of its
inputs."(1) Brian Kernighan suggests that "an alternative approach
to state machines is to just read the input into an array, then use
indexing. It's almost always easier code, and for most inputs
where you would use this, just as fast." Rewrite the logic to
follow this suggestion.
6. Why can't the 'wc.awk' program (*note Wc Program::) just use the
value of 'FNR' in 'endfile()'? Hint: Examine the code in *note
Filetrans Function::.
7. Manipulation of individual characters in the 'translate' program
(*note Translate Program::) is painful using standard 'awk'
functions. Given that 'gawk' can split strings into individual
characters using '""' as the separator, how might you use this
feature to simplify the program?
8. The 'extract.awk' program (*note Extract Program::) was written
before 'gawk' had the 'gensub()' function. Use it to simplify the
code.
9. Compare the performance of the 'awksed.awk' program (*note Simple
Sed::) with the more straightforward:
BEGIN {
pat = ARGV[1]
repl = ARGV[2]
ARGV[1] = ARGV[2] = ""
}
{ gsub(pat, repl); print }
10. What are the advantages and disadvantages of 'awksed.awk' versus
the real 'sed' utility?
11. In *note Igawk Program::, we mentioned that not trying to save the
line read with 'getline' in the 'pathto()' function when testing
for the file's accessibility for use with the main program
simplifies things considerably. What problem does this engender
though?
12. As an additional example of the idea that it is not always
necessary to add new features to a program, consider the idea of
having two files in a directory in the search path:
'default.awk'
This file contains a set of default library functions, such as
'getopt()' and 'assert()'.
'site.awk'
This file contains library functions that are specific to a
site or installation; i.e., locally developed functions.
Having a separate file allows 'default.awk' to change with new
'gawk' releases, without requiring the system administrator to
update it each time by adding the local functions.
One user suggested that 'gawk' be modified to automatically read
these files upon startup. Instead, it would be very simple to
modify 'igawk' to do this. Since 'igawk' can process nested
'@include' directives, 'default.awk' could simply contain
'@include' statements for the desired library functions. Make this
change.
13. Modify 'anagram.awk' (*note Anagram Program::), to avoid the use
of the external 'sort' utility.
---------- Footnotes ----------
(1) This is the definition returned from entering 'define: state
machine' into Google.
File: gawk.info, Node: Advanced Features, Next: Internationalization, Prev: Sample Programs, Up: Top
12 Advanced Features of 'gawk'
******************************
Write documentation as if whoever reads it is a violent psychopath
who knows where you live.
-- _Steve English, as quoted by Peter Langston_
This major node discusses advanced features in 'gawk'. It's a bit of
a "grab bag" of items that are otherwise unrelated to each other.
First, we look at a command-line option that allows 'gawk' to recognize
nondecimal numbers in input data, not just in 'awk' programs. Then,
'gawk''s special features for sorting arrays are presented. Next,
two-way I/O, discussed briefly in earlier parts of this Info file, is
described in full detail, along with the basics of TCP/IP networking.
Finally, we see how 'gawk' can "profile" an 'awk' program, making it
possible to tune it for performance.
Additional advanced features are discussed in separate major nodes of
their own:
* *note Internationalization::, discusses how to internationalize
your 'awk' programs, so that they can speak multiple national
languages.
* *note Debugger::, describes 'gawk''s built-in command-line debugger
for debugging 'awk' programs.
* *note Arbitrary Precision Arithmetic::, describes how you can use
'gawk' to perform arbitrary-precision arithmetic.
* *note Dynamic Extensions::, discusses the ability to dynamically
add new built-in functions to 'gawk'.
* Menu:
* Nondecimal Data:: Allowing nondecimal input data.
* Array Sorting:: Facilities for controlling array traversal and
sorting arrays.
* Two-way I/O:: Two-way communications with another process.
* TCP/IP Networking:: Using 'gawk' for network programming.
* Profiling:: Profiling your 'awk' programs.
* Advanced Features Summary:: Summary of advanced features.
File: gawk.info, Node: Nondecimal Data, Next: Array Sorting, Up: Advanced Features
12.1 Allowing Nondecimal Input Data
===================================
If you run 'gawk' with the '--non-decimal-data' option, you can have
nondecimal values in your input data:
$ echo 0123 123 0x123 |
> gawk --non-decimal-data '{ printf "%d, %d, %d\n", $1, $2, $3 }'
-| 83, 123, 291
For this feature to work, write your program so that 'gawk' treats
your data as numeric:
$ echo 0123 123 0x123 | gawk '{ print $1, $2, $3 }'
-| 0123 123 0x123
The 'print' statement treats its expressions as strings. Although the
fields can act as numbers when necessary, they are still strings, so
'print' does not try to treat them numerically. You need to add zero to
a field to force it to be treated as a number. For example:
$ echo 0123 123 0x123 | gawk --non-decimal-data '
> { print $1, $2, $3
> print $1 + 0, $2 + 0, $3 + 0 }'
-| 0123 123 0x123
-| 83 123 291
Because it is common to have decimal data with leading zeros, and
because using this facility could lead to surprising results, the
default is to leave it disabled. If you want it, you must explicitly
request it.
CAUTION: _Use of this option is not recommended._ It can break old
programs very badly. Instead, use the 'strtonum()' function to
convert your data (*note String Functions::). This makes your
programs easier to write and easier to read, and leads to less
surprising results.
This option may disappear in a future version of 'gawk'.
File: gawk.info, Node: Array Sorting, Next: Two-way I/O, Prev: Nondecimal Data, Up: Advanced Features
12.2 Controlling Array Traversal and Array Sorting
==================================================
'gawk' lets you control the order in which a 'for (INDX in ARRAY)' loop
traverses an array.
In addition, two built-in functions, 'asort()' and 'asorti()', let
you sort arrays based on the array values and indices, respectively.
These two functions also provide control over the sorting criteria used
to order the elements during sorting.
* Menu:
* Controlling Array Traversal:: How to use PROCINFO["sorted_in"].
* Array Sorting Functions:: How to use 'asort()' and 'asorti()'.
File: gawk.info, Node: Controlling Array Traversal, Next: Array Sorting Functions, Up: Array Sorting
12.2.1 Controlling Array Traversal
----------------------------------
By default, the order in which a 'for (INDX in ARRAY)' loop scans an
array is not defined; it is generally based upon the internal
implementation of arrays inside 'awk'.
Often, though, it is desirable to be able to loop over the elements
in a particular order that you, the programmer, choose. 'gawk' lets you
do this.
*note Controlling Scanning:: describes how you can assign special,
predefined values to 'PROCINFO["sorted_in"]' in order to control the
order in which 'gawk' traverses an array during a 'for' loop.
In addition, the value of 'PROCINFO["sorted_in"]' can be a function
name.(1) This lets you traverse an array based on any custom criterion.
The array elements are ordered according to the return value of this
function. The comparison function should be defined with at least four
arguments:
function comp_func(i1, v1, i2, v2)
{
COMPARE ELEMENTS 1 AND 2 IN SOME FASHION
RETURN < 0; 0; OR > 0
}
Here, 'i1' and 'i2' are the indices, and 'v1' and 'v2' are the
corresponding values of the two elements being compared. Either 'v1' or
'v2', or both, can be arrays if the array being traversed contains
subarrays as values. (*Note Arrays of Arrays:: for more information
about subarrays.) The three possible return values are interpreted as
follows:
'comp_func(i1, v1, i2, v2) < 0'
Index 'i1' comes before index 'i2' during loop traversal.
'comp_func(i1, v1, i2, v2) == 0'
Indices 'i1' and 'i2' come together, but the relative order with
respect to each other is undefined.
'comp_func(i1, v1, i2, v2) > 0'
Index 'i1' comes after index 'i2' during loop traversal.
Our first comparison function can be used to scan an array in
numerical order of the indices:
function cmp_num_idx(i1, v1, i2, v2)
{
# numerical index comparison, ascending order
return (i1 - i2)
}
Our second function traverses an array based on the string order of
the element values rather than by indices:
function cmp_str_val(i1, v1, i2, v2)
{
# string value comparison, ascending order
v1 = v1 ""
v2 = v2 ""
if (v1 < v2)
return -1
return (v1 != v2)
}
The third comparison function makes all numbers, and numeric strings
without any leading or trailing spaces, come out first during loop
traversal:
function cmp_num_str_val(i1, v1, i2, v2, n1, n2)
{
# numbers before string value comparison, ascending order
n1 = v1 + 0
n2 = v2 + 0
if (n1 == v1)
return (n2 == v2) ? (n1 - n2) : -1
else if (n2 == v2)
return 1
return (v1 < v2) ? -1 : (v1 != v2)
}
Here is a main program to demonstrate how 'gawk' behaves using each
of the previous functions:
BEGIN {
data["one"] = 10
data["two"] = 20
data[10] = "one"
data[100] = 100
data[20] = "two"
f[1] = "cmp_num_idx"
f[2] = "cmp_str_val"
f[3] = "cmp_num_str_val"
for (i = 1; i <= 3; i++) {
printf("Sort function: %s\n", f[i])
PROCINFO["sorted_in"] = f[i]
for (j in data)
printf("\tdata[%s] = %s\n", j, data[j])
print ""
}
}
Here are the results when the program is run:
$ gawk -f compdemo.awk
-| Sort function: cmp_num_idx Sort by numeric index
-| data[two] = 20
-| data[one] = 10 Both strings are numerically zero
-| data[10] = one
-| data[20] = two
-| data[100] = 100
-|
-| Sort function: cmp_str_val Sort by element values as strings
-| data[one] = 10
-| data[100] = 100 String 100 is less than string 20
-| data[two] = 20
-| data[10] = one
-| data[20] = two
-|
-| Sort function: cmp_num_str_val Sort all numeric values before all strings
-| data[one] = 10
-| data[two] = 20
-| data[100] = 100
-| data[10] = one
-| data[20] = two
Consider sorting the entries of a GNU/Linux system password file
according to login name. The following program sorts records by a
specific field position and can be used for this purpose:
# passwd-sort.awk --- simple program to sort by field position
# field position is specified by the global variable POS
function cmp_field(i1, v1, i2, v2)
{
# comparison by value, as string, and ascending order
return v1[POS] < v2[POS] ? -1 : (v1[POS] != v2[POS])
}
{
for (i = 1; i <= NF; i++)
a[NR][i] = $i
}
END {
PROCINFO["sorted_in"] = "cmp_field"
if (POS < 1 || POS > NF)
POS = 1
for (i in a) {
for (j = 1; j <= NF; j++)
printf("%s%c", a[i][j], j < NF ? ":" : "")
print ""
}
}
The first field in each entry of the password file is the user's
login name, and the fields are separated by colons. Each record defines
a subarray, with each field as an element in the subarray. Running the
program produces the following output:
$ gawk -v POS=1 -F: -f sort.awk /etc/passwd
-| adm:x:3:4:adm:/var/adm:/sbin/nologin
-| apache:x:48:48:Apache:/var/www:/sbin/nologin
-| avahi:x:70:70:Avahi daemon:/:/sbin/nologin
...
The comparison should normally always return the same value when
given a specific pair of array elements as its arguments. If
inconsistent results are returned, then the order is undefined. This
behavior can be exploited to introduce random order into otherwise
seemingly ordered data:
function cmp_randomize(i1, v1, i2, v2)
{
# random order (caution: this may never terminate!)
return (2 - 4 * rand())
}
As already mentioned, the order of the indices is arbitrary if two
elements compare equal. This is usually not a problem, but letting the
tied elements come out in arbitrary order can be an issue, especially
when comparing item values. The partial ordering of the equal elements
may change the next time the array is traversed, if other elements are
added to or removed from the array. One way to resolve ties when
comparing elements with otherwise equal values is to include the indices
in the comparison rules. Note that doing this may make the loop
traversal less efficient, so consider it only if necessary. The
following comparison functions force a deterministic order, and are
based on the fact that the (string) indices of two elements are never
equal:
function cmp_numeric(i1, v1, i2, v2)
{
# numerical value (and index) comparison, descending order
return (v1 != v2) ? (v2 - v1) : (i2 - i1)
}
function cmp_string(i1, v1, i2, v2)
{
# string value (and index) comparison, descending order
v1 = v1 i1
v2 = v2 i2
return (v1 > v2) ? -1 : (v1 != v2)
}
A custom comparison function can often simplify ordered loop
traversal, and the sky is really the limit when it comes to designing
such a function.
When string comparisons are made during a sort, either for element
values where one or both aren't numbers, or for element indices handled
as strings, the value of 'IGNORECASE' (*note Built-in Variables::)
controls whether the comparisons treat corresponding upper- and
lowercase letters as equivalent or distinct.
Another point to keep in mind is that in the case of subarrays, the
element values can themselves be arrays; a production comparison
function should use the 'isarray()' function (*note Type Functions::) to
check for this, and choose a defined sorting order for subarrays.
All sorting based on 'PROCINFO["sorted_in"]' is disabled in POSIX
mode, because the 'PROCINFO' array is not special in that case.
As a side note, sorting the array indices before traversing the array
has been reported to add a 15% to 20% overhead to the execution time of
'awk' programs. For this reason, sorted array traversal is not the
default.
---------- Footnotes ----------
(1) This is why the predefined sorting orders start with an '@'
character, which cannot be part of an identifier.
File: gawk.info, Node: Array Sorting Functions, Prev: Controlling Array Traversal, Up: Array Sorting
12.2.2 Sorting Array Values and Indices with 'gawk'
---------------------------------------------------
In most 'awk' implementations, sorting an array requires writing a
'sort()' function. This can be educational for exploring different
sorting algorithms, but usually that's not the point of the program.
'gawk' provides the built-in 'asort()' and 'asorti()' functions (*note
String Functions::) for sorting arrays. For example:
POPULATE THE ARRAY data
n = asort(data)
for (i = 1; i <= n; i++)
DO SOMETHING WITH data[i]
After the call to 'asort()', the array 'data' is indexed from 1 to
some number N, the total number of elements in 'data'. (This count is
'asort()''s return value.) 'data[1]' <= 'data[2]' <= 'data[3]', and so
on. The default comparison is based on the type of the elements (*note
Typing and Comparison::). All numeric values come before all string
values, which in turn come before all subarrays.
An important side effect of calling 'asort()' is that _the array's
original indices are irrevocably lost_. As this isn't always desirable,
'asort()' accepts a second argument:
POPULATE THE ARRAY source
n = asort(source, dest)
for (i = 1; i <= n; i++)
DO SOMETHING WITH dest[i]
In this case, 'gawk' copies the 'source' array into the 'dest' array
and then sorts 'dest', destroying its indices. However, the 'source'
array is not affected.
Often, what's needed is to sort on the values of the _indices_
instead of the values of the elements. To do that, use the 'asorti()'
function. The interface and behavior are identical to that of
'asort()', except that the index values are used for sorting and become
the values of the result array:
{ source[$0] = some_func($0) }
END {
n = asorti(source, dest)
for (i = 1; i <= n; i++) {
Work with sorted indices directly:
DO SOMETHING WITH dest[i]
...
Access original array via sorted indices:
DO SOMETHING WITH source[dest[i]]
}
}
So far, so good. Now it starts to get interesting. Both 'asort()'
and 'asorti()' accept a third string argument to control comparison of
array elements. When we introduced 'asort()' and 'asorti()' in *note
String Functions::, we ignored this third argument; however, now is the
time to describe how this argument affects these two functions.
Basically, the third argument specifies how the array is to be
sorted. There are two possibilities. As with 'PROCINFO["sorted_in"]',
this argument may be one of the predefined names that 'gawk' provides
(*note Controlling Scanning::), or it may be the name of a user-defined
function (*note Controlling Array Traversal::).
In the latter case, _the function can compare elements in any way it
chooses_, taking into account just the indices, just the values, or
both. This is extremely powerful.
Once the array is sorted, 'asort()' takes the _values_ in their final
order and uses them to fill in the result array, whereas 'asorti()'
takes the _indices_ in their final order and uses them to fill in the
result array.
NOTE: Copying array indices and elements isn't expensive in terms
of memory. Internally, 'gawk' maintains "reference counts" to
data. For example, when 'asort()' copies the first array to the
second one, there is only one copy of the original array elements'
data, even though both arrays use the values.
Because 'IGNORECASE' affects string comparisons, the value of
'IGNORECASE' also affects sorting for both 'asort()' and 'asorti()'.
Note also that the locale's sorting order does _not_ come into play;
comparisons are based on character values only.(1)
The following example demonstrates the use of a comparison function
with 'asort()'. The comparison function, 'case_fold_compare()', maps
both values to lowercase in order to compare them ignoring case.
# case_fold_compare --- compare as strings, ignoring case
function case_fold_compare(i1, v1, i2, v2, l, r)
{
l = tolower(v1)
r = tolower(v2)
if (l < r)
return -1
else if (l == r)
return 0
else
return 1
}
And here is the test program for it:
# Test program
BEGIN {
Letters = "abcdefghijklmnopqrstuvwxyz" \
"ABCDEFGHIJKLMNOPQRSTUVWXYZ"
split(Letters, data, "")
asort(data, result, "case_fold_compare")
j = length(result)
for (i = 1; i <= j; i++) {
printf("%s", result[i])
if (i % (j/2) == 0)
printf("\n")
else
printf(" ")
}
}
When run, we get the following:
$ gawk -f case_fold_compare.awk
-| A a B b c C D d e E F f g G H h i I J j k K l L M m
-| n N O o p P Q q r R S s t T u U V v w W X x y Y z Z
---------- Footnotes ----------
(1) This is true because locale-based comparison occurs only when in
POSIX-compatibility mode, and because 'asort()' and 'asorti()' are
'gawk' extensions, they are not available in that case.
File: gawk.info, Node: Two-way I/O, Next: TCP/IP Networking, Prev: Array Sorting, Up: Advanced Features
12.3 Two-Way Communications with Another Process
================================================
It is often useful to be able to send data to a separate program for
processing and then read the result. This can always be done with
temporary files:
# Write the data for processing
tempfile = ("mydata." PROCINFO["pid"])
while (NOT DONE WITH DATA)
print DATA | ("subprogram > " tempfile)
close("subprogram > " tempfile)
# Read the results, remove tempfile when done
while ((getline newdata < tempfile) > 0)
PROCESS newdata APPROPRIATELY
close(tempfile)
system("rm " tempfile)
This works, but not elegantly. Among other things, it requires that the
program be run in a directory that cannot be shared among users; for
example, '/tmp' will not do, as another user might happen to be using a
temporary file with the same name.(1)
However, with 'gawk', it is possible to open a _two-way_ pipe to
another process. The second process is termed a "coprocess", as it runs
in parallel with 'gawk'. The two-way connection is created using the
'|&' operator (borrowed from the Korn shell, 'ksh'):(2)
do {
print DATA |& "subprogram"
"subprogram" |& getline results
} while (DATA LEFT TO PROCESS)
close("subprogram")
The first time an I/O operation is executed using the '|&' operator,
'gawk' creates a two-way pipeline to a child process that runs the other
program. Output created with 'print' or 'printf' is written to the
program's standard input, and output from the program's standard output
can be read by the 'gawk' program using 'getline'. As is the case with
processes started by '|', the subprogram can be any program, or pipeline
of programs, that can be started by the shell.
There are some cautionary items to be aware of:
* As the code inside 'gawk' currently stands, the coprocess's
standard error goes to the same place that the parent 'gawk''s
standard error goes. It is not possible to read the child's
standard error separately.
* I/O buffering may be a problem. 'gawk' automatically flushes all
output down the pipe to the coprocess. However, if the coprocess
does not flush its output, 'gawk' may hang when doing a 'getline'
in order to read the coprocess's results. This could lead to a
situation known as "deadlock", where each process is waiting for
the other one to do something.
It is possible to close just one end of the two-way pipe to a
coprocess, by supplying a second argument to the 'close()' function of
either '"to"' or '"from"' (*note Close Files And Pipes::). These
strings tell 'gawk' to close the end of the pipe that sends data to the
coprocess or the end that reads from it, respectively.
This is particularly necessary in order to use the system 'sort'
utility as part of a coprocess; 'sort' must read _all_ of its input data
before it can produce any output. The 'sort' program does not receive
an end-of-file indication until 'gawk' closes the write end of the pipe.
When you have finished writing data to the 'sort' utility, you can
close the '"to"' end of the pipe, and then start reading sorted data via
'getline'. For example:
BEGIN {
command = "LC_ALL=C sort"
n = split("abcdefghijklmnopqrstuvwxyz", a, "")
for (i = n; i > 0; i--)
print a[i] |& command
close(command, "to")
while ((command |& getline line) > 0)
print "got", line
close(command)
}
This program writes the letters of the alphabet in reverse order, one
per line, down the two-way pipe to 'sort'. It then closes the write end
of the pipe, so that 'sort' receives an end-of-file indication. This
causes 'sort' to sort the data and write the sorted data back to the
'gawk' program. Once all of the data has been read, 'gawk' terminates
the coprocess and exits.
As a side note, the assignment 'LC_ALL=C' in the 'sort' command
ensures traditional Unix (ASCII) sorting from 'sort'. This is not
strictly necessary here, but it's good to know how to do this.
Be careful when closing the '"from"' end of a two-way pipe; in this
case 'gawk' waits for the child process to exit, which may cause your
program to hang. (Thus, this particular feature is of much less use in
practice than being able to close the '"to"' end.)
CAUTION: Normally, it is a fatal error to write to the '"to"' end
of a two-way pipe which has been closed, and it is also a fatal
error to read from the '"from"' end of a two-way pipe that has been
closed.
You may set 'PROCINFO["COMMAND", "NONFATAL"]' to make such
operations become nonfatal. If you do so, you then need to check
'ERRNO' after each 'print', 'printf', or 'getline'. *Note
Nonfatal::, for more information.
You may also use pseudo-ttys (ptys) for two-way communication instead
of pipes, if your system supports them. This is done on a per-command
basis, by setting a special element in the 'PROCINFO' array (*note
Auto-set::), like so:
command = "sort -nr" # command, save in convenience variable
PROCINFO[command, "pty"] = 1 # update PROCINFO
print ... |& command # start two-way pipe
...
If your system does not have ptys, or if all the system's ptys are in
use, 'gawk' automatically falls back to using regular pipes.
Using ptys usually avoids the buffer deadlock issues described
earlier, at some loss in performance. This is because the tty driver
buffers and sends data line-by-line. On systems with the 'stdbuf' (part
of the GNU Coreutils package
(https://www.gnu.org/software/coreutils/coreutils.html)), you can use
that program instead of ptys.
Note also that ptys are not fully transparent. Certain binary
control codes, such 'Ctrl-d' for end-of-file, are interpreted by the tty
driver and not passed through.
CAUTION: Finally, coprocesses open up the possibility of "deadlock"
between 'gawk' and the program running in the coprocess. This can
occur if you send "too much" data to the coprocess before reading
any back; each process is blocked writing data with noone available
to read what they've already written. There is no workaround for
deadlock; careful programming and knowledge of the behavior of the
coprocess are required.
---------- Footnotes ----------
(1) Michael Brennan suggests the use of 'rand()' to generate unique
file names. This is a valid point; nevertheless, temporary files remain
more difficult to use than two-way pipes.
(2) This is very different from the same operator in the C shell and
in Bash.
File: gawk.info, Node: TCP/IP Networking, Next: Profiling, Prev: Two-way I/O, Up: Advanced Features
12.4 Using 'gawk' for Network Programming
=========================================
'EMRED':
A host is a host from coast to coast,
and nobody talks to a host that's close,
unless the host that isn't close
is busy, hung, or dead.
-- _Mike O'Brien (aka Mr. Protocol)_
In addition to being able to open a two-way pipeline to a coprocess
on the same system (*note Two-way I/O::), it is possible to make a
two-way connection to another process on another system across an IP
network connection.
You can think of this as just a _very long_ two-way pipeline to a
coprocess. The way 'gawk' decides that you want to use TCP/IP
networking is by recognizing special file names that begin with one of
'/inet/', '/inet4/', or '/inet6/'.
The full syntax of the special file name is
'/NET-TYPE/PROTOCOL/LOCAL-PORT/REMOTE-HOST/REMOTE-PORT'. The components
are:
NET-TYPE
Specifies the kind of Internet connection to make. Use '/inet4/'
to force IPv4, and '/inet6/' to force IPv6. Plain '/inet/' (which
used to be the only option) uses the system default, most likely
IPv4.
PROTOCOL
The protocol to use over IP. This must be either 'tcp', or 'udp',
for a TCP or UDP IP connection, respectively. TCP should be used
for most applications.
LOCAL-PORT
The local TCP or UDP port number to use. Use a port number of '0'
when you want the system to pick a port. This is what you should
do when writing a TCP or UDP client. You may also use a well-known
service name, such as 'smtp' or 'http', in which case 'gawk'
attempts to determine the predefined port number using the C
'getaddrinfo()' function.
REMOTE-HOST
The IP address or fully qualified domain name of the Internet host
to which you want to connect.
REMOTE-PORT
The TCP or UDP port number to use on the given REMOTE-HOST. Again,
use '0' if you don't care, or else a well-known service name.
NOTE: Failure in opening a two-way socket will result in a nonfatal
error being returned to the calling code. The value of 'ERRNO'
indicates the error (*note Auto-set::).
Consider the following very simple example:
BEGIN {
Service = "/inet/tcp/0/localhost/daytime"
Service |& getline
print $0
close(Service)
}
This program reads the current date and time from the local system's
TCP 'daytime' server. It then prints the results and closes the
connection.
Because this topic is extensive, the use of 'gawk' for TCP/IP
programming is documented separately. See *note (General Introduction,
gawkinet, TCP/IP Internetworking with 'gawk')Top::, for a much more
complete introduction and discussion, as well as extensive examples.
NOTE: 'gawk' can only open direct sockets. There is currently no
way to access services available over Secure Socket Layer (SSL);
this includes any web service whose URL starts with 'https://'.
File: gawk.info, Node: Profiling, Next: Advanced Features Summary, Prev: TCP/IP Networking, Up: Advanced Features
12.5 Profiling Your 'awk' Programs
==================================
You may produce execution traces of your 'awk' programs. This is done
by passing the option '--profile' to 'gawk'. When 'gawk' has finished
running, it creates a profile of your program in a file named
'awkprof.out'. Because it is profiling, it also executes up to 45%
slower than 'gawk' normally does.
As shown in the following example, the '--profile' option can be used
to change the name of the file where 'gawk' will write the profile:
gawk --profile=myprog.prof -f myprog.awk data1 data2
In the preceding example, 'gawk' places the profile in 'myprog.prof'
instead of in 'awkprof.out'.
Here is a sample session showing a simple 'awk' program, its input
data, and the results from running 'gawk' with the '--profile' option.
First, the 'awk' program:
BEGIN { print "First BEGIN rule" }
END { print "First END rule" }
/foo/ {
print "matched /foo/, gosh"
for (i = 1; i <= 3; i++)
sing()
}
{
if (/foo/)
print "if is true"
else
print "else is true"
}
BEGIN { print "Second BEGIN rule" }
END { print "Second END rule" }
function sing( dummy)
{
print "I gotta be me!"
}
Following is the input data:
foo
bar
baz
foo
junk
Here is the 'awkprof.out' that results from running the 'gawk'
profiler on this program and data (this example also illustrates that
'awk' programmers sometimes get up very early in the morning to work):
# gawk profile, created Mon Sep 29 05:16:21 2014
# BEGIN rule(s)
BEGIN {
1 print "First BEGIN rule"
}
BEGIN {
1 print "Second BEGIN rule"
}
# Rule(s)
5 /foo/ { # 2
2 print "matched /foo/, gosh"
6 for (i = 1; i <= 3; i++) {
6 sing()
}
}
5 {
5 if (/foo/) { # 2
2 print "if is true"
3 } else {
3 print "else is true"
}
}
# END rule(s)
END {
1 print "First END rule"
}
END {
1 print "Second END rule"
}
# Functions, listed alphabetically
6 function sing(dummy)
{
6 print "I gotta be me!"
}
This example illustrates many of the basic features of profiling
output. They are as follows:
* The program is printed in the order 'BEGIN' rules, 'BEGINFILE'
rules, pattern-action rules, 'ENDFILE' rules, 'END' rules, and
functions, listed alphabetically. Multiple 'BEGIN' and 'END' rules
retain their separate identities, as do multiple 'BEGINFILE' and
'ENDFILE' rules.
* Pattern-action rules have two counts. The first count, to the left
of the rule, shows how many times the rule's pattern was _tested_.
The second count, to the right of the rule's opening left brace in
a comment, shows how many times the rule's action was _executed_.
The difference between the two indicates how many times the rule's
pattern evaluated to false.
* Similarly, the count for an 'if'-'else' statement shows how many
times the condition was tested. To the right of the opening left
brace for the 'if''s body is a count showing how many times the
condition was true. The count for the 'else' indicates how many
times the test failed.
* The count for a loop header (such as 'for' or 'while') shows how
many times the loop test was executed. (Because of this, you can't
just look at the count on the first statement in a rule to
determine how many times the rule was executed. If the first
statement is a loop, the count is misleading.)
* For user-defined functions, the count next to the 'function'
keyword indicates how many times the function was called. The
counts next to the statements in the body show how many times those
statements were executed.
* The layout uses "K&R" style with TABs. Braces are used everywhere,
even when the body of an 'if', 'else', or loop is only a single
statement.
* Parentheses are used only where needed, as indicated by the
structure of the program and the precedence rules. For example,
'(3 + 5) * 4' means add three and five, then multiply the total by
four. However, '3 + 5 * 4' has no parentheses, and means '3 + (5 *
4)'.
* Parentheses are used around the arguments to 'print' and 'printf'
only when the 'print' or 'printf' statement is followed by a
redirection. Similarly, if the target of a redirection isn't a
scalar, it gets parenthesized.
* 'gawk' supplies leading comments in front of the 'BEGIN' and 'END'
rules, the 'BEGINFILE' and 'ENDFILE' rules, the pattern-action
rules, and the functions.
The profiled version of your program may not look exactly like what
you typed when you wrote it. This is because 'gawk' creates the
profiled version by "pretty-printing" its internal representation of the
program. The advantage to this is that 'gawk' can produce a standard
representation. Also, things such as:
/foo/
come out as:
/foo/ {
print $0
}
which is correct, but possibly unexpected.
Besides creating profiles when a program has completed, 'gawk' can
produce a profile while it is running. This is useful if your 'awk'
program goes into an infinite loop and you want to see what has been
executed. To use this feature, run 'gawk' with the '--profile' option
in the background:
$ gawk --profile -f myprog &
[1] 13992
The shell prints a job number and process ID number; in this case,
13992. Use the 'kill' command to send the 'USR1' signal to 'gawk':
$ kill -USR1 13992
As usual, the profiled version of the program is written to
'awkprof.out', or to a different file if one was specified with the
'--profile' option.
Along with the regular profile, as shown earlier, the profile file
includes a trace of any active functions:
# Function Call Stack:
# 3. baz
# 2. bar
# 1. foo
# -- main --
You may send 'gawk' the 'USR1' signal as many times as you like.
Each time, the profile and function call trace are appended to the
output profile file.
If you use the 'HUP' signal instead of the 'USR1' signal, 'gawk'
produces the profile and the function call trace and then exits.
When 'gawk' runs on MS-Windows systems, it uses the 'INT' and 'QUIT'
signals for producing the profile, and in the case of the 'INT' signal,
'gawk' exits. This is because these systems don't support the 'kill'
command, so the only signals you can deliver to a program are those
generated by the keyboard. The 'INT' signal is generated by the
'Ctrl-c' or 'Ctrl-BREAK' key, while the 'QUIT' signal is generated by
the 'Ctrl-\' key.
Finally, 'gawk' also accepts another option, '--pretty-print'. When
called this way, 'gawk' "pretty-prints" the program into 'awkprof.out',
without any execution counts.
NOTE: Once upon a time, the '--pretty-print' option would also run
your program. This is is no longer the case.
There is a significant difference between the output created when
profiling, and that created when pretty-printing. Pretty-printed output
preserves the original comments that were in the program, although their
placement may not correspond exactly to their original locations in the
source code.(1)
However, as a deliberate design decision, profiling output _omits_
the original program's comments. This allows you to focus on the
execution count data and helps you avoid the temptation to use the
profiler for pretty-printing.
Additionally, pretty-printed output does not have the leading
indentation that the profiling output does. This makes it easy to
pretty-print your code once development is completed, and then use the
result as the final version of your program.
Because the internal representation of your program is formatted to
recreate an 'awk' program, profiling and pretty-printing automatically
disable 'gawk''s default optimizations.
Pretty printing also preserves the original format of numeric
constants; if you used an octal or hexadecimal value in your source
code, it will appear that way in the output.
---------- Footnotes ----------
(1) 'gawk' does the best it can to preserve the distinction between
comments at the end of a statement and comments on lines by themselves.
Due to implementation constraints, it does not always do so correctly,
particularly for 'switch' statements. The 'gawk' maintainers hope to
improve this in a subsequent release.
File: gawk.info, Node: Advanced Features Summary, Prev: Profiling, Up: Advanced Features
12.6 Summary
============
* The '--non-decimal-data' option causes 'gawk' to treat octal- and
hexadecimal-looking input data as octal and hexadecimal. This
option should be used with caution or not at all; use of
'strtonum()' is preferable. Note that this option may disappear in
a future version of 'gawk'.
* You can take over complete control of sorting in 'for (INDX in
ARRAY)' array traversal by setting 'PROCINFO["sorted_in"]' to the
name of a user-defined function that does the comparison of array
elements based on index and value.
* Similarly, you can supply the name of a user-defined comparison
function as the third argument to either 'asort()' or 'asorti()' to
control how those functions sort arrays. Or you may provide one of
the predefined control strings that work for
'PROCINFO["sorted_in"]'.
* You can use the '|&' operator to create a two-way pipe to a
coprocess. You read from the coprocess with 'getline' and write to
it with 'print' or 'printf'. Use 'close()' to close off the
coprocess completely, or optionally, close off one side of the
two-way communications.
* By using special file names with the '|&' operator, you can open a
TCP/IP (or UDP/IP) connection to remote hosts on the Internet.
'gawk' supports both IPv4 and IPv6.
* You can generate statement count profiles of your program. This
can help you determine which parts of your program may be taking
the most time and let you tune them more easily. Sending the
'USR1' signal while profiling causes 'gawk' to dump the profile and
keep going, including a function call stack.
* You can also just "pretty-print" the program.
File: gawk.info, Node: Internationalization, Next: Debugger, Prev: Advanced Features, Up: Top
13 Internationalization with 'gawk'
***********************************
Once upon a time, computer makers wrote software that worked only in
English. Eventually, hardware and software vendors noticed that if
their systems worked in the native languages of non-English-speaking
countries, they were able to sell more systems. As a result,
internationalization and localization of programs and software systems
became a common practice.
For many years, the ability to provide internationalization was
largely restricted to programs written in C and C++. This major node
describes the underlying library 'gawk' uses for internationalization,
as well as how 'gawk' makes internationalization features available at
the 'awk' program level. Having internationalization available at the
'awk' level gives software developers additional flexibility--they are
no longer forced to write in C or C++ when internationalization is a
requirement.
* Menu:
* I18N and L10N:: Internationalization and Localization.
* Explaining gettext:: How GNU 'gettext' works.
* Programmer i18n:: Features for the programmer.
* Translator i18n:: Features for the translator.
* I18N Example:: A simple i18n example.
* Gawk I18N:: 'gawk' is also internationalized.
* I18N Summary:: Summary of I18N stuff.
File: gawk.info, Node: I18N and L10N, Next: Explaining gettext, Up: Internationalization
13.1 Internationalization and Localization
==========================================
"Internationalization" means writing (or modifying) a program once, in
such a way that it can use multiple languages without requiring further
source code changes. "Localization" means providing the data necessary
for an internationalized program to work in a particular language. Most
typically, these terms refer to features such as the language used for
printing error messages, the language used to read responses, and
information related to how numerical and monetary values are printed and
read.
File: gawk.info, Node: Explaining gettext, Next: Programmer i18n, Prev: I18N and L10N, Up: Internationalization
13.2 GNU 'gettext'
==================
'gawk' uses GNU 'gettext' to provide its internationalization features.
The facilities in GNU 'gettext' focus on messages: strings printed by a
program, either directly or via formatting with 'printf' or
'sprintf()'.(1)
When using GNU 'gettext', each application has its own "text domain".
This is a unique name, such as 'kpilot' or 'gawk', that identifies the
application. A complete application may have multiple
components--programs written in C or C++, as well as scripts written in
'sh' or 'awk'. All of the components use the same text domain.
To make the discussion concrete, assume we're writing an application
named 'guide'. Internationalization consists of the following steps, in
this order:
1. The programmer reviews the source for all of 'guide''s components
and marks each string that is a candidate for translation. For
example, '"`-F': option required"' is a good candidate for
translation. A table with strings of option names is not (e.g.,
'gawk''s '--profile' option should remain the same, no matter what
the local language).
2. The programmer indicates the application's text domain ('"guide"')
to the 'gettext' library, by calling the 'textdomain()' function.
3. Messages from the application are extracted from the source code
and collected into a portable object template file ('guide.pot'),
which lists the strings and their translations. The translations
are initially empty. The original (usually English) messages serve
as the key for lookup of the translations.
4. For each language with a translator, 'guide.pot' is copied to a
portable object file ('.po') and translations are created and
shipped with the application. For example, there might be a
'fr.po' for a French translation.
5. Each language's '.po' file is converted into a binary message
object ('.gmo') file. A message object file contains the original
messages and their translations in a binary format that allows fast
lookup of translations at runtime.
6. When 'guide' is built and installed, the binary translation files
are installed in a standard place.
7. For testing and development, it is possible to tell 'gettext' to
use '.gmo' files in a different directory than the standard one by
using the 'bindtextdomain()' function.
8. At runtime, 'guide' looks up each string via a call to 'gettext()'.
The returned string is the translated string if available, or the
original string if not.
9. If necessary, it is possible to access messages from a different
text domain than the one belonging to the application, without
having to switch the application's default text domain back and
forth.
In C (or C++), the string marking and dynamic translation lookup are
accomplished by wrapping each string in a call to 'gettext()':
printf("%s", gettext("Don't Panic!\n"));
The tools that extract messages from source code pull out all strings
enclosed in calls to 'gettext()'.
The GNU 'gettext' developers, recognizing that typing 'gettext(...)'
over and over again is both painful and ugly to look at, use the macro
'_' (an underscore) to make things easier:
/* In the standard header file: */
#define _(str) gettext(str)
/* In the program text: */
printf("%s", _("Don't Panic!\n"));
This reduces the typing overhead to just three extra characters per
string and is considerably easier to read as well.
There are locale "categories" for different types of locale-related
information. The defined locale categories that 'gettext' knows about
are:
'LC_MESSAGES'
Text messages. This is the default category for 'gettext'
operations, but it is possible to supply a different one
explicitly, if necessary. (It is almost never necessary to supply
a different category.)
'LC_COLLATE'
Text-collation information (i.e., how different characters and/or
groups of characters sort in a given language).
'LC_CTYPE'
Character-type information (alphabetic, digit, upper- or lowercase,
and so on) as well as character encoding. This information is
accessed via the POSIX character classes in regular expressions,
such as '/[[:alnum:]]/' (*note Bracket Expressions::).
'LC_MONETARY'
Monetary information, such as the currency symbol, and whether the
symbol goes before or after a number.
'LC_NUMERIC'
Numeric information, such as which characters to use for the
decimal point and the thousands separator.(2)
'LC_TIME'
Time- and date-related information, such as 12- or 24-hour clock,
month printed before or after the day in a date, local month
abbreviations, and so on.
'LC_ALL'
All of the above. (Not too useful in the context of 'gettext'.)
NOTE: As described in *note Locales::, environment variables with
the same name as the locale categories ('LC_CTYPE', 'LC_ALL', etc.)
influence 'gawk''s behavior (and that of other utilities).
Normally, these variables also affect how the 'gettext' library
finds translations. However, the 'LANGUAGE' environment variable
overrides the 'LC_XXX' variables. Many GNU/Linux systems may
define this variable without your knowledge, causing 'gawk' to not
find the correct translations. If this happens to you, look to see
if 'LANGUAGE' is defined, and if so, use the shell's 'unset'
command to remove it.
For testing translations of 'gawk' itself, you can set the
'GAWK_LOCALE_DIR' environment variable. See the documentation for the C
'bindtextdomain()' function and also see *note Other Environment
Variables::.
---------- Footnotes ----------
(1) For some operating systems, the 'gawk' port doesn't support GNU
'gettext'. Therefore, these features are not available if you are using
one of those operating systems. Sorry.
(2) Americans use a comma every three decimal places and a period for
the decimal point, while many Europeans do exactly the opposite:
1,234.56 versus 1.234,56.
File: gawk.info, Node: Programmer i18n, Next: Translator i18n, Prev: Explaining gettext, Up: Internationalization
13.3 Internationalizing 'awk' Programs
======================================
'gawk' provides the following variables for internationalization:
'TEXTDOMAIN'
This variable indicates the application's text domain. For
compatibility with GNU 'gettext', the default value is
'"messages"'.
'_"your message here"'
String constants marked with a leading underscore are candidates
for translation at runtime. String constants without a leading
underscore are not translated.
'gawk' provides the following functions for internationalization:
'dcgettext(STRING [, DOMAIN [, CATEGORY]])'
Return the translation of STRING in text domain DOMAIN for locale
category CATEGORY. The default value for DOMAIN is the current
value of 'TEXTDOMAIN'. The default value for CATEGORY is
'"LC_MESSAGES"'.
If you supply a value for CATEGORY, it must be a string equal to
one of the known locale categories described in *note Explaining
gettext::. You must also supply a text domain. Use 'TEXTDOMAIN'
if you want to use the current domain.
CAUTION: The order of arguments to the 'awk' version of the
'dcgettext()' function is purposely different from the order
for the C version. The 'awk' version's order was chosen to be
simple and to allow for reasonable 'awk'-style default
arguments.
'dcngettext(STRING1, STRING2, NUMBER [, DOMAIN [, CATEGORY]])'
Return the plural form used for NUMBER of the translation of
STRING1 and STRING2 in text domain DOMAIN for locale category
CATEGORY. STRING1 is the English singular variant of a message,
and STRING2 is the English plural variant of the same message. The
default value for DOMAIN is the current value of 'TEXTDOMAIN'. The
default value for CATEGORY is '"LC_MESSAGES"'.
The same remarks about argument order as for the 'dcgettext()'
function apply.
'bindtextdomain(DIRECTORY [, DOMAIN ])'
Change the directory in which 'gettext' looks for '.gmo' files, in
case they will not or cannot be placed in the standard locations
(e.g., during testing). Return the directory in which DOMAIN is
"bound."
The default DOMAIN is the value of 'TEXTDOMAIN'. If DIRECTORY is
the null string ('""'), then 'bindtextdomain()' returns the current
binding for the given DOMAIN.
To use these facilities in your 'awk' program, follow these steps:
1. Set the variable 'TEXTDOMAIN' to the text domain of your program.
This is best done in a 'BEGIN' rule (*note BEGIN/END::), or it can
also be done via the '-v' command-line option (*note Options::):
BEGIN {
TEXTDOMAIN = "guide"
...
}
2. Mark all translatable strings with a leading underscore ('_')
character. It _must_ be adjacent to the opening quote of the
string. For example:
print _"hello, world"
x = _"you goofed"
printf(_"Number of users is %d\n", nusers)
3. If you are creating strings dynamically, you can still translate
them, using the 'dcgettext()' built-in function:(1)
if (groggy)
message = dcgettext("%d customers disturbing me\n", "adminprog")
else
message = dcgettext("enjoying %d customers\n", "adminprog")
printf(message, ncustomers)
Here, the call to 'dcgettext()' supplies a different text domain
('"adminprog"') in which to find the message, but it uses the
default '"LC_MESSAGES"' category.
The previous example only works if 'ncustomers' is greater than
one. This example would be better done with 'dcngettext()':
if (groggy)
message = dcngettext("%d customer disturbing me\n",
"%d customers disturbing me\n",
ncustomers, "adminprog")
else
message = dcngettext("enjoying %d customer\n",
"enjoying %d customers\n",
ncustomers, "adminprog")
printf(message, ncustomers)
4. During development, you might want to put the '.gmo' file in a
private directory for testing. This is done with the
'bindtextdomain()' built-in function:
BEGIN {
TEXTDOMAIN = "guide" # our text domain
if (Testing) {
# where to find our files
bindtextdomain("testdir")
# joe is in charge of adminprog
bindtextdomain("../joe/testdir", "adminprog")
}
...
}
*Note I18N Example:: for an example program showing the steps to
create and use translations from 'awk'.
---------- Footnotes ----------
(1) Thanks to Bruno Haible for this example.
File: gawk.info, Node: Translator i18n, Next: I18N Example, Prev: Programmer i18n, Up: Internationalization
13.4 Translating 'awk' Programs
===============================
Once a program's translatable strings have been marked, they must be
extracted to create the initial '.pot' file. As part of translation, it
is often helpful to rearrange the order in which arguments to 'printf'
are output.
'gawk''s '--gen-pot' command-line option extracts the messages and is
discussed next. After that, 'printf''s ability to rearrange the order
for 'printf' arguments at runtime is covered.
* Menu:
* String Extraction:: Extracting marked strings.
* Printf Ordering:: Rearranging 'printf' arguments.
* I18N Portability:: 'awk'-level portability issues.
File: gawk.info, Node: String Extraction, Next: Printf Ordering, Up: Translator i18n
13.4.1 Extracting Marked Strings
--------------------------------
Once your 'awk' program is working, and all the strings have been marked
and you've set (and perhaps bound) the text domain, it is time to
produce translations. First, use the '--gen-pot' command-line option to
create the initial '.pot' file:
gawk --gen-pot -f guide.awk > guide.pot
When run with '--gen-pot', 'gawk' does not execute your program.
Instead, it parses it as usual and prints all marked strings to standard
output in the format of a GNU 'gettext' Portable Object file. Also
included in the output are any constant strings that appear as the first
argument to 'dcgettext()' or as the first and second argument to
'dcngettext()'.(1) You should distribute the generated '.pot' file with
your 'awk' program; translators will eventually use it to provide you
translations that you can also then distribute. *Note I18N Example::
for the full list of steps to go through to create and test translations
for 'guide'.
---------- Footnotes ----------
(1) The 'xgettext' utility that comes with GNU 'gettext' can handle
'.awk' files.
File: gawk.info, Node: Printf Ordering, Next: I18N Portability, Prev: String Extraction, Up: Translator i18n
13.4.2 Rearranging 'printf' Arguments
-------------------------------------
Format strings for 'printf' and 'sprintf()' (*note Printf::) present a
special problem for translation. Consider the following:(1)
printf(_"String `%s' has %d characters\n",
string, length(string)))
A possible German translation for this might be:
"%d Zeichen lang ist die Zeichenkette `%s'\n"
The problem should be obvious: the order of the format specifications
is different from the original! Even though 'gettext()' can return the
translated string at runtime, it cannot change the argument order in the
call to 'printf'.
To solve this problem, 'printf' format specifiers may have an
additional optional element, which we call a "positional specifier".
For example:
"%2$d Zeichen lang ist die Zeichenkette `%1$s'\n"
Here, the positional specifier consists of an integer count, which
indicates which argument to use, and a '$'. Counts are one-based, and
the format string itself is _not_ included. Thus, in the following
example, 'string' is the first argument and 'length(string)' is the
second:
$ gawk 'BEGIN {
> string = "Don\47t Panic"
> printf "%2$d characters live in \"%1$s\"\n",
> string, length(string)
> }'
-| 11 characters live in "Don't Panic"
If present, positional specifiers come first in the format
specification, before the flags, the field width, and/or the precision.
Positional specifiers can be used with the dynamic field width and
precision capability:
$ gawk 'BEGIN {
> printf("%*.*s\n", 10, 20, "hello")
> printf("%3$*2$.*1$s\n", 20, 10, "hello")
> }'
-| hello
-| hello
NOTE: When using '*' with a positional specifier, the '*' comes
first, then the integer position, and then the '$'. This is
somewhat counterintuitive.
'gawk' does not allow you to mix regular format specifiers and those
with positional specifiers in the same string:
$ gawk 'BEGIN { printf "%d %3$s\n", 1, 2, "hi" }'
error-> gawk: cmd. line:1: fatal: must use `count$' on all formats or none
NOTE: There are some pathological cases that 'gawk' may fail to
diagnose. In such cases, the output may not be what you expect.
It's still a bad idea to try mixing them, even if 'gawk' doesn't
detect it.
Although positional specifiers can be used directly in 'awk'
programs, their primary purpose is to help in producing correct
translations of format strings into languages different from the one in
which the program is first written.
---------- Footnotes ----------
(1) This example is borrowed from the GNU 'gettext' manual.
File: gawk.info, Node: I18N Portability, Prev: Printf Ordering, Up: Translator i18n
13.4.3 'awk' Portability Issues
-------------------------------
'gawk''s internationalization features were purposely chosen to have as
little impact as possible on the portability of 'awk' programs that use
them to other versions of 'awk'. Consider this program:
BEGIN {
TEXTDOMAIN = "guide"
if (Test_Guide) # set with -v
bindtextdomain("/test/guide/messages")
print _"don't panic!"
}
As written, it won't work on other versions of 'awk'. However, it is
actually almost portable, requiring very little change:
* Assignments to 'TEXTDOMAIN' won't have any effect, because
'TEXTDOMAIN' is not special in other 'awk' implementations.
* Non-GNU versions of 'awk' treat marked strings as the concatenation
of a variable named '_' with the string following it.(1)
Typically, the variable '_' has the null string ('""') as its
value, leaving the original string constant as the result.
* By defining "dummy" functions to replace 'dcgettext()',
'dcngettext()', and 'bindtextdomain()', the 'awk' program can be
made to run, but all the messages are output in the original
language. For example:
function bindtextdomain(dir, domain)
{
return dir
}
function dcgettext(string, domain, category)
{
return string
}
function dcngettext(string1, string2, number, domain, category)
{
return (number == 1 ? string1 : string2)
}
* The use of positional specifications in 'printf' or 'sprintf()' is
_not_ portable. To support 'gettext()' at the C level, many
systems' C versions of 'sprintf()' do support positional
specifiers. But it works only if enough arguments are supplied in
the function call. Many versions of 'awk' pass 'printf' formats
and arguments unchanged to the underlying C library version of
'sprintf()', but only one format and argument at a time. What
happens if a positional specification is used is anybody's guess.
However, because the positional specifications are primarily for
use in _translated_ format strings, and because non-GNU 'awk's
never retrieve the translated string, this should not be a problem
in practice.
---------- Footnotes ----------
(1) This is good fodder for an "Obfuscated 'awk'" contest.
File: gawk.info, Node: I18N Example, Next: Gawk I18N, Prev: Translator i18n, Up: Internationalization
13.5 A Simple Internationalization Example
==========================================
Now let's look at a step-by-step example of how to internationalize and
localize a simple 'awk' program, using 'guide.awk' as our original
source:
BEGIN {
TEXTDOMAIN = "guide"
bindtextdomain(".") # for testing
print _"Don't Panic"
print _"The Answer Is", 42
print "Pardon me, Zaphod who?"
}
Run 'gawk --gen-pot' to create the '.pot' file:
$ gawk --gen-pot -f guide.awk > guide.pot
This produces:
#: guide.awk:4
msgid "Don't Panic"
msgstr ""
#: guide.awk:5
msgid "The Answer Is"
msgstr ""
This original portable object template file is saved and reused for
each language into which the application is translated. The 'msgid' is
the original string and the 'msgstr' is the translation.
NOTE: Strings not marked with a leading underscore do not appear in
the 'guide.pot' file.
Next, the messages must be translated. Here is a translation to a
hypothetical dialect of English, called "Mellow":(1)
$ cp guide.pot guide-mellow.po
ADD TRANSLATIONS TO guide-mellow.po ...
Following are the translations:
#: guide.awk:4
msgid "Don't Panic"
msgstr "Hey man, relax!"
#: guide.awk:5
msgid "The Answer Is"
msgstr "Like, the scoop is"
The next step is to make the directory to hold the binary message
object file and then to create the 'guide.mo' file. We pretend that our
file is to be used in the 'en_US.UTF-8' locale, because we have to use a
locale name known to the C 'gettext' routines. The directory layout
shown here is standard for GNU 'gettext' on GNU/Linux systems. Other
versions of 'gettext' may use a different layout:
$ mkdir en_US.UTF-8 en_US.UTF-8/LC_MESSAGES
The 'msgfmt' utility does the conversion from human-readable '.po'
file to machine-readable '.mo' file. By default, 'msgfmt' creates a
file named 'messages'. This file must be renamed and placed in the
proper directory (using the '-o' option) so that 'gawk' can find it:
$ msgfmt guide-mellow.po -o en_US.UTF-8/LC_MESSAGES/guide.mo
Finally, we run the program to test it:
$ gawk -f guide.awk
-| Hey man, relax!
-| Like, the scoop is 42
-| Pardon me, Zaphod who?
If the three replacement functions for 'dcgettext()', 'dcngettext()',
and 'bindtextdomain()' (*note I18N Portability::) are in a file named
'libintl.awk', then we can run 'guide.awk' unchanged as follows:
$ gawk --posix -f guide.awk -f libintl.awk
-| Don't Panic
-| The Answer Is 42
-| Pardon me, Zaphod who?
---------- Footnotes ----------
(1) Perhaps it would be better if it were called "Hippy." Ah, well.
File: gawk.info, Node: Gawk I18N, Next: I18N Summary, Prev: I18N Example, Up: Internationalization
13.6 'gawk' Can Speak Your Language
===================================
'gawk' itself has been internationalized using the GNU 'gettext'
package. (GNU 'gettext' is described in complete detail in *note (GNU
'gettext' utilities, gettext, GNU 'gettext' utilities)Top::.) As of
this writing, the latest version of GNU 'gettext' is version 0.19.4
(ftp://ftp.gnu.org/gnu/gettext/gettext-0.19.4.tar.gz).
If a translation of 'gawk''s messages exists, then 'gawk' produces
usage messages, warnings, and fatal errors in the local language.
File: gawk.info, Node: I18N Summary, Prev: Gawk I18N, Up: Internationalization
13.7 Summary
============
* Internationalization means writing a program such that it can use
multiple languages without requiring source code changes.
Localization means providing the data necessary for an
internationalized program to work in a particular language.
* 'gawk' uses GNU 'gettext' to let you internationalize and localize
'awk' programs. A program's text domain identifies the program for
grouping all messages and other data together.
* You mark a program's strings for translation by preceding them with
an underscore. Once that is done, the strings are extracted into a
'.pot' file. This file is copied for each language into a '.po'
file, and the '.po' files are compiled into '.gmo' files for use at
runtime.
* You can use positional specifications with 'sprintf()' and 'printf'
to rearrange the placement of argument values in formatted strings
and output. This is useful for the translation of format control
strings.
* The internationalization features have been designed so that they
can be easily worked around in a standard 'awk'.
* 'gawk' itself has been internationalized and ships with a number of
translations for its messages.
File: gawk.info, Node: Debugger, Next: Arbitrary Precision Arithmetic, Prev: Internationalization, Up: Top
14 Debugging 'awk' Programs
***************************
It would be nice if computer programs worked perfectly the first time
they were run, but in real life, this rarely happens for programs of any
complexity. Thus, most programming languages have facilities available
for "debugging" programs, and now 'awk' is no exception.
The 'gawk' debugger is purposely modeled after the GNU Debugger (GDB)
(https://www.gnu.org/software/gdb/) command-line debugger. If you are
familiar with GDB, learning how to use 'gawk' for debugging your program
is easy.
* Menu:
* Debugging:: Introduction to 'gawk' debugger.
* Sample Debugging Session:: Sample debugging session.
* List of Debugger Commands:: Main debugger commands.
* Readline Support:: Readline support.
* Limitations:: Limitations and future plans.
* Debugging Summary:: Debugging summary.
File: gawk.info, Node: Debugging, Next: Sample Debugging Session, Up: Debugger
14.1 Introduction to the 'gawk' Debugger
========================================
This minor node introduces debugging in general and begins the
discussion of debugging in 'gawk'.
* Menu:
* Debugging Concepts:: Debugging in General.
* Debugging Terms:: Additional Debugging Concepts.
* Awk Debugging:: Awk Debugging.
File: gawk.info, Node: Debugging Concepts, Next: Debugging Terms, Up: Debugging
14.1.1 Debugging in General
---------------------------
(If you have used debuggers in other languages, you may want to skip
ahead to *note Awk Debugging::.)
Of course, a debugging program cannot remove bugs for you, because it
has no way of knowing what you or your users consider a "bug" versus a
"feature." (Sometimes, we humans have a hard time with this ourselves.)
In that case, what can you expect from such a tool? The answer to that
depends on the language being debugged, but in general, you can expect
at least the following:
* The ability to watch a program execute its instructions one by one,
giving you, the programmer, the opportunity to think about what is
happening on a time scale of seconds, minutes, or hours, rather
than the nanosecond time scale at which the code usually runs.
* The opportunity to not only passively observe the operation of your
program, but to control it and try different paths of execution,
without having to change your source files.
* The chance to see the values of data in the program at any point in
execution, and also to change that data on the fly, to see how that
affects what happens afterward. (This often includes the ability
to look at internal data structures besides the variables you
actually defined in your code.)
* The ability to obtain additional information about your program's
state or even its internal structure.
All of these tools provide a great amount of help in using your own
skills and understanding of the goals of your program to find where it
is going wrong (or, for that matter, to better comprehend a perfectly
functional program that you or someone else wrote).
File: gawk.info, Node: Debugging Terms, Next: Awk Debugging, Prev: Debugging Concepts, Up: Debugging
14.1.2 Debugging Concepts
-------------------------
Before diving in to the details, we need to introduce several important
concepts that apply to just about all debuggers. The following list
defines terms used throughout the rest of this major node:
"Stack frame"
Programs generally call functions during the course of their
execution. One function can call another, or a function can call
itself (recursion). You can view the chain of called functions
(main program calls A, which calls B, which calls C), as a stack of
executing functions: the currently running function is the topmost
one on the stack, and when it finishes (returns), the next one down
then becomes the active function. Such a stack is termed a "call
stack".
For each function on the call stack, the system maintains a data
area that contains the function's parameters, local variables, and
return value, as well as any other "bookkeeping" information needed
to manage the call stack. This data area is termed a "stack
frame".
'gawk' also follows this model, and gives you access to the call
stack and to each stack frame. You can see the call stack, as well
as from where each function on the stack was invoked. Commands
that print the call stack print information about each stack frame
(as detailed later on).
"Breakpoint"
During debugging, you often wish to let the program run until it
reaches a certain point, and then continue execution from there one
statement (or instruction) at a time. The way to do this is to set
a "breakpoint" within the program. A breakpoint is where the
execution of the program should break off (stop), so that you can
take over control of the program's execution. You can add and
remove as many breakpoints as you like.
"Watchpoint"
A watchpoint is similar to a breakpoint. The difference is that
breakpoints are oriented around the code: stop when a certain point
in the code is reached. A watchpoint, however, specifies that
program execution should stop when a _data value_ is changed. This
is useful, as sometimes it happens that a variable receives an
erroneous value, and it's hard to track down where this happens
just by looking at the code. By using a watchpoint, you can stop
whenever a variable is assigned to, and usually find the errant
code quite quickly.
File: gawk.info, Node: Awk Debugging, Prev: Debugging Terms, Up: Debugging
14.1.3 'awk' Debugging
----------------------
Debugging an 'awk' program has some specific aspects that are not shared
with programs written in other languages.
First of all, the fact that 'awk' programs usually take input line by
line from a file or files and operate on those lines using specific
rules makes it especially useful to organize viewing the execution of
the program in terms of these rules. As we will see, each 'awk' rule is
treated almost like a function call, with its own specific block of
instructions.
In addition, because 'awk' is by design a very concise language, it
is easy to lose sight of everything that is going on "inside" each line
of 'awk' code. The debugger provides the opportunity to look at the
individual primitive instructions carried out by the higher-level 'awk'
commands.
File: gawk.info, Node: Sample Debugging Session, Next: List of Debugger Commands, Prev: Debugging, Up: Debugger
14.2 Sample 'gawk' Debugging Session
====================================
In order to illustrate the use of 'gawk' as a debugger, let's look at a
sample debugging session. We will use the 'awk' implementation of the
POSIX 'uniq' command described earlier (*note Uniq Program::) as our
example.
* Menu:
* Debugger Invocation:: How to Start the Debugger.
* Finding The Bug:: Finding the Bug.
File: gawk.info, Node: Debugger Invocation, Next: Finding The Bug, Up: Sample Debugging Session
14.2.1 How to Start the Debugger
--------------------------------
Starting the debugger is almost exactly like running 'gawk' normally,
except you have to pass an additional option, '--debug', or the
corresponding short option, '-D'. The file(s) containing the program
and any supporting code are given on the command line as arguments to
one or more '-f' options. ('gawk' is not designed to debug command-line
programs, only programs contained in files.) In our case, we invoke the
debugger like this:
$ gawk -D -f getopt.awk -f join.awk -f uniq.awk -1 inputfile
where both 'getopt.awk' and 'uniq.awk' are in '$AWKPATH'. (Experienced
users of GDB or similar debuggers should note that this syntax is
slightly different from what you are used to. With the 'gawk' debugger,
you give the arguments for running the program in the command line to
the debugger rather than as part of the 'run' command at the debugger
prompt.) The '-1' is an option to 'uniq.awk'.
Instead of immediately running the program on 'inputfile', as 'gawk'
would ordinarily do, the debugger merely loads all the program source
files, compiles them internally, and then gives us a prompt:
gawk>
from which we can issue commands to the debugger. At this point, no
code has been executed.
File: gawk.info, Node: Finding The Bug, Prev: Debugger Invocation, Up: Sample Debugging Session
14.2.2 Finding the Bug
----------------------
Let's say that we are having a problem using (a faulty version of)
'uniq.awk' in the "field-skipping" mode, and it doesn't seem to be
catching lines which should be identical when skipping the first field,
such as:
awk is a wonderful program!
gawk is a wonderful program!
This could happen if we were thinking (C-like) of the fields in a
record as being numbered in a zero-based fashion, so instead of the
lines:
clast = join(alast, fcount+1, n)
cline = join(aline, fcount+1, m)
we wrote:
clast = join(alast, fcount, n)
cline = join(aline, fcount, m)
The first thing we usually want to do when trying to investigate a
problem like this is to put a breakpoint in the program so that we can
watch it at work and catch what it is doing wrong. A reasonable spot
for a breakpoint in 'uniq.awk' is at the beginning of the function
'are_equal()', which compares the current line with the previous one.
To set the breakpoint, use the 'b' (breakpoint) command:
gawk> b are_equal
-| Breakpoint 1 set at file `awklib/eg/prog/uniq.awk', line 63
The debugger tells us the file and line number where the breakpoint
is. Now type 'r' or 'run' and the program runs until it hits the
breakpoint for the first time:
gawk> r
-| Starting program:
-| Stopping in Rule ...
-| Breakpoint 1, are_equal(n, m, clast, cline, alast, aline)
at `awklib/eg/prog/uniq.awk':63
-| 63 if (fcount == 0 && charcount == 0)
gawk>
Now we can look at what's going on inside our program. First of all,
let's see how we got to where we are. At the prompt, we type 'bt'
(short for "backtrace"), and the debugger responds with a listing of the
current stack frames:
gawk> bt
-| #0 are_equal(n, m, clast, cline, alast, aline)
at `awklib/eg/prog/uniq.awk':68
-| #1 in main() at `awklib/eg/prog/uniq.awk':88
This tells us that 'are_equal()' was called by the main program at
line 88 of 'uniq.awk'. (This is not a big surprise, because this is the
only call to 'are_equal()' in the program, but in more complex programs,
knowing who called a function and with what parameters can be the key to
finding the source of the problem.)
Now that we're in 'are_equal()', we can start looking at the values
of some variables. Let's say we type 'p n' ('p' is short for "print").
We would expect to see the value of 'n', a parameter to 'are_equal()'.
Actually, the debugger gives us:
gawk> p n
-| n = untyped variable
In this case, 'n' is an uninitialized local variable, because the
function was called without arguments (*note Function Calls::).
A more useful variable to display might be the current record:
gawk> p $0
-| $0 = "gawk is a wonderful program!"
This might be a bit puzzling at first, as this is the second line of our
test input. Let's look at 'NR':
gawk> p NR
-| NR = 2
So we can see that 'are_equal()' was only called for the second record
of the file. Of course, this is because our program contains a rule for
'NR == 1':
NR == 1 {
last = $0
next
}
OK, let's just check that that rule worked correctly:
gawk> p last
-| last = "awk is a wonderful program!"
Everything we have done so far has verified that the program has
worked as planned, up to and including the call to 'are_equal()', so the
problem must be inside this function. To investigate further, we must
begin "stepping through" the lines of 'are_equal()'. We start by typing
'n' (for "next"):
gawk> n
-| 66 if (fcount > 0) {
This tells us that 'gawk' is now ready to execute line 66, which
decides whether to give the lines the special "field-skipping" treatment
indicated by the '-1' command-line option. (Notice that we skipped from
where we were before, at line 63, to here, because the condition in line
63, 'if (fcount == 0 && charcount == 0)', was false.)
Continuing to step, we now get to the splitting of the current and
last records:
gawk> n
-| 67 n = split(last, alast)
gawk> n
-| 68 m = split($0, aline)
At this point, we should be curious to see what our records were
split into, so we try to look:
gawk> p n m alast aline
-| n = 5
-| m = untyped variable
-| alast = array, 5 elements
-| aline = untyped variable
(The 'p' command can take more than one argument, similar to 'awk''s
'print' statement.)
This is kind of disappointing, though. All we found out is that
there are five elements in 'alast'; 'm' and 'aline' don't have values
because we are at line 68 but haven't executed it yet. This information
is useful enough (we now know that none of the words were accidentally
left out), but what if we want to see inside the array?
The first choice would be to use subscripts:
gawk> p alast[0]
-| "0" not in array `alast'
Oops!
gawk> p alast[1]
-| alast["1"] = "awk"
This would be kind of slow for a 100-member array, though, so 'gawk'
provides a shortcut (reminiscent of another language not to be
mentioned):
gawk> p @alast
-| alast["1"] = "awk"
-| alast["2"] = "is"
-| alast["3"] = "a"
-| alast["4"] = "wonderful"
-| alast["5"] = "program!"
It looks like we got this far OK. Let's take another step or two:
gawk> n
-| 69 clast = join(alast, fcount, n)
gawk> n
-| 70 cline = join(aline, fcount, m)
Well, here we are at our error (sorry to spoil the suspense). What
we had in mind was to join the fields starting from the second one to
make the virtual record to compare, and if the first field were numbered
zero, this would work. Let's look at what we've got:
gawk> p cline clast
-| cline = "gawk is a wonderful program!"
-| clast = "awk is a wonderful program!"
Hey, those look pretty familiar! They're just our original,
unaltered input records. A little thinking (the human brain is still
the best debugging tool), and we realize that we were off by one!
We get out of the debugger:
gawk> q
-| The program is running. Exit anyway (y/n)? y
Then we get into an editor:
clast = join(alast, fcount+1, n)
cline = join(aline, fcount+1, m)
and problem solved!
File: gawk.info, Node: List of Debugger Commands, Next: Readline Support, Prev: Sample Debugging Session, Up: Debugger
14.3 Main Debugger Commands
===========================
The 'gawk' debugger command set can be divided into the following
categories:
* Breakpoint control
* Execution control
* Viewing and changing data
* Working with the stack
* Getting information
* Miscellaneous
Each of these are discussed in the following subsections. In the
following descriptions, commands that may be abbreviated show the
abbreviation on a second description line. A debugger command name may
also be truncated if that partial name is unambiguous. The debugger has
the built-in capability to automatically repeat the previous command
just by hitting 'Enter'. This works for the commands 'list', 'next',
'nexti', 'step', 'stepi', and 'continue' executed without any argument.
* Menu:
* Breakpoint Control:: Control of Breakpoints.
* Debugger Execution Control:: Control of Execution.
* Viewing And Changing Data:: Viewing and Changing Data.
* Execution Stack:: Dealing with the Stack.
* Debugger Info:: Obtaining Information about the Program and
the Debugger State.
* Miscellaneous Debugger Commands:: Miscellaneous Commands.
File: gawk.info, Node: Breakpoint Control, Next: Debugger Execution Control, Up: List of Debugger Commands
14.3.1 Control of Breakpoints
-----------------------------
As we saw earlier, the first thing you probably want to do in a
debugging session is to get your breakpoints set up, because your
program will otherwise just run as if it was not under the debugger.
The commands for controlling breakpoints are:
'break' [[FILENAME':']N | FUNCTION] ['"EXPRESSION"']
'b' [[FILENAME':']N | FUNCTION] ['"EXPRESSION"']
Without any argument, set a breakpoint at the next instruction to
be executed in the selected stack frame. Arguments can be one of
the following:
N
Set a breakpoint at line number N in the current source file.
FILENAME':'N
Set a breakpoint at line number N in source file FILENAME.
FUNCTION
Set a breakpoint at entry to (the first instruction of)
function FUNCTION.
Each breakpoint is assigned a number that can be used to delete it
from the breakpoint list using the 'delete' command.
With a breakpoint, you may also supply a condition. This is an
'awk' expression (enclosed in double quotes) that the debugger
evaluates whenever the breakpoint is reached. If the condition is
true, then the debugger stops execution and prompts for a command.
Otherwise, it continues executing the program.
'clear' [[FILENAME':']N | FUNCTION]
Without any argument, delete any breakpoint at the next instruction
to be executed in the selected stack frame. If the program stops
at a breakpoint, this deletes that breakpoint so that the program
does not stop at that location again. Arguments can be one of the
following:
N
Delete breakpoint(s) set at line number N in the current
source file.
FILENAME':'N
Delete breakpoint(s) set at line number N in source file
FILENAME.
FUNCTION
Delete breakpoint(s) set at entry to function FUNCTION.
'condition' N '"EXPRESSION"'
Add a condition to existing breakpoint or watchpoint N. The
condition is an 'awk' expression _enclosed in double quotes_ that
the debugger evaluates whenever the breakpoint or watchpoint is
reached. If the condition is true, then the debugger stops
execution and prompts for a command. Otherwise, the debugger
continues executing the program. If the condition expression is
not specified, any existing condition is removed (i.e., the
breakpoint or watchpoint is made unconditional).
'delete' [N1 N2 ...] [N-M]
'd' [N1 N2 ...] [N-M]
Delete specified breakpoints or a range of breakpoints. Delete all
defined breakpoints if no argument is supplied.
'disable' [N1 N2 ... | N-M]
Disable specified breakpoints or a range of breakpoints. Without
any argument, disable all breakpoints.
'enable' ['del' | 'once'] [N1 N2 ...] [N-M]
'e' ['del' | 'once'] [N1 N2 ...] [N-M]
Enable specified breakpoints or a range of breakpoints. Without
any argument, enable all breakpoints. Optionally, you can specify
how to enable the breakpoints:
'del'
Enable the breakpoints temporarily, then delete each one when
the program stops at it.
'once'
Enable the breakpoints temporarily, then disable each one when
the program stops at it.
'ignore' N COUNT
Ignore breakpoint number N the next COUNT times it is hit.
'tbreak' [[FILENAME':']N | FUNCTION]
't' [[FILENAME':']N | FUNCTION]
Set a temporary breakpoint (enabled for only one stop). The
arguments are the same as for 'break'.
File: gawk.info, Node: Debugger Execution Control, Next: Viewing And Changing Data, Prev: Breakpoint Control, Up: List of Debugger Commands
14.3.2 Control of Execution
---------------------------
Now that your breakpoints are ready, you can start running the program
and observing its behavior. There are more commands for controlling
execution of the program than we saw in our earlier example:
'commands' [N]
'silent'
...
'end'
Set a list of commands to be executed upon stopping at a breakpoint
or watchpoint. N is the breakpoint or watchpoint number. Without
a number, the last one set is used. The actual commands follow,
starting on the next line, and terminated by the 'end' command. If
the command 'silent' is in the list, the usual messages about
stopping at a breakpoint and the source line are not printed. Any
command in the list that resumes execution (e.g., 'continue')
terminates the list (an implicit 'end'), and subsequent commands
are ignored. For example:
gawk> commands
> silent
> printf "A silent breakpoint; i = %d\n", i
> info locals
> set i = 10
> continue
> end
gawk>
'continue' [COUNT]
'c' [COUNT]
Resume program execution. If continued from a breakpoint and COUNT
is specified, ignore the breakpoint at that location the next COUNT
times before stopping.
'finish'
Execute until the selected stack frame returns. Print the returned
value.
'next' [COUNT]
'n' [COUNT]
Continue execution to the next source line, stepping over function
calls. The argument COUNT controls how many times to repeat the
action, as in 'step'.
'nexti' [COUNT]
'ni' [COUNT]
Execute one (or COUNT) instruction(s), stepping over function
calls.
'return' [VALUE]
Cancel execution of a function call. If VALUE (either a string or
a number) is specified, it is used as the function's return value.
If used in a frame other than the innermost one (the currently
executing function; i.e., frame number 0), discard all inner frames
in addition to the selected one, and the caller of that frame
becomes the innermost frame.
'run'
'r'
Start/restart execution of the program. When restarting, the
debugger retains the current breakpoints, watchpoints, command
history, automatic display variables, and debugger options.
'step' [COUNT]
's' [COUNT]
Continue execution until control reaches a different source line in
the current stack frame, stepping inside any function called within
the line. If the argument COUNT is supplied, steps that many times
before stopping, unless it encounters a breakpoint or watchpoint.
'stepi' [COUNT]
'si' [COUNT]
Execute one (or COUNT) instruction(s), stepping inside function
calls. (For illustration of what is meant by an "instruction" in
'gawk', see the output shown under 'dump' in *note Miscellaneous
Debugger Commands::.)
'until' [[FILENAME':']N | FUNCTION]
'u' [[FILENAME':']N | FUNCTION]
Without any argument, continue execution until a line past the
current line in the current stack frame is reached. With an
argument, continue execution until the specified location is
reached, or the current stack frame returns.
File: gawk.info, Node: Viewing And Changing Data, Next: Execution Stack, Prev: Debugger Execution Control, Up: List of Debugger Commands
14.3.3 Viewing and Changing Data
--------------------------------
The commands for viewing and changing variables inside of 'gawk' are:
'display' [VAR | '$'N]
Add variable VAR (or field '$N') to the display list. The value of
the variable or field is displayed each time the program stops.
Each variable added to the list is identified by a unique number:
gawk> display x
-| 10: x = 1
This displays the assigned item number, the variable name, and its
current value. If the display variable refers to a function
parameter, it is silently deleted from the list as soon as the
execution reaches a context where no such variable of the given
name exists. Without argument, 'display' displays the current
values of items on the list.
'eval "AWK STATEMENTS"'
Evaluate AWK STATEMENTS in the context of the running program. You
can do anything that an 'awk' program would do: assign values to
variables, call functions, and so on.
'eval' PARAM, ...
AWK STATEMENTS
'end'
This form of 'eval' is similar, but it allows you to define "local
variables" that exist in the context of the AWK STATEMENTS, instead
of using variables or function parameters defined by the program.
'print' VAR1[',' VAR2 ...]
'p' VAR1[',' VAR2 ...]
Print the value of a 'gawk' variable or field. Fields must be
referenced by constants:
gawk> print $3
This prints the third field in the input record (if the specified
field does not exist, it prints 'Null field'). A variable can be
an array element, with the subscripts being constant string values.
To print the contents of an array, prefix the name of the array
with the '@' symbol:
gawk> print @a
This prints the indices and the corresponding values for all
elements in the array 'a'.
'printf' FORMAT [',' ARG ...]
Print formatted text. The FORMAT may include escape sequences,
such as '\n' (*note Escape Sequences::). No newline is printed
unless one is specified.
'set' VAR'='VALUE
Assign a constant (number or string) value to an 'awk' variable or
field. String values must be enclosed between double quotes
('"'...'"').
You can also set special 'awk' variables, such as 'FS', 'NF', 'NR',
and so on.
'watch' VAR | '$'N ['"EXPRESSION"']
'w' VAR | '$'N ['"EXPRESSION"']
Add variable VAR (or field '$N') to the watch list. The debugger
then stops whenever the value of the variable or field changes.
Each watched item is assigned a number that can be used to delete
it from the watch list using the 'unwatch' command.
With a watchpoint, you may also supply a condition. This is an
'awk' expression (enclosed in double quotes) that the debugger
evaluates whenever the watchpoint is reached. If the condition is
true, then the debugger stops execution and prompts for a command.
Otherwise, 'gawk' continues executing the program.
'undisplay' [N]
Remove item number N (or all items, if no argument) from the
automatic display list.
'unwatch' [N]
Remove item number N (or all items, if no argument) from the watch
list.
File: gawk.info, Node: Execution Stack, Next: Debugger Info, Prev: Viewing And Changing Data, Up: List of Debugger Commands
14.3.4 Working with the Stack
-----------------------------
Whenever you run a program that contains any function calls, 'gawk'
maintains a stack of all of the function calls leading up to where the
program is right now. You can see how you got to where you are, and
also move around in the stack to see what the state of things was in the
functions that called the one you are in. The commands for doing this
are:
'backtrace' [COUNT]
'bt' [COUNT]
'where' [COUNT]
Print a backtrace of all function calls (stack frames), or
innermost COUNT frames if COUNT > 0. Print the outermost COUNT
frames if COUNT < 0. The backtrace displays the name and arguments
to each function, the source file name, and the line number. The
alias 'where' for 'backtrace' is provided for longtime GDB users
who may be used to that command.
'down' [COUNT]
Move COUNT (default 1) frames down the stack toward the innermost
frame. Then select and print the frame.
'frame' [N]
'f' [N]
Select and print stack frame N. Frame 0 is the currently
executing, or "innermost", frame (function call); frame 1 is the
frame that called the innermost one. The highest-numbered frame is
the one for the main program. The printed information consists of
the frame number, function and argument names, source file, and the
source line.
'up' [COUNT]
Move COUNT (default 1) frames up the stack toward the outermost
frame. Then select and print the frame.
File: gawk.info, Node: Debugger Info, Next: Miscellaneous Debugger Commands, Prev: Execution Stack, Up: List of Debugger Commands
14.3.5 Obtaining Information About the Program and the Debugger State
---------------------------------------------------------------------
Besides looking at the values of variables, there is often a need to get
other sorts of information about the state of your program and of the
debugging environment itself. The 'gawk' debugger has one command that
provides this information, appropriately called 'info'. 'info' is used
with one of a number of arguments that tell it exactly what you want to
know:
'info' WHAT
'i' WHAT
The value for WHAT should be one of the following:
'args'
List arguments of the selected frame.
'break'
List all currently set breakpoints.
'display'
List all items in the automatic display list.
'frame'
Give a description of the selected stack frame.
'functions'
List all function definitions including source file names and
line numbers.
'locals'
List local variables of the selected frame.
'source'
Print the name of the current source file. Each time the
program stops, the current source file is the file containing
the current instruction. When the debugger first starts, the
current source file is the first file included via the '-f'
option. The 'list FILENAME:LINENO' command can be used at any
time to change the current source.
'sources'
List all program sources.
'variables'
List all global variables.
'watch'
List all items in the watch list.
Additional commands give you control over the debugger, the ability
to save the debugger's state, and the ability to run debugger commands
from a file. The commands are:
'option' [NAME['='VALUE]]
'o' [NAME['='VALUE]]
Without an argument, display the available debugger options and
their current values. 'option NAME' shows the current value of the
named option. 'option NAME=VALUE' assigns a new value to the named
option. The available options are:
'history_size'
Set the maximum number of lines to keep in the history file
'./.gawk_history'. The default is 100.
'listsize'
Specify the number of lines that 'list' prints. The default
is 15.
'outfile'
Send 'gawk' output to a file; debugger output still goes to
standard output. An empty string ('""') resets output to
standard output.
'prompt'
Change the debugger prompt. The default is 'gawk> '.
'save_history' ['on' | 'off']
Save command history to file './.gawk_history'. The default
is 'on'.
'save_options' ['on' | 'off']
Save current options to file './.gawkrc' upon exit. The
default is 'on'. Options are read back into the next session
upon startup.
'trace' ['on' | 'off']
Turn instruction tracing on or off. The default is 'off'.
'save' FILENAME
Save the commands from the current session to the given file name,
so that they can be replayed using the 'source' command.
'source' FILENAME
Run command(s) from a file; an error in any command does not
terminate execution of subsequent commands. Comments (lines
starting with '#') are allowed in a command file. Empty lines are
ignored; they do _not_ repeat the last command. You can't restart
the program by having more than one 'run' command in the file.
Also, the list of commands may include additional 'source'
commands; however, the 'gawk' debugger will not source the same
file more than once in order to avoid infinite recursion.
In addition to, or instead of, the 'source' command, you can use
the '-D FILE' or '--debug=FILE' command-line options to execute
commands from a file non-interactively (*note Options::).
File: gawk.info, Node: Miscellaneous Debugger Commands, Prev: Debugger Info, Up: List of Debugger Commands
14.3.6 Miscellaneous Commands
-----------------------------
There are a few more commands that do not fit into the previous
categories, as follows:
'dump' [FILENAME]
Dump byte code of the program to standard output or to the file
named in FILENAME. This prints a representation of the internal
instructions that 'gawk' executes to implement the 'awk' commands
in a program. This can be very enlightening, as the following
partial dump of Davide Brini's obfuscated code (*note Signature
Program::) demonstrates:
gawk> dump
-| # BEGIN
-|
-| [ 1:0xfcd340] Op_rule : [in_rule = BEGIN] [source_file = brini.awk]
-| [ 1:0xfcc240] Op_push_i : "~" [MALLOC|STRING|STRCUR]
-| [ 1:0xfcc2a0] Op_push_i : "~" [MALLOC|STRING|STRCUR]
-| [ 1:0xfcc280] Op_match :
-| [ 1:0xfcc1e0] Op_store_var : O
-| [ 1:0xfcc2e0] Op_push_i : "==" [MALLOC|STRING|STRCUR]
-| [ 1:0xfcc340] Op_push_i : "==" [MALLOC|STRING|STRCUR]
-| [ 1:0xfcc320] Op_equal :
-| [ 1:0xfcc200] Op_store_var : o
-| [ 1:0xfcc380] Op_push : o
-| [ 1:0xfcc360] Op_plus_i : 0 [MALLOC|NUMCUR|NUMBER]
-| [ 1:0xfcc220] Op_push_lhs : o [do_reference = true]
-| [ 1:0xfcc300] Op_assign_plus :
-| [ :0xfcc2c0] Op_pop :
-| [ 1:0xfcc400] Op_push : O
-| [ 1:0xfcc420] Op_push_i : "" [MALLOC|STRING|STRCUR]
-| [ :0xfcc4a0] Op_no_op :
-| [ 1:0xfcc480] Op_push : O
-| [ :0xfcc4c0] Op_concat : [expr_count = 3] [concat_flag = 0]
-| [ 1:0xfcc3c0] Op_store_var : x
-| [ 1:0xfcc440] Op_push_lhs : X [do_reference = true]
-| [ 1:0xfcc3a0] Op_postincrement :
-| [ 1:0xfcc4e0] Op_push : x
-| [ 1:0xfcc540] Op_push : o
-| [ 1:0xfcc500] Op_plus :
-| [ 1:0xfcc580] Op_push : o
-| [ 1:0xfcc560] Op_plus :
-| [ 1:0xfcc460] Op_leq :
-| [ :0xfcc5c0] Op_jmp_false : [target_jmp = 0xfcc5e0]
-| [ 1:0xfcc600] Op_push_i : "%c" [MALLOC|STRING|STRCUR]
-| [ :0xfcc660] Op_no_op :
-| [ 1:0xfcc520] Op_assign_concat : c
-| [ :0xfcc620] Op_jmp : [target_jmp = 0xfcc440]
...
-| [ 2:0xfcc5a0] Op_K_printf : [expr_count = 17] [redir_type = ""]
-| [ :0xfcc140] Op_no_op :
-| [ :0xfcc1c0] Op_atexit :
-| [ :0xfcc640] Op_stop :
-| [ :0xfcc180] Op_no_op :
-| [ :0xfcd150] Op_after_beginfile :
-| [ :0xfcc160] Op_no_op :
-| [ :0xfcc1a0] Op_after_endfile :
gawk>
'exit'
Exit the debugger. See the entry for 'quit', later in this list.
'help'
'h'
Print a list of all of the 'gawk' debugger commands with a short
summary of their usage. 'help COMMAND' prints the information
about the command COMMAND.
'list' ['-' | '+' | N | FILENAME':'N | N-M | FUNCTION]
'l' ['-' | '+' | N | FILENAME':'N | N-M | FUNCTION]
Print the specified lines (default 15) from the current source file
or the file named FILENAME. The possible arguments to 'list' are
as follows:
'-' (Minus)
Print lines before the lines last printed.
'+'
Print lines after the lines last printed. 'list' without any
argument does the same thing.
N
Print lines centered around line number N.
N-M
Print lines from N to M.
FILENAME':'N
Print lines centered around line number N in source file
FILENAME. This command may change the current source file.
FUNCTION
Print lines centered around the beginning of the function
FUNCTION. This command may change the current source file.
'quit'
'q'
Exit the debugger. Debugging is great fun, but sometimes we all
have to tend to other obligations in life, and sometimes we find
the bug and are free to go on to the next one! As we saw earlier,
if you are running a program, the debugger warns you when you type
'q' or 'quit', to make sure you really want to quit.
'trace' ['on' | 'off']
Turn on or off continuous printing of the instructions that are
about to be executed, along with the 'awk' lines they implement.
The default is 'off'.
It is to be hoped that most of the "opcodes" in these instructions
are fairly self-explanatory, and using 'stepi' and 'nexti' while
'trace' is on will make them into familiar friends.
File: gawk.info, Node: Readline Support, Next: Limitations, Prev: List of Debugger Commands, Up: Debugger
14.4 Readline Support
=====================
If 'gawk' is compiled with the GNU Readline library
(http://cnswww.cns.cwru.edu/php/chet/readline/readline.html), you can
take advantage of that library's command completion and history
expansion features. The following types of completion are available:
Command completion
Command names.
Source file name completion
Source file names. Relevant commands are 'break', 'clear', 'list',
'tbreak', and 'until'.
Argument completion
Non-numeric arguments to a command. Relevant commands are 'enable'
and 'info'.
Variable name completion
Global variable names, and function arguments in the current
context if the program is running. Relevant commands are
'display', 'print', 'set', and 'watch'.
File: gawk.info, Node: Limitations, Next: Debugging Summary, Prev: Readline Support, Up: Debugger
14.5 Limitations
================
We hope you find the 'gawk' debugger useful and enjoyable to work with,
but as with any program, especially in its early releases, it still has
some limitations. A few that it's worth being aware of are:
* At this point, the debugger does not give a detailed explanation of
what you did wrong when you type in something it doesn't like.
Rather, it just responds 'syntax error'. When you do figure out
what your mistake was, though, you'll feel like a real guru.
* If you perused the dump of opcodes in *note Miscellaneous Debugger
Commands:: (or if you are already familiar with 'gawk' internals),
you will realize that much of the internal manipulation of data in
'gawk', as in many interpreters, is done on a stack. 'Op_push',
'Op_pop', and the like are the "bread and butter" of most 'gawk'
code.
Unfortunately, as of now, the 'gawk' debugger does not allow you to
examine the stack's contents. That is, the intermediate results of
expression evaluation are on the stack, but cannot be printed.
Rather, only variables that are defined in the program can be
printed. Of course, a workaround for this is to use more explicit
variables at the debugging stage and then change back to obscure,
perhaps more optimal code later.
* There is no way to look "inside" the process of compiling regular
expressions to see if you got it right. As an 'awk' programmer,
you are expected to know the meaning of '/[^[:alnum:][:blank:]]/'.
* The 'gawk' debugger is designed to be used by running a program
(with all its parameters) on the command line, as described in
*note Debugger Invocation::. There is no way (as of now) to attach
or "break into" a running program. This seems reasonable for a
language that is used mainly for quickly executing, short programs.
* The 'gawk' debugger only accepts source code supplied with the '-f'
option.
File: gawk.info, Node: Debugging Summary, Prev: Limitations, Up: Debugger
14.6 Summary
============
* Programs rarely work correctly the first time. Finding bugs is
called debugging, and a program that helps you find bugs is a
debugger. 'gawk' has a built-in debugger that works very similarly
to the GNU Debugger, GDB.
* Debuggers let you step through your program one statement at a
time, examine and change variable and array values, and do a number
of other things that let you understand what your program is
actually doing (as opposed to what it is supposed to do).
* Like most debuggers, the 'gawk' debugger works in terms of stack
frames, and lets you set both breakpoints (stop at a point in the
code) and watchpoints (stop when a data value changes).
* The debugger command set is fairly complete, providing control over
breakpoints, execution, viewing and changing data, working with the
stack, getting information, and other tasks.
* If the GNU Readline library is available when 'gawk' is compiled,
it is used by the debugger to provide command-line history and
editing.
* Usually, the debugger does not not affect the program being
debugged, but occasionally it can.
File: gawk.info, Node: Arbitrary Precision Arithmetic, Next: Dynamic Extensions, Prev: Debugger, Up: Top
15 Arithmetic and Arbitrary-Precision Arithmetic with 'gawk'
************************************************************
This major node introduces some basic concepts relating to how computers
do arithmetic and defines some important terms. It then proceeds to
describe floating-point arithmetic, which is what 'awk' uses for all its
computations, including a discussion of arbitrary-precision
floating-point arithmetic, which is a feature available only in 'gawk'.
It continues on to present arbitrary-precision integers, and concludes
with a description of some points where 'gawk' and the POSIX standard
are not quite in agreement.
NOTE: Most users of 'gawk' can safely skip this chapter. But if
you want to do scientific calculations with 'gawk', this is the
place to be.
* Menu:
* Computer Arithmetic:: A quick intro to computer math.
* Math Definitions:: Defining terms used.
* MPFR features:: The MPFR features in 'gawk'.
* FP Math Caution:: Things to know.
* Arbitrary Precision Integers:: Arbitrary Precision Integer Arithmetic with
'gawk'.
* Checking for MPFR:: How to check if MPFR is available.
* POSIX Floating Point Problems:: Standards Versus Existing Practice.
* Floating point summary:: Summary of floating point discussion.
File: gawk.info, Node: Computer Arithmetic, Next: Math Definitions, Up: Arbitrary Precision Arithmetic
15.1 A General Description of Computer Arithmetic
=================================================
Until now, we have worked with data as either numbers or strings.
Ultimately, however, computers represent everything in terms of "binary
digits", or "bits". A decimal digit can take on any of 10 values: zero
through nine. A binary digit can take on any of two values, zero or
one. Using binary, computers (and computer software) can represent and
manipulate numerical and character data. In general, the more bits you
can use to represent a particular thing, the greater the range of
possible values it can take on.
Modern computers support at least two, and often more, ways to do
arithmetic. Each kind of arithmetic uses a different representation
(organization of the bits) for the numbers. The kinds of arithmetic
that interest us are:
Decimal arithmetic
This is the kind of arithmetic you learned in elementary school,
using paper and pencil (and/or a calculator). In theory, numbers
can have an arbitrary number of digits on either side (or both
sides) of the decimal point, and the results of a computation are
always exact.
Some modern systems can do decimal arithmetic in hardware, but
usually you need a special software library to provide access to
these instructions. There are also libraries that do decimal
arithmetic entirely in software.
Despite the fact that some users expect 'gawk' to be performing
decimal arithmetic,(1) it does not do so.
Integer arithmetic
In school, integer values were referred to as "whole" numbers--that
is, numbers without any fractional part, such as 1, 42, or -17.
The advantage to integer numbers is that they represent values
exactly. The disadvantage is that their range is limited.
In computers, integer values come in two flavors: "signed" and
"unsigned". Signed values may be negative or positive, whereas
unsigned values are always greater than or equal to zero.
In computer systems, integer arithmetic is exact, but the possible
range of values is limited. Integer arithmetic is generally faster
than floating-point arithmetic.
Floating-point arithmetic
Floating-point numbers represent what were called in school "real"
numbers (i.e., those that have a fractional part, such as
3.1415927). The advantage to floating-point numbers is that they
can represent a much larger range of values than can integers. The
disadvantage is that there are numbers that they cannot represent
exactly.
Modern systems support floating-point arithmetic in hardware, with
a limited range of values. There are software libraries that allow
the use of arbitrary-precision floating-point calculations.
POSIX 'awk' uses "double-precision" floating-point numbers, which
can hold more digits than "single-precision" floating-point
numbers. 'gawk' has facilities for performing arbitrary-precision
floating-point arithmetic, which we describe in more detail
shortly.
Computers work with integer and floating-point values of different
ranges. Integer values are usually either 32 or 64 bits in size.
Single-precision floating-point values occupy 32 bits, whereas
double-precision floating-point values occupy 64 bits.
(Quadruple-precision floating point values also exist. They occupy 128
bits, but such numbers are not available in 'awk'.) Floating-point
values are always signed. The possible ranges of values are shown in
*note Table 15.1: table-numeric-ranges. and *note Table 15.2:
table-floating-point-ranges.
Representation Minimum value Maximum value
---------------------------------------------------------------------------
32-bit signed integer -2,147,483,648 2,147,483,647
32-bit unsigned 0 4,294,967,295
integer
64-bit signed integer -9,223,372,036,854,775,8089,223,372,036,854,775,807
64-bit unsigned 0 18,446,744,073,709,551,615
integer
Table 15.1: Value ranges for integer representations
Representation Minimum Minimum finite Maximum finite
positive value value
nonzero value
--------------------------------------------------------------------------------
Single-precision 1.175494e-38 -3.402823e+38 3.402823e+38
floating-point
Double-precision 2.225074e-308 -1.797693e+308 1.797693e+308
floating-point
Quadruple-precision 3.362103e-4932 -1.189731e+4932 1.189731e+4932
floating-point
Table 15.2: Approximate value ranges for floating-point number
representations
---------- Footnotes ----------
(1) We don't know why they expect this, but they do.
File: gawk.info, Node: Math Definitions, Next: MPFR features, Prev: Computer Arithmetic, Up: Arbitrary Precision Arithmetic
15.2 Other Stuff to Know
========================
The rest of this major node uses a number of terms. Here are some
informal definitions that should help you work your way through the
material here:
"Accuracy"
A floating-point calculation's accuracy is how close it comes to
the real (paper and pencil) value.
"Error"
The difference between what the result of a computation "should be"
and what it actually is. It is best to minimize error as much as
possible.
"Exponent"
The order of magnitude of a value; some number of bits in a
floating-point value store the exponent.
"Inf"
A special value representing infinity. Operations involving
another number and infinity produce infinity.
"NaN"
"Not a number."(1) A special value that results from attempting a
calculation that has no answer as a real number. In such a case,
programs can either receive a floating-point exception, or get
'NaN' back as the result. The IEEE 754 standard recommends that
systems return 'NaN'. Some examples:
'sqrt(-1)'
This makes sense in the range of complex numbers, but not in
the range of real numbers, so the result is 'NaN'.
'log(-8)'
-8 is out of the domain of 'log()', so the result is 'NaN'.
"Normalized"
How the significand (see later in this list) is usually stored.
The value is adjusted so that the first bit is one, and then that
leading one is assumed instead of physically stored. This provides
one extra bit of precision.
"Precision"
The number of bits used to represent a floating-point number. The
more bits, the more digits you can represent. Binary and decimal
precisions are related approximately, according to the formula:
PREC = 3.322 * DPS
Here, _prec_ denotes the binary precision (measured in bits) and
_dps_ (short for decimal places) is the decimal digits.
"Rounding mode"
How numbers are rounded up or down when necessary. More details
are provided later.
"Significand"
A floating-point value consists of the significand multiplied by 10
to the power of the exponent. For example, in '1.2345e67', the
significand is '1.2345'.
"Stability"
From the Wikipedia article on numerical stability
(https://en.wikipedia.org/wiki/Numerical_stability): "Calculations
that can be proven not to magnify approximation errors are called
"numerically stable"."
See the Wikipedia article on accuracy and precision
(https://en.wikipedia.org/wiki/Accuracy_and_precision) for more
information on some of those terms.
On modern systems, floating-point hardware uses the representation
and operations defined by the IEEE 754 standard. Three of the standard
IEEE 754 types are 32-bit single precision, 64-bit double precision, and
128-bit quadruple precision. The standard also specifies extended
precision formats to allow greater precisions and larger exponent
ranges. ('awk' uses only the 64-bit double-precision format.)
*note Table 15.3: table-ieee-formats. lists the precision and
exponent field values for the basic IEEE 754 binary formats.
Name Total bits Precision Minimum Maximum
exponent exponent
---------------------------------------------------------------------------
Single 32 24 -126 +127
Double 64 53 -1022 +1023
Quadruple 128 113 -16382 +16383
Table 15.3: Basic IEEE format values
NOTE: The precision numbers include the implied leading one that
gives them one extra bit of significand.
---------- Footnotes ----------
(1) Thanks to Michael Brennan for this description, which we have
paraphrased, and for the examples.
File: gawk.info, Node: MPFR features, Next: FP Math Caution, Prev: Math Definitions, Up: Arbitrary Precision Arithmetic
15.3 Arbitrary-Precision Arithmetic Features in 'gawk'
======================================================
By default, 'gawk' uses the double-precision floating-point values
supplied by the hardware of the system it runs on. However, if it was
compiled to do so, and the '-M' command-line option is supplied, 'gawk'
uses the GNU MPFR (http://www.mpfr.org) and GNU MP (https://gmplib.org)
(GMP) libraries for arbitrary-precision arithmetic on numbers. You can
see if MPFR support is available like so:
$ gawk --version
-| GNU Awk 4.1.2, API: 1.1 (GNU MPFR 3.1.0-p3, GNU MP 5.0.2)
-| Copyright (C) 1989, 1991-2015 Free Software Foundation.
...
(You may see different version numbers than what's shown here. That's
OK; what's important is to see that GNU MPFR and GNU MP are listed in
the output.)
Additionally, there are a few elements available in the 'PROCINFO'
array to provide information about the MPFR and GMP libraries (*note
Auto-set::).
The MPFR library provides precise control over precisions and
rounding modes, and gives correctly rounded, reproducible,
platform-independent results. With the '-M' command-line option, all
floating-point arithmetic operators and numeric functions can yield
results to any desired precision level supported by MPFR.
Two predefined variables, 'PREC' and 'ROUNDMODE', provide control
over the working precision and the rounding mode. The precision and the
rounding mode are set globally for every operation to follow. *Note
Setting precision:: and *note Setting the rounding mode:: for more
information.
File: gawk.info, Node: FP Math Caution, Next: Arbitrary Precision Integers, Prev: MPFR features, Up: Arbitrary Precision Arithmetic
15.4 Floating-Point Arithmetic: Caveat Emptor!
==============================================
Math class is tough!
-- _Teen Talk Barbie, July 1992_
This minor node provides a high-level overview of the issues involved
when doing lots of floating-point arithmetic.(1) The discussion applies
to both hardware and arbitrary-precision floating-point arithmetic.
CAUTION: The material here is purposely general. If you need to do
serious computer arithmetic, you should do some research first, and
not rely just on what we tell you.
* Menu:
* Inexactness of computations:: Floating point math is not exact.
* Getting Accuracy:: Getting more accuracy takes some work.
* Try To Round:: Add digits and round.
* Setting precision:: How to set the precision.
* Setting the rounding mode:: How to set the rounding mode.
---------- Footnotes ----------
(1) There is a very nice paper on floating-point arithmetic
(http://www.validlab.com/goldberg/paper.pdf) by David Goldberg, "What
Every Computer Scientist Should Know About Floating-Point Arithmetic,"
'ACM Computing Surveys' *23*, 1 (1991-03): 5-48. This is worth reading
if you are interested in the details, but it does require a background
in computer science.
File: gawk.info, Node: Inexactness of computations, Next: Getting Accuracy, Up: FP Math Caution
15.4.1 Floating-Point Arithmetic Is Not Exact
---------------------------------------------
Binary floating-point representations and arithmetic are inexact.
Simple values like 0.1 cannot be precisely represented using binary
floating-point numbers, and the limited precision of floating-point
numbers means that slight changes in the order of operations or the
precision of intermediate storage can change the result. To make
matters worse, with arbitrary-precision floating-point arithmetic, you
can set the precision before starting a computation, but then you cannot
be sure of the number of significant decimal places in the final result.
* Menu:
* Inexact representation:: Numbers are not exactly represented.
* Comparing FP Values:: How to compare floating point values.
* Errors accumulate:: Errors get bigger as they go.
File: gawk.info, Node: Inexact representation, Next: Comparing FP Values, Up: Inexactness of computations
15.4.1.1 Many Numbers Cannot Be Represented Exactly
...................................................
So, before you start to write any code, you should think about what you
really want and what's really happening. Consider the two numbers in
the following example:
x = 0.875 # 1/2 + 1/4 + 1/8
y = 0.425
Unlike the number in 'y', the number stored in 'x' is exactly
representable in binary because it can be written as a finite sum of one
or more fractions whose denominators are all powers of two. When 'gawk'
reads a floating-point number from program source, it automatically
rounds that number to whatever precision your machine supports. If you
try to print the numeric content of a variable using an output format
string of '"%.17g"', it may not produce the same number as you assigned
to it:
$ gawk 'BEGIN { x = 0.875; y = 0.425
> printf("%0.17g, %0.17g\n", x, y) }'
-| 0.875, 0.42499999999999999
Often the error is so small you do not even notice it, and if you do,
you can always specify how much precision you would like in your output.
Usually this is a format string like '"%.15g"', which, when used in the
previous example, produces an output identical to the input.
File: gawk.info, Node: Comparing FP Values, Next: Errors accumulate, Prev: Inexact representation, Up: Inexactness of computations
15.4.1.2 Be Careful Comparing Values
....................................
Because the underlying representation can be a little bit off from the
exact value, comparing floating-point values to see if they are exactly
equal is generally a bad idea. Here is an example where it does not
work like you would expect:
$ gawk 'BEGIN { print (0.1 + 12.2 == 12.3) }'
-| 0
The general wisdom when comparing floating-point values is to see if
they are within some small range of each other (called a "delta", or
"tolerance"). You have to decide how small a delta is important to you.
Code to do this looks something like the following:
delta = 0.00001 # for example
difference = abs(a) - abs(b) # subtract the two values
if (difference < delta)
# all ok
else
# not ok
(We assume that you have a simple absolute value function named 'abs()'
defined elsewhere in your program.)
File: gawk.info, Node: Errors accumulate, Prev: Comparing FP Values, Up: Inexactness of computations
15.4.1.3 Errors Accumulate
..........................
The loss of accuracy during a single computation with floating-point
numbers usually isn't enough to worry about. However, if you compute a
value that is the result of a sequence of floating-point operations, the
error can accumulate and greatly affect the computation itself. Here is
an attempt to compute the value of pi using one of its many series
representations:
BEGIN {
x = 1.0 / sqrt(3.0)
n = 6
for (i = 1; i < 30; i++) {
n = n * 2.0
x = (sqrt(x * x + 1) - 1) / x
printf("%.15f\n", n * x)
}
}
When run, the early errors propagate through later computations,
causing the loop to terminate prematurely after attempting to divide by
zero:
$ gawk -f pi.awk
-| 3.215390309173475
-| 3.159659942097510
-| 3.146086215131467
-| 3.142714599645573
...
-| 3.224515243534819
-| 2.791117213058638
-| 0.000000000000000
error-> gawk: pi.awk:6: fatal: division by zero attempted
Here is an additional example where the inaccuracies in internal
representations yield an unexpected result:
$ gawk 'BEGIN {
> for (d = 1.1; d <= 1.5; d += 0.1) # loop five times (?)
> i++
> print i
> }'
-| 4
File: gawk.info, Node: Getting Accuracy, Next: Try To Round, Prev: Inexactness of computations, Up: FP Math Caution
15.4.2 Getting the Accuracy You Need
------------------------------------
Can arbitrary-precision arithmetic give exact results? There are no
easy answers. The standard rules of algebra often do not apply when
using floating-point arithmetic. Among other things, the distributive
and associative laws do not hold completely, and order of operation may
be important for your computation. Rounding error, cumulative precision
loss, and underflow are often troublesome.
When 'gawk' tests the expressions '0.1 + 12.2' and '12.3' for
equality using the machine double-precision arithmetic, it decides that
they are not equal! (*Note Comparing FP Values::.) You can get the
result you want by increasing the precision; 56 bits in this case does
the job:
$ gawk -M -v PREC=56 'BEGIN { print (0.1 + 12.2 == 12.3) }'
-| 1
If adding more bits is good, perhaps adding even more bits of
precision is better? Here is what happens if we use an even larger
value of 'PREC':
$ gawk -M -v PREC=201 'BEGIN { print (0.1 + 12.2 == 12.3) }'
-| 0
This is not a bug in 'gawk' or in the MPFR library. It is easy to
forget that the finite number of bits used to store the value is often
just an approximation after proper rounding. The test for equality
succeeds if and only if _all_ bits in the two operands are exactly the
same. Because this is not necessarily true after floating-point
computations with a particular precision and effective rounding mode, a
straight test for equality may not work. Instead, compare the two
numbers to see if they are within the desirable delta of each other.
In applications where 15 or fewer decimal places suffice, hardware
double-precision arithmetic can be adequate, and is usually much faster.
But you need to keep in mind that every floating-point operation can
suffer a new rounding error with catastrophic consequences, as
illustrated by our earlier attempt to compute the value of pi. Extra
precision can greatly enhance the stability and the accuracy of your
computation in such cases.
Additionally, you should understand that repeated addition is not
necessarily equivalent to multiplication in floating-point arithmetic.
In the example in *note Errors accumulate:::
$ gawk 'BEGIN {
> for (d = 1.1; d <= 1.5; d += 0.1) # loop five times (?)
> i++
> print i
> }'
-| 4
you may or may not succeed in getting the correct result by choosing an
arbitrarily large value for 'PREC'. Reformulation of the problem at
hand is often the correct approach in such situations.
File: gawk.info, Node: Try To Round, Next: Setting precision, Prev: Getting Accuracy, Up: FP Math Caution
15.4.3 Try a Few Extra Bits of Precision and Rounding
-----------------------------------------------------
Instead of arbitrary-precision floating-point arithmetic, often all you
need is an adjustment of your logic or a different order for the
operations in your calculation. The stability and the accuracy of the
computation of pi in the earlier example can be enhanced by using the
following simple algebraic transformation:
(sqrt(x * x + 1) - 1) / x == x / (sqrt(x * x + 1) + 1)
After making this change, the program converges to pi in under 30
iterations:
$ gawk -f pi2.awk
-| 3.215390309173473
-| 3.159659942097501
-| 3.146086215131436
-| 3.142714599645370
-| 3.141873049979825
...
-| 3.141592653589797
-| 3.141592653589797
File: gawk.info, Node: Setting precision, Next: Setting the rounding mode, Prev: Try To Round, Up: FP Math Caution
15.4.4 Setting the Precision
----------------------------
'gawk' uses a global working precision; it does not keep track of the
precision or accuracy of individual numbers. Performing an arithmetic
operation or calling a built-in function rounds the result to the
current working precision. The default working precision is 53 bits,
which you can modify using the predefined variable 'PREC'. You can also
set the value to one of the predefined case-insensitive strings shown in
*note Table 15.4: table-predefined-precision-strings, to emulate an IEEE
754 binary format.
'PREC' IEEE 754 binary format
---------------------------------------------------
'"half"' 16-bit half-precision
'"single"' Basic 32-bit single precision
'"double"' Basic 64-bit double precision
'"quad"' Basic 128-bit quadruple precision
'"oct"' 256-bit octuple precision
Table 15.4: Predefined precision strings for 'PREC'
The following example illustrates the effects of changing precision
on arithmetic operations:
$ gawk -M -v PREC=100 'BEGIN { x = 1.0e-400; print x + 0
> PREC = "double"; print x + 0 }'
-| 1e-400
-| 0
CAUTION: Be wary of floating-point constants! When reading a
floating-point constant from program source code, 'gawk' uses the
default precision (that of a C 'double'), unless overridden by an
assignment to the special variable 'PREC' on the command line, to
store it internally as an MPFR number. Changing the precision
using 'PREC' in the program text does _not_ change the precision of
a constant.
If you need to represent a floating-point constant at a higher
precision than the default and cannot use a command-line assignment
to 'PREC', you should either specify the constant as a string, or
as a rational number, whenever possible. The following example
illustrates the differences among various ways to print a
floating-point constant:
$ gawk -M 'BEGIN { PREC = 113; printf("%0.25f\n", 0.1) }'
-| 0.1000000000000000055511151
$ gawk -M -v PREC=113 'BEGIN { printf("%0.25f\n", 0.1) }'
-| 0.1000000000000000000000000
$ gawk -M 'BEGIN { PREC = 113; printf("%0.25f\n", "0.1") }'
-| 0.1000000000000000000000000
$ gawk -M 'BEGIN { PREC = 113; printf("%0.25f\n", 1/10) }'
-| 0.1000000000000000000000000
File: gawk.info, Node: Setting the rounding mode, Prev: Setting precision, Up: FP Math Caution
15.4.5 Setting the Rounding Mode
--------------------------------
The 'ROUNDMODE' variable provides program-level control over the
rounding mode. The correspondence between 'ROUNDMODE' and the IEEE
rounding modes is shown in *note Table 15.5: table-gawk-rounding-modes.
Rounding mode IEEE name 'ROUNDMODE'
---------------------------------------------------------------------------
Round to nearest, ties to even 'roundTiesToEven' '"N"' or '"n"'
Round toward positive infinity 'roundTowardPositive' '"U"' or '"u"'
Round toward negative infinity 'roundTowardNegative' '"D"' or '"d"'
Round toward zero 'roundTowardZero' '"Z"' or '"z"'
Round away from zero '"A"' or '"a"'
Table 15.5: 'gawk' rounding modes
'ROUNDMODE' has the default value '"N"', which selects the IEEE 754
rounding mode 'roundTiesToEven'. In *note Table 15.5:
table-gawk-rounding-modes, the value '"A"' selects rounding away from
zero. This is only available if your version of the MPFR library
supports it; otherwise, setting 'ROUNDMODE' to '"A"' has no effect.
The default mode 'roundTiesToEven' is the most preferred, but the
least intuitive. This method does the obvious thing for most values, by
rounding them up or down to the nearest digit. For example, rounding
1.132 to two digits yields 1.13, and rounding 1.157 yields 1.16.
However, when it comes to rounding a value that is exactly halfway
between, things do not work the way you probably learned in school. In
this case, the number is rounded to the nearest even digit. So rounding
0.125 to two digits rounds down to 0.12, but rounding 0.6875 to three
digits rounds up to 0.688. You probably have already encountered this
rounding mode when using 'printf' to format floating-point numbers. For
example:
BEGIN {
x = -4.5
for (i = 1; i < 10; i++) {
x += 1.0
printf("%4.1f => %2.0f\n", x, x)
}
}
produces the following output when run on the author's system:(1)
-3.5 => -4
-2.5 => -2
-1.5 => -2
-0.5 => 0
0.5 => 0
1.5 => 2
2.5 => 2
3.5 => 4
4.5 => 4
The theory behind 'roundTiesToEven' is that it more or less evenly
distributes upward and downward rounds of exact halves, which might
cause any accumulating round-off error to cancel itself out. This is
the default rounding mode for IEEE 754 computing functions and
operators.
Rounding Modes and Conversion
It's important to understand that, along with 'CONVFMT' and 'OFMT',
the rounding mode affects how numbers are converted to strings. For
example, consider the following program:
BEGIN {
pi = 3.1416
OFMT = "%.f" # Print value as integer
print pi # ROUNDMODE = "N" by default.
ROUNDMODE = "U" # Now change ROUNDMODE
print pi
}
Running this program produces this output:
$ gawk -M -f roundmode.awk
-| 3
-| 4
The other rounding modes are rarely used. Rounding toward positive
infinity ('roundTowardPositive') and toward negative infinity
('roundTowardNegative') are often used to implement interval arithmetic,
where you adjust the rounding mode to calculate upper and lower bounds
for the range of output. The 'roundTowardZero' mode can be used for
converting floating-point numbers to integers. When rounding away from
zero, the nearest number with magnitude greater than or equal to the
value is selected.
Some numerical analysts will tell you that your choice of rounding
style has tremendous impact on the final outcome, and advise you to wait
until final output for any rounding. Instead, you can often avoid
round-off error problems by setting the precision initially to some
value sufficiently larger than the final desired precision, so that the
accumulation of round-off error does not influence the outcome. If you
suspect that results from your computation are sensitive to accumulation
of round-off error, look for a significant difference in output when you
change the rounding mode to be sure.
---------- Footnotes ----------
(1) It is possible for the output to be completely different if the C
library in your system does not use the IEEE 754 even-rounding rule to
round halfway cases for 'printf'.
File: gawk.info, Node: Arbitrary Precision Integers, Next: Checking for MPFR, Prev: FP Math Caution, Up: Arbitrary Precision Arithmetic
15.5 Arbitrary-Precision Integer Arithmetic with 'gawk'
=======================================================
When given the '-M' option, 'gawk' performs all integer arithmetic using
GMP arbitrary-precision integers. Any number that looks like an integer
in a source or data file is stored as an arbitrary-precision integer.
The size of the integer is limited only by the available memory. For
example, the following computes 5^4^3^2, the result of which is beyond
the limits of ordinary hardware double-precision floating-point values:
$ gawk -M 'BEGIN {
> x = 5^4^3^2
> print "number of digits =", length(x)
> print substr(x, 1, 20), "...", substr(x, length(x) - 19, 20)
> }'
-| number of digits = 183231
-| 62060698786608744707 ... 92256259918212890625
If instead you were to compute the same value using
arbitrary-precision floating-point values, the precision needed for
correct output (using the formula 'prec = 3.322 * dps') would be 3.322 x
183231, or 608693.
The result from an arithmetic operation with an integer and a
floating-point value is a floating-point value with a precision equal to
the working precision. The following program calculates the eighth term
in Sylvester's sequence(1) using a recurrence:
$ gawk -M 'BEGIN {
> s = 2.0
> for (i = 1; i <= 7; i++)
> s = s * (s - 1) + 1
> print s
> }'
-| 113423713055421845118910464
The output differs from the actual number,
113,423,713,055,421,844,361,000,443, because the default precision of 53
bits is not enough to represent the floating-point results exactly. You
can either increase the precision (100 bits is enough in this case), or
replace the floating-point constant '2.0' with an integer, to perform
all computations using integer arithmetic to get the correct output.
Sometimes 'gawk' must implicitly convert an arbitrary-precision
integer into an arbitrary-precision floating-point value. This is
primarily because the MPFR library does not always provide the relevant
interface to process arbitrary-precision integers or mixed-mode numbers
as needed by an operation or function. In such a case, the precision is
set to the minimum value necessary for exact conversion, and the working
precision is not used for this purpose. If this is not what you need or
want, you can employ a subterfuge and convert the integer to floating
point first, like this:
gawk -M 'BEGIN { n = 13; print (n + 0.0) % 2.0 }'
You can avoid this issue altogether by specifying the number as a
floating-point value to begin with:
gawk -M 'BEGIN { n = 13.0; print n % 2.0 }'
Note that for this particular example, it is likely best to just use
the following:
gawk -M 'BEGIN { n = 13; print n % 2 }'
When dividing two arbitrary precision integers with either '/' or
'%', the result is typically an arbitrary precision floating point value
(unless the denominator evenly divides into the numerator).
---------- Footnotes ----------
(1) Weisstein, Eric W. 'Sylvester's Sequence'. From MathWorld--A
Wolfram Web Resource
(<http://mathworld.wolfram.com/SylvestersSequence.html>;).
File: gawk.info, Node: Checking for MPFR, Next: POSIX Floating Point Problems, Prev: Arbitrary Precision Integers, Up: Arbitrary Precision Arithmetic
15.6 How To Check If MPFR Is Available
======================================
Occasionally, you might like to be able to check if 'gawk' was invoked
with the '-M' option, enabling arbitrary-precision arithmetic. You can
do so with the following function, contributed by Andrew Schorr:
# adequate_math_precision --- return true if we have enough bits
function adequate_math_precision(n)
{
return (1 != (1+(1/(2^(n-1)))))
}
Here is code that invokes the function in order to check if
arbitrary-precision arithmetic is available:
BEGIN {
# How many bits of mantissa precision are required
# for this program to function properly?
fpbits = 123
# We hope that we were invoked with MPFR enabled. If so, the
# following statement should configure calculations to our desired
# precision.
PREC = fpbits
if (! adequate_math_precision(fpbits)) {
print("Error: insufficient computation precision available.\n" \
"Try again with the -M argument?") > "/dev/stderr"
# Note: you may need to set a flag here to bail out of END rules
exit 1
}
}
Please be aware that 'exit' will jump to the 'END' rules, if present
(*note Exit Statement::).
File: gawk.info, Node: POSIX Floating Point Problems, Next: Floating point summary, Prev: Checking for MPFR, Up: Arbitrary Precision Arithmetic
15.7 Standards Versus Existing Practice
=======================================
Historically, 'awk' has converted any nonnumeric-looking string to the
numeric value zero, when required. Furthermore, the original definition
of the language and the original POSIX standards specified that 'awk'
only understands decimal numbers (base 10), and not octal (base 8) or
hexadecimal numbers (base 16).
Changes in the language of the 2001 and 2004 POSIX standards can be
interpreted to imply that 'awk' should support additional features.
These features are:
* Interpretation of floating-point data values specified in
hexadecimal notation (e.g., '0xDEADBEEF'). (Note: data values,
_not_ source code constants.)
* Support for the special IEEE 754 floating-point values "not a
number" (NaN), positive infinity ("inf"), and negative infinity
("-inf"). In particular, the format for these values is as
specified by the ISO 1999 C standard, which ignores case and can
allow implementation-dependent additional characters after the
'nan' and allow either 'inf' or 'infinity'.
The first problem is that both of these are clear changes to
historical practice:
* The 'gawk' maintainer feels that supporting hexadecimal
floating-point values, in particular, is ugly, and was never
intended by the original designers to be part of the language.
* Allowing completely alphabetic strings to have valid numeric values
is also a very severe departure from historical practice.
The second problem is that the 'gawk' maintainer feels that this
interpretation of the standard, which required a certain amount of
"language lawyering" to arrive at in the first place, was not even
intended by the standard developers. In other words, "We see how you
got where you are, but we don't think that that's where you want to be."
Recognizing these issues, but attempting to provide compatibility
with the earlier versions of the standard, the 2008 POSIX standard added
explicit wording to allow, but not require, that 'awk' support
hexadecimal floating-point values and special values for "not a number"
and infinity.
Although the 'gawk' maintainer continues to feel that providing those
features is inadvisable, nevertheless, on systems that support IEEE
floating point, it seems reasonable to provide _some_ way to support NaN
and infinity values. The solution implemented in 'gawk' is as follows:
* With the '--posix' command-line option, 'gawk' becomes "hands off."
String values are passed directly to the system library's
'strtod()' function, and if it successfully returns a numeric
value, that is what's used.(1) By definition, the results are not
portable across different systems. They are also a little
surprising:
$ echo nanny | gawk --posix '{ print $1 + 0 }'
-| nan
$ echo 0xDeadBeef | gawk --posix '{ print $1 + 0 }'
-| 3735928559
* Without '--posix', 'gawk' interprets the four string values '+inf',
'-inf', '+nan', and '-nan' specially, producing the corresponding
special numeric values. The leading sign acts a signal to 'gawk'
(and the user) that the value is really numeric. Hexadecimal
floating point is not supported (unless you also use
'--non-decimal-data', which is _not_ recommended). For example:
$ echo nanny | gawk '{ print $1 + 0 }'
-| 0
$ echo +nan | gawk '{ print $1 + 0 }'
-| nan
$ echo 0xDeadBeef | gawk '{ print $1 + 0 }'
-| 0
'gawk' ignores case in the four special values. Thus, '+nan' and
'+NaN' are the same.
---------- Footnotes ----------
(1) You asked for it, you got it.
File: gawk.info, Node: Floating point summary, Prev: POSIX Floating Point Problems, Up: Arbitrary Precision Arithmetic
15.8 Summary
============
* Most computer arithmetic is done using either integers or
floating-point values. Standard 'awk' uses double-precision
floating-point values.
* In the early 1990s Barbie mistakenly said, "Math class is tough!"
Although math isn't tough, floating-point arithmetic isn't the same
as pencil-and-paper math, and care must be taken:
- Not all numbers can be represented exactly.
- Comparing values should use a delta, instead of being done
directly with '==' and '!='.
- Errors accumulate.
- Operations are not always truly associative or distributive.
* Increasing the accuracy can help, but it is not a panacea.
* Often, increasing the accuracy and then rounding to the desired
number of digits produces reasonable results.
* Use '-M' (or '--bignum') to enable MPFR arithmetic. Use 'PREC' to
set the precision in bits, and 'ROUNDMODE' to set the IEEE 754
rounding mode.
* With '-M', 'gawk' performs arbitrary-precision integer arithmetic
using the GMP library. This is faster and more space-efficient
than using MPFR for the same calculations.
* There are several areas with respect to floating-point numbers
where 'gawk' disagrees with the POSIX standard. It pays to be
aware of them.
* Overall, there is no need to be unduly suspicious about the results
from floating-point arithmetic. The lesson to remember is that
floating-point arithmetic is always more complex than arithmetic
using pencil and paper. In order to take advantage of the power of
floating-point arithmetic, you need to know its limitations and
work within them. For most casual use of floating-point
arithmetic, you will often get the expected result if you simply
round the display of your final results to the correct number of
significant decimal digits.
* As general advice, avoid presenting numerical data in a manner that
implies better precision than is actually the case.
File: gawk.info, Node: Dynamic Extensions, Next: Language History, Prev: Arbitrary Precision Arithmetic, Up: Top
16 Writing Extensions for 'gawk'
********************************
It is possible to add new functions written in C or C++ to 'gawk' using
dynamically loaded libraries. This facility is available on systems
that support the C 'dlopen()' and 'dlsym()' functions. This major node
describes how to create extensions using code written in C or C++.
If you don't know anything about C programming, you can safely skip
this major node, although you may wish to review the documentation on
the extensions that come with 'gawk' (*note Extension Samples::), and
the information on the 'gawkextlib' project (*note gawkextlib::). The
sample extensions are automatically built and installed when 'gawk' is.
NOTE: When '--sandbox' is specified, extensions are disabled (*note
Options::).
* Menu:
* Extension Intro:: What is an extension.
* Plugin License:: A note about licensing.
* Extension Mechanism Outline:: An outline of how it works.
* Extension API Description:: A full description of the API.
* Finding Extensions:: How 'gawk' finds compiled extensions.
* Extension Example:: Example C code for an extension.
* Extension Samples:: The sample extensions that ship with
'gawk'.
* gawkextlib:: The 'gawkextlib' project.
* Extension summary:: Extension summary.
* Extension Exercises:: Exercises.
File: gawk.info, Node: Extension Intro, Next: Plugin License, Up: Dynamic Extensions
16.1 Introduction
=================
An "extension" (sometimes called a "plug-in") is a piece of external
compiled code that 'gawk' can load at runtime to provide additional
functionality, over and above the built-in capabilities described in the
rest of this Info file.
Extensions are useful because they allow you (of course) to extend
'gawk''s functionality. For example, they can provide access to system
calls (such as 'chdir()' to change directory) and to other C library
routines that could be of use. As with most software, "the sky is the
limit"; if you can imagine something that you might want to do and can
write in C or C++, you can write an extension to do it!
Extensions are written in C or C++, using the "application
programming interface" (API) defined for this purpose by the 'gawk'
developers. The rest of this major node explains the facilities that
the API provides and how to use them, and presents a small example
extension. In addition, it documents the sample extensions included in
the 'gawk' distribution and describes the 'gawkextlib' project. *Note
Extension Design::, for a discussion of the extension mechanism goals
and design.
File: gawk.info, Node: Plugin License, Next: Extension Mechanism Outline, Prev: Extension Intro, Up: Dynamic Extensions
16.2 Extension Licensing
========================
Every dynamic extension must be distributed under a license that is
compatible with the GNU GPL (*note Copying::).
In order for the extension to tell 'gawk' that it is properly
licensed, the extension must define the global symbol
'plugin_is_GPL_compatible'. If this symbol does not exist, 'gawk' emits
a fatal error and exits when it tries to load your extension.
The declared type of the symbol should be 'int'. It does not need to
be in any allocated section, though. The code merely asserts that the
symbol exists in the global scope. Something like this is enough:
int plugin_is_GPL_compatible;
File: gawk.info, Node: Extension Mechanism Outline, Next: Extension API Description, Prev: Plugin License, Up: Dynamic Extensions
16.3 How It Works at a High Level
=================================
Communication between 'gawk' and an extension is two-way. First, when
an extension is loaded, 'gawk' passes it a pointer to a 'struct' whose
fields are function pointers. This is shown in *note Figure 16.1:
figure-load-extension.
API
Struct
+---+
| |
+---+
+---------------| |
| +---+ dl_load(api_p, id);
| | | ___________________
| +---+ |
| +---------| | __________________ |
| | +---+ ||
| | | | ||
| | +---+ ||
| | +---| | ||
| | | +---+ \ || /
| | | \ /
v v v \/
+-------+-+---+-+---+-+------------------+--------------------+
| |x| |x| |x| |OOOOOOOOOOOOOOOOOOOO|
| |x| |x| |x| |OOOOOOOOOOOOOOOOOOOO|
| |x| |x| |x| |OOOOOOOOOOOOOOOOOOOO|
+-------+-+---+-+---+-+------------------+--------------------+
gawk Main Program Address Space Extension
Figure 16.1: Loading the extension
The extension can call functions inside 'gawk' through these function
pointers, at runtime, without needing (link-time) access to 'gawk''s
symbols. One of these function pointers is to a function for
"registering" new functions. This is shown in *note Figure 16.2:
figure-register-new-function.
register_ext_func({ "chdir", do_chdir, 1 });
+--------------------------------------------+
| |
V |
+-------+-+---+-+---+-+------------------+--------------+-+---+
| |x| |x| |x| |OOOOOOOOOOOOOO|X|OOO|
| |x| |x| |x| |OOOOOOOOOOOOOO|X|OOO|
| |x| |x| |x| |OOOOOOOOOOOOOO|X|OOO|
+-------+-+---+-+---+-+------------------+--------------+-+---+
gawk Main Program Address Space Extension
Figure 16.2: Registering a new function
In the other direction, the extension registers its new functions
with 'gawk' by passing function pointers to the functions that provide
the new feature ('do_chdir()', for example). 'gawk' associates the
function pointer with a name and can then call it, using a defined
calling convention. This is shown in *note Figure 16.3:
figure-call-new-function.
BEGIN {
chdir("/path") (*fnptr)(1);
}
+--------------------------------------------+
| |
| V
+-------+-+---+-+---+-+------------------+--------------+-+---+
| |x| |x| |x| |OOOOOOOOOOOOOO|X|OOO|
| |x| |x| |x| |OOOOOOOOOOOOOO|X|OOO|
| |x| |x| |x| |OOOOOOOOOOOOOO|X|OOO|
+-------+-+---+-+---+-+------------------+--------------+-+---+
gawk Main Program Address Space Extension
Figure 16.3: Calling the new function
The 'do_XXX()' function, in turn, then uses the function pointers in
the API 'struct' to do its work, such as updating variables or arrays,
printing messages, setting 'ERRNO', and so on.
Convenience macros make calling through the function pointers look
like regular function calls so that extension code is quite readable and
understandable.
Although all of this sounds somewhat complicated, the result is that
extension code is quite straightforward to write and to read. You can
see this in the sample extension 'filefuncs.c' (*note Extension
Example::) and also in the 'testext.c' code for testing the APIs.
Some other bits and pieces:
* The API provides access to 'gawk''s 'do_XXX' values, reflecting
command-line options, like 'do_lint', 'do_profiling', and so on
(*note Extension API Variables::). These are informational: an
extension cannot affect their values inside 'gawk'. In addition,
attempting to assign to them produces a compile-time error.
* The API also provides major and minor version numbers, so that an
extension can check if the 'gawk' it is loaded with supports the
facilities it was compiled with. (Version mismatches "shouldn't"
happen, but we all know how _that_ goes.) *Note Extension
Versioning:: for details.
File: gawk.info, Node: Extension API Description, Next: Finding Extensions, Prev: Extension Mechanism Outline, Up: Dynamic Extensions
16.4 API Description
====================
C or C++ code for an extension must include the header file 'gawkapi.h',
which declares the functions and defines the data types used to
communicate with 'gawk'. This (rather large) minor node describes the
API in detail.
* Menu:
* Extension API Functions Introduction:: Introduction to the API functions.
* General Data Types:: The data types.
* Memory Allocation Functions:: Functions for allocating memory.
* Constructor Functions:: Functions for creating values.
* Registration Functions:: Functions to register things with
'gawk'.
* Printing Messages:: Functions for printing messages.
* Updating ERRNO:: Functions for updating 'ERRNO'.
* Requesting Values:: How to get a value.
* Accessing Parameters:: Functions for accessing parameters.
* Symbol Table Access:: Functions for accessing global
variables.
* Array Manipulation:: Functions for working with arrays.
* Redirection API:: How to access and manipulate
redirections.
* Extension API Variables:: Variables provided by the API.
* Extension API Boilerplate:: Boilerplate code for using the API.
* Changes from API V1:: Changes from V1 of the API.
File: gawk.info, Node: Extension API Functions Introduction, Next: General Data Types, Up: Extension API Description
16.4.1 Introduction
-------------------
Access to facilities within 'gawk' is achieved by calling through
function pointers passed into your extension.
API function pointers are provided for the following kinds of
operations:
* Allocating, reallocating, and releasing memory.
* Registration functions. You may register:
- Extension functions
- Exit callbacks
- A version string
- Input parsers
- Output wrappers
- Two-way processors
All of these are discussed in detail later in this major node.
* Printing fatal, warning, and "lint" warning messages.
* Updating 'ERRNO', or unsetting it.
* Accessing parameters, including converting an undefined parameter
into an array.
* Symbol table access: retrieving a global variable, creating one, or
changing one.
* Creating and releasing cached values; this provides an efficient
way to use values for multiple variables and can be a big
performance win.
* Manipulating arrays:
- Retrieving, adding, deleting, and modifying elements
- Getting the count of elements in an array
- Creating a new array
- Clearing an array
- Flattening an array for easy C-style looping over all its
indices and elements
* Accessing and manipulating redirections.
Some points about using the API:
* The following types, macros, and/or functions are referenced in
'gawkapi.h'. For correct use, you must therefore include the
corresponding standard header file _before_ including 'gawkapi.h':
C entity Header file
-------------------------------------------
'EOF' '<stdio.h>'
Values for 'errno' '<errno.h>'
'FILE' '<stdio.h>'
'NULL' '<stddef.h>'
'memcpy()' '<string.h>'
'memset()' '<string.h>'
'size_t' '<sys/types.h>'
'struct stat' '<sys/stat.h>'
Due to portability concerns, especially to systems that are not
fully standards-compliant, it is your responsibility to include the
correct files in the correct way. This requirement is necessary in
order to keep 'gawkapi.h' clean, instead of becoming a portability
hodge-podge as can be seen in some parts of the 'gawk' source code.
* If your extension uses MPFR facilities, and you wish to receive
such values from 'gawk' and/or pass such values to it, you must
include the '<mpfr.h>' header before including '<gawkapi.h>'.
* The 'gawkapi.h' file may be included more than once without ill
effect. Doing so, however, is poor coding practice.
* Although the API only uses ISO C 90 features, there is an
exception; the "constructor" functions use the 'inline' keyword.
If your compiler does not support this keyword, you should either
place '-Dinline=''' on your command line or use the GNU Autotools
and include a 'config.h' file in your extensions.
* All pointers filled in by 'gawk' point to memory managed by 'gawk'
and should be treated by the extension as read-only. Memory for
_all_ strings passed into 'gawk' from the extension _must_ come
from calling one of 'gawk_malloc()', 'gawk_calloc()', or
'gawk_realloc()', and is managed by 'gawk' from then on.
* The API defines several simple 'struct's that map values as seen
from 'awk'. A value can be a 'double', a string, or an array (as
in multidimensional arrays, or when creating a new array).
String values maintain both pointer and length, because embedded
NUL characters are allowed.
NOTE: By intent, 'gawk' maintains strings using the current
multibyte encoding (as defined by 'LC_XXX' environment
variables) and not using wide characters. This matches how
'gawk' stores strings internally and also how characters are
likely to be input into and output from files.
NOTE: String values passed to an extension by 'gawk' are
always NUL-terminated. Thus it is safe to pass such string
values to standard library and system routines. However,
because 'gawk' allows embedded NUL characters in string data,
before using the data as a regular C string, you should check
that the length for that string passed to the extension
matches the return value of 'strlen()' for it.
* When retrieving a value (such as a parameter or that of a global
variable or array element), the extension requests a specific type
(number, string, scalar, value cookie, array, or "undefined").
When the request is "undefined," the returned value will have the
real underlying type.
However, if the request and actual type don't match, the access
function returns "false" and fills in the type of the actual value
that is there, so that the extension can, e.g., print an error
message (such as "scalar passed where array expected").
You may call the API functions by using the function pointers
directly, but the interface is not so pretty. To make extension code
look more like regular code, the 'gawkapi.h' header file defines several
macros that you should use in your code. This minor node presents the
macros as if they were functions.
File: gawk.info, Node: General Data Types, Next: Memory Allocation Functions, Prev: Extension API Functions Introduction, Up: Extension API Description
16.4.2 General-Purpose Data Types
---------------------------------
I have a true love/hate relationship with unions.
-- _Arnold Robbins_
That's the thing about unions: the compiler will arrange things so
they can accommodate both love and hate.
-- _Chet Ramey_
The extension API defines a number of simple types and structures for
general-purpose use. Additional, more specialized, data structures are
introduced in subsequent minor nodes, together with the functions that
use them.
The general-purpose types and structures are as follows:
'typedef void *awk_ext_id_t;'
A value of this type is received from 'gawk' when an extension is
loaded. That value must then be passed back to 'gawk' as the first
parameter of each API function.
'#define awk_const ...'
This macro expands to 'const' when compiling an extension, and to
nothing when compiling 'gawk' itself. This makes certain fields in
the API data structures unwritable from extension code, while
allowing 'gawk' to use them as it needs to.
'typedef enum awk_bool {'
' awk_false = 0,'
' awk_true'
'} awk_bool_t;'
A simple Boolean type.
'typedef struct awk_string {'
' char *str; /* data */'
' size_t len; /* length thereof, in chars */'
'} awk_string_t;'
This represents a mutable string. 'gawk' owns the memory pointed
to if it supplied the value. Otherwise, it takes ownership of the
memory pointed to. _Such memory must come from calling one of the
'gawk_malloc()', 'gawk_calloc()', or 'gawk_realloc()' functions!_
As mentioned earlier, strings are maintained using the current
multibyte encoding.
'typedef enum {'
' AWK_UNDEFINED,'
' AWK_NUMBER,'
' AWK_STRING,'
' AWK_REGEX,'
' AWK_STRNUM,'
' AWK_ARRAY,'
' AWK_SCALAR, /* opaque access to a variable */'
' AWK_VALUE_COOKIE /* for updating a previously created value */'
'} awk_valtype_t;'
This 'enum' indicates the type of a value. It is used in the
following 'struct'.
'typedef struct awk_value {'
' awk_valtype_t val_type;'
' union {'
' awk_string_t s;'
' awknum_t n;'
' awk_array_t a;'
' awk_scalar_t scl;'
' awk_value_cookie_t vc;'
' } u;'
'} awk_value_t;'
An "'awk' value." The 'val_type' member indicates what kind of
value the 'union' holds, and each member is of the appropriate
type.
'#define str_value u.s'
'#define strnum_value str_value'
'#define regex_value str_value'
'#define num_value u.n.d'
'#define num_type u.n.type'
'#define num_ptr u.n.ptr'
'#define array_cookie u.a'
'#define scalar_cookie u.scl'
'#define value_cookie u.vc'
Using these macros makes accessing the fields of the 'awk_value_t'
more readable.
'enum AWK_NUMBER_TYPE {'
' AWK_NUMBER_TYPE_DOUBLE,'
' AWK_NUMBER_TYPE_MPFR,'
' AWK_NUMBER_TYPE_MPZ'
'};'
This 'enum' is used in the following structure for defining the
type of numeric value that is being worked with. It is declared at
the top level of the file so that it works correctly for C++ as
well as for C.
'typedef struct awk_number {'
' double d;'
' enum AWK_NUMBER_TYPE type;'
' void *ptr;'
'} awk_number_t;'
This represents a numeric value. Internally, 'gawk' stores every
number as either a C 'double', a GMP integer, or an MPFR
arbitrary-precision floating-point value. In order to allow
extensions to also support GMP and MPFR values, numeric values are
passed in this structure.
The double-precision 'd' element is always populated in data
received from 'gawk'. In addition, by examining the 'type' member,
an extension can determine if the 'ptr' member is either a GMP
integer (type 'mpz_ptr'), or an MPFR floating-point value (type
'mpfr_ptr_t'), and cast it appropriately.
'typedef void *awk_scalar_t;'
Scalars can be represented as an opaque type. These values are
obtained from 'gawk' and then passed back into it. This is
discussed in a general fashion in the text following this list, and
in more detail in *note Symbol table by cookie::.
'typedef void *awk_value_cookie_t;'
A "value cookie" is an opaque type representing a cached value.
This is also discussed in a general fashion in the text following
this list, and in more detail in *note Cached values::.
Scalar values in 'awk' are numbers, strings, strnums, or typed
regexps. The 'awk_value_t' struct represents values. The 'val_type'
member indicates what is in the 'union'.
Representing numbers is easy--the API uses a C 'double'. Strings
require more work. Because 'gawk' allows embedded NUL bytes in string
values, a string must be represented as a pair containing a data pointer
and length. This is the 'awk_string_t' type.
A strnum (numeric string) value is represented as a string and
consists of user input data that appears to be numeric. When an
extension creates a strnum value, the result is a string flagged as user
input. Subsequent parsing by 'gawk' then determines whether it looks
like a number and should be treated as a strnum, or as a regular string.
This is useful in cases where an extension function would like to do
something comparable to the 'split()' function which sets the strnum
attribute on the array elements it creates. For example, an extension
that implements CSV splitting would want to use this feature. This is
also useful for a function that retrieves a data item from a database.
The PostgreSQL 'PQgetvalue()' function, for example, returns a string
that may be numeric or textual depending on the contents.
Typed regexp values (*note Strong Regexp Constants::) are not of much
use to extension functions. Extension functions can tell that they've
received them, and create them for scalar values. Otherwise, they can
examine the text of the regexp through 'regex_value.str' and
'regex_value.len'.
Identifiers (i.e., the names of global variables) can be associated
with either scalar values or with arrays. In addition, 'gawk' provides
true arrays of arrays, where any given array element can itself be an
array. Discussion of arrays is delayed until *note Array
Manipulation::.
The various macros listed earlier make it easier to use the elements
of the 'union' as if they were fields in a 'struct'; this is a common
coding practice in C. Such code is easier to write and to read, but it
remains _your_ responsibility to make sure that the 'val_type' member
correctly reflects the type of the value in the 'awk_value_t' struct.
Conceptually, the first three members of the 'union' (number, string,
and array) are all that is needed for working with 'awk' values.
However, because the API provides routines for accessing and changing
the value of a global scalar variable only by using the variable's name,
there is a performance penalty: 'gawk' must find the variable each time
it is accessed and changed. This turns out to be a real issue, not just
a theoretical one.
Thus, if you know that your extension will spend considerable time
reading and/or changing the value of one or more scalar variables, you
can obtain a "scalar cookie"(1) object for that variable, and then use
the cookie for getting the variable's value or for changing the
variable's value. The 'awk_scalar_t' type holds a scalar cookie, and
the 'scalar_cookie' macro provides access to the value of that type in
the 'awk_value_t' struct. Given a scalar cookie, 'gawk' can directly
retrieve or modify the value, as required, without having to find it
first.
The 'awk_value_cookie_t' type and 'value_cookie' macro are similar.
If you know that you wish to use the same numeric or string _value_ for
one or more variables, you can create the value once, retaining a "value
cookie" for it, and then pass in that value cookie whenever you wish to
set the value of a variable. This saves storage space within the
running 'gawk' process and reduces the time needed to create the value.
---------- Footnotes ----------
(1) See the "cookie" entry in the Jargon file
(http://catb.org/jargon/html/C/cookie.html) for a definition of
"cookie", and the "magic cookie" entry in the Jargon file
(http://catb.org/jargon/html/M/magic-cookie.html) for a nice example.
See also the entry for "Cookie" in the *note Glossary::.
File: gawk.info, Node: Memory Allocation Functions, Next: Constructor Functions, Prev: General Data Types, Up: Extension API Description
16.4.3 Memory Allocation Functions and Convenience Macros
---------------------------------------------------------
The API provides a number of "memory allocation" functions for
allocating memory that can be passed to 'gawk', as well as a number of
convenience macros. This node presents them all as function prototypes,
in the way that extension code would use them:
'void *gawk_malloc(size_t size);'
Call the correct version of 'malloc()' to allocate storage that may
be passed to 'gawk'.
'void *gawk_calloc(size_t nmemb, size_t size);'
Call the correct version of 'calloc()' to allocate storage that may
be passed to 'gawk'.
'void *gawk_realloc(void *ptr, size_t size);'
Call the correct version of 'realloc()' to allocate storage that
may be passed to 'gawk'.
'void gawk_free(void *ptr);'
Call the correct version of 'free()' to release storage that was
allocated with 'gawk_malloc()', 'gawk_calloc()', or
'gawk_realloc()'.
The API has to provide these functions because it is possible for an
extension to be compiled and linked against a different version of the C
library than was used for the 'gawk' executable.(1) If 'gawk' were to
use its version of 'free()' when the memory came from an unrelated
version of 'malloc()', unexpected behavior would likely result.
Three convenience macros may be used for allocating storage from
'gawk_malloc()', 'gawk_calloc', and 'gawk_realloc()'. If the allocation
fails, they cause 'gawk' to exit with a fatal error message. They
should be used as if they were procedure calls that do not return a
value:
'#define emalloc(pointer, type, size, message) ...'
The arguments to this macro are as follows:
'pointer'
The pointer variable to point at the allocated storage.
'type'
The type of the pointer variable. This is used to create a
cast for the call to 'gawk_malloc()'.
'size'
The total number of bytes to be allocated.
'message'
A message to be prefixed to the fatal error message.
Typically this is the name of the function using the macro.
For example, you might allocate a string value like so:
awk_value_t result;
char *message;
const char greet[] = "Don't Panic!";
emalloc(message, char *, sizeof(greet), "myfunc");
strcpy(message, greet);
make_malloced_string(message, strlen(message), & result);
'#define ezalloc(pointer, type, size, message) ...'
This is like 'emalloc()', but it calls 'gawk_calloc()' instead of
'gawk_malloc()'. The arguments are the same as for the 'emalloc()'
macro, but this macro guarantees that the memory returned is
initialized to zero.
'#define erealloc(pointer, type, size, message) ...'
This is like 'emalloc()', but it calls 'gawk_realloc()' instead of
'gawk_malloc()'. The arguments are the same as for the 'emalloc()'
macro.
Two additional functions allocate MPFR and GMP objects for use by
extension functions that need to create and then return such values:
'void *get_mpfr_ptr();'
Allocate and initialize an MPFR object and return a pointer to it.
If the allocation fails, 'gawk' exits with a fatal "out of memory"
error. If 'gawk' was compiled without MPFR support, calling this
function causes a fatal error.
'void *get_mpz_ptr();'
Allocate and initialize a GMP object and return a pointer to it.
If the allocation fails, 'gawk' exits with a fatal "out of memory"
error. If 'gawk' was compiled without MPFR support, calling this
function causes a fatal error.
Both of these functions return 'void *', since the 'gawkapi.h' header
file should not have dependency upon '<mpfr.h>' (and '<gmp.h>', which is
included from '<mpfr.h>'). The actual return values are of types
'mpfr_ptr' and 'mpz_ptr' respectively, and you should cast the return
values appropriately before assigning the results to variables of the
correct types.
---------- Footnotes ----------
(1) This is more common on MS-Windows systems, but it can happen on
Unix-like systems as well.
File: gawk.info, Node: Constructor Functions, Next: Registration Functions, Prev: Memory Allocation Functions, Up: Extension API Description
16.4.4 Constructor Functions
----------------------------
The API provides a number of "constructor" functions for creating string
and numeric values, as well as a number of convenience macros. This
node presents them all as function prototypes, in the way that extension
code would use them:
'static inline awk_value_t *'
'make_const_string(const char *string, size_t length, awk_value_t *result);'
This function creates a string value in the 'awk_value_t' variable
pointed to by 'result'. It expects 'string' to be a C string
constant (or other string data), and automatically creates a _copy_
of the data for storage in 'result'. It returns 'result'.
'static inline awk_value_t *'
'make_malloced_string(const char *string, size_t length, awk_value_t *result);'
This function creates a string value in the 'awk_value_t' variable
pointed to by 'result'. It expects 'string' to be a 'char *' value
pointing to data previously obtained from 'gawk_malloc()',
'gawk_calloc()', or 'gawk_realloc()'. The idea here is that the
data is passed directly to 'gawk', which assumes responsibility for
it. It returns 'result'.
'static inline awk_value_t *'
'make_null_string(awk_value_t *result);'
This specialized function creates a null string (the "undefined"
value) in the 'awk_value_t' variable pointed to by 'result'. It
returns 'result'.
'static inline awk_value_t *'
'make_number(double num, awk_value_t *result);'
This function simply creates a numeric value in the 'awk_value_t'
variable pointed to by 'result'.
'static inline awk_value_t *'
'make_number_mpz(void *mpz, awk_value_t *result);'
This function creates a GMP number value in 'result'. The 'mpz'
must be from a call to 'get_mpz_ptr()' (and thus be of real
underlying type 'mpz_ptr'). 'gawk' takes ownership of this memory.
'static inline awk_value_t *'
'make_number_mpfr(void *mpfr, awk_value_t *result);'
This function creates an MPFR number value in 'result'. The 'mpfr'
must be from a call to 'get_mpfr_ptr()'. (and thus be of real
underlying type 'mpfr_ptr') 'gawk' takes ownership of this memory.
'static inline awk_value_t *'
'make_const_user_input(const char *string, size_t length, awk_value_t *result);'
This function is identical to 'make_const_string()', but the string
is flagged as user input that should be treated as a strnum value
if the contents of the string are numeric.
'static inline awk_value_t *'
'make_malloced_user_input(const char *string, size_t length, awk_value_t *result);'
This function is identical to 'make_malloced_string()', but the
string is flagged as user input that should be treated as a strnum
value if the contents of the string are numeric.
'static inline awk_value_t *'
'make_const_regex(const char *string, size_t length, awk_value_t *result);'
This function creates a strongly typed regexp value by allocating a
copy of the string. 'string' is the regular expression of length
'len'.
'static inline awk_value_t *'
'make_malloced_regex(const char *string, size_t length, awk_value_t *result);'
This function creates a strongly typed regexp value. 'string' is
the regular expression of length 'len'. It expects 'string' to be
a 'char *' value pointing to data previously obtained from
'gawk_malloc()', 'gawk_calloc()', or 'gawk_realloc()'.
File: gawk.info, Node: Registration Functions, Next: Printing Messages, Prev: Constructor Functions, Up: Extension API Description
16.4.5 Registration Functions
-----------------------------
This minor node describes the API functions for registering parts of
your extension with 'gawk'.
* Menu:
* Extension Functions:: Registering extension functions.
* Exit Callback Functions:: Registering an exit callback.
* Extension Version String:: Registering a version string.
* Input Parsers:: Registering an input parser.
* Output Wrappers:: Registering an output wrapper.
* Two-way processors:: Registering a two-way processor.
File: gawk.info, Node: Extension Functions, Next: Exit Callback Functions, Up: Registration Functions
16.4.5.1 Registering An Extension Function
..........................................
Extension functions are described by the following record:
typedef struct awk_ext_func {
const char *name;
awk_value_t *(*const function)(int num_actual_args,
awk_value_t *result,
struct awk_ext_func *finfo);
const size_t max_expected_args;
const size_t min_required_args;
awk_bool_t suppress_lint;
void *data; /* opaque pointer to any extra state */
} awk_ext_func_t;
The fields are:
'const char *name;'
The name of the new function. 'awk'-level code calls the function
by this name. This is a regular C string.
Function names must obey the rules for 'awk' identifiers. That is,
they must begin with either an English letter or an underscore,
which may be followed by any number of letters, digits, and
underscores. Letter case in function names is significant.
'awk_value_t *(*const function)(int num_actual_args,'
' awk_value_t *result,'
' struct awk_ext_func *finfo);'
This is a pointer to the C function that provides the extension's
functionality. The function must fill in '*result' with either a
number, a string, or a regexp. 'gawk' takes ownership of any
string memory. As mentioned earlier, string memory _must_ come
from one of 'gawk_malloc()', 'gawk_calloc()', or 'gawk_realloc()'.
The 'num_actual_args' argument tells the C function how many actual
parameters were passed from the calling 'awk' code.
The 'finfo' parameter is a pointer to the 'awk_ext_func_t' for this
function. The called function may access data within it as
desired, or not.
The function must return the value of 'result'. This is for the
convenience of the calling code inside 'gawk'.
'const size_t max_expected_args;'
This is the maximum number of arguments the function expects to
receive. If called with more arguments than this, and if lint
checking has been enabled, then 'gawk' prints a warning message.
For more information, see the entry for 'suppress_lint', later in
this list.
'const size_t min_required_args;'
This is the minimum number of arguments the function expects to
receive. If called with fewer arguments, 'gawk' prints a fatal
error message and exits.
'awk_bool_t suppress_lint;'
This flag tells 'gawk' not to print a lint message if lint checking
has been enabled and if more arguments were supplied in the call
than expected. An extension function can tell if 'gawk' already
printed at least one such message by checking if 'num_actual_args >
finfo->max_expected_args'. If so, and the function does not want
more lint messages to be printed, it should set
'finfo->suppress_lint' to 'awk_true'.
'void *data;'
This is an opaque pointer to any data that an extension function
may wish to have available when called. Passing the
'awk_ext_func_t' structure to the extension function, and having
this pointer available in it enable writing a single C or C++
function that implements multiple 'awk'-level extension functions.
Once you have a record representing your extension function, you
register it with 'gawk' using this API function:
'awk_bool_t add_ext_func(const char *name_space, awk_ext_func_t *func);'
This function returns true upon success, false otherwise. The
'name_space' parameter is currently not used; you should pass in an
empty string ('""'). The 'func' pointer is the address of a
'struct' representing your function, as just described.
'gawk' does not modify what 'func' points to, but the extension
function itself receives this pointer and can modify what it points
to, thus it is purposely not declared to be 'const'.
The combination of 'min_required_args', 'max_expected_args', and
'suppress_lint' may be confusing. Here is how you should set things up.
Any number of arguments is valid
Set 'min_required_args' and 'max_expected_args' to zero and set
'suppress_lint' to 'awk_true'.
A minimum number of arguments is required, no limit on maximum number of arguments
Set 'min_required_args' to the minimum required. Set
'max_expected_args' to zero and set 'suppress_lint' to 'awk_true'.
A minimum number of arguments is required, a maximum number is expected
Set 'min_required_args' to the minimum required. Set
'max_expected_args' to the maximum expected. Set 'suppress_lint'
to 'awk_false'.
A minimum number of arguments is required, and no more than a maximum is allowed
Set 'min_required_args' to the minimum required. Set
'max_expected_args' to the maximum expected. Set 'suppress_lint'
to 'awk_false'. In your extension function, check that
'num_actual_args' does not exceed 'f->max_expected_args'. If it
does, issue a fatal error message.
File: gawk.info, Node: Exit Callback Functions, Next: Extension Version String, Prev: Extension Functions, Up: Registration Functions
16.4.5.2 Registering An Exit Callback Function
..............................................
An "exit callback" function is a function that 'gawk' calls before it
exits. Such functions are useful if you have general "cleanup" tasks
that should be performed in your extension (such as closing database
connections or other resource deallocations). You can register such a
function with 'gawk' using the following function:
'void awk_atexit(void (*funcp)(void *data, int exit_status),'
' void *arg0);'
The parameters are:
'funcp'
A pointer to the function to be called before 'gawk' exits.
The 'data' parameter will be the original value of 'arg0'.
The 'exit_status' parameter is the exit status value that
'gawk' intends to pass to the 'exit()' system call.
'arg0'
A pointer to private data that 'gawk' saves in order to pass
to the function pointed to by 'funcp'.
Exit callback functions are called in last-in, first-out (LIFO)
order--that is, in the reverse order in which they are registered with
'gawk'.
File: gawk.info, Node: Extension Version String, Next: Input Parsers, Prev: Exit Callback Functions, Up: Registration Functions
16.4.5.3 Registering An Extension Version String
................................................
You can register a version string that indicates the name and version of
your extension with 'gawk', as follows:
'void register_ext_version(const char *version);'
Register the string pointed to by 'version' with 'gawk'. Note that
'gawk' does _not_ copy the 'version' string, so it should not be
changed.
'gawk' prints all registered extension version strings when it is
invoked with the '--version' option.
File: gawk.info, Node: Input Parsers, Next: Output Wrappers, Prev: Extension Version String, Up: Registration Functions
16.4.5.4 Customized Input Parsers
.................................
By default, 'gawk' reads text files as its input. It uses the value of
'RS' to find the end of the record, and then uses 'FS' (or 'FIELDWIDTHS'
or 'FPAT') to split it into fields (*note Reading Files::).
Additionally, it sets the value of 'RT' (*note Built-in Variables::).
If you want, you can provide your own custom input parser. An input
parser's job is to return a record to the 'gawk' record-processing code,
along with indicators for the value and length of the data to be used
for 'RT', if any.
To provide an input parser, you must first provide two functions
(where XXX is a prefix name for your extension):
'awk_bool_t XXX_can_take_file(const awk_input_buf_t *iobuf);'
This function examines the information available in 'iobuf' (which
we discuss shortly). Based on the information there, it decides if
the input parser should be used for this file. If so, it should
return true. Otherwise, it should return false. It should not
change any state (variable values, etc.) within 'gawk'.
'awk_bool_t XXX_take_control_of(awk_input_buf_t *iobuf);'
When 'gawk' decides to hand control of the file over to the input
parser, it calls this function. This function in turn must fill in
certain fields in the 'awk_input_buf_t' structure and ensure that
certain conditions are true. It should then return true. If an
error of some kind occurs, it should not fill in any fields and
should return false; then 'gawk' will not use the input parser.
The details are presented shortly.
Your extension should package these functions inside an
'awk_input_parser_t', which looks like this:
typedef struct awk_input_parser {
const char *name; /* name of parser */
awk_bool_t (*can_take_file)(const awk_input_buf_t *iobuf);
awk_bool_t (*take_control_of)(awk_input_buf_t *iobuf);
awk_const struct awk_input_parser *awk_const next; /* for gawk */
} awk_input_parser_t;
The fields are:
'const char *name;'
The name of the input parser. This is a regular C string.
'awk_bool_t (*can_take_file)(const awk_input_buf_t *iobuf);'
A pointer to your 'XXX_can_take_file()' function.
'awk_bool_t (*take_control_of)(awk_input_buf_t *iobuf);'
A pointer to your 'XXX_take_control_of()' function.
'awk_const struct input_parser *awk_const next;'
This is for use by 'gawk'; therefore it is marked 'awk_const' so
that the extension cannot modify it.
The steps are as follows:
1. Create a 'static awk_input_parser_t' variable and initialize it
appropriately.
2. When your extension is loaded, register your input parser with
'gawk' using the 'register_input_parser()' API function (described
next).
An 'awk_input_buf_t' looks like this:
typedef struct awk_input {
const char *name; /* filename */
int fd; /* file descriptor */
#define INVALID_HANDLE (-1)
void *opaque; /* private data for input parsers */
int (*get_record)(char **out, struct awk_input *iobuf,
int *errcode, char **rt_start, size_t *rt_len,
const awk_fieldwidth_info_t **field_width);
ssize_t (*read_func)();
void (*close_func)(struct awk_input *iobuf);
struct stat sbuf; /* stat buf */
} awk_input_buf_t;
The fields can be divided into two categories: those for use
(initially, at least) by 'XXX_can_take_file()', and those for use by
'XXX_take_control_of()'. The first group of fields and their uses are
as follows:
'const char *name;'
The name of the file.
'int fd;'
A file descriptor for the file. If 'gawk' was able to open the
file, then 'fd' will _not_ be equal to 'INVALID_HANDLE'.
Otherwise, it will.
'struct stat sbuf;'
If the file descriptor is valid, then 'gawk' will have filled in
this structure via a call to the 'fstat()' system call.
The 'XXX_can_take_file()' function should examine these fields and
decide if the input parser should be used for the file. The decision
can be made based upon 'gawk' state (the value of a variable defined
previously by the extension and set by 'awk' code), the name of the
file, whether or not the file descriptor is valid, the information in
the 'struct stat', or any combination of these factors.
Once 'XXX_can_take_file()' has returned true, and 'gawk' has decided
to use your input parser, it calls 'XXX_take_control_of()'. That
function then fills either the 'get_record' field or the 'read_func'
field in the 'awk_input_buf_t'. It must also ensure that 'fd' is _not_
set to 'INVALID_HANDLE'. The following list describes the fields that
may be filled by 'XXX_take_control_of()':
'void *opaque;'
This is used to hold any state information needed by the input
parser for this file. It is "opaque" to 'gawk'. The input parser
is not required to use this pointer.
'int (*get_record)(char **out,'
' struct awk_input *iobuf,'
' int *errcode,'
' char **rt_start,'
' size_t *rt_len,'
' const awk_fieldwidth_info_t **field_width);'
This function pointer should point to a function that creates the
input records. Said function is the core of the input parser. Its
behavior is described in the text following this list.
'ssize_t (*read_func)();'
This function pointer should point to a function that has the same
behavior as the standard POSIX 'read()' system call. It is an
alternative to the 'get_record' pointer. Its behavior is also
described in the text following this list.
'void (*close_func)(struct awk_input *iobuf);'
This function pointer should point to a function that does the
"teardown." It should release any resources allocated by
'XXX_take_control_of()'. It may also close the file. If it does
so, it should set the 'fd' field to 'INVALID_HANDLE'.
If 'fd' is still not 'INVALID_HANDLE' after the call to this
function, 'gawk' calls the regular 'close()' system call.
Having a "teardown" function is optional. If your input parser
does not need it, do not set this field. Then, 'gawk' calls the
regular 'close()' system call on the file descriptor, so it should
be valid.
The 'XXX_get_record()' function does the work of creating input
records. The parameters are as follows:
'char **out'
This is a pointer to a 'char *' variable that is set to point to
the record. 'gawk' makes its own copy of the data, so the
extension must manage this storage.
'struct awk_input *iobuf'
This is the 'awk_input_buf_t' for the file. The fields should be
used for reading data ('fd') and for managing private state
('opaque'), if any.
'int *errcode'
If an error occurs, '*errcode' should be set to an appropriate code
from '<errno.h>'.
'char **rt_start'
'size_t *rt_len'
If the concept of a "record terminator" makes sense, then
'*rt_start' should be set to point to the data to be used for 'RT',
and '*rt_len' should be set to the length of the data. Otherwise,
'*rt_len' should be set to zero. 'gawk' makes its own copy of this
data, so the extension must manage this storage.
'const awk_fieldwidth_info_t **field_width'
If 'field_width' is not 'NULL', then '*field_width' will be
initialized to 'NULL', and the function may set it to point to a
structure supplying field width information to override the default
field parsing mechanism. Note that this structure will not be
copied by 'gawk'; it must persist at least until the next call to
'get_record' or 'close_func'. Note also that 'field_width' is
'NULL' when 'getline' is assigning the results to a variable, thus
field parsing is not needed. If the parser does set
'*field_width', then 'gawk' uses this layout to parse the input
record, and the 'PROCINFO["FS"]' value will be '"API"' while this
record is active in '$0'. The 'awk_fieldwidth_info_t' data
structure is described below.
The return value is the length of the buffer pointed to by '*out', or
'EOF' if end-of-file was reached or an error occurred.
It is guaranteed that 'errcode' is a valid pointer, so there is no
need to test for a 'NULL' value. 'gawk' sets '*errcode' to zero, so
there is no need to set it unless an error occurs.
If an error does occur, the function should return 'EOF' and set
'*errcode' to a value greater than zero. In that case, if '*errcode'
does not equal zero, 'gawk' automatically updates the 'ERRNO' variable
based on the value of '*errcode'. (In general, setting '*errcode =
errno' should do the right thing.)
As an alternative to supplying a function that returns an input
record, you may instead supply a function that simply reads bytes, and
let 'gawk' parse the data into records. If you do so, the data should
be returned in the multibyte encoding of the current locale. Such a
function should follow the same behavior as the 'read()' system call,
and you fill in the 'read_func' pointer with its address in the
'awk_input_buf_t' structure.
By default, 'gawk' sets the 'read_func' pointer to point to the
'read()' system call. So your extension need not set this field
explicitly.
NOTE: You must choose one method or the other: either a function
that returns a record, or one that returns raw data. In
particular, if you supply a function to get a record, 'gawk' will
call it, and will never call the raw read function.
'gawk' ships with a sample extension that reads directories,
returning records for each entry in a directory (*note Extension Sample
Readdir::). You may wish to use that code as a guide for writing your
own input parser.
When writing an input parser, you should think about (and document)
how it is expected to interact with 'awk' code. You may want it to
always be called, and to take effect as appropriate (as the 'readdir'
extension does). Or you may want it to take effect based upon the value
of an 'awk' variable, as the XML extension from the 'gawkextlib' project
does (*note gawkextlib::). In the latter case, code in a 'BEGINFILE'
rule can look at 'FILENAME' and 'ERRNO' to decide whether or not to
activate an input parser (*note BEGINFILE/ENDFILE::).
You register your input parser with the following function:
'void register_input_parser(awk_input_parser_t *input_parser);'
Register the input parser pointed to by 'input_parser' with 'gawk'.
If you would like to override the default field parsing mechanism for
a given record, then you must populate an 'awk_fieldwidth_info_t'
structure, which looks like this:
typedef struct {
awk_bool_t use_chars; /* false ==> use bytes */
size_t nf; /* number of fields in record (NF) */
struct awk_field_info {
size_t skip; /* amount to skip before field starts */
size_t len; /* length of field */
} fields[1]; /* actual dimension should be nf */
} awk_fieldwidth_info_t;
The fields are:
'awk_bool_t use_chars;'
Set this to 'awk_true' if the field lengths are specified in terms
of potentially multi-byte characters, and set it to 'awk_false' if
the lengths are in terms of bytes. Performance will be better if
the values are supplied in terms of bytes.
'size_t nf;'
Set this to the number of fields in the input record, i.e. 'NF'.
'struct awk_field_info fields[nf];'
This is a variable-length array whose actual dimension should be
'nf'. For each field, the 'skip' element should be set to the
number of characters or bytes, as controlled by the 'use_chars'
flag, to skip before the start of this field. The 'len' element
provides the length of the field. The values in 'fields[0]'
provide the information for '$1', and so on through the
'fields[nf-1]' element containing the information for '$NF'.
A convenience macro 'awk_fieldwidth_info_size(numfields)' is provided
to calculate the appropriate size of a variable-length
'awk_fieldwidth_info_t' structure containing 'numfields' fields. This
can be used as an argument to 'malloc()' or in a union to allocate space
statically. Please refer to the 'readdir_test' sample extension for an
example.
File: gawk.info, Node: Output Wrappers, Next: Two-way processors, Prev: Input Parsers, Up: Registration Functions
16.4.5.5 Customized Output Wrappers
...................................
An "output wrapper" is the mirror image of an input parser. It allows
an extension to take over the output to a file opened with the '>' or
'>>' I/O redirection operators (*note Redirection::).
The output wrapper is very similar to the input parser structure:
typedef struct awk_output_wrapper {
const char *name; /* name of the wrapper */
awk_bool_t (*can_take_file)(const awk_output_buf_t *outbuf);
awk_bool_t (*take_control_of)(awk_output_buf_t *outbuf);
awk_const struct awk_output_wrapper *awk_const next; /* for gawk */
} awk_output_wrapper_t;
The members are as follows:
'const char *name;'
This is the name of the output wrapper.
'awk_bool_t (*can_take_file)(const awk_output_buf_t *outbuf);'
This points to a function that examines the information in the
'awk_output_buf_t' structure pointed to by 'outbuf'. It should
return true if the output wrapper wants to take over the file, and
false otherwise. It should not change any state (variable values,
etc.) within 'gawk'.
'awk_bool_t (*take_control_of)(awk_output_buf_t *outbuf);'
The function pointed to by this field is called when 'gawk' decides
to let the output wrapper take control of the file. It should fill
in appropriate members of the 'awk_output_buf_t' structure, as
described next, and return true if successful, false otherwise.
'awk_const struct output_wrapper *awk_const next;'
This is for use by 'gawk'; therefore it is marked 'awk_const' so
that the extension cannot modify it.
The 'awk_output_buf_t' structure looks like this:
typedef struct awk_output_buf {
const char *name; /* name of output file */
const char *mode; /* mode argument to fopen */
FILE *fp; /* stdio file pointer */
awk_bool_t redirected; /* true if a wrapper is active */
void *opaque; /* for use by output wrapper */
size_t (*gawk_fwrite)(const void *buf, size_t size, size_t count,
FILE *fp, void *opaque);
int (*gawk_fflush)(FILE *fp, void *opaque);
int (*gawk_ferror)(FILE *fp, void *opaque);
int (*gawk_fclose)(FILE *fp, void *opaque);
} awk_output_buf_t;
Here too, your extension will define 'XXX_can_take_file()' and
'XXX_take_control_of()' functions that examine and update data members
in the 'awk_output_buf_t'. The data members are as follows:
'const char *name;'
The name of the output file.
'const char *mode;'
The mode string (as would be used in the second argument to
'fopen()') with which the file was opened.
'FILE *fp;'
The 'FILE' pointer from '<stdio.h>'. 'gawk' opens the file before
attempting to find an output wrapper.
'awk_bool_t redirected;'
This field must be set to true by the 'XXX_take_control_of()'
function.
'void *opaque;'
This pointer is opaque to 'gawk'. The extension should use it to
store a pointer to any private data associated with the file.
'size_t (*gawk_fwrite)(const void *buf, size_t size, size_t count,'
' FILE *fp, void *opaque);'
'int (*gawk_fflush)(FILE *fp, void *opaque);'
'int (*gawk_ferror)(FILE *fp, void *opaque);'
'int (*gawk_fclose)(FILE *fp, void *opaque);'
These pointers should be set to point to functions that perform the
equivalent function as the '<stdio.h>' functions do, if
appropriate. 'gawk' uses these function pointers for all output.
'gawk' initializes the pointers to point to internal "pass-through"
functions that just call the regular '<stdio.h>' functions, so an
extension only needs to redefine those functions that are
appropriate for what it does.
The 'XXX_can_take_file()' function should make a decision based upon
the 'name' and 'mode' fields, and any additional state (such as 'awk'
variable values) that is appropriate.
When 'gawk' calls 'XXX_take_control_of()', that function should fill
in the other fields as appropriate, except for 'fp', which it should
just use normally.
You register your output wrapper with the following function:
'void register_output_wrapper(awk_output_wrapper_t *output_wrapper);'
Register the output wrapper pointed to by 'output_wrapper' with
'gawk'.
File: gawk.info, Node: Two-way processors, Prev: Output Wrappers, Up: Registration Functions
16.4.5.6 Customized Two-way Processors
......................................
A "two-way processor" combines an input parser and an output wrapper for
two-way I/O with the '|&' operator (*note Redirection::). It makes
identical use of the 'awk_input_parser_t' and 'awk_output_buf_t'
structures as described earlier.
A two-way processor is represented by the following structure:
typedef struct awk_two_way_processor {
const char *name; /* name of the two-way processor */
awk_bool_t (*can_take_two_way)(const char *name);
awk_bool_t (*take_control_of)(const char *name,
awk_input_buf_t *inbuf,
awk_output_buf_t *outbuf);
awk_const struct awk_two_way_processor *awk_const next; /* for gawk */
} awk_two_way_processor_t;
The fields are as follows:
'const char *name;'
The name of the two-way processor.
'awk_bool_t (*can_take_two_way)(const char *name);'
The function pointed to by this field should return true if it
wants to take over two-way I/O for this file name. It should not
change any state (variable values, etc.) within 'gawk'.
'awk_bool_t (*take_control_of)(const char *name,'
' awk_input_buf_t *inbuf,'
' awk_output_buf_t *outbuf);'
The function pointed to by this field should fill in the
'awk_input_buf_t' and 'awk_output_buf_t' structures pointed to by
'inbuf' and 'outbuf', respectively. These structures were
described earlier.
'awk_const struct two_way_processor *awk_const next;'
This is for use by 'gawk'; therefore it is marked 'awk_const' so
that the extension cannot modify it.
As with the input parser and output processor, you provide "yes I can
take this" and "take over for this" functions, 'XXX_can_take_two_way()'
and 'XXX_take_control_of()'.
You register your two-way processor with the following function:
'void register_two_way_processor(awk_two_way_processor_t *two_way_processor);'
Register the two-way processor pointed to by 'two_way_processor'
with 'gawk'.
File: gawk.info, Node: Printing Messages, Next: Updating ERRNO, Prev: Registration Functions, Up: Extension API Description
16.4.6 Printing Messages
------------------------
You can print different kinds of warning messages from your extension,
as described here. Note that for these functions, you must pass in the
extension ID received from 'gawk' when the extension was loaded:(1)
'void fatal(awk_ext_id_t id, const char *format, ...);'
Print a message and then cause 'gawk' to exit immediately.
'void nonfatal(awk_ext_id_t id, const char *format, ...);'
Print a nonfatal error message.
'void warning(awk_ext_id_t id, const char *format, ...);'
Print a warning message.
'void lintwarn(awk_ext_id_t id, const char *format, ...);'
Print a "lint warning." Normally this is the same as printing a
warning message, but if 'gawk' was invoked with '--lint=fatal',
then lint warnings become fatal error messages.
All of these functions are otherwise like the C 'printf()' family of
functions, where the 'format' parameter is a string with literal
characters and formatting codes intermixed.
---------- Footnotes ----------
(1) Because the API uses only ISO C 90 features, it cannot make use
of the ISO C 99 variadic macro feature to hide that parameter. More's
the pity.
File: gawk.info, Node: Updating ERRNO, Next: Requesting Values, Prev: Printing Messages, Up: Extension API Description
16.4.7 Updating 'ERRNO'
-----------------------
The following functions allow you to update the 'ERRNO' variable:
'void update_ERRNO_int(int errno_val);'
Set 'ERRNO' to the string equivalent of the error code in
'errno_val'. The value should be one of the defined error codes in
'<errno.h>', and 'gawk' turns it into a (possibly translated)
string using the C 'strerror()' function.
'void update_ERRNO_string(const char *string);'
Set 'ERRNO' directly to the string value of 'ERRNO'. 'gawk' makes
a copy of the value of 'string'.
'void unset_ERRNO(void);'
Unset 'ERRNO'.
File: gawk.info, Node: Requesting Values, Next: Accessing Parameters, Prev: Updating ERRNO, Up: Extension API Description
16.4.8 Requesting Values
------------------------
All of the functions that return values from 'gawk' work in the same
way. You pass in an 'awk_valtype_t' value to indicate what kind of
value you expect. If the actual value matches what you requested, the
function returns true and fills in the 'awk_value_t' result. Otherwise,
the function returns false, and the 'val_type' member indicates the type
of the actual value. You may then print an error message or reissue the
request for the actual value type, as appropriate. This behavior is
summarized in *note Table 16.1: table-value-types-returned.
Type of Actual Value
--------------------------------------------------------------------------
String Strnum Number Regex Array Undefined
-------------------------------------------------------------------------------
String String String String String false false
Strnum false Strnum Strnum false false false
Number Number Number Number false false false
Type Regex false false false Regex false false
Requested Array false false false false Array false
Scalar Scalar Scalar Scalar Scalar false false
Undefined String Strnum Number Regex Array Undefined
Value false false false false false false
cookie
Table 16.1: API value types returned
File: gawk.info, Node: Accessing Parameters, Next: Symbol Table Access, Prev: Requesting Values, Up: Extension API Description
16.4.9 Accessing and Updating Parameters
----------------------------------------
Two functions give you access to the arguments (parameters) passed to
your extension function. They are:
'awk_bool_t get_argument(size_t count,'
' awk_valtype_t wanted,'
' awk_value_t *result);'
Fill in the 'awk_value_t' structure pointed to by 'result' with the
'count'th argument. Return true if the actual type matches
'wanted', and false otherwise. In the latter case,
'result->val_type' indicates the actual type (*note Table 16.1:
table-value-types-returned.). Counts are zero-based--the first
argument is numbered zero, the second one, and so on. 'wanted'
indicates the type of value expected.
'awk_bool_t set_argument(size_t count, awk_array_t array);'
Convert a parameter that was undefined into an array; this provides
call by reference for arrays. Return false if 'count' is too big,
or if the argument's type is not undefined. *Note Array
Manipulation:: for more information on creating arrays.
File: gawk.info, Node: Symbol Table Access, Next: Array Manipulation, Prev: Accessing Parameters, Up: Extension API Description
16.4.10 Symbol Table Access
---------------------------
Two sets of routines provide access to global variables, and one set
allows you to create and release cached values.
* Menu:
* Symbol table by name:: Accessing variables by name.
* Symbol table by cookie:: Accessing variables by "cookie".
* Cached values:: Creating and using cached values.
File: gawk.info, Node: Symbol table by name, Next: Symbol table by cookie, Up: Symbol Table Access
16.4.10.1 Variable Access and Update by Name
............................................
The following routines provide the ability to access and update global
'awk'-level variables by name. In compiler terminology, identifiers of
different kinds are termed "symbols", thus the "sym" in the routines'
names. The data structure that stores information about symbols is
termed a "symbol table". The functions are as follows:
'awk_bool_t sym_lookup(const char *name,'
' awk_valtype_t wanted,'
' awk_value_t *result);'
Fill in the 'awk_value_t' structure pointed to by 'result' with the
value of the variable named by the string 'name', which is a
regular C string. 'wanted' indicates the type of value expected.
Return true if the actual type matches 'wanted', and false
otherwise. In the latter case, 'result->val_type' indicates the
actual type (*note Table 16.1: table-value-types-returned.).
'awk_bool_t sym_update(const char *name, awk_value_t *value);'
Update the variable named by the string 'name', which is a regular
C string. The variable is added to 'gawk''s symbol table if it is
not there. Return true if everything worked, and false otherwise.
Changing types (scalar to array or vice versa) of an existing
variable is _not_ allowed, nor may this routine be used to update
an array. This routine cannot be used to update any of the
predefined variables (such as 'ARGC' or 'NF').
An extension can look up the value of 'gawk''s special variables.
However, with the exception of the 'PROCINFO' array, an extension cannot
change any of those variables.
File: gawk.info, Node: Symbol table by cookie, Next: Cached values, Prev: Symbol table by name, Up: Symbol Table Access
16.4.10.2 Variable Access and Update by Cookie
..............................................
A "scalar cookie" is an opaque handle that provides access to a global
variable or array. It is an optimization that avoids looking up
variables in 'gawk''s symbol table every time access is needed. This
was discussed earlier, in *note General Data Types::.
The following functions let you work with scalar cookies:
'awk_bool_t sym_lookup_scalar(awk_scalar_t cookie,'
' awk_valtype_t wanted,'
' awk_value_t *result);'
Retrieve the current value of a scalar cookie. Once you have
obtained a scalar cookie using 'sym_lookup()', you can use this
function to get its value more efficiently. Return false if the
value cannot be retrieved.
'awk_bool_t sym_update_scalar(awk_scalar_t cookie, awk_value_t *value);'
Update the value associated with a scalar cookie. Return false if
the new value is not of type 'AWK_STRING', 'AWK_STRNUM',
'AWK_REGEX', or 'AWK_NUMBER'. Here too, the predefined variables
may not be updated.
It is not obvious at first glance how to work with scalar cookies or
what their raison d'e^tre really is. In theory, the 'sym_lookup()' and
'sym_update()' routines are all you really need to work with variables.
For example, you might have code that looks up the value of a variable,
evaluates a condition, and then possibly changes the value of the
variable based on the result of that evaluation, like so:
/* do_magic --- do something really great */
static awk_value_t *
do_magic(int nargs, awk_value_t *result)
{
awk_value_t value;
if ( sym_lookup("MAGIC_VAR", AWK_NUMBER, & value)
&& some_condition(value.num_value)) {
value.num_value += 42;
sym_update("MAGIC_VAR", & value);
}
return make_number(0.0, result);
}
This code looks (and is) simple and straightforward. So what's the
problem?
Well, consider what happens if 'awk'-level code associated with your
extension calls the 'magic()' function (implemented in C by
'do_magic()'), once per record, while processing hundreds of thousands
or millions of records. The 'MAGIC_VAR' variable is looked up in the
symbol table once or twice per function call!
The symbol table lookup is really pure overhead; it is considerably
more efficient to get a cookie that represents the variable, and use
that to get the variable's value and update it as needed.(1)
Thus, the way to use cookies is as follows. First, install your
extension's variable in 'gawk''s symbol table using 'sym_update()', as
usual. Then get a scalar cookie for the variable using 'sym_lookup()':
static awk_scalar_t magic_var_cookie; /* cookie for MAGIC_VAR */
static void
my_extension_init()
{
awk_value_t value;
/* install initial value */
sym_update("MAGIC_VAR", make_number(42.0, & value));
/* get the cookie */
sym_lookup("MAGIC_VAR", AWK_SCALAR, & value);
/* save the cookie */
magic_var_cookie = value.scalar_cookie;
...
}
Next, use the routines in this minor node for retrieving and updating
the value through the cookie. Thus, 'do_magic()' now becomes something
like this:
/* do_magic --- do something really great */
static awk_value_t *
do_magic(int nargs, awk_value_t *result)
{
awk_value_t value;
if ( sym_lookup_scalar(magic_var_cookie, AWK_NUMBER, & value)
&& some_condition(value.num_value)) {
value.num_value += 42;
sym_update_scalar(magic_var_cookie, & value);
}
...
return make_number(0.0, result);
}
NOTE: The previous code omitted error checking for presentation
purposes. Your extension code should be more robust and carefully
check the return values from the API functions.
---------- Footnotes ----------
(1) The difference is measurable and quite real. Trust us.
File: gawk.info, Node: Cached values, Prev: Symbol table by cookie, Up: Symbol Table Access
16.4.10.3 Creating and Using Cached Values
..........................................
The routines in this minor node allow you to create and release cached
values. Like scalar cookies, in theory, cached values are not
necessary. You can create numbers and strings using the functions in
*note Constructor Functions::. You can then assign those values to
variables using 'sym_update()' or 'sym_update_scalar()', as you like.
However, you can understand the point of cached values if you
remember that _every_ string value's storage _must_ come from
'gawk_malloc()', 'gawk_calloc()', or 'gawk_realloc()'. If you have 20
variables, all of which have the same string value, you must create 20
identical copies of the string.(1)
It is clearly more efficient, if possible, to create a value once,
and then tell 'gawk' to reuse the value for multiple variables. That is
what the routines in this minor node let you do. The functions are as
follows:
'awk_bool_t create_value(awk_value_t *value, awk_value_cookie_t *result);'
Create a cached string or numeric value from 'value' for efficient
later assignment. Only values of type 'AWK_NUMBER', 'AWK_REGEX',
'AWK_STRNUM', and 'AWK_STRING' are allowed. Any other type is
rejected. 'AWK_UNDEFINED' could be allowed, but doing so would
result in inferior performance.
'awk_bool_t release_value(awk_value_cookie_t vc);'
Release the memory associated with a value cookie obtained from
'create_value()'.
You use value cookies in a fashion similar to the way you use scalar
cookies. In the extension initialization routine, you create the value
cookie:
static awk_value_cookie_t answer_cookie; /* static value cookie */
static void
my_extension_init()
{
awk_value_t value;
char *long_string;
size_t long_string_len;
/* code from earlier */
...
/* ... fill in long_string and long_string_len ... */
make_malloced_string(long_string, long_string_len, & value);
create_value(& value, & answer_cookie); /* create cookie */
...
}
Once the value is created, you can use it as the value of any number
of variables:
static awk_value_t *
do_magic(int nargs, awk_value_t *result)
{
awk_value_t new_value;
... /* as earlier */
value.val_type = AWK_VALUE_COOKIE;
value.value_cookie = answer_cookie;
sym_update("VAR1", & value);
sym_update("VAR2", & value);
...
sym_update("VAR100", & value);
...
}
Using value cookies in this way saves considerable storage, as all of
'VAR1' through 'VAR100' share the same value.
You might be wondering, "Is this sharing problematic? What happens
if 'awk' code assigns a new value to 'VAR1'; are all the others changed
too?"
That's a great question. The answer is that no, it's not a problem.
Internally, 'gawk' uses "reference-counted strings". This means that
many variables can share the same string value, and 'gawk' keeps track
of the usage. When a variable's value changes, 'gawk' simply decrements
the reference count on the old value and updates the variable to use the
new value.
Finally, as part of your cleanup action (*note Exit Callback
Functions::) you should release any cached values that you created,
using 'release_value()'.
---------- Footnotes ----------
(1) Numeric values are clearly less problematic, requiring only a C
'double' to store. But of course, GMP and MPFR values _do_ take up more
memory.
File: gawk.info, Node: Array Manipulation, Next: Redirection API, Prev: Symbol Table Access, Up: Extension API Description
16.4.11 Array Manipulation
--------------------------
The primary data structure(1) in 'awk' is the associative array (*note
Arrays::). Extensions need to be able to manipulate 'awk' arrays. The
API provides a number of data structures for working with arrays,
functions for working with individual elements, and functions for
working with arrays as a whole. This includes the ability to "flatten"
an array so that it is easy for C code to traverse every element in an
array. The array data structures integrate nicely with the data
structures for values to make it easy to both work with and create true
arrays of arrays (*note General Data Types::).
* Menu:
* Array Data Types:: Data types for working with arrays.
* Array Functions:: Functions for working with arrays.
* Flattening Arrays:: How to flatten arrays.
* Creating Arrays:: How to create and populate arrays.
---------- Footnotes ----------
(1) OK, the only data structure.
File: gawk.info, Node: Array Data Types, Next: Array Functions, Up: Array Manipulation
16.4.11.1 Array Data Types
..........................
The data types associated with arrays are as follows:
'typedef void *awk_array_t;'
If you request the value of an array variable, you get back an
'awk_array_t' value. This value is opaque(1) to the extension; it
uniquely identifies the array but can only be used by passing it
into API functions or receiving it from API functions. This is
very similar to way 'FILE *' values are used with the '<stdio.h>'
library routines.
'typedef struct awk_element {'
' /* convenience linked list pointer, not used by gawk */'
' struct awk_element *next;'
' enum {'
' AWK_ELEMENT_DEFAULT = 0, /* set by gawk */'
' AWK_ELEMENT_DELETE = 1 /* set by extension */'
' } flags;'
' awk_value_t index;'
' awk_value_t value;'
'} awk_element_t;'
The 'awk_element_t' is a "flattened" array element. 'awk' produces
an array of these inside the 'awk_flat_array_t' (see the next
item). Individual elements may be marked for deletion. New
elements must be added individually, one at a time, using the
separate API for that purpose. The fields are as follows:
'struct awk_element *next;'
This pointer is for the convenience of extension writers. It
allows an extension to create a linked list of new elements
that can then be added to an array in a loop that traverses
the list.
'enum { ... } flags;'
A set of flag values that convey information between the
extension and 'gawk'. Currently there is only one:
'AWK_ELEMENT_DELETE'. Setting it causes 'gawk' to delete the
element from the original array upon release of the flattened
array.
'index'
'value'
The index and value of the element, respectively. _All_
memory pointed to by 'index' and 'value' belongs to 'gawk'.
'typedef struct awk_flat_array {'
' awk_const void *awk_const opaque1; /* for use by gawk */'
' awk_const void *awk_const opaque2; /* for use by gawk */'
' awk_const size_t count; /* how many elements */'
' awk_element_t elements[1]; /* will be extended */'
'} awk_flat_array_t;'
This is a flattened array. When an extension gets one of these
from 'gawk', the 'elements' array is of actual size 'count'. The
'opaque1' and 'opaque2' pointers are for use by 'gawk'; therefore
they are marked 'awk_const' so that the extension cannot modify
them.
---------- Footnotes ----------
(1) It is also a "cookie," but the 'gawk' developers did not wish to
overuse this term.
File: gawk.info, Node: Array Functions, Next: Flattening Arrays, Prev: Array Data Types, Up: Array Manipulation
16.4.11.2 Array Functions
.........................
The following functions relate to individual array elements:
'awk_bool_t get_element_count(awk_array_t a_cookie, size_t *count);'
For the array represented by 'a_cookie', place in '*count' the
number of elements it contains. A subarray counts as a single
element. Return false if there is an error.
'awk_bool_t get_array_element(awk_array_t a_cookie,'
' const awk_value_t *const index,'
' awk_valtype_t wanted,'
' awk_value_t *result);'
For the array represented by 'a_cookie', return in '*result' the
value of the element whose index is 'index'. 'wanted' specifies
the type of value you wish to retrieve. Return false if 'wanted'
does not match the actual type or if 'index' is not in the array
(*note Table 16.1: table-value-types-returned.).
The value for 'index' can be numeric, in which case 'gawk' converts
it to a string. Using nonintegral values is possible, but requires
that you understand how such values are converted to strings (*note
Conversion::); thus, using integral values is safest.
As with _all_ strings passed into 'gawk' from an extension, the
string value of 'index' must come from 'gawk_malloc()',
'gawk_calloc()', or 'gawk_realloc()', and 'gawk' releases the
storage.
'awk_bool_t set_array_element(awk_array_t a_cookie,'
' const awk_value_t *const index,'
' const awk_value_t *const value);'
In the array represented by 'a_cookie', create or modify the
element whose index is given by 'index'. The 'ARGV' and 'ENVIRON'
arrays may not be changed, although the 'PROCINFO' array can be.
'awk_bool_t set_array_element_by_elem(awk_array_t a_cookie,'
' awk_element_t element);'
Like 'set_array_element()', but take the 'index' and 'value' from
'element'. This is a convenience macro.
'awk_bool_t del_array_element(awk_array_t a_cookie,'
' const awk_value_t* const index);'
Remove the element with the given index from the array represented
by 'a_cookie'. Return true if the element was removed, or false if
the element did not exist in the array.
The following functions relate to arrays as a whole:
'awk_array_t create_array(void);'
Create a new array to which elements may be added. *Note Creating
Arrays:: for a discussion of how to create a new array and add
elements to it.
'awk_bool_t clear_array(awk_array_t a_cookie);'
Clear the array represented by 'a_cookie'. Return false if there
was some kind of problem, true otherwise. The array remains an
array, but after calling this function, it has no elements. This
is equivalent to using the 'delete' statement (*note Delete::).
'awk_bool_t flatten_array_typed(awk_array_t a_cookie,'
' awk_flat_array_t **data,'
' awk_valtype_t index_type,'
' awk_valtype_t value_type);'
For the array represented by 'a_cookie', create an
'awk_flat_array_t' structure and fill it in with indices and values
of the requested types. Set the pointer whose address is passed as
'data' to point to this structure. Return true upon success, or
false otherwise. *Note Flattening Arrays::, for a discussion of
how to flatten an array and work with it.
'awk_bool_t flatten_array(awk_array_t a_cookie, awk_flat_array_t **data);'
For the array represented by 'a_cookie', create an
'awk_flat_array_t' structure and fill it in with 'AWK_STRING'
indices and 'AWK_UNDEFINED' values. This is superseded by
'flatten_array_typed()'. It is provided as a macro, and remains
for convenience and for source code compatibility with the previous
version of the API.
'awk_bool_t release_flattened_array(awk_array_t a_cookie,'
' awk_flat_array_t *data);'
When done with a flattened array, release the storage using this
function. You must pass in both the original array cookie and the
address of the created 'awk_flat_array_t' structure. The function
returns true upon success, false otherwise.
File: gawk.info, Node: Flattening Arrays, Next: Creating Arrays, Prev: Array Functions, Up: Array Manipulation
16.4.11.3 Working With All The Elements of an Array
...................................................
To "flatten" an array is to create a structure that represents the full
array in a fashion that makes it easy for C code to traverse the entire
array. Some of the code in 'extension/testext.c' does this, and also
serves as a nice example showing how to use the APIs.
We walk through that part of the code one step at a time. First, the
'gawk' script that drives the test extension:
@load "testext"
BEGIN {
n = split("blacky rusty sophie raincloud lucky", pets)
printf("pets has %d elements\n", length(pets))
ret = dump_array_and_delete("pets", "3")
printf("dump_array_and_delete(pets) returned %d\n", ret)
if ("3" in pets)
printf("dump_array_and_delete() did NOT remove index \"3\"!\n")
else
printf("dump_array_and_delete() did remove index \"3\"!\n")
print ""
}
This code creates an array with 'split()' (*note String Functions::) and
then calls 'dump_array_and_delete()'. That function looks up the array
whose name is passed as the first argument, and deletes the element at
the index passed in the second argument. The 'awk' code then prints the
return value and checks if the element was indeed deleted. Here is the
C code that implements 'dump_array_and_delete()'. It has been edited
slightly for presentation.
The first part declares variables, sets up the default return value
in 'result', and checks that the function was called with the correct
number of arguments:
static awk_value_t *
dump_array_and_delete(int nargs, awk_value_t *result)
{
awk_value_t value, value2, value3;
awk_flat_array_t *flat_array;
size_t count;
char *name;
int i;
assert(result != NULL);
make_number(0.0, result);
if (nargs != 2) {
printf("dump_array_and_delete: nargs not right "
"(%d should be 2)\n", nargs);
goto out;
}
The function then proceeds in steps, as follows. First, retrieve the
name of the array, passed as the first argument, followed by the array
itself. If either operation fails, print an error message and return:
/* get argument named array as flat array and print it */
if (get_argument(0, AWK_STRING, & value)) {
name = value.str_value.str;
if (sym_lookup(name, AWK_ARRAY, & value2))
printf("dump_array_and_delete: sym_lookup of %s passed\n",
name);
else {
printf("dump_array_and_delete: sym_lookup of %s failed\n",
name);
goto out;
}
} else {
printf("dump_array_and_delete: get_argument(0) failed\n");
goto out;
}
For testing purposes and to make sure that the C code sees the same
number of elements as the 'awk' code, the second step is to get the
count of elements in the array and print it:
if (! get_element_count(value2.array_cookie, & count)) {
printf("dump_array_and_delete: get_element_count failed\n");
goto out;
}
printf("dump_array_and_delete: incoming size is %lu\n",
(unsigned long) count);
The third step is to actually flatten the array, and then to
double-check that the count in the 'awk_flat_array_t' is the same as the
count just retrieved:
if (! flatten_array_typed(value2.array_cookie, & flat_array,
AWK_STRING, AWK_UNDEFINED)) {
printf("dump_array_and_delete: could not flatten array\n");
goto out;
}
if (flat_array->count != count) {
printf("dump_array_and_delete: flat_array->count (%lu)"
" != count (%lu)\n",
(unsigned long) flat_array->count,
(unsigned long) count);
goto out;
}
The fourth step is to retrieve the index of the element to be
deleted, which was passed as the second argument. Remember that
argument counts passed to 'get_argument()' are zero-based, and thus the
second argument is numbered one:
if (! get_argument(1, AWK_STRING, & value3)) {
printf("dump_array_and_delete: get_argument(1) failed\n");
goto out;
}
The fifth step is where the "real work" is done. The function loops
over every element in the array, printing the index and element values.
In addition, upon finding the element with the index that is supposed to
be deleted, the function sets the 'AWK_ELEMENT_DELETE' bit in the
'flags' field of the element. When the array is released, 'gawk'
traverses the flattened array, and deletes any elements that have this
flag bit set:
for (i = 0; i < flat_array->count; i++) {
printf("\t%s[\"%.*s\"] = %s\n",
name,
(int) flat_array->elements[i].index.str_value.len,
flat_array->elements[i].index.str_value.str,
valrep2str(& flat_array->elements[i].value));
if (strcmp(value3.str_value.str,
flat_array->elements[i].index.str_value.str) == 0) {
flat_array->elements[i].flags |= AWK_ELEMENT_DELETE;
printf("dump_array_and_delete: marking element \"%s\" "
"for deletion\n",
flat_array->elements[i].index.str_value.str);
}
}
The sixth step is to release the flattened array. This tells 'gawk'
that the extension is no longer using the array, and that it should
delete any elements marked for deletion. 'gawk' also frees any storage
that was allocated, so you should not use the pointer ('flat_array' in
this code) once you have called 'release_flattened_array()':
if (! release_flattened_array(value2.array_cookie, flat_array)) {
printf("dump_array_and_delete: could not release flattened array\n");
goto out;
}
Finally, because everything was successful, the function sets the
return value to success, and returns:
make_number(1.0, result);
out:
return result;
}
Here is the output from running this part of the test:
pets has 5 elements
dump_array_and_delete: sym_lookup of pets passed
dump_array_and_delete: incoming size is 5
pets["1"] = "blacky"
pets["2"] = "rusty"
pets["3"] = "sophie"
dump_array_and_delete: marking element "3" for deletion
pets["4"] = "raincloud"
pets["5"] = "lucky"
dump_array_and_delete(pets) returned 1
dump_array_and_delete() did remove index "3"!
File: gawk.info, Node: Creating Arrays, Prev: Flattening Arrays, Up: Array Manipulation
16.4.11.4 How To Create and Populate Arrays
...........................................
Besides working with arrays created by 'awk' code, you can create arrays
and populate them as you see fit, and then 'awk' code can access them
and manipulate them.
There are two important points about creating arrays from extension
code:
* You must install a new array into 'gawk''s symbol table immediately
upon creating it. Once you have done so, you can then populate the
array.
Similarly, if installing a new array as a subarray of an existing
array, you must add the new array to its parent before adding any
elements to it.
Thus, the correct way to build an array is to work "top down."
Create the array, and immediately install it in 'gawk''s symbol
table using 'sym_update()', or install it as an element in a
previously existing array using 'set_array_element()'. We show
example code shortly.
* Due to 'gawk' internals, after using 'sym_update()' to install an
array into 'gawk', you have to retrieve the array cookie from the
value passed in to 'sym_update()' before doing anything else with
it, like so:
awk_value_t val;
awk_array_t new_array;
new_array = create_array();
val.val_type = AWK_ARRAY;
val.array_cookie = new_array;
/* install array in the symbol table */
sym_update("array", & val);
new_array = val.array_cookie; /* YOU MUST DO THIS */
If installing an array as a subarray, you must also retrieve the
value of the array cookie after the call to 'set_element()'.
The following C code is a simple test extension to create an array
with two regular elements and with a subarray. The leading '#include'
directives and boilerplate variable declarations (*note Extension API
Boilerplate::) are omitted for brevity. The first step is to create a
new array and then install it in the symbol table:
/* create_new_array --- create a named array */
static void
create_new_array()
{
awk_array_t a_cookie;
awk_array_t subarray;
awk_value_t index, value;
a_cookie = create_array();
value.val_type = AWK_ARRAY;
value.array_cookie = a_cookie;
if (! sym_update("new_array", & value))
printf("create_new_array: sym_update(\"new_array\") failed!\n");
a_cookie = value.array_cookie;
Note how 'a_cookie' is reset from the 'array_cookie' field in the
'value' structure.
The second step is to install two regular values into 'new_array':
(void) make_const_string("hello", 5, & index);
(void) make_const_string("world", 5, & value);
if (! set_array_element(a_cookie, & index, & value)) {
printf("fill_in_array: set_array_element failed\n");
return;
}
(void) make_const_string("answer", 6, & index);
(void) make_number(42.0, & value);
if (! set_array_element(a_cookie, & index, & value)) {
printf("fill_in_array: set_array_element failed\n");
return;
}
The third step is to create the subarray and install it:
(void) make_const_string("subarray", 8, & index);
subarray = create_array();
value.val_type = AWK_ARRAY;
value.array_cookie = subarray;
if (! set_array_element(a_cookie, & index, & value)) {
printf("fill_in_array: set_array_element failed\n");
return;
}
subarray = value.array_cookie;
The final step is to populate the subarray with its own element:
(void) make_const_string("foo", 3, & index);
(void) make_const_string("bar", 3, & value);
if (! set_array_element(subarray, & index, & value)) {
printf("fill_in_array: set_array_element failed\n");
return;
}
}
Here is a sample script that loads the extension and then dumps the
array:
@load "subarray"
function dumparray(name, array, i)
{
for (i in array)
if (isarray(array[i]))
dumparray(name "[\"" i "\"]", array[i])
else
printf("%s[\"%s\"] = %s\n", name, i, array[i])
}
BEGIN {
dumparray("new_array", new_array);
}
Here is the result of running the script:
$ AWKLIBPATH=$PWD gawk -f subarray.awk
-| new_array["subarray"]["foo"] = bar
-| new_array["hello"] = world
-| new_array["answer"] = 42
(*Note Finding Extensions:: for more information on the 'AWKLIBPATH'
environment variable.)
File: gawk.info, Node: Redirection API, Next: Extension API Variables, Prev: Array Manipulation, Up: Extension API Description
16.4.12 Accessing and Manipulating Redirections
-----------------------------------------------
The following function allows extensions to access and manipulate
redirections.
'awk_bool_t get_file(const char *name,'
' size_t name_len,'
' const char *filetype,'
' int fd,'
' const awk_input_buf_t **ibufp,'
' const awk_output_buf_t **obufp);'
Look up file 'name' in 'gawk''s internal redirection table. If
'name' is 'NULL' or 'name_len' is zero, return data for the
currently open input file corresponding to 'FILENAME'. (This does
not access the 'filetype' argument, so that may be undefined). If
the file is not already open, attempt to open it. The 'filetype'
argument must be zero-terminated and should be one of:
'">"'
A file opened for output.
'">>"'
A file opened for append.
'"<"'
A file opened for input.
'"|>"'
A pipe opened for output.
'"|<"'
A pipe opened for input.
'"|&"'
A two-way coprocess.
On error, return 'awk_false'. Otherwise, return 'awk_true', and
return additional information about the redirection in the 'ibufp'
and 'obufp' pointers.
For input redirections, the '*ibufp' value should be non-'NULL',
and '*obufp' should be 'NULL'. For output redirections, the
'*obufp' value should be non-'NULL', and '*ibufp' should be 'NULL'.
For two-way coprocesses, both values should be non-'NULL'.
In the usual case, the extension is interested in '(*ibufp)->fd'
and/or 'fileno((*obufp)->fp)'. If the file is not already open,
and the 'fd' argument is nonnegative, 'gawk' will use that file
descriptor instead of opening the file in the usual way. If 'fd'
is nonnegative, but the file exists already, 'gawk' ignores 'fd'
and returns the existing file. It is the caller's responsibility
to notice that neither the 'fd' in the returned 'awk_input_buf_t'
nor the 'fd' in the returned 'awk_output_buf_t' matches the
requested value.
Note that supplying a file descriptor is currently _not_ supported
for pipes. However, supplying a file descriptor should work for
input, output, append, and two-way (coprocess) sockets. If
'filetype' is two-way, 'gawk' assumes that it is a socket! Note
that in the two-way case, the input and output file descriptors may
differ. To check for success, you must check whether either
matches.
It is anticipated that this API function will be used to implement
I/O multiplexing and a socket library.
File: gawk.info, Node: Extension API Variables, Next: Extension API Boilerplate, Prev: Redirection API, Up: Extension API Description
16.4.13 API Variables
---------------------
The API provides two sets of variables. The first provides information
about the version of the API (both with which the extension was
compiled, and with which 'gawk' was compiled). The second provides
information about how 'gawk' was invoked.
* Menu:
* Extension Versioning:: API Version information.
* Extension GMP/MPFR Versioning:: Version information about GMP and MPFR.
* Extension API Informational Variables:: Variables providing information about
'gawk''s invocation.
File: gawk.info, Node: Extension Versioning, Next: Extension GMP/MPFR Versioning, Up: Extension API Variables
16.4.13.1 API Version Constants and Variables
.............................................
The API provides both a "major" and a "minor" version number. The API
versions are available at compile time as C preprocessor defines to
support conditional compilation, and as enum constants to facilitate
debugging:
API Version C Preprocessor Define enum constant
--------------------------------------------------------------------
Major 'gawk_api_major_version' 'GAWK_API_MAJOR_VERSION'
Minor 'gawk_api_minor_version' 'GAWK_API_MINOR_VERSION'
Table 16.2: gawk API version constants
The minor version increases when new functions are added to the API.
Such new functions are always added to the end of the API 'struct'.
The major version increases (and the minor version is reset to zero)
if any of the data types change size or member order, or if any of the
existing functions change signature.
It could happen that an extension may be compiled against one version
of the API but loaded by a version of 'gawk' using a different version.
For this reason, the major and minor API versions of the running 'gawk'
are included in the API 'struct' as read-only constant integers:
'api->major_version'
The major version of the running 'gawk'.
'api->minor_version'
The minor version of the running 'gawk'.
It is up to the extension to decide if there are API
incompatibilities. Typically, a check like this is enough:
if ( api->major_version != GAWK_API_MAJOR_VERSION
|| api->minor_version < GAWK_API_MINOR_VERSION) {
fprintf(stderr, "foo_extension: version mismatch with gawk!\n");
fprintf(stderr, "\tmy version (%d, %d), gawk version (%d, %d)\n",
GAWK_API_MAJOR_VERSION, GAWK_API_MINOR_VERSION,
api->major_version, api->minor_version);
exit(1);
}
Such code is included in the boilerplate 'dl_load_func()' macro
provided in 'gawkapi.h' (discussed in *note Extension API
Boilerplate::).
File: gawk.info, Node: Extension GMP/MPFR Versioning, Next: Extension API Informational Variables, Prev: Extension Versioning, Up: Extension API Variables
16.4.13.2 GMP and MPFR Version Information
..........................................
The API also includes information about the versions of GMP and MPFR
with which the running 'gawk' was compiled (if any). They are included
in the API 'struct' as read-only constant integers:
'api->gmp_major_version'
The major version of the GMP library used to compile 'gawk'.
'api->gmp_minor_version'
The minor version of the GMP library used to compile 'gawk'.
'api->mpfr_major_version'
The major version of the MPFR library used to compile 'gawk'.
'api->mpfr_minor_version'
The minor version of the MPFR library used to compile 'gawk'.
These fields are set to zero if 'gawk' was compiled without MPFR
support.
You can check if the versions of MPFR and GMP that you are using
match those of 'gawk' with the following macro:
'check_mpfr_version(extension)'
The 'extension' is the extension id passed to all the other macros
and functions defined in 'gawkapi.h'. If you have not included the
'<mpfr.h>' header file, then this macro will be defined to do
nothing.
If you have included that file, then this macro compares the MPFR
and GMP major and minor versions against those of the library you
are compiling against. If your libraries are newer than 'gawk''s,
it produces a fatal error message.
The 'dl_load_func()' macro (*note Extension API Boilerplate::)
calls 'check_mpfr_version()'.
File: gawk.info, Node: Extension API Informational Variables, Prev: Extension GMP/MPFR Versioning, Up: Extension API Variables
16.4.13.3 Informational Variables
.................................
The API provides access to several variables that describe whether the
corresponding command-line options were enabled when 'gawk' was invoked.
The variables are:
'do_debug'
This variable is true if 'gawk' was invoked with '--debug' option.
'do_lint'
This variable is true if 'gawk' was invoked with '--lint' option.
'do_mpfr'
This variable is true if 'gawk' was invoked with '--bignum' option.
'do_profile'
This variable is true if 'gawk' was invoked with '--profile'
option.
'do_sandbox'
This variable is true if 'gawk' was invoked with '--sandbox'
option.
'do_traditional'
This variable is true if 'gawk' was invoked with '--traditional'
option.
The value of 'do_lint' can change if 'awk' code modifies the 'LINT'
predefined variable (*note Built-in Variables::). The others should not
change during execution.
File: gawk.info, Node: Extension API Boilerplate, Next: Changes from API V1, Prev: Extension API Variables, Up: Extension API Description
16.4.14 Boilerplate Code
------------------------
As mentioned earlier (*note Extension Mechanism Outline::), the function
definitions as presented are really macros. To use these macros, your
extension must provide a small amount of boilerplate code (variables and
functions) toward the top of your source file, using predefined names as
described here. The boilerplate needed is also provided in comments in
the 'gawkapi.h' header file:
/* Boilerplate code: */
int plugin_is_GPL_compatible;
static gawk_api_t *const api;
static awk_ext_id_t ext_id;
static const char *ext_version = NULL; /* or ... = "some string" */
static awk_ext_func_t func_table[] = {
{ "name", do_name, 1, 0, awk_false, NULL },
/* ... */
};
/* EITHER: */
static awk_bool_t (*init_func)(void) = NULL;
/* OR: */
static awk_bool_t
init_my_extension(void)
{
...
}
static awk_bool_t (*init_func)(void) = init_my_extension;
dl_load_func(func_table, some_name, "name_space_in_quotes")
These variables and functions are as follows:
'int plugin_is_GPL_compatible;'
This asserts that the extension is compatible with the GNU GPL
(*note Copying::). If your extension does not have this, 'gawk'
will not load it (*note Plugin License::).
'static gawk_api_t *const api;'
This global 'static' variable should be set to point to the
'gawk_api_t' pointer that 'gawk' passes to your 'dl_load()'
function. This variable is used by all of the macros.
'static awk_ext_id_t ext_id;'
This global static variable should be set to the 'awk_ext_id_t'
value that 'gawk' passes to your 'dl_load()' function. This
variable is used by all of the macros.
'static const char *ext_version = NULL; /* or ... = "some string" */'
This global 'static' variable should be set either to 'NULL', or to
point to a string giving the name and version of your extension.
'static awk_ext_func_t func_table[] = { ... };'
This is an array of one or more 'awk_ext_func_t' structures, as
described earlier (*note Extension Functions::). It can then be
looped over for multiple calls to 'add_ext_func()'.
'static awk_bool_t (*init_func)(void) = NULL;'
' OR'
'static awk_bool_t init_my_extension(void) { ... }'
'static awk_bool_t (*init_func)(void) = init_my_extension;'
If you need to do some initialization work, you should define a
function that does it (creates variables, opens files, etc.) and
then define the 'init_func' pointer to point to your function. The
function should return 'awk_false' upon failure, or 'awk_true' if
everything goes well.
If you don't need to do any initialization, define the pointer and
initialize it to 'NULL'.
'dl_load_func(func_table, some_name, "name_space_in_quotes")'
This macro expands to a 'dl_load()' function that performs all the
necessary initializations.
The point of all the variables and arrays is to let the 'dl_load()'
function (from the 'dl_load_func()' macro) do all the standard work. It
does the following:
1. Check the API versions. If the extension major version does not
match 'gawk''s, or if the extension minor version is greater than
'gawk''s, it prints a fatal error message and exits.
2. Check the MPFR and GMP versions. If there is a mismatch, it prints
a fatal error message and exits.
3. Load the functions defined in 'func_table'. If any of them fails
to load, it prints a warning message but continues on.
4. If the 'init_func' pointer is not 'NULL', call the function it
points to. If it returns 'awk_false', print a warning message.
5. If 'ext_version' is not 'NULL', register the version string with
'gawk'.
File: gawk.info, Node: Changes from API V1, Prev: Extension API Boilerplate, Up: Extension API Description
16.4.15 Changes From Version 1 of the API
-----------------------------------------
The current API is _not_ binary compatible with version 1 of the API.
You will have to recompile your extensions in order to use them with the
current version of 'gawk'.
Fortunately, at the possible expense of some compile-time warnings,
the API remains source-code-compatible with the previous API. The major
differences are the additional members in the 'awk_ext_func_t'
structure, and the addition of the third argument to the C
implementation function (*note Extension Functions::).
Here is a list of individual features that changed from version 1 to
version 2 of the API:
* Numeric values can now have MPFR/MPZ variants (*note General Data
Types::).
* There are new string types: 'AWK_REGEX' and 'AWK_STRNUM' (*note
General Data Types::).
* The 'ezalloc()' macro is new (*note Memory Allocation Functions::).
* The 'awk_ext_func_t' structure changed. Instead of
'num_expected_args', it now has 'max_expected' and 'min_required'
(*note Extension Functions::).
* For 'get_record()', an input parser can now specify field widths
(*note Input Parsers::).
* Extensions can now produce nonfatal error messages (*note Printing
Messages::).
* When flattening an array, you can now specify the index and value
types (*note Array Functions::).
* The 'get_file()' API is new (*note Redirection API::).
File: gawk.info, Node: Finding Extensions, Next: Extension Example, Prev: Extension API Description, Up: Dynamic Extensions
16.5 How 'gawk' Finds Extensions
================================
Compiled extensions have to be installed in a directory where 'gawk' can
find them. If 'gawk' is configured and built in the default fashion,
the directory in which to find extensions is '/usr/local/lib/gawk'. You
can also specify a search path with a list of directories to search for
compiled extensions. *Note AWKLIBPATH Variable:: for more information.
File: gawk.info, Node: Extension Example, Next: Extension Samples, Prev: Finding Extensions, Up: Dynamic Extensions
16.6 Example: Some File Functions
=================================
No matter where you go, there you are.
-- _Buckaroo Banzai_
Two useful functions that are not in 'awk' are 'chdir()' (so that an
'awk' program can change its directory) and 'stat()' (so that an 'awk'
program can gather information about a file). In order to illustrate
the API in action, this minor node implements these functions for 'gawk'
in an extension.
* Menu:
* Internal File Description:: What the new functions will do.
* Internal File Ops:: The code for internal file operations.
* Using Internal File Ops:: How to use an external extension.
File: gawk.info, Node: Internal File Description, Next: Internal File Ops, Up: Extension Example
16.6.1 Using 'chdir()' and 'stat()'
-----------------------------------
This minor node shows how to use the new functions at the 'awk' level
once they've been integrated into the running 'gawk' interpreter. Using
'chdir()' is very straightforward. It takes one argument, the new
directory to change to:
@load "filefuncs"
...
newdir = "/home/arnold/funstuff"
ret = chdir(newdir)
if (ret < 0) {
printf("could not change to %s: %s\n", newdir, ERRNO) > "/dev/stderr"
exit 1
}
...
The return value is negative if the 'chdir()' failed, and 'ERRNO'
(*note Built-in Variables::) is set to a string indicating the error.
Using 'stat()' is a bit more complicated. The C 'stat()' function
fills in a structure that has a fair amount of information. The right
way to model this in 'awk' is to fill in an associative array with the
appropriate information:
file = "/home/arnold/.profile"
ret = stat(file, fdata)
if (ret < 0) {
printf("could not stat %s: %s\n",
file, ERRNO) > "/dev/stderr"
exit 1
}
printf("size of %s is %d bytes\n", file, fdata["size"])
The 'stat()' function always clears the data array, even if the
'stat()' fails. It fills in the following elements:
'"name"'
The name of the file that was 'stat()'ed.
'"dev"'
'"ino"'
The file's device and inode numbers, respectively.
'"mode"'
The file's mode, as a numeric value. This includes both the file's
type and its permissions.
'"nlink"'
The number of hard links (directory entries) the file has.
'"uid"'
'"gid"'
The numeric user and group ID numbers of the file's owner.
'"size"'
The size in bytes of the file.
'"blocks"'
The number of disk blocks the file actually occupies. This may not
be a function of the file's size if the file has holes.
'"atime"'
'"mtime"'
'"ctime"'
The file's last access, modification, and inode update times,
respectively. These are numeric timestamps, suitable for
formatting with 'strftime()' (*note Time Functions::).
'"pmode"'
The file's "printable mode." This is a string representation of
the file's type and permissions, such as is produced by 'ls
-l'--for example, '"drwxr-xr-x"'.
'"type"'
A printable string representation of the file's type. The value is
one of the following:
'"blockdev"'
'"chardev"'
The file is a block or character device ("special file").
'"directory"'
The file is a directory.
'"fifo"'
The file is a named pipe (also known as a FIFO).
'"file"'
The file is just a regular file.
'"socket"'
The file is an 'AF_UNIX' ("Unix domain") socket in the
filesystem.
'"symlink"'
The file is a symbolic link.
'"devbsize"'
The size of a block for the element indexed by '"blocks"'. This
information is derived from either the 'DEV_BSIZE' constant defined
in '<sys/param.h>' on most systems, or the 'S_BLKSIZE' constant in
'<sys/stat.h>' on BSD systems. For some other systems, "a priori"
knowledge is used to provide a value. Where no value can be
determined, it defaults to 512.
Several additional elements may be present, depending upon the
operating system and the type of the file. You can test for them in
your 'awk' program by using the 'in' operator (*note Reference to
Elements::):
'"blksize"'
The preferred block size for I/O to the file. This field is not
present on all POSIX-like systems in the C 'stat' structure.
'"linkval"'
If the file is a symbolic link, this element is the name of the
file the link points to (i.e., the value of the link).
'"rdev"'
'"major"'
'"minor"'
If the file is a block or character device file, then these values
represent the numeric device number and the major and minor
components of that number, respectively.
File: gawk.info, Node: Internal File Ops, Next: Using Internal File Ops, Prev: Internal File Description, Up: Extension Example
16.6.2 C Code for 'chdir()' and 'stat()'
----------------------------------------
Here is the C code for these extensions.(1)
The file includes a number of standard header files, and then
includes the 'gawkapi.h' header file, which provides the API
definitions. Those are followed by the necessary variable declarations
to make use of the API macros and boilerplate code (*note Extension API
Boilerplate::):
#ifdef HAVE_CONFIG_H
#include <config.h>
#endif
#include <stdio.h>
#include <assert.h>
#include <errno.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include "gawkapi.h"
#include "gettext.h"
#define _(msgid) gettext(msgid)
#define N_(msgid) msgid
#include "gawkfts.h"
#include "stack.h"
static const gawk_api_t *api; /* for convenience macros to work */
static awk_ext_id_t ext_id;
static awk_bool_t init_filefuncs(void);
static awk_bool_t (*init_func)(void) = init_filefuncs;
static const char *ext_version = "filefuncs extension: version 1.0";
int plugin_is_GPL_compatible;
By convention, for an 'awk' function 'foo()', the C function that
implements it is called 'do_foo()'. The function should have two
arguments. The first is an 'int', usually called 'nargs', that
represents the number of actual arguments for the function. The second
is a pointer to an 'awk_value_t' structure, usually named 'result':
/* do_chdir --- provide dynamically loaded chdir() function for gawk */
static awk_value_t *
do_chdir(int nargs, awk_value_t *result, struct awk_ext_func *unused)
{
awk_value_t newdir;
int ret = -1;
assert(result != NULL);
The 'newdir' variable represents the new directory to change to,
which is retrieved with 'get_argument()'. Note that the first argument
is numbered zero.
If the argument is retrieved successfully, the function calls the
'chdir()' system call. Otherwise, if the 'chdir()' fails, it updates
'ERRNO':
if (get_argument(0, AWK_STRING, & newdir)) {
ret = chdir(newdir.str_value.str);
if (ret < 0)
update_ERRNO_int(errno);
}
Finally, the function returns the return value to the 'awk' level:
return make_number(ret, result);
}
The 'stat()' extension is more involved. First comes a function that
turns a numeric mode into a printable representation (e.g., octal '0644'
becomes '-rw-r--r--'). This is omitted here for brevity:
/* format_mode --- turn a stat mode field into something readable */
static char *
format_mode(unsigned long fmode)
{
...
}
Next comes a function for reading symbolic links, which is also
omitted here for brevity:
/* read_symlink --- read a symbolic link into an allocated buffer.
... */
static char *
read_symlink(const char *fname, size_t bufsize, ssize_t *linksize)
{
...
}
Two helper functions simplify entering values in the array that will
contain the result of the 'stat()':
/* array_set --- set an array element */
static void
array_set(awk_array_t array, const char *sub, awk_value_t *value)
{
awk_value_t index;
set_array_element(array,
make_const_string(sub, strlen(sub), & index),
value);
}
/* array_set_numeric --- set an array element with a number */
static void
array_set_numeric(awk_array_t array, const char *sub, double num)
{
awk_value_t tmp;
array_set(array, sub, make_number(num, & tmp));
}
The following function does most of the work to fill in the
'awk_array_t' result array with values obtained from a valid 'struct
stat'. This work is done in a separate function to support the 'stat()'
function for 'gawk' and also to support the 'fts()' extension, which is
included in the same file but whose code is not shown here (*note
Extension Sample File Functions::).
The first part of the function is variable declarations, including a
table to map file types to strings:
/* fill_stat_array --- do the work to fill an array with stat info */
static int
fill_stat_array(const char *name, awk_array_t array, struct stat *sbuf)
{
char *pmode; /* printable mode */
const char *type = "unknown";
awk_value_t tmp;
static struct ftype_map {
unsigned int mask;
const char *type;
} ftype_map[] = {
{ S_IFREG, "file" },
{ S_IFBLK, "blockdev" },
{ S_IFCHR, "chardev" },
{ S_IFDIR, "directory" },
#ifdef S_IFSOCK
{ S_IFSOCK, "socket" },
#endif
#ifdef S_IFIFO
{ S_IFIFO, "fifo" },
#endif
#ifdef S_IFLNK
{ S_IFLNK, "symlink" },
#endif
#ifdef S_IFDOOR /* Solaris weirdness */
{ S_IFDOOR, "door" },
#endif
};
int j, k;
The destination array is cleared, and then code fills in various
elements based on values in the 'struct stat':
/* empty out the array */
clear_array(array);
/* fill in the array */
array_set(array, "name", make_const_string(name, strlen(name),
& tmp));
array_set_numeric(array, "dev", sbuf->st_dev);
array_set_numeric(array, "ino", sbuf->st_ino);
array_set_numeric(array, "mode", sbuf->st_mode);
array_set_numeric(array, "nlink", sbuf->st_nlink);
array_set_numeric(array, "uid", sbuf->st_uid);
array_set_numeric(array, "gid", sbuf->st_gid);
array_set_numeric(array, "size", sbuf->st_size);
array_set_numeric(array, "blocks", sbuf->st_blocks);
array_set_numeric(array, "atime", sbuf->st_atime);
array_set_numeric(array, "mtime", sbuf->st_mtime);
array_set_numeric(array, "ctime", sbuf->st_ctime);
/* for block and character devices, add rdev,
major and minor numbers */
if (S_ISBLK(sbuf->st_mode) || S_ISCHR(sbuf->st_mode)) {
array_set_numeric(array, "rdev", sbuf->st_rdev);
array_set_numeric(array, "major", major(sbuf->st_rdev));
array_set_numeric(array, "minor", minor(sbuf->st_rdev));
}
The latter part of the function makes selective additions to the
destination array, depending upon the availability of certain members
and/or the type of the file. It then returns zero, for success:
#ifdef HAVE_STRUCT_STAT_ST_BLKSIZE
array_set_numeric(array, "blksize", sbuf->st_blksize);
#endif
pmode = format_mode(sbuf->st_mode);
array_set(array, "pmode", make_const_string(pmode, strlen(pmode),
& tmp));
/* for symbolic links, add a linkval field */
if (S_ISLNK(sbuf->st_mode)) {
char *buf;
ssize_t linksize;
if ((buf = read_symlink(name, sbuf->st_size,
& linksize)) != NULL)
array_set(array, "linkval",
make_malloced_string(buf, linksize, & tmp));
else
warning(ext_id, _("stat: unable to read symbolic link `%s'"),
name);
}
/* add a type field */
type = "unknown"; /* shouldn't happen */
for (j = 0, k = sizeof(ftype_map)/sizeof(ftype_map[0]); j < k; j++) {
if ((sbuf->st_mode & S_IFMT) == ftype_map[j].mask) {
type = ftype_map[j].type;
break;
}
}
array_set(array, "type", make_const_string(type, strlen(type), & tmp));
return 0;
}
The third argument to 'stat()' was not discussed previously. This
argument is optional. If present, it causes 'do_stat()' to use the
'stat()' system call instead of the 'lstat()' system call. This is done
by using a function pointer: 'statfunc'. 'statfunc' is initialized to
point to 'lstat()' (instead of 'stat()') to get the file information, in
case the file is a symbolic link. However, if the third argument is
included, 'statfunc' is set to point to 'stat()', instead.
Here is the 'do_stat()' function, which starts with variable
declarations and argument checking:
/* do_stat --- provide a stat() function for gawk */
static awk_value_t *
do_stat(int nargs, awk_value_t *result, struct awk_ext_func *unused)
{
awk_value_t file_param, array_param;
char *name;
awk_array_t array;
int ret;
struct stat sbuf;
/* default is lstat() */
int (*statfunc)(const char *path, struct stat *sbuf) = lstat;
assert(result != NULL);
Then comes the actual work. First, the function gets the arguments.
Next, it gets the information for the file. If the called function
('lstat()' or 'stat()') returns an error, the code sets 'ERRNO' and
returns:
/* file is first arg, array to hold results is second */
if ( ! get_argument(0, AWK_STRING, & file_param)
|| ! get_argument(1, AWK_ARRAY, & array_param)) {
warning(ext_id, _("stat: bad parameters"));
return make_number(-1, result);
}
if (nargs == 3) {
statfunc = stat;
}
name = file_param.str_value.str;
array = array_param.array_cookie;
/* always empty out the array */
clear_array(array);
/* stat the file; if error, set ERRNO and return */
ret = statfunc(name, & sbuf);
if (ret < 0) {
update_ERRNO_int(errno);
return make_number(ret, result);
}
The tedious work is done by 'fill_stat_array()', shown earlier. When
done, the function returns the result from 'fill_stat_array()':
ret = fill_stat_array(name, array, & sbuf);
return make_number(ret, result);
}
Finally, it's necessary to provide the "glue" that loads the new
function(s) into 'gawk'.
The 'filefuncs' extension also provides an 'fts()' function, which we
omit here (*note Extension Sample File Functions::). For its sake,
there is an initialization function:
/* init_filefuncs --- initialization routine */
static awk_bool_t
init_filefuncs(void)
{
...
}
We are almost done. We need an array of 'awk_ext_func_t' structures
for loading each function into 'gawk':
static awk_ext_func_t func_table[] = {
{ "chdir", do_chdir, 1, 1, awk_false, NULL },
{ "stat", do_stat, 3, 2, awk_false, NULL },
...
};
Each extension must have a routine named 'dl_load()' to load
everything that needs to be loaded. It is simplest to use the
'dl_load_func()' macro in 'gawkapi.h':
/* define the dl_load() function using the boilerplate macro */
dl_load_func(func_table, filefuncs, "")
And that's it!
---------- Footnotes ----------
(1) This version is edited slightly for presentation. See
'extension/filefuncs.c' in the 'gawk' distribution for the complete
version.
File: gawk.info, Node: Using Internal File Ops, Prev: Internal File Ops, Up: Extension Example
16.6.3 Integrating the Extensions
---------------------------------
Now that the code is written, it must be possible to add it at runtime
to the running 'gawk' interpreter. First, the code must be compiled.
Assuming that the functions are in a file named 'filefuncs.c', and IDIR
is the location of the 'gawkapi.h' header file, the following steps(1)
create a GNU/Linux shared library:
$ gcc -fPIC -shared -DHAVE_CONFIG_H -c -O -g -IIDIR filefuncs.c
$ gcc -o filefuncs.so -shared filefuncs.o
Once the library exists, it is loaded by using the '@load' keyword:
# file testff.awk
@load "filefuncs"
BEGIN {
"pwd" | getline curdir # save current directory
close("pwd")
chdir("/tmp")
system("pwd") # test it
chdir(curdir) # go back
print "Info for testff.awk"
ret = stat("testff.awk", data)
print "ret =", ret
for (i in data)
printf "data[\"%s\"] = %s\n", i, data[i]
print "testff.awk modified:",
strftime("%m %d %Y %H:%M:%S", data["mtime"])
print "\nInfo for JUNK"
ret = stat("JUNK", data)
print "ret =", ret
for (i in data)
printf "data[\"%s\"] = %s\n", i, data[i]
print "JUNK modified:", strftime("%m %d %Y %H:%M:%S", data["mtime"])
}
The 'AWKLIBPATH' environment variable tells 'gawk' where to find
extensions (*note Finding Extensions::). We set it to the current
directory and run the program:
$ AWKLIBPATH=$PWD gawk -f testff.awk
-| /tmp
-| Info for testff.awk
-| ret = 0
-| data["blksize"] = 4096
-| data["devbsize"] = 512
-| data["mtime"] = 1412004710
-| data["mode"] = 33204
-| data["type"] = file
-| data["dev"] = 2053
-| data["gid"] = 1000
-| data["ino"] = 10358899
-| data["ctime"] = 1412004710
-| data["blocks"] = 8
-| data["nlink"] = 1
-| data["name"] = testff.awk
-| data["atime"] = 1412004716
-| data["pmode"] = -rw-rw-r--
-| data["size"] = 666
-| data["uid"] = 1000
-| testff.awk modified: 09 29 2014 18:31:50
-|
-| Info for JUNK
-| ret = -1
-| JUNK modified: 01 01 1970 02:00:00
---------- Footnotes ----------
(1) In practice, you would probably want to use the GNU Autotools
(Automake, Autoconf, Libtool, and 'gettext') to configure and build your
libraries. Instructions for doing so are beyond the scope of this Info
file. *Note gawkextlib:: for Internet links to the tools.
File: gawk.info, Node: Extension Samples, Next: gawkextlib, Prev: Extension Example, Up: Dynamic Extensions
16.7 The Sample Extensions in the 'gawk' Distribution
=====================================================
This minor node provides a brief overview of the sample extensions that
come in the 'gawk' distribution. Some of them are intended for
production use (e.g., the 'filefuncs', 'readdir', and 'inplace'
extensions). Others mainly provide example code that shows how to use
the extension API.
* Menu:
* Extension Sample File Functions:: The file functions sample.
* Extension Sample Fnmatch:: An interface to 'fnmatch()'.
* Extension Sample Fork:: An interface to 'fork()' and other
process functions.
* Extension Sample Inplace:: Enabling in-place file editing.
* Extension Sample Ord:: Character to value to character
conversions.
* Extension Sample Readdir:: An interface to 'readdir()'.
* Extension Sample Revout:: Reversing output sample output wrapper.
* Extension Sample Rev2way:: Reversing data sample two-way processor.
* Extension Sample Read write array:: Serializing an array to a file.
* Extension Sample Readfile:: Reading an entire file into a string.
* Extension Sample Time:: An interface to 'gettimeofday()'
and 'sleep()'.
* Extension Sample API Tests:: Tests for the API.
File: gawk.info, Node: Extension Sample File Functions, Next: Extension Sample Fnmatch, Up: Extension Samples
16.7.1 File-Related Functions
-----------------------------
The 'filefuncs' extension provides three different functions, as
follows. The usage is:
'@load "filefuncs"'
This is how you load the extension.
'result = chdir("/some/directory")'
The 'chdir()' function is a direct hook to the 'chdir()' system
call to change the current directory. It returns zero upon success
or a value less than zero upon error. In the latter case, it
updates 'ERRNO'.
'result = stat("/some/path", statdata' [', follow']')'
The 'stat()' function provides a hook into the 'stat()' system
call. It returns zero upon success or a value less than zero upon
error. In the latter case, it updates 'ERRNO'.
By default, it uses the 'lstat()' system call. However, if passed
a third argument, it uses 'stat()' instead.
In all cases, it clears the 'statdata' array. When the call is
successful, 'stat()' fills the 'statdata' array with information
retrieved from the filesystem, as follows:
Subscript Field in 'struct stat' File type
----------------------------------------------------------------
'"name"' The file name All
'"dev"' 'st_dev' All
'"ino"' 'st_ino' All
'"mode"' 'st_mode' All
'"nlink"' 'st_nlink' All
'"uid"' 'st_uid' All
'"gid"' 'st_gid' All
'"size"' 'st_size' All
'"atime"' 'st_atime' All
'"mtime"' 'st_mtime' All
'"ctime"' 'st_ctime' All
'"rdev"' 'st_rdev' Device files
'"major"' 'st_major' Device files
'"minor"' 'st_minor' Device files
'"blksize"' 'st_blksize' All
'"pmode"' A human-readable version of the All
mode value, like that printed by
'ls' (for example, '"-rwxr-xr-x"')
'"linkval"' The value of the symbolic link Symbolic
links
'"type"' The type of the file as a All
string--one of '"file"',
'"blockdev"', '"chardev"',
'"directory"', '"socket"',
'"fifo"', '"symlink"', '"door"',
or '"unknown"' (not all systems
support all file types)
'flags = or(FTS_PHYSICAL, ...)'
'result = fts(pathlist, flags, filedata)'
Walk the file trees provided in 'pathlist' and fill in the
'filedata' array, as described next. 'flags' is the bitwise OR of
several predefined values, also described in a moment. Return zero
if there were no errors, otherwise return -1.
The 'fts()' function provides a hook to the C library 'fts()'
routines for traversing file hierarchies. Instead of returning data
about one file at a time in a stream, it fills in a multidimensional
array with data about each file and directory encountered in the
requested hierarchies.
The arguments are as follows:
'pathlist'
An array of file names. The element values are used; the index
values are ignored.
'flags'
This should be the bitwise OR of one or more of the following
predefined constant flag values. At least one of 'FTS_LOGICAL' or
'FTS_PHYSICAL' must be provided; otherwise 'fts()' returns an error
value and sets 'ERRNO'. The flags are:
'FTS_LOGICAL'
Do a "logical" file traversal, where the information returned
for a symbolic link refers to the linked-to file, and not to
the symbolic link itself. This flag is mutually exclusive
with 'FTS_PHYSICAL'.
'FTS_PHYSICAL'
Do a "physical" file traversal, where the information returned
for a symbolic link refers to the symbolic link itself. This
flag is mutually exclusive with 'FTS_LOGICAL'.
'FTS_NOCHDIR'
As a performance optimization, the C library 'fts()' routines
change directory as they traverse a file hierarchy. This flag
disables that optimization.
'FTS_COMFOLLOW'
Immediately follow a symbolic link named in 'pathlist',
whether or not 'FTS_LOGICAL' is set.
'FTS_SEEDOT'
By default, the C library 'fts()' routines do not return
entries for '.' (dot) and '..' (dot-dot). This option causes
entries for dot-dot to also be included. (The extension
always includes an entry for dot; more on this in a moment.)
'FTS_XDEV'
During a traversal, do not cross onto a different mounted
filesystem.
'filedata'
The 'filedata' array holds the results. 'fts()' first clears it.
Then it creates an element in 'filedata' for every element in
'pathlist'. The index is the name of the directory or file given
in 'pathlist'. The element for this index is itself an array.
There are two cases:
_The path is a file_
In this case, the array contains two or three elements:
'"path"'
The full path to this file, starting from the "root" that
was given in the 'pathlist' array.
'"stat"'
This element is itself an array, containing the same
information as provided by the 'stat()' function
described earlier for its 'statdata' argument. The
element may not be present if the 'stat()' system call
for the file failed.
'"error"'
If some kind of error was encountered, the array will
also contain an element named '"error"', which is a
string describing the error.
_The path is a directory_
In this case, the array contains one element for each entry in
the directory. If an entry is a file, that element is the
same as for files, just described. If the entry is a
directory, that element is (recursively) an array describing
the subdirectory. If 'FTS_SEEDOT' was provided in the flags,
then there will also be an element named '".."'. This element
will be an array containing the data as provided by 'stat()'.
In addition, there will be an element whose index is '"."'.
This element is an array containing the same two or three
elements as for a file: '"path"', '"stat"', and '"error"'.
The 'fts()' function returns zero if there were no errors.
Otherwise, it returns -1.
NOTE: The 'fts()' extension does not exactly mimic the interface of
the C library 'fts()' routines, choosing instead to provide an
interface that is based on associative arrays, which is more
comfortable to use from an 'awk' program. This includes the lack
of a comparison function, because 'gawk' already provides powerful
array sorting facilities. Although an 'fts_read()'-like interface
could have been provided, this felt less natural than simply
creating a multidimensional array to represent the file hierarchy
and its information.
See 'test/fts.awk' in the 'gawk' distribution for an example use of
the 'fts()' extension function.
File: gawk.info, Node: Extension Sample Fnmatch, Next: Extension Sample Fork, Prev: Extension Sample File Functions, Up: Extension Samples
16.7.2 Interface to 'fnmatch()'
-------------------------------
This extension provides an interface to the C library 'fnmatch()'
function. The usage is:
'@load "fnmatch"'
This is how you load the extension.
'result = fnmatch(pattern, string, flags)'
The return value is zero on success, 'FNM_NOMATCH' if the string
did not match the pattern, or a different nonzero value if an error
occurred.
In addition to the 'fnmatch()' function, the 'fnmatch' extension adds
one constant ('FNM_NOMATCH'), and an array of flag values named 'FNM'.
The arguments to 'fnmatch()' are:
'pattern'
The file name wildcard to match
'string'
The file name string
'flag'
Either zero, or the bitwise OR of one or more of the flags in the
'FNM' array
The flags are as follows:
Array element Corresponding flag defined by 'fnmatch()'
--------------------------------------------------------------------------
'FNM["CASEFOLD"]' 'FNM_CASEFOLD'
'FNM["FILE_NAME"]' 'FNM_FILE_NAME'
'FNM["LEADING_DIR"]''FNM_LEADING_DIR'
'FNM["NOESCAPE"]' 'FNM_NOESCAPE'
'FNM["PATHNAME"]' 'FNM_PATHNAME'
'FNM["PERIOD"]' 'FNM_PERIOD'
Here is an example:
@load "fnmatch"
...
flags = or(FNM["PERIOD"], FNM["NOESCAPE"])
if (fnmatch("*.a", "foo.c", flags) == FNM_NOMATCH)
print "no match"
File: gawk.info, Node: Extension Sample Fork, Next: Extension Sample Inplace, Prev: Extension Sample Fnmatch, Up: Extension Samples
16.7.3 Interface to 'fork()', 'wait()', and 'waitpid()'
-------------------------------------------------------
The 'fork' extension adds three functions, as follows:
'@load "fork"'
This is how you load the extension.
'pid = fork()'
This function creates a new process. The return value is zero in
the child and the process ID number of the child in the parent, or
-1 upon error. In the latter case, 'ERRNO' indicates the problem.
In the child, 'PROCINFO["pid"]' and 'PROCINFO["ppid"]' are updated
to reflect the correct values.
'ret = waitpid(pid)'
This function takes a numeric argument, which is the process ID to
wait for. The return value is that of the 'waitpid()' system call.
'ret = wait()'
This function waits for the first child to die. The return value
is that of the 'wait()' system call.
There is no corresponding 'exec()' function.
Here is an example:
@load "fork"
...
if ((pid = fork()) == 0)
print "hello from the child"
else
print "hello from the parent"
File: gawk.info, Node: Extension Sample Inplace, Next: Extension Sample Ord, Prev: Extension Sample Fork, Up: Extension Samples
16.7.4 Enabling In-Place File Editing
-------------------------------------
The 'inplace' extension emulates GNU 'sed''s '-i' option, which performs
"in-place" editing of each input file. It uses the bundled
'inplace.awk' include file to invoke the extension properly:
# inplace --- load and invoke the inplace extension.
@load "inplace"
# Please set INPLACE_SUFFIX to make a backup copy. For example, you may
# want to set INPLACE_SUFFIX to .bak on the command line or in a BEGIN rule.
# By default, each filename on the command line will be edited inplace.
# But you can selectively disable this by adding an inplace=0 argument
# prior to files that you do not want to process this way. You can then
# reenable it later on the commandline by putting inplace=1 before files
# that you wish to be subject to inplace editing.
# N.B. We call inplace_end() in the BEGINFILE and END rules so that any
# actions in an ENDFILE rule will be redirected as expected.
BEGIN {
inplace = 1 # enabled by default
}
BEGINFILE {
if (_inplace_filename != "")
inplace_end(_inplace_filename, INPLACE_SUFFIX)
if (inplace)
inplace_begin(_inplace_filename = FILENAME, INPLACE_SUFFIX)
else
_inplace_filename = ""
}
END {
if (_inplace_filename != "")
inplace_end(_inplace_filename, INPLACE_SUFFIX)
}
For each regular file that is processed, the extension redirects
standard output to a temporary file configured to have the same owner
and permissions as the original. After the file has been processed, the
extension restores standard output to its original destination. If
'INPLACE_SUFFIX' is not an empty string, the original file is linked to
a backup file name created by appending that suffix. Finally, the
temporary file is renamed to the original file name.
Note that the use of this feature can be controlled by placing
'inplace=0' on the command-line prior to listing files that should not
be processed this way. You can reenable inplace editing by adding an
'inplace=1' argument prior to files that should be subject to inplace
editing.
The '_inplace_filename' variable serves to keep track of the current
filename so as to not invoke 'inplace_end()' before processing the first
file.
If any error occurs, the extension issues a fatal error to terminate
processing immediately without damaging the original file.
Here are some simple examples:
$ gawk -i inplace '{ gsub(/foo/, "bar") }; { print }' file1 file2 file3
To keep a backup copy of the original files, try this:
$ gawk -i inplace -v INPLACE_SUFFIX=.bak '{ gsub(/foo/, "bar") }
> { print }' file1 file2 file3
Please note that, while the extension does attempt to preserve
ownership and permissions, it makes no attempt to copy the ACLs from the
original file.
If the program dies prematurely, as might happen if an unhandled
signal is received, a temporary file may be left behind.
File: gawk.info, Node: Extension Sample Ord, Next: Extension Sample Readdir, Prev: Extension Sample Inplace, Up: Extension Samples
16.7.5 Character and Numeric values: 'ord()' and 'chr()'
--------------------------------------------------------
The 'ordchr' extension adds two functions, named 'ord()' and 'chr()', as
follows:
'@load "ordchr"'
This is how you load the extension.
'number = ord(string)'
Return the numeric value of the first character in 'string'.
'char = chr(number)'
Return a string whose first character is that represented by
'number'.
These functions are inspired by the Pascal language functions of the
same name. Here is an example:
@load "ordchr"
...
printf("The numeric value of 'A' is %d\n", ord("A"))
printf("The string value of 65 is %s\n", chr(65))
File: gawk.info, Node: Extension Sample Readdir, Next: Extension Sample Revout, Prev: Extension Sample Ord, Up: Extension Samples
16.7.6 Reading Directories
--------------------------
The 'readdir' extension adds an input parser for directories. The usage
is as follows:
@load "readdir"
When this extension is in use, instead of skipping directories named
on the command line (or with 'getline'), they are read, with each entry
returned as a record.
The record consists of three fields. The first two are the inode
number and the file name, separated by a forward slash character. On
systems where the directory entry contains the file type, the record has
a third field (also separated by a slash), which is a single letter
indicating the type of the file. The letters and their corresponding
file types are shown in *note Table 16.3: table-readdir-file-types.
Letter File type
--------------------------------------------------------------------------
'b' Block device
'c' Character device
'd' Directory
'f' Regular file
'l' Symbolic link
'p' Named pipe (FIFO)
's' Socket
'u' Anything else (unknown)
Table 16.3: File types returned by the 'readdir' extension
On systems without the file type information, the third field is
always 'u'.
NOTE: On GNU/Linux systems, there are filesystems that don't
support the 'd_type' entry (see the readdir(3) manual page), and so
the file type is always 'u'. You can use the 'filefuncs' extension
to call 'stat()' in order to get correct type information.
Here is an example:
@load "readdir"
...
BEGIN { FS = "/" }
{ print "file name is", $2 }
File: gawk.info, Node: Extension Sample Revout, Next: Extension Sample Rev2way, Prev: Extension Sample Readdir, Up: Extension Samples
16.7.7 Reversing Output
-----------------------
The 'revoutput' extension adds a simple output wrapper that reverses the
characters in each output line. Its main purpose is to show how to
write an output wrapper, although it may be mildly amusing for the
unwary. Here is an example:
@load "revoutput"
BEGIN {
REVOUT = 1
print "don't panic" > "/dev/stdout"
}
The output from this program is 'cinap t'nod'.
File: gawk.info, Node: Extension Sample Rev2way, Next: Extension Sample Read write array, Prev: Extension Sample Revout, Up: Extension Samples
16.7.8 Two-Way I/O Example
--------------------------
The 'revtwoway' extension adds a simple two-way processor that reverses
the characters in each line sent to it for reading back by the 'awk'
program. Its main purpose is to show how to write a two-way processor,
although it may also be mildly amusing. The following example shows how
to use it:
@load "revtwoway"
BEGIN {
cmd = "/magic/mirror"
print "don't panic" |& cmd
cmd |& getline result
print result
close(cmd)
}
The output from this program is: 'cinap t'nod'.
File: gawk.info, Node: Extension Sample Read write array, Next: Extension Sample Readfile, Prev: Extension Sample Rev2way, Up: Extension Samples
16.7.9 Dumping and Restoring an Array
-------------------------------------
The 'rwarray' extension adds two functions, named 'writea()' and
'reada()', as follows:
'@load "rwarray"'
This is how you load the extension.
'ret = writea(file, array)'
This function takes a string argument, which is the name of the
file to which to dump the array, and the array itself as the second
argument. 'writea()' understands arrays of arrays. It returns one
on success, or zero upon failure.
'ret = reada(file, array)'
'reada()' is the inverse of 'writea()'; it reads the file named as
its first argument, filling in the array named as the second
argument. It clears the array first. Here too, the return value
is one on success, or zero upon failure.
The array created by 'reada()' is identical to that written by
'writea()' in the sense that the contents are the same. However, due to
implementation issues, the array traversal order of the re-created array
is likely to be different from that of the original array. As array
traversal order in 'awk' is by default undefined, this is (technically)
not a problem. If you need to guarantee a particular traversal order,
use the array sorting features in 'gawk' to do so (*note Array
Sorting::).
The file contains binary data. All integral values are written in
network byte order. However, double-precision floating-point values are
written as native binary data. Thus, arrays containing only string data
can theoretically be dumped on systems with one byte order and restored
on systems with a different one, but this has not been tried.
Here is an example:
@load "rwarray"
...
ret = writea("arraydump.bin", array)
...
ret = reada("arraydump.bin", array)
File: gawk.info, Node: Extension Sample Readfile, Next: Extension Sample Time, Prev: Extension Sample Read write array, Up: Extension Samples
16.7.10 Reading an Entire File
------------------------------
The 'readfile' extension adds a single function named 'readfile()', and
an input parser:
'@load "readfile"'
This is how you load the extension.
'result = readfile("/some/path")'
The argument is the name of the file to read. The return value is
a string containing the entire contents of the requested file.
Upon error, the function returns the empty string and sets 'ERRNO'.
'BEGIN { PROCINFO["readfile"] = 1 }'
In addition, the extension adds an input parser that is activated
if 'PROCINFO["readfile"]' exists. When activated, each input file
is returned in its entirety as '$0'. 'RT' is set to the null
string.
Here is an example:
@load "readfile"
...
contents = readfile("/path/to/file");
if (contents == "" && ERRNO != "") {
print("problem reading file", ERRNO) > "/dev/stderr"
...
}
File: gawk.info, Node: Extension Sample Time, Next: Extension Sample API Tests, Prev: Extension Sample Readfile, Up: Extension Samples
16.7.11 Extension Time Functions
--------------------------------
The 'time' extension adds two functions, named 'gettimeofday()' and
'sleep()', as follows:
'@load "time"'
This is how you load the extension.
'the_time = gettimeofday()'
Return the time in seconds that has elapsed since 1970-01-01 UTC as
a floating-point value. If the time is unavailable on this
platform, return -1 and set 'ERRNO'. The returned time should have
sub-second precision, but the actual precision may vary based on
the platform. If the standard C 'gettimeofday()' system call is
available on this platform, then it simply returns the value.
Otherwise, if on MS-Windows, it tries to use
'GetSystemTimeAsFileTime()'.
'result = sleep(SECONDS)'
Attempt to sleep for SECONDS seconds. If SECONDS is negative, or
the attempt to sleep fails, return -1 and set 'ERRNO'. Otherwise,
return zero after sleeping for the indicated amount of time. Note
that SECONDS may be a floating-point (nonintegral) value.
Implementation details: depending on platform availability, this
function tries to use 'nanosleep()' or 'select()' to implement the
delay.
File: gawk.info, Node: Extension Sample API Tests, Prev: Extension Sample Time, Up: Extension Samples
16.7.12 API Tests
-----------------
The 'testext' extension exercises parts of the extension API that are
not tested by the other samples. The 'extension/testext.c' file
contains both the C code for the extension and 'awk' test code inside C
comments that run the tests. The testing framework extracts the 'awk'
code and runs the tests. See the source file for more information.
File: gawk.info, Node: gawkextlib, Next: Extension summary, Prev: Extension Samples, Up: Dynamic Extensions
16.8 The 'gawkextlib' Project
=============================
The 'gawkextlib' (https://sourceforge.net/projects/gawkextlib/) project
provides a number of 'gawk' extensions, including one for processing XML
files. This is the evolution of the original 'xgawk' (XML 'gawk')
project.
There are a number of extensions. Some of the more interesting ones
are:
* 'abort' extension. It allows you to exit immediately from your
'awk' program without running the 'END' rules.
* 'json' extension. This serializes a multidimensional array into a
JSON string, and can deserialize a JSON string into a 'gawk' array.
This extension is interesting since it is written in C++ instead of
C.
* MPFR library extension. This provides access to a number of MPFR
functions that 'gawk''s native MPFR support does not.
* Select extension. It provides functionality based on the
'select()' system call.
* XML parser extension, using the Expat
(https://expat.sourceforge.net) XML parsing library
You can check out the code for the 'gawkextlib' project using the Git
(https://git-scm.com) distributed source code control system. The
command is as follows:
git clone git://git.code.sf.net/p/gawkextlib/code gawkextlib-code
You will need to have the RapidJson (http://www.rapidjson.org) JSON
parser library installed in order to build and use the 'json' extension.
You will need to have the Expat (https://expat.sourceforge.net) XML
parser library installed in order to build and use the XML extension.
In addition, you must have the GNU Autotools installed (Autoconf
(https://www.gnu.org/software/autoconf), Automake
(https://www.gnu.org/software/automake), Libtool
(https://www.gnu.org/software/libtool), and GNU 'gettext'
(https://www.gnu.org/software/gettext)).
The simple recipe for building and testing 'gawkextlib' is as
follows. First, build and install 'gawk':
cd .../path/to/gawk/code
./configure --prefix=/tmp/newgawk Install in /tmp/newgawk for now
make && make check Build and check that all is OK
make install Install gawk
Next, go to <https://sourceforge.net/projects/gawkextlib/files>; to
download 'gawkextlib' and any extensions that you would like to build.
The 'README' file at that site explains how to build the code. If you
installed 'gawk' in a non-standard location, you will need to specify
'./configure --with-gawk=/PATH/TO/GAWK' to find it. You may need to use
the 'sudo' utility to install both 'gawk' and 'gawkextlib', depending
upon how your system works.
If you write an extension that you wish to share with other 'gawk'
users, consider doing so through the 'gawkextlib' project. See the
project's website for more information.
File: gawk.info, Node: Extension summary, Next: Extension Exercises, Prev: gawkextlib, Up: Dynamic Extensions
16.9 Summary
============
* You can write extensions (sometimes called plug-ins) for 'gawk' in
C or C++ using the application programming interface (API) defined
by the 'gawk' developers.
* Extensions must have a license compatible with the GNU General
Public License (GPL), and they must assert that fact by declaring a
variable named 'plugin_is_GPL_compatible'.
* Communication between 'gawk' and an extension is two-way. 'gawk'
passes a 'struct' to the extension that contains various data
fields and function pointers. The extension can then call into
'gawk' via the supplied function pointers to accomplish certain
tasks.
* One of these tasks is to "register" the name and implementation of
new 'awk'-level functions with 'gawk'. The implementation takes
the form of a C function pointer with a defined signature. By
convention, implementation functions are named 'do_XXXX()' for some
'awk'-level function 'XXXX()'.
* The API is defined in a header file named 'gawkapi.h'. You must
include a number of standard header files _before_ including it in
your source file.
* API function pointers are provided for the following kinds of
operations:
* Allocating, reallocating, and releasing memory
* Registration functions (you may register extension functions,
exit callbacks, a version string, input parsers, output
wrappers, and two-way processors)
* Printing fatal, nonfatal, warning, and "lint" warning messages
* Updating 'ERRNO', or unsetting it
* Accessing parameters, including converting an undefined
parameter into an array
* Symbol table access (retrieving a global variable, creating
one, or changing one)
* Creating and releasing cached values; this provides an
efficient way to use values for multiple variables and can be
a big performance win
* Manipulating arrays (retrieving, adding, deleting, and
modifying elements; getting the count of elements in an array;
creating a new array; clearing an array; and flattening an
array for easy C-style looping over all its indices and
elements)
* The API defines a number of standard data types for representing
'awk' values, array elements, and arrays.
* The API provides convenience functions for constructing values. It
also provides memory management functions to ensure compatibility
between memory allocated by 'gawk' and memory allocated by an
extension.
* _All_ memory passed from 'gawk' to an extension must be treated as
read-only by the extension.
* _All_ memory passed from an extension to 'gawk' must come from the
API's memory allocation functions. 'gawk' takes responsibility for
the memory and releases it when appropriate.
* The API provides information about the running version of 'gawk' so
that an extension can make sure it is compatible with the 'gawk'
that loaded it.
* It is easiest to start a new extension by copying the boilerplate
code described in this major node. Macros in the 'gawkapi.h'
header file make this easier to do.
* The 'gawk' distribution includes a number of small but useful
sample extensions. The 'gawkextlib' project includes several more
(larger) extensions. If you wish to write an extension and
contribute it to the community of 'gawk' users, the 'gawkextlib'
project is the place to do so.
File: gawk.info, Node: Extension Exercises, Prev: Extension summary, Up: Dynamic Extensions
16.10 Exercises
===============
1. Add functions to implement system calls such as 'chown()',
'chmod()', and 'umask()' to the file operations extension presented
in *note Internal File Ops::.
2. Write an input parser that prints a prompt if the input is a from a
"terminal" device. You can use the 'isatty()' function to tell if
the input file is a terminal. (Hint: this function is usually
expensive to call; try to call it just once.) The content of the
prompt should come from a variable settable by 'awk'-level code.
You can write the prompt to standard error. However, for best
results, open a new file descriptor (or file pointer) on '/dev/tty'
and print the prompt there, in case standard error has been
redirected.
Why is standard error a better choice than standard output for
writing the prompt? Which reading mechanism should you replace,
the one to get a record, or the one to read raw bytes?
3. (Hard.) How would you provide namespaces in 'gawk', so that the
names of functions in different extensions don't conflict with each
other? If you come up with a really good scheme, contact the
'gawk' maintainer to tell him about it.
4. Write a wrapper script that provides an interface similar to 'sed
-i' for the "inplace" extension presented in *note Extension Sample
Inplace::.
File: gawk.info, Node: Language History, Next: Installation, Prev: Dynamic Extensions, Up: Top
Appendix A The Evolution of the 'awk' Language
**********************************************
This Info file describes the GNU implementation of 'awk', which follows
the POSIX specification. Many longtime 'awk' users learned 'awk'
programming with the original 'awk' implementation in Version 7 Unix.
(This implementation was the basis for 'awk' in Berkeley Unix, through
4.3-Reno. Subsequent versions of Berkeley Unix, and, for a while, some
systems derived from 4.4BSD-Lite, used various versions of 'gawk' for
their 'awk'.) This major node briefly describes the evolution of the
'awk' language, with cross-references to other parts of the Info file
where you can find more information.
* Menu:
* V7/SVR3.1:: The major changes between V7 and System V
Release 3.1.
* SVR4:: Minor changes between System V Releases 3.1
and 4.
* POSIX:: New features from the POSIX standard.
* BTL:: New features from Brian Kernighan's version of
'awk'.
* POSIX/GNU:: The extensions in 'gawk' not in POSIX
'awk'.
* Feature History:: The history of the features in 'gawk'.
* Common Extensions:: Common Extensions Summary.
* Ranges and Locales:: How locales used to affect regexp ranges.
* Contributors:: The major contributors to 'gawk'.
* History summary:: History summary.
File: gawk.info, Node: V7/SVR3.1, Next: SVR4, Up: Language History
A.1 Major Changes Between V7 and SVR3.1
=======================================
The 'awk' language evolved considerably between the release of Version 7
Unix (1978) and the new version that was first made generally available
in System V Release 3.1 (1987). This minor node summarizes the changes,
with cross-references to further details:
* The requirement for ';' to separate rules on a line (*note
Statements/Lines::)
* User-defined functions and the 'return' statement (*note
User-defined::)
* The 'delete' statement (*note Delete::)
* The 'do'-'while' statement (*note Do Statement::)
* The built-in functions 'atan2()', 'cos()', 'sin()', 'rand()', and
'srand()' (*note Numeric Functions::)
* The built-in functions 'gsub()', 'sub()', and 'match()' (*note
String Functions::)
* The built-in functions 'close()' and 'system()' (*note I/O
Functions::)
* The 'ARGC', 'ARGV', 'FNR', 'RLENGTH', 'RSTART', and 'SUBSEP'
predefined variables (*note Built-in Variables::)
* Assignable '$0' (*note Changing Fields::)
* The conditional expression using the ternary operator '?:' (*note
Conditional Exp::)
* The expression 'INDX in ARRAY' outside of 'for' statements (*note
Reference to Elements::)
* The exponentiation operator '^' (*note Arithmetic Ops::) and its
assignment operator form '^=' (*note Assignment Ops::)
* C-compatible operator precedence, which breaks some old 'awk'
programs (*note Precedence::)
* Regexps as the value of 'FS' (*note Field Separators::) and as the
third argument to the 'split()' function (*note String
Functions::), rather than using only the first character of 'FS'
* Dynamic regexps as operands of the '~' and '!~' operators (*note
Computed Regexps::)
* The escape sequences '\b', '\f', and '\r' (*note Escape
Sequences::)
* Redirection of input for the 'getline' function (*note Getline::)
* Multiple 'BEGIN' and 'END' rules (*note BEGIN/END::)
* Multidimensional arrays (*note Multidimensional::)
File: gawk.info, Node: SVR4, Next: POSIX, Prev: V7/SVR3.1, Up: Language History
A.2 Changes Between SVR3.1 and SVR4
===================================
The System V Release 4 (1989) version of Unix 'awk' added these features
(some of which originated in 'gawk'):
* The 'ENVIRON' array (*note Built-in Variables::)
* Multiple '-f' options on the command line (*note Options::)
* The '-v' option for assigning variables before program execution
begins (*note Options::)
* The '--' signal for terminating command-line options
* The '\a', '\v', and '\x' escape sequences (*note Escape
Sequences::)
* A defined return value for the 'srand()' built-in function (*note
Numeric Functions::)
* The 'toupper()' and 'tolower()' built-in string functions for case
translation (*note String Functions::)
* A cleaner specification for the '%c' format-control letter in the
'printf' function (*note Control Letters::)
* The ability to dynamically pass the field width and precision
('"%*.*d"') in the argument list of 'printf' and 'sprintf()' (*note
Control Letters::)
* The use of regexp constants, such as '/foo/', as expressions, where
they are equivalent to using the matching operator, as in '$0 ~
/foo/' (*note Using Constant Regexps::)
* Processing of escape sequences inside command-line variable
assignments (*note Assignment Options::)
File: gawk.info, Node: POSIX, Next: BTL, Prev: SVR4, Up: Language History
A.3 Changes Between SVR4 and POSIX 'awk'
========================================
The POSIX Command Language and Utilities standard for 'awk' (1992)
introduced the following changes into the language:
* The use of '-W' for implementation-specific options (*note
Options::)
* The use of 'CONVFMT' for controlling the conversion of numbers to
strings (*note Conversion::)
* The concept of a numeric string and tighter comparison rules to go
with it (*note Typing and Comparison::)
* The use of predefined variables as function parameter names is
forbidden (*note Definition Syntax::)
* More complete documentation of many of the previously undocumented
features of the language
In 2012, a number of extensions that had been commonly available for
many years were finally added to POSIX. They are:
* The 'fflush()' built-in function for flushing buffered output
(*note I/O Functions::)
* The 'nextfile' statement (*note Nextfile Statement::)
* The ability to delete all of an array at once with 'delete ARRAY'
(*note Delete::)
*Note Common Extensions:: for a list of common extensions not
permitted by the POSIX standard.
The 2008 POSIX standard can be found online at
<http://pubs.opengroup.org/onlinepubs/9699919799/>;.
File: gawk.info, Node: BTL, Next: POSIX/GNU, Prev: POSIX, Up: Language History
A.4 Extensions in Brian Kernighan's 'awk'
=========================================
Brian Kernighan has made his version available via his home page (*note
Other Versions::).
This minor node describes common extensions that originally appeared
in his version of 'awk':
* The '**' and '**=' operators (*note Arithmetic Ops:: and *note
Assignment Ops::)
* The use of 'func' as an abbreviation for 'function' (*note
Definition Syntax::)
* The 'fflush()' built-in function for flushing buffered output
(*note I/O Functions::)
*Note Common Extensions:: for a full list of the extensions available
in his 'awk'.
File: gawk.info, Node: POSIX/GNU, Next: Feature History, Prev: BTL, Up: Language History
A.5 Extensions in 'gawk' Not in POSIX 'awk'
===========================================
The GNU implementation, 'gawk', adds a large number of features. They
can all be disabled with either the '--traditional' or '--posix' options
(*note Options::).
A number of features have come and gone over the years. This minor
node summarizes the additional features over POSIX 'awk' that are in the
current version of 'gawk'.
* Additional predefined variables:
- The 'ARGIND', 'BINMODE', 'ERRNO', 'FIELDWIDTHS', 'FPAT',
'IGNORECASE', 'LINT', 'PROCINFO', 'RT', and 'TEXTDOMAIN'
variables (*note Built-in Variables::)
* Special files in I/O redirections:
- The '/dev/stdin', '/dev/stdout', '/dev/stderr', and
'/dev/fd/N' special file names (*note Special Files::)
- The '/inet', '/inet4', and '/inet6' special files for TCP/IP
networking using '|&' to specify which version of the IP
protocol to use (*note TCP/IP Networking::)
* Changes and/or additions to the language:
- The '\x' escape sequence (*note Escape Sequences::)
- Full support for both POSIX and GNU regexps (*note Regexp::)
- The ability for 'FS' and for the third argument to 'split()'
to be null strings (*note Single Character Fields::)
- The ability for 'RS' to be a regexp (*note Records::)
- The ability to use octal and hexadecimal constants in 'awk'
program source code (*note Nondecimal-numbers::)
- The '|&' operator for two-way I/O to a coprocess (*note
Two-way I/O::)
- Indirect function calls (*note Indirect Calls::)
- Directories on the command line produce a warning and are
skipped (*note Command-line directories::)
- Output with 'print' and 'printf' need not be fatal (*note
Nonfatal::)
* New keywords:
- The 'BEGINFILE' and 'ENDFILE' special patterns (*note
BEGINFILE/ENDFILE::)
- The 'switch' statement (*note Switch Statement::)
* Changes to standard 'awk' functions:
- The optional second argument to 'close()' that allows closing
one end of a two-way pipe to a coprocess (*note Two-way I/O::)
- POSIX compliance for 'gsub()' and 'sub()' with '--posix'
- The 'length()' function accepts an array argument and returns
the number of elements in the array (*note String Functions::)
- The optional third argument to the 'match()' function for
capturing text-matching subexpressions within a regexp (*note
String Functions::)
- Positional specifiers in 'printf' formats for making
translations easier (*note Printf Ordering::)
- The 'split()' function's additional optional fourth argument,
which is an array to hold the text of the field separators
(*note String Functions::)
* Additional functions only in 'gawk':
- The 'gensub()', 'patsplit()', and 'strtonum()' functions for
more powerful text manipulation (*note String Functions::)
- The 'asort()' and 'asorti()' functions for sorting arrays
(*note Array Sorting::)
- The 'mktime()', 'systime()', and 'strftime()' functions for
working with timestamps (*note Time Functions::)
- The 'and()', 'compl()', 'lshift()', 'or()', 'rshift()', and
'xor()' functions for bit manipulation (*note Bitwise
Functions::)
- The 'isarray()' function to check if a variable is an array or
not (*note Type Functions::)
- The 'bindtextdomain()', 'dcgettext()', and 'dcngettext()'
functions for internationalization (*note Programmer i18n::)
* Changes and/or additions in the command-line options:
- The 'AWKPATH' environment variable for specifying a path
search for the '-f' command-line option (*note Options::)
- The 'AWKLIBPATH' environment variable for specifying a path
search for the '-l' command-line option (*note Options::)
- The '-b', '-c', '-C', '-d', '-D', '-e', '-E', '-g', '-h',
'-i', '-l', '-L', '-M', '-n', '-N', '-o', '-O', '-p', '-P',
'-r', '-s', '-S', '-t', and '-V' short options. Also, the
ability to use GNU-style long-named options that start with
'--', and the '--assign', '--bignum', '--characters-as-bytes',
'--copyright', '--debug', '--dump-variables', '--exec',
'--field-separator', '--file', '--gen-pot', '--help',
'--include', '--lint', '--lint-old', '--load',
'--non-decimal-data', '--optimize', '--no-optimize',
'--posix', '--pretty-print', '--profile', '--re-interval',
'--sandbox', '--source', '--traditional', '--use-lc-numeric',
and '--version' long options (*note Options::).
* Support for the following obsolete systems was removed from the
code and the documentation for 'gawk' version 4.0:
- Amiga
- Atari
- BeOS
- Cray
- MIPS RiscOS
- MS-DOS with the Microsoft Compiler
- MS-Windows with the Microsoft Compiler
- NeXT
- SunOS 3.x, Sun 386 (Road Runner)
- Tandem (non-POSIX)
- Prestandard VAX C compiler for VAX/VMS
- GCC for VAX and Alpha has not been tested for a while.
* Support for the following obsolete system was removed from the code
for 'gawk' version 4.1:
- Ultrix
* Support for the following systems was removed from the code for
'gawk' version 4.2:
- MirBSD
- GNU/Linux on Alpha
File: gawk.info, Node: Feature History, Next: Common Extensions, Prev: POSIX/GNU, Up: Language History
A.6 History of 'gawk' Features
==============================
This minor node describes the features in 'gawk' over and above those in
POSIX 'awk', in the order they were added to 'gawk'.
Version 2.10 of 'gawk' introduced the following features:
* The 'AWKPATH' environment variable for specifying a path search for
the '-f' command-line option (*note Options::).
* The 'IGNORECASE' variable and its effects (*note
Case-sensitivity::).
* The '/dev/stdin', '/dev/stdout', '/dev/stderr' and '/dev/fd/N'
special file names (*note Special Files::).
Version 2.13 of 'gawk' introduced the following features:
* The 'FIELDWIDTHS' variable and its effects (*note Constant Size::).
* The 'systime()' and 'strftime()' built-in functions for obtaining
and printing timestamps (*note Time Functions::).
* Additional command-line options (*note Options::):
- The '-W lint' option to provide error and portability checking
for both the source code and at runtime.
- The '-W compat' option to turn off the GNU extensions.
- The '-W posix' option for full POSIX compliance.
Version 2.14 of 'gawk' introduced the following feature:
* The 'next file' statement for skipping to the next data file (*note
Nextfile Statement::).
Version 2.15 of 'gawk' introduced the following features:
* New variables (*note Built-in Variables::):
- 'ARGIND', which tracks the movement of 'FILENAME' through
'ARGV'.
- 'ERRNO', which contains the system error message when
'getline' returns -1 or 'close()' fails.
* The '/dev/pid', '/dev/ppid', '/dev/pgrpid', and '/dev/user' special
file names. These have since been removed.
* The ability to delete all of an array at once with 'delete ARRAY'
(*note Delete::).
* Command-line option changes (*note Options::):
- The ability to use GNU-style long-named options that start
with '--'.
- The '--source' option for mixing command-line and library-file
source code.
Version 3.0 of 'gawk' introduced the following features:
* New or changed variables:
- 'IGNORECASE' changed, now applying to string comparison as
well as regexp operations (*note Case-sensitivity::).
- 'RT', which contains the input text that matched 'RS' (*note
Records::).
* Full support for both POSIX and GNU regexps (*note Regexp::).
* The 'gensub()' function for more powerful text manipulation (*note
String Functions::).
* The 'strftime()' function acquired a default time format, allowing
it to be called with no arguments (*note Time Functions::).
* The ability for 'FS' and for the third argument to 'split()' to be
null strings (*note Single Character Fields::).
* The ability for 'RS' to be a regexp (*note Records::).
* The 'next file' statement became 'nextfile' (*note Nextfile
Statement::).
* The 'fflush()' function from BWK 'awk' (then at Bell Laboratories;
*note I/O Functions::).
* New command-line options:
- The '--lint-old' option to warn about constructs that are not
available in the original Version 7 Unix version of 'awk'
(*note V7/SVR3.1::).
- The '-m' option from BWK 'awk'. (Brian was still at Bell
Laboratories at the time.) This was later removed from both
his 'awk' and from 'gawk'.
- The '--re-interval' option to provide interval expressions in
regexps (*note Regexp Operators::).
- The '--traditional' option was added as a better name for
'--compat' (*note Options::).
* The use of GNU Autoconf to control the configuration process (*note
Quick Installation::).
* Amiga support. This has since been removed.
Version 3.1 of 'gawk' introduced the following features:
* New variables (*note Built-in Variables::):
- 'BINMODE', for non-POSIX systems, which allows binary I/O for
input and/or output files (*note PC Using::).
- 'LINT', which dynamically controls lint warnings.
- 'PROCINFO', an array for providing process-related
information.
- 'TEXTDOMAIN', for setting an application's
internationalization text domain (*note
Internationalization::).
* The ability to use octal and hexadecimal constants in 'awk' program
source code (*note Nondecimal-numbers::).
* The '|&' operator for two-way I/O to a coprocess (*note Two-way
I/O::).
* The '/inet' special files for TCP/IP networking using '|&' (*note
TCP/IP Networking::).
* The optional second argument to 'close()' that allows closing one
end of a two-way pipe to a coprocess (*note Two-way I/O::).
* The optional third argument to the 'match()' function for capturing
text-matching subexpressions within a regexp (*note String
Functions::).
* Positional specifiers in 'printf' formats for making translations
easier (*note Printf Ordering::).
* A number of new built-in functions:
- The 'asort()' and 'asorti()' functions for sorting arrays
(*note Array Sorting::).
- The 'bindtextdomain()', 'dcgettext()' and 'dcngettext()'
functions for internationalization (*note Programmer i18n::).
- The 'extension()' function and the ability to add new built-in
functions dynamically (*note Dynamic Extensions::).
- The 'mktime()' function for creating timestamps (*note Time
Functions::).
- The 'and()', 'or()', 'xor()', 'compl()', 'lshift()',
'rshift()', and 'strtonum()' functions (*note Bitwise
Functions::).
* The support for 'next file' as two words was removed completely
(*note Nextfile Statement::).
* Additional command-line options (*note Options::):
- The '--dump-variables' option to print a list of all global
variables.
- The '--exec' option, for use in CGI scripts.
- The '--gen-po' command-line option and the use of a leading
underscore to mark strings that should be translated (*note
String Extraction::).
- The '--non-decimal-data' option to allow non-decimal input
data (*note Nondecimal Data::).
- The '--profile' option and 'pgawk', the profiling version of
'gawk', for producing execution profiles of 'awk' programs
(*note Profiling::).
- The '--use-lc-numeric' option to force 'gawk' to use the
locale's decimal point for parsing input data (*note
Conversion::).
* The use of GNU Automake to help in standardizing the configuration
process (*note Quick Installation::).
* The use of GNU 'gettext' for 'gawk''s own message output (*note
Gawk I18N::).
* BeOS support. This was later removed.
* Tandem support. This was later removed.
* The Atari port became officially unsupported and was later removed
entirely.
* The source code changed to use ISO C standard-style function
definitions.
* POSIX compliance for 'sub()' and 'gsub()' (*note Gory Details::).
* The 'length()' function was extended to accept an array argument
and return the number of elements in the array (*note String
Functions::).
* The 'strftime()' function acquired a third argument to enable
printing times as UTC (*note Time Functions::).
Version 4.0 of 'gawk' introduced the following features:
* Variable additions:
- 'FPAT', which allows you to specify a regexp that matches the
fields, instead of matching the field separator (*note
Splitting By Content::).
- If 'PROCINFO["sorted_in"]' exists, 'for(iggy in foo)' loops
sort the indices before looping over them. The value of this
element provides control over how the indices are sorted
before the loop traversal starts (*note Controlling
Scanning::).
- 'PROCINFO["strftime"]', which holds the default format for
'strftime()' (*note Time Functions::).
* The special files '/dev/pid', '/dev/ppid', '/dev/pgrpid' and
'/dev/user' were removed.
* Support for IPv6 was added via the '/inet6' special file. '/inet4'
forces IPv4 and '/inet' chooses the system default, which is
probably IPv4 (*note TCP/IP Networking::).
* The use of '\s' and '\S' escape sequences in regular expressions
(*note GNU Regexp Operators::).
* Interval expressions became part of default regular expressions
(*note Regexp Operators::).
* POSIX character classes work even with '--traditional' (*note
Regexp Operators::).
* 'break' and 'continue' became invalid outside a loop, even with
'--traditional' (*note Break Statement::, and also see *note
Continue Statement::).
* 'fflush()', 'nextfile', and 'delete ARRAY' are allowed if '--posix'
or '--traditional', since they are all now part of POSIX.
* An optional third argument to 'asort()' and 'asorti()', specifying
how to sort (*note String Functions::).
* The behavior of 'fflush()' changed to match BWK 'awk' and for
POSIX; now both 'fflush()' and 'fflush("")' flush all open output
redirections (*note I/O Functions::).
* The 'isarray()' function which distinguishes if an item is an array
or not, to make it possible to traverse arrays of arrays (*note
Type Functions::).
* The 'patsplit()' function which gives the same capability as
'FPAT', for splitting (*note String Functions::).
* An optional fourth argument to the 'split()' function, which is an
array to hold the values of the separators (*note String
Functions::).
* Arrays of arrays (*note Arrays of Arrays::).
* The 'BEGINFILE' and 'ENDFILE' special patterns (*note
BEGINFILE/ENDFILE::).
* Indirect function calls (*note Indirect Calls::).
* 'switch' / 'case' are enabled by default (*note Switch
Statement::).
* Command-line option changes (*note Options::):
- The '-b' and '--characters-as-bytes' options which prevent
'gawk' from treating input as a multibyte string.
- The redundant '--compat', '--copyleft', and '--usage' long
options were removed.
- The '--gen-po' option was finally renamed to the correct
'--gen-pot'.
- The '--sandbox' option which disables certain features.
- All long options acquired corresponding short options, for use
in '#!' scripts.
* Directories named on the command line now produce a warning, not a
fatal error, unless '--posix' or '--traditional' are used (*note
Command-line directories::).
* The 'gawk' internals were rewritten, bringing the 'dgawk' debugger
and possibly improved performance (*note Debugger::).
* Per the GNU Coding Standards, dynamic extensions must now define a
global symbol indicating that they are GPL-compatible (*note Plugin
License::).
* In POSIX mode, string comparisons use 'strcoll()' / 'wcscoll()'
(*note POSIX String Comparison::).
* The option for raw sockets was removed, since it was never
implemented (*note TCP/IP Networking::).
* Ranges of the form '[d-h]' are treated as if they were in the C
locale, no matter what kind of regexp is being used, and even if
'--posix' (*note Ranges and Locales::).
* Support was removed for the following systems:
- Atari
- Amiga
- BeOS
- Cray
- MIPS RiscOS
- MS-DOS with the Microsoft Compiler
- MS-Windows with the Microsoft Compiler
- NeXT
- SunOS 3.x, Sun 386 (Road Runner)
- Tandem (non-POSIX)
- Prestandard VAX C compiler for VAX/VMS
Version 4.1 of 'gawk' introduced the following features:
* Three new arrays: 'SYMTAB', 'FUNCTAB', and
'PROCINFO["identifiers"]' (*note Auto-set::).
* The three executables 'gawk', 'pgawk', and 'dgawk', were merged
into one, named just 'gawk'. As a result the command-line options
changed.
* Command-line option changes (*note Options::):
- The '-D' option invokes the debugger.
- The '-i' and '--include' options load 'awk' library files.
- The '-l' and '--load' options load compiled dynamic
extensions.
- The '-M' and '--bignum' options enable MPFR.
- The '-o' option only does pretty-printing.
- The '-p' option is used for profiling.
- The '-R' option was removed.
* Support for high precision arithmetic with MPFR (*note Arbitrary
Precision Arithmetic::).
* The 'and()', 'or()' and 'xor()' functions changed to allow any
number of arguments, with a minimum of two (*note Bitwise
Functions::).
* The dynamic extension interface was completely redone (*note
Dynamic Extensions::).
* Redirected 'getline' became allowed inside 'BEGINFILE' and
'ENDFILE' (*note BEGINFILE/ENDFILE::).
* The 'where' command was added to the debugger (*note Execution
Stack::).
* Support for Ultrix was removed.
Version 4.2 of 'gawk' introduced the following changes:
* Changes to 'ENVIRON' are reflected into 'gawk''s environment and
that of programs that it runs. *Note Auto-set::.
* 'FIELDWIDTHS' was enhanced to allow skipping characters before
assigning a value to a field (*note Splitting By Content::).
* The 'PROCINFO["argv"]' array. *Note Auto-set::.
* The maximum number of hexadecimal digits in '\x' escapes is now
two. *Note Escape Sequences::.
* Strongly typed regexp constants of the form '@/.../' (*note Strong
Regexp Constants::).
* The bitwise functions changed, making negative arguments into a
fatal error (*note Bitwise Functions::).
* The 'mktime()' function now accepts an optional second argument
(*note Time Functions::).
* The 'typeof()' function (*note Type Functions::).
* Optimizations are enabled by default. Use '-s' / '--no-optimize'
to disable optimizations.
* For many years, POSIX specified that default field splitting only
allowed spaces and tabs to separate fields, and this was how 'gawk'
behaved with '--posix'. As of 2013, the standard restored
historical behavior, and now default field splitting with '--posix'
also allows newlines to separate fields.
* Nonfatal output with 'print' and 'printf'. *Note Nonfatal::.
* Retryable I/O via 'PROCINFO[INPUT-FILE, "RETRY"]'; (*note Retrying
Input::).
* Changes to the pretty-printer (*note Profiling::):
- The '--pretty-print' option no longer runs the 'awk' program
too.
- Comments in the source program are preserved and placed into
the output file.
- Explicit parentheses for expressions in the input are
preserved in the generated output.
* Improvements to the extension API (*note Dynamic Extensions::):
- The 'get_file()' function to access open redirections.
- The 'nonfatal()' function for generating nonfatal error
messages.
- Support for GMP and MPFR values.
- Input parsers can now override the default field parsing
mechanism by specifying explicit locations.
* Shell startup files are supplied with the distribution and
installed by 'make install' (*note Shell Startup Files::).
* The 'igawk' program and its manual page are no longer installed
when 'gawk' is built. *Note Igawk Program::.
* Support for MirBSD was removed.
* Support for GNU/Linux on Alpha was removed.
File: gawk.info, Node: Common Extensions, Next: Ranges and Locales, Prev: Feature History, Up: Language History
A.7 Common Extensions Summary
=============================
The following table summarizes the common extensions supported by
'gawk', Brian Kernighan's 'awk', and 'mawk', the three most widely used
freely available versions of 'awk' (*note Other Versions::).
Feature BWK 'awk' 'mawk' 'gawk' Now standard
--------------------------------------------------------------------------
'\x' escape sequence X X X
'FS' as null string X X X
'/dev/stdin' special file X X X
'/dev/stdout' special file X X X
'/dev/stderr' special file X X X
'delete' without subscript X X X X
'fflush()' function X X X X
'length()' of an array X X X
'nextfile' statement X X X X
'**' and '**=' operators X X
'func' keyword X X
'BINMODE' variable X X
'RS' as regexp X X
Time-related functions X X
File: gawk.info, Node: Ranges and Locales, Next: Contributors, Prev: Common Extensions, Up: Language History
A.8 Regexp Ranges and Locales: A Long Sad Story
===============================================
This minor node describes the confusing history of ranges within regular
expressions and their interactions with locales, and how this affected
different versions of 'gawk'.
The original Unix tools that worked with regular expressions defined
character ranges (such as '[a-z]') to match any character between the
first character in the range and the last character in the range,
inclusive. Ordering was based on the numeric value of each character in
the machine's native character set. Thus, on ASCII-based systems,
'[a-z]' matched all the lowercase letters, and only the lowercase
letters, as the numeric values for the letters from 'a' through 'z' were
contiguous. (On an EBCDIC system, the range '[a-z]' includes additional
nonalphabetic characters as well.)
Almost all introductory Unix literature explained range expressions
as working in this fashion, and in particular, would teach that the
"correct" way to match lowercase letters was with '[a-z]', and that
'[A-Z]' was the "correct" way to match uppercase letters. And indeed,
this was true.(1)
The 1992 POSIX standard introduced the idea of locales (*note
Locales::). Because many locales include other letters besides the
plain 26 letters of the English alphabet, the POSIX standard added
character classes (*note Bracket Expressions::) as a way to match
different kinds of characters besides the traditional ones in the ASCII
character set.
However, the standard _changed_ the interpretation of range
expressions. In the '"C"' and '"POSIX"' locales, a range expression
like '[a-dx-z]' is still equivalent to '[abcdxyz]', as in ASCII. But
outside those locales, the ordering was defined to be based on
"collation order".
What does that mean? In many locales, 'A' and 'a' are both less than
'B'. In other words, these locales sort characters in dictionary order,
and '[a-dx-z]' is typically not equivalent to '[abcdxyz]'; instead, it
might be equivalent to '[ABCXYabcdxyz]', for example.
This point needs to be emphasized: much literature teaches that you
should use '[a-z]' to match a lowercase character. But on systems with
non-ASCII locales, this also matches all of the uppercase characters
except 'A' or 'Z'! This was a continuous cause of confusion, even well
into the twenty-first century.
To demonstrate these issues, the following example uses the 'sub()'
function, which does text replacement (*note String Functions::). Here,
the intent is to remove trailing uppercase characters:
$ echo something1234abc | gawk-3.1.8 '{ sub("[A-Z]*$", ""); print }'
-| something1234a
This output is unexpected, as the 'bc' at the end of 'something1234abc'
should not normally match '[A-Z]*'. This result is due to the locale
setting (and thus you may not see it on your system).
Similar considerations apply to other ranges. For example, '["-/]'
is perfectly valid in ASCII, but is not valid in many Unicode locales,
such as 'en_US.UTF-8'.
Early versions of 'gawk' used regexp matching code that was not
locale-aware, so ranges had their traditional interpretation.
When 'gawk' switched to using locale-aware regexp matchers, the
problems began; especially as both GNU/Linux and commercial Unix vendors
started implementing non-ASCII locales, _and making them the default_.
Perhaps the most frequently asked question became something like, "Why
does '[A-Z]' match lowercase letters?!?"
This situation existed for close to 10 years, if not more, and the
'gawk' maintainer grew weary of trying to explain that 'gawk' was being
nicely standards-compliant, and that the issue was in the user's locale.
During the development of version 4.0, he modified 'gawk' to always
treat ranges in the original, pre-POSIX fashion, unless '--posix' was
used (*note Options::).(2)
Fortunately, shortly before the final release of 'gawk' 4.0, the
maintainer learned that the 2008 standard had changed the definition of
ranges, such that outside the '"C"' and '"POSIX"' locales, the meaning
of range expressions was _undefined_.(3)
By using this lovely technical term, the standard gives license to
implementers to implement ranges in whatever way they choose. The
'gawk' maintainer chose to apply the pre-POSIX meaning both with the
default regexp matching and when '--traditional' or '--posix' are used.
In all cases 'gawk' remains POSIX-compliant.
---------- Footnotes ----------
(1) And Life was good.
(2) And thus was born the Campaign for Rational Range Interpretation
(or RRI). A number of GNU tools have already implemented this change, or
will soon. Thanks to Karl Berry for coining the phrase "Rational Range
Interpretation."
(3) See the standard
(http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_03_05)
and its rationale
(http://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xbd_chap09.html#tag_21_09_03_05).
File: gawk.info, Node: Contributors, Next: History summary, Prev: Ranges and Locales, Up: Language History
A.9 Major Contributors to 'gawk'
================================
Always give credit where credit is due.
-- _Anonymous_
This minor node names the major contributors to 'gawk' and/or this
Info file, in approximate chronological order:
* Dr. Alfred V. Aho, Dr. Peter J. Weinberger, and Dr. Brian W.
Kernighan, all of Bell Laboratories, designed and implemented Unix
'awk', from which 'gawk' gets the majority of its feature set.
* Paul Rubin did the initial design and implementation in 1986, and
wrote the first draft (around 40 pages) of this Info file.
* Jay Fenlason finished the initial implementation.
* Diane Close revised the first draft of this Info file, bringing it
to around 90 pages.
* Richard Stallman helped finish the implementation and the initial
draft of this Info file. He is also the founder of the FSF and the
GNU Project.
* John Woods contributed parts of the code (mostly fixes) in the
initial version of 'gawk'.
* In 1988, David Trueman took over primary maintenance of 'gawk',
making it compatible with "new" 'awk', and greatly improving its
performance.
* Conrad Kwok, Scott Garfinkle, and Kent Williams did the initial
ports to MS-DOS with various versions of MSC.
* Pat Rankin provided the VMS port and its documentation.
* Hal Peterson provided help in porting 'gawk' to Cray systems.
(This is no longer supported.)
* Kai Uwe Rommel provided the initial port to OS/2 and its
documentation.
* Michal Jaegermann provided the port to Atari systems and its
documentation. (This port is no longer supported.) He continues
to provide portability checking, and has done a lot of work to make
sure 'gawk' works on non-32-bit systems.
* Fred Fish provided the port to Amiga systems and its documentation.
(With Fred's sad passing, this is no longer supported.)
* Scott Deifik formerly maintained the MS-DOS port using DJGPP.
* Eli Zaretskii currently maintains the MS-Windows port using MinGW.
* Juan Grigera provided a port to Windows32 systems. (This is no
longer supported.)
* For many years, Dr. Darrel Hankerson acted as coordinator for the
various ports to different PC platforms and created binary
distributions for various PC operating systems. He was also
instrumental in keeping the documentation up to date for the
various PC platforms.
* Christos Zoulas provided the 'extension()' built-in function for
dynamically adding new functions. (This was obsoleted at 'gawk'
4.1.)
* Ju"rgen Kahrs contributed the initial version of the TCP/IP
networking code and documentation, and motivated the inclusion of
the '|&' operator.
* Stephen Davies provided the initial port to Tandem systems and its
documentation. (However, this is no longer supported.) He was
also instrumental in the initial work to integrate the byte-code
internals into the 'gawk' code base. Additionally, he did most of
the work enabling the pretty-printer to preserve and output
comments.
* Matthew Woehlke provided improvements for Tandem's POSIX-compliant
systems.
* Martin Brown provided the port to BeOS and its documentation.
(This is no longer supported.)
* Arno Peters did the initial work to convert 'gawk' to use GNU
Automake and GNU 'gettext'.
* Alan J. Broder provided the initial version of the 'asort()'
function as well as the code for the optional third argument to the
'match()' function.
* Andreas Buening updated the 'gawk' port for OS/2.
* Isamu Hasegawa, of IBM in Japan, contributed support for multibyte
characters.
* Michael Benzinger contributed the initial code for 'switch'
statements.
* Patrick T.J. McPhee contributed the code for dynamic loading in
Windows32 environments. (This is no longer supported.)
* Anders Wallin helped keep the VMS port going for several years.
* Assaf Gordon contributed the code to implement the '--sandbox'
option.
* John Haque made the following contributions:
- The modifications to convert 'gawk' into a byte-code
interpreter, including the debugger
- The addition of true arrays of arrays
- The additional modifications for support of
arbitrary-precision arithmetic
- The initial text of *note Arbitrary Precision Arithmetic::
- The work to merge the three versions of 'gawk' into one, for
the 4.1 release
- Improved array internals for arrays indexed by integers
- The improved array sorting features were also driven by John,
together with Pat Rankin
* Panos Papadopoulos contributed the original text for *note Include
Files::.
* Efraim Yawitz contributed the original text for *note Debugger::.
* The development of the extension API first released with 'gawk' 4.1
was driven primarily by Arnold Robbins and Andrew Schorr, with
notable contributions from the rest of the development team.
* John Malmberg contributed significant improvements to the OpenVMS
port and the related documentation.
* Antonio Giovanni Colombo rewrote a number of examples in the early
chapters that were severely dated, for which I am incredibly
grateful. He also provided and maintains the Italian translation.
* Marco Curreli, together with Antonio Colombo, translated this Info
file into Italian. It is included in the 'gawk' distribution.
* Juan Manuel Guerrero took over maintenance of the DJGPP port.
* Arnold Robbins has been working on 'gawk' since 1988, at first
helping David Trueman, and as the primary maintainer since around
1994.
File: gawk.info, Node: History summary, Prev: Contributors, Up: Language History
A.10 Summary
============
* The 'awk' language has evolved over time. The first release was
with V7 Unix, circa 1978. In 1987, for System V Release 3.1, major
additions, including user-defined functions, were made to the
language. Additional changes were made for System V Release 4, in
1989. Since then, further minor changes have happened under the
auspices of the POSIX standard.
* Brian Kernighan's 'awk' provides a small number of extensions that
are implemented in common with other versions of 'awk'.
* 'gawk' provides a large number of extensions over POSIX 'awk'.
They can be disabled with either the '--traditional' or '--posix'
options.
* The interaction of POSIX locales and regexp matching in 'gawk' has
been confusing over the years. Today, 'gawk' implements Rational
Range Interpretation, where ranges of the form '[a-z]' match _only_
the characters numerically between 'a' through 'z' in the machine's
native character set. Usually this is ASCII, but it can be EBCDIC
on IBM S/390 systems.
* Many people have contributed to 'gawk' development over the years.
We hope that the list provided in this major node is complete and
gives the appropriate credit where credit is due.
File: gawk.info, Node: Installation, Next: Notes, Prev: Language History, Up: Top
Appendix B Installing 'gawk'
****************************
This appendix provides instructions for installing 'gawk' on the various
platforms that are supported by the developers. The primary developer
supports GNU/Linux (and Unix), whereas the other ports are contributed.
*Note Bugs:: for the email addresses of the people who maintain the
respective ports.
* Menu:
* Gawk Distribution:: What is in the 'gawk' distribution.
* Unix Installation:: Installing 'gawk' under various
versions of Unix.
* Non-Unix Installation:: Installation on Other Operating Systems.
* Bugs:: Reporting Problems and Bugs.
* Other Versions:: Other freely available 'awk'
implementations.
* Installation summary:: Summary of installation.
File: gawk.info, Node: Gawk Distribution, Next: Unix Installation, Up: Installation
B.1 The 'gawk' Distribution
===========================
This minor node describes how to get the 'gawk' distribution, how to
extract it, and then what is in the various files and subdirectories.
* Menu:
* Getting:: How to get the distribution.
* Extracting:: How to extract the distribution.
* Distribution contents:: What is in the distribution.
File: gawk.info, Node: Getting, Next: Extracting, Up: Gawk Distribution
B.1.1 Getting the 'gawk' Distribution
-------------------------------------
There are two ways to get GNU software:
* Copy it from someone else who already has it.
* Retrieve 'gawk' from the Internet host 'ftp.gnu.org', in the
directory '/gnu/gawk'. Both anonymous 'ftp' and 'http' access are
supported. If you have the 'wget' program, you can use a command
like the following:
wget https://ftp.gnu.org/gnu/gawk/gawk-4.2.1.tar.gz
The GNU software archive is mirrored around the world. The
up-to-date list of mirror sites is available from the main FSF website
(https://www.gnu.org/order/ftp.html). Try to use one of the mirrors;
they will be less busy, and you can usually find one closer to your
site.
You may also retrieve the 'gawk' source code from the official Git
repository; for more information see *note Accessing The Source::.
File: gawk.info, Node: Extracting, Next: Distribution contents, Prev: Getting, Up: Gawk Distribution
B.1.2 Extracting the Distribution
---------------------------------
'gawk' is distributed as several 'tar' files compressed with different
compression programs: 'gzip', 'bzip2', and 'xz'. For simplicity, the
rest of these instructions assume you are using the one compressed with
the GNU Gzip program ('gzip').
Once you have the distribution (e.g., 'gawk-4.2.1.tar.gz'), use
'gzip' to expand the file and then use 'tar' to extract it. You can use
the following pipeline to produce the 'gawk' distribution:
gzip -d -c gawk-4.2.1.tar.gz | tar -xvpf -
On a system with GNU 'tar', you can let 'tar' do the decompression
for you:
tar -xvpzf gawk-4.2.1.tar.gz
Extracting the archive creates a directory named 'gawk-4.2.1' in the
current directory.
The distribution file name is of the form 'gawk-V.R.P.tar.gz'. The V
represents the major version of 'gawk', the R represents the current
release of version V, and the P represents a "patch level", meaning that
minor bugs have been fixed in the release. The current patch level is
1, but when retrieving distributions, you should get the version with
the highest version, release, and patch level. (Note, however, that
patch levels greater than or equal to 70 denote "beta" or nonproduction
software; you might not want to retrieve such a version unless you don't
mind experimenting.) If you are not on a Unix or GNU/Linux system, you
need to make other arrangements for getting and extracting the 'gawk'
distribution. You should consult a local expert.
File: gawk.info, Node: Distribution contents, Prev: Extracting, Up: Gawk Distribution
B.1.3 Contents of the 'gawk' Distribution
-----------------------------------------
The 'gawk' distribution has a number of C source files, documentation
files, subdirectories, and files related to the configuration process
(*note Unix Installation::), as well as several subdirectories related
to different non-Unix operating systems:
Various '.c', '.y', and '.h' files
These files contain the actual 'gawk' source code.
'support/*'
C header and source files for routines that 'gawk' uses, but that
are not part of its core functionality. For example, argument
parsing, regular expression matching, and random number generating
routines are all kept here.
'ABOUT-NLS'
A file containing information about GNU 'gettext' and translations.
'AUTHORS'
A file with some information about the authorship of 'gawk'. It
exists only to satisfy the pedants at the Free Software Foundation.
'README'
'README_d/README.*'
Descriptive files: 'README' for 'gawk' under Unix and the rest for
the various hardware and software combinations.
'INSTALL'
A file providing an overview of the configuration and installation
process.
'ChangeLog'
A detailed list of source code changes as bugs are fixed or
improvements made.
'ChangeLog.0'
An older list of source code changes.
'NEWS'
A list of changes to 'gawk' since the last release or patch.
'NEWS.0'
An older list of changes to 'gawk'.
'COPYING'
The GNU General Public License.
'POSIX.STD'
A description of behaviors in the POSIX standard for 'awk' that are
left undefined, or where 'gawk' may not comply fully, as well as a
list of things that the POSIX standard should describe but does
not.
'doc/awkforai.txt'
Pointers to the original draft of a short article describing why
'gawk' is a good language for artificial intelligence (AI)
programming.
'doc/bc_notes'
A brief description of 'gawk''s "byte code" internals.
'doc/README.card'
'doc/ad.block'
'doc/awkcard.in'
'doc/cardfonts'
'doc/colors'
'doc/macros'
'doc/no.colors'
'doc/setter.outline'
The 'troff' source for a five-color 'awk' reference card. A modern
version of 'troff' such as GNU 'troff' ('groff') is needed to
produce the color version. See the file 'README.card' for
instructions if you have an older 'troff'.
'doc/gawk.1'
The 'troff' source for a manual page describing 'gawk'. This is
distributed for the convenience of Unix users.
'doc/gawktexi.in'
'doc/sidebar.awk'
The Texinfo source file for this Info file. It should be processed
by 'doc/sidebar.awk' before processing with 'texi2dvi' or
'texi2pdf' to produce a printed document, and with 'makeinfo' to
produce an Info or HTML file. The 'Makefile' takes care of this
processing and produces printable output via 'texi2dvi' or
'texi2pdf'.
'doc/gawk.texi'
The file produced after processing 'gawktexi.in' with
'sidebar.awk'.
'doc/gawk.info'
The generated Info file for this Info file.
'doc/gawkinet.texi'
The Texinfo source file for *note (General Introduction, gawkinet,
TCP/IP Internetworking with 'gawk')Top::. It should be processed
with TeX (via 'texi2dvi' or 'texi2pdf') to produce a printed
document and with 'makeinfo' to produce an Info or HTML file.
'doc/gawkinet.info'
The generated Info file for 'TCP/IP Internetworking with 'gawk''.
'doc/igawk.1'
The 'troff' source for a manual page describing the 'igawk' program
presented in *note Igawk Program::. (Since 'gawk' can do its own
'@include' processing, neither 'igawk' nor 'igawk.1' are
installed.)
'doc/it/*'
Files for the Italian translation of this Info file, produced and
contributed by Antonio Colombo and Marco Curreli.
'doc/Makefile.in'
The input file used during the configuration process to generate
the actual 'Makefile' for creating the documentation.
'Makefile.am'
'*/Makefile.am'
Files used by the GNU Automake software for generating the
'Makefile.in' files used by Autoconf and 'configure'.
'Makefile.in'
'aclocal.m4'
'bisonfix.awk'
'config.guess'
'configh.in'
'configure.ac'
'configure'
'custom.h'
'depcomp'
'install-sh'
'missing_d/*'
'mkinstalldirs'
'm4/*'
These files and subdirectories are used when configuring and
compiling 'gawk' for various Unix systems. Most of them are
explained in *note Unix Installation::. The rest are there to
support the main infrastructure.
'po/*'
The 'po' library contains message translations.
'awklib/extract.awk'
'awklib/Makefile.am'
'awklib/Makefile.in'
'awklib/eg/*'
The 'awklib' directory contains a copy of 'extract.awk' (*note
Extract Program::), which can be used to extract the sample
programs from the Texinfo source file for this Info file. It also
contains a 'Makefile.in' file, which 'configure' uses to generate a
'Makefile'. 'Makefile.am' is used by GNU Automake to create
'Makefile.in'. The library functions from *note Library
Functions::, are included as ready-to-use files in the 'gawk'
distribution. They are installed as part of the installation
process. The rest of the programs in this Info file are available
in appropriate subdirectories of 'awklib/eg'.
'extension/*'
The source code, manual pages, and infrastructure files for the
sample extensions included with 'gawk'. *Note Dynamic
Extensions::, for more information.
'extras/*'
Additional non-essential files. Currently, this directory contains
some shell startup files to be installed in '/etc/profile.d' to aid
in manipulating the 'AWKPATH' and 'AWKLIBPATH' environment
variables. *Note Shell Startup Files::, for more information.
'posix/*'
Files needed for building 'gawk' on POSIX-compliant systems.
'pc/*'
Files needed for building 'gawk' under MS-Windows (*note PC
Installation:: for details).
'vms/*'
Files needed for building 'gawk' under Vax/VMS and OpenVMS (*note
VMS Installation:: for details).
'test/*'
A test suite for 'gawk'. You can use 'make check' from the
top-level 'gawk' directory to run your version of 'gawk' against
the test suite. If 'gawk' successfully passes 'make check', then
you can be confident of a successful port.
File: gawk.info, Node: Unix Installation, Next: Non-Unix Installation, Prev: Gawk Distribution, Up: Installation
B.2 Compiling and Installing 'gawk' on Unix-Like Systems
========================================================
Usually, you can compile and install 'gawk' by typing only two commands.
However, if you use an unusual system, you may need to configure 'gawk'
for your system yourself.
* Menu:
* Quick Installation:: Compiling 'gawk' under Unix.
* Shell Startup Files:: Shell convenience functions.
* Additional Configuration Options:: Other compile-time options.
* Configuration Philosophy:: How it's all supposed to work.
File: gawk.info, Node: Quick Installation, Next: Shell Startup Files, Up: Unix Installation
B.2.1 Compiling 'gawk' for Unix-Like Systems
--------------------------------------------
The normal installation steps should work on all modern commercial
Unix-derived systems, GNU/Linux, BSD-based systems, and the Cygwin
environment for MS-Windows.
After you have extracted the 'gawk' distribution, 'cd' to
'gawk-4.2.1'. As with most GNU software, you configure 'gawk' for your
system by running the 'configure' program. This program is a Bourne
shell script that is generated automatically using GNU Autoconf. (The
Autoconf software is described fully starting with *note (Autoconf,
autoconf,Autoconf---Generating Automatic Configuration Scripts)Top::.)
To configure 'gawk', simply run 'configure':
sh ./configure
This produces a 'Makefile' and 'config.h' tailored to your system.
The 'config.h' file describes various facts about your system. You
might want to edit the 'Makefile' to change the 'CFLAGS' variable, which
controls the command-line options that are passed to the C compiler
(such as optimization levels or compiling for debugging).
Alternatively, you can add your own values for most 'make' variables
on the command line, such as 'CC' and 'CFLAGS', when running
'configure':
CC=cc CFLAGS=-g sh ./configure
See the file 'INSTALL' in the 'gawk' distribution for all the details.
After you have run 'configure' and possibly edited the 'Makefile',
type:
make
Shortly thereafter, you should have an executable version of 'gawk'.
That's all there is to it! To verify that 'gawk' is working properly,
run 'make check'. All of the tests should succeed. If these steps do
not work, or if any of the tests fail, check the files in the 'README_d'
directory to see if you've found a known problem. If the failure is not
described there, send in a bug report (*note Bugs::).
Of course, once you've built 'gawk', it is likely that you will wish
to install it. To do so, you need to run the command 'make install', as
a user with the appropriate permissions. How to do this varies by
system, but on many systems you can use the 'sudo' command to do so.
The command then becomes 'sudo make install'. It is likely that you
will be asked for your password, and you will have to have been set up
previously as a user who is allowed to run the 'sudo' command.
File: gawk.info, Node: Shell Startup Files, Next: Additional Configuration Options, Prev: Quick Installation, Up: Unix Installation
B.2.2 Shell Startup Files
-------------------------
The distribution contains shell startup files 'gawk.sh' and 'gawk.csh',
containing functions to aid in manipulating the 'AWKPATH' and
'AWKLIBPATH' environment variables. On a Fedora GNU/Linux system, these
files should be installed in '/etc/profile.d'; on other platforms, the
appropriate location may be different.
'gawkpath_default'
Reset the 'AWKPATH' environment variable to its default value.
'gawkpath_prepend'
Add the argument to the front of the 'AWKPATH' environment
variable.
'gawkpath_append'
Add the argument to the end of the 'AWKPATH' environment variable.
'gawklibpath_default'
Reset the 'AWKLIBPATH' environment variable to its default value.
'gawklibpath_prepend'
Add the argument to the front of the 'AWKLIBPATH' environment
variable.
'gawklibpath_append'
Add the argument to the end of the 'AWKLIBPATH' environment
variable.
File: gawk.info, Node: Additional Configuration Options, Next: Configuration Philosophy, Prev: Shell Startup Files, Up: Unix Installation
B.2.3 Additional Configuration Options
--------------------------------------
There are several additional options you may use on the 'configure'
command line when compiling 'gawk' from scratch, including:
'--disable-extensions'
Disable configuring and building the sample extensions in the
'extension' directory. This is useful for cross-compiling. The
default action is to dynamically check if the extensions can be
configured and compiled.
'--disable-lint'
Disable all lint checking within 'gawk'. The '--lint' and
'--lint-old' options (*note Options::) are accepted, but silently
do nothing. Similarly, setting the 'LINT' variable (*note
User-modified::) has no effect on the running 'awk' program.
When used with the GNU Compiler Collection's (GCC's) automatic
dead-code-elimination, this option cuts almost 23K bytes off the
size of the 'gawk' executable on GNU/Linux x86_64 systems. Results
on other systems and with other compilers are likely to vary.
Using this option may bring you some slight performance
improvement.
CAUTION: Using this option will cause some of the tests in the
test suite to fail. This option may be removed at a later
date.
'--disable-mpfr'
Skip checking for the MPFR and GMP libraries. This is useful
mainly for the developers, to make sure nothing breaks if MPFR
support is not available.
'--disable-nls'
Disable all message-translation facilities. This is usually not
desirable, but it may bring you some slight performance
improvement.
'--enable-versioned-extension-dir'
Use a versioned directory for extensions. The directory name will
include the major and minor API versions in it. This makes it
possible to keep extensions for different API versions on the same
system without their conflicting with one another.
'--with-whiny-user-strftime'
Force use of the included version of the C 'strftime()' function
for deficient systems.
Use the command './configure --help' to see the full list of options
supplied by 'configure'.
File: gawk.info, Node: Configuration Philosophy, Prev: Additional Configuration Options, Up: Unix Installation
B.2.4 The Configuration Process
-------------------------------
This minor node is of interest only if you know something about using
the C language and Unix-like operating systems.
The source code for 'gawk' generally attempts to adhere to formal
standards wherever possible. This means that 'gawk' uses library
routines that are specified by the ISO C standard and by the POSIX
operating system interface standard. The 'gawk' source code requires
using an ISO C compiler (the 1990 standard).
Many Unix systems do not support all of either the ISO or the POSIX
standards. The 'missing_d' subdirectory in the 'gawk' distribution
contains replacement versions of those functions that are most likely to
be missing.
The 'config.h' file that 'configure' creates contains definitions
that describe features of the particular operating system where you are
attempting to compile 'gawk'. The three things described by this file
are: what header files are available, so that they can be correctly
included, what (supposedly) standard functions are actually available in
your C libraries, and various miscellaneous facts about your operating
system. For example, there may not be an 'st_blksize' element in the
'stat' structure. In this case, 'HAVE_STRUCT_STAT_ST_BLKSIZE' is
undefined.
It is possible for your C compiler to lie to 'configure'. It may do
so by not exiting with an error when a library function is not
available. To get around this, edit the 'custom.h' file. Use an
'#ifdef' that is appropriate for your system, and either '#define' any
constants that 'configure' should have defined but didn't, or '#undef'
any constants that 'configure' defined and should not have. The
'custom.h' file is automatically included by the 'config.h' file.
It is also possible that the 'configure' program generated by
Autoconf will not work on your system in some other fashion. If you do
have a problem, the 'configure.ac' file is the input for Autoconf. You
may be able to change this file and generate a new version of
'configure' that works on your system (*note Bugs:: for information on
how to report problems in configuring 'gawk'). The same mechanism may
be used to send in updates to 'configure.ac' and/or 'custom.h'.
File: gawk.info, Node: Non-Unix Installation, Next: Bugs, Prev: Unix Installation, Up: Installation
B.3 Installation on Other Operating Systems
===========================================
This minor node describes how to install 'gawk' on various non-Unix
systems.
* Menu:
* PC Installation:: Installing and Compiling 'gawk' on
Microsoft Windows.
* VMS Installation:: Installing 'gawk' on VMS.
File: gawk.info, Node: PC Installation, Next: VMS Installation, Up: Non-Unix Installation
B.3.1 Installation on MS-Windows
--------------------------------
This minor node covers installation and usage of 'gawk' on Intel
architecture machines running any version of MS-Windows. In this minor
node, the term "Windows32" refers to any of Microsoft Windows
95/98/ME/NT/2000/XP/Vista/7/8/10.
See also the 'README_d/README.pc' file in the distribution.
* Menu:
* PC Binary Installation:: Installing a prepared distribution.
* PC Compiling:: Compiling 'gawk' for Windows32.
* PC Using:: Running 'gawk' on Windows32.
* Cygwin:: Building and running 'gawk' for
Cygwin.
* MSYS:: Using 'gawk' In The MSYS Environment.
File: gawk.info, Node: PC Binary Installation, Next: PC Compiling, Up: PC Installation
B.3.1.1 Installing a Prepared Distribution for MS-Windows Systems
.................................................................
The only supported binary distribution for MS-Windows systems is that
provided by Eli Zaretskii's "ezwinports"
(https://sourceforge.net/projects/ezwinports/) project. Install the
compiled 'gawk' from there.
File: gawk.info, Node: PC Compiling, Next: PC Using, Prev: PC Binary Installation, Up: PC Installation
B.3.1.2 Compiling 'gawk' for PC Operating Systems
.................................................
'gawk' can be compiled for Windows32 using MinGW (Windows32). The file
'README_d/README.pc' in the 'gawk' distribution contains additional
notes, and 'pc/Makefile' contains important information on compilation
options.
To build 'gawk' for Windows32, copy the files in the 'pc' directory
(_except_ for 'ChangeLog') to the directory with the rest of the 'gawk'
sources, then invoke 'make' with the appropriate target name as an
argument to build 'gawk'. The 'Makefile' copied from the 'pc' directory
contains a configuration section with comments and may need to be edited
in order to work with your 'make' utility.
The 'Makefile' supports a number of targets for building various
MS-DOS and Windows32 versions. A list of targets is printed if the
'make' command is given without a target. As an example, to build a
native MS-Windows binary of 'gawk' using the MinGW tools, type 'make
mingw32'.
File: gawk.info, Node: PC Using, Next: Cygwin, Prev: PC Compiling, Up: PC Installation
B.3.1.3 Using 'gawk' on PC Operating Systems
............................................
Information in this section applies to the MinGW and DJGPP ports of
'gawk'. *Note Cygwin:: for information about the Cygwin port.
Under MS-Windows, the MinGW environment supports both the '|&'
operator and TCP/IP networking (*note TCP/IP Networking::). The DJGPP
environment does not support '|&'.
The MS-Windows version of 'gawk' searches for program files as
described in *note AWKPATH Variable::. However, semicolons (rather than
colons) separate elements in the 'AWKPATH' variable. If 'AWKPATH' is
not set or is empty, then the default search path is
'.;c:/lib/awk;c:/gnu/lib/awk'.
Under MS-Windows, 'gawk' (and many other text programs) silently
translates end-of-line '\r\n' to '\n' on input and '\n' to '\r\n' on
output. A special 'BINMODE' variable (c.e.) allows control over these
translations and is interpreted as follows:
* If 'BINMODE' is '"r"' or one, then binary mode is set on read
(i.e., no translations on reads).
* If 'BINMODE' is '"w"' or two, then binary mode is set on write
(i.e., no translations on writes).
* If 'BINMODE' is '"rw"' or '"wr"' or three, binary mode is set for
both read and write.
* 'BINMODE=NON-NULL-STRING' is the same as 'BINMODE=3' (i.e., no
translations on reads or writes). However, 'gawk' issues a warning
message if the string is not one of '"rw"' or '"wr"'.
The modes for standard input and standard output are set one time only
(after the command line is read, but before processing any of the 'awk'
program). Setting 'BINMODE' for standard input or standard output is
accomplished by using an appropriate '-v BINMODE=N' option on the
command line. 'BINMODE' is set at the time a file or pipe is opened and
cannot be changed midstream.
The name 'BINMODE' was chosen to match 'mawk' (*note Other
Versions::). 'mawk' and 'gawk' handle 'BINMODE' similarly; however,
'mawk' adds a '-W BINMODE=N' option and an environment variable that can
set 'BINMODE', 'RS', and 'ORS'. The files 'binmode[1-3].awk' (under
'gnu/lib/awk' in some of the prepared binary distributions) have been
chosen to match 'mawk''s '-W BINMODE=N' option. These can be changed or
discarded; in particular, the setting of 'RS' giving the fewest
"surprises" is open to debate. 'mawk' uses 'RS = "\r\n"' if binary mode
is set on read, which is appropriate for files with the MS-DOS-style
end-of-line.
To illustrate, the following examples set binary mode on writes for
standard output and other files, and set 'ORS' as the "usual"
MS-DOS-style end-of-line:
gawk -v BINMODE=2 -v ORS="\r\n" ...
or:
gawk -v BINMODE=w -f binmode2.awk ...
These give the same result as the '-W BINMODE=2' option in 'mawk'. The
following changes the record separator to '"\r\n"' and sets binary mode
on reads, but does not affect the mode on standard input:
gawk -v RS="\r\n" -e "BEGIN { BINMODE = 1 }" ...
or:
gawk -f binmode1.awk ...
With proper quoting, in the first example the setting of 'RS' can be
moved into the 'BEGIN' rule.
File: gawk.info, Node: Cygwin, Next: MSYS, Prev: PC Using, Up: PC Installation
B.3.1.4 Using 'gawk' In The Cygwin Environment
..............................................
'gawk' can be built and used "out of the box" under MS-Windows if you
are using the Cygwin environment (http://www.cygwin.com). This
environment provides an excellent simulation of GNU/Linux, using Bash,
GCC, GNU Make, and other GNU programs. Compilation and installation for
Cygwin is the same as for a Unix system:
tar -xvpzf gawk-4.2.1.tar.gz
cd gawk-4.2.1
./configure
make && make check
When compared to GNU/Linux on the same system, the 'configure' step
on Cygwin takes considerably longer. However, it does finish, and then
the 'make' proceeds as usual.
Recent versions of Cygwin open all files in binary mode. This means
that you should use 'RS = "\r?\n"' in order to be able to handle
standard MS-Windows text files with carriage-return plus line-feed line
endings.
The Cygwin environment supports both the '|&' operator and TCP/IP
networking (*note TCP/IP Networking::).
File: gawk.info, Node: MSYS, Prev: Cygwin, Up: PC Installation
B.3.1.5 Using 'gawk' In The MSYS Environment
............................................
In the MSYS environment under MS-Windows, 'gawk' automatically uses
binary mode for reading and writing files. Thus, there is no need to
use the 'BINMODE' variable.
This can cause problems with other Unix-like components that have
been ported to MS-Windows that expect 'gawk' to do automatic translation
of '"\r\n"', because it won't.
File: gawk.info, Node: VMS Installation, Prev: PC Installation, Up: Non-Unix Installation
B.3.2 Compiling and Installing 'gawk' on Vax/VMS and OpenVMS
------------------------------------------------------------
This node describes how to compile and install 'gawk' under VMS. The
older designation "VMS" is used throughout to refer to OpenVMS.
* Menu:
* VMS Compilation:: How to compile 'gawk' under VMS.
* VMS Dynamic Extensions:: Compiling 'gawk' dynamic extensions on
VMS.
* VMS Installation Details:: How to install 'gawk' under VMS.
* VMS Running:: How to run 'gawk' under VMS.
* VMS GNV:: The VMS GNV Project.
* VMS Old Gawk:: An old version comes with some VMS systems.
File: gawk.info, Node: VMS Compilation, Next: VMS Dynamic Extensions, Up: VMS Installation
B.3.2.1 Compiling 'gawk' on VMS
...............................
To compile 'gawk' under VMS, there is a 'DCL' command procedure that
issues all the necessary 'CC' and 'LINK' commands. There is also a
'Makefile' for use with the 'MMS' and 'MMK' utilities. From the source
directory, use either:
$ @[.vms]vmsbuild.com
or:
$ MMS/DESCRIPTION=[.vms]descrip.mms gawk
or:
$ MMK/DESCRIPTION=[.vms]descrip.mms gawk
'MMK' is an open source, free, near-clone of 'MMS' and can better
handle ODS-5 volumes with upper- and lowercase file names. 'MMK' is
available from <https://github.com/endlesssoftware/mmk>;.
With ODS-5 volumes and extended parsing enabled, the case of the
target parameter may need to be exact.
'gawk' has been tested under VAX/VMS 7.3 and Alpha/VMS 7.3-1 using
Compaq C V6.4, and under Alpha/VMS 7.3, Alpha/VMS 7.3-2, and IA64/VMS
8.3. The most recent builds used HP C V7.3 on Alpha VMS 8.3 and both
Alpha and IA64 VMS 8.4 used HP C 7.3.(1)
*Note VMS GNV:: for information on building 'gawk' as a PCSI kit that
is compatible with the GNV product.
---------- Footnotes ----------
(1) The IA64 architecture is also known as "Itanium."
File: gawk.info, Node: VMS Dynamic Extensions, Next: VMS Installation Details, Prev: VMS Compilation, Up: VMS Installation
B.3.2.2 Compiling 'gawk' Dynamic Extensions on VMS
..................................................
The extensions that have been ported to VMS can be built using one of
the following commands:
$ MMS/DESCRIPTION=[.vms]descrip.mms extensions
or:
$ MMK/DESCRIPTION=[.vms]descrip.mms extensions
'gawk' uses 'AWKLIBPATH' as either an environment variable or a
logical name to find the dynamic extensions.
Dynamic extensions need to be compiled with the same compiler options
for floating-point, pointer size, and symbol name handling as were used
to compile 'gawk' itself. Alpha and Itanium should use IEEE floating
point. The pointer size is 32 bits, and the symbol name handling should
be exact case with CRC shortening for symbols longer than 32 bits.
For Alpha and Itanium:
/name=(as_is,short)
/float=ieee/ieee_mode=denorm_results
For VAX:
/name=(as_is,short)
Compile-time macros need to be defined before the first VMS-supplied
header file is included, as follows:
#if (__CRTL_VER >= 70200000) && !defined (__VAX)
#define _LARGEFILE 1
#endif
#ifndef __VAX
#ifdef __CRTL_VER
#if __CRTL_VER >= 80200000
#define _USE_STD_STAT 1
#endif
#endif
#endif
If you are writing your own extensions to run on VMS, you must supply
these definitions yourself. The 'config.h' file created when building
'gawk' on VMS does this for you; if instead you use that file or a
similar one, then you must remember to include it before any
VMS-supplied header files.
File: gawk.info, Node: VMS Installation Details, Next: VMS Running, Prev: VMS Dynamic Extensions, Up: VMS Installation
B.3.2.3 Installing 'gawk' on VMS
................................
To use 'gawk', all you need is a "foreign" command, which is a 'DCL'
symbol whose value begins with a dollar sign. For example:
$ GAWK :== $disk1:[gnubin]gawk
Substitute the actual location of 'gawk.exe' for '$disk1:[gnubin]'. The
symbol should be placed in the 'login.com' of any user who wants to run
'gawk', so that it is defined every time the user logs on.
Alternatively, the symbol may be placed in the system-wide 'sylogin.com'
procedure, which allows all users to run 'gawk'.
If your 'gawk' was installed by a PCSI kit into the 'GNV$GNU:'
directory tree, the program will be known as 'GNV$GNU:[bin]gnv$gawk.exe'
and the help file will be 'GNV$GNU:[vms_help]gawk.hlp'.
The PCSI kit also installs a 'GNV$GNU:[vms_bin]gawk_verb.cld' file
that can be used to add 'gawk' and 'awk' as DCL commands.
For just the current process you can use:
$ set command gnv$gnu:[vms_bin]gawk_verb.cld
Or the system manager can use 'GNV$GNU:[vms_bin]gawk_verb.cld' to add
the 'gawk' and 'awk' to the system-wide 'DCLTABLES'.
The DCL syntax is documented in the 'gawk.hlp' file.
Optionally, the 'gawk.hlp' entry can be loaded into a VMS help
library:
$ LIBRARY/HELP sys$help:helplib [.vms]gawk.hlp
(You may want to substitute a site-specific help library rather than the
standard VMS library 'HELPLIB'.) After loading the help text, the
command:
$ HELP GAWK
provides information about both the 'gawk' implementation and the 'awk'
programming language.
The logical name 'AWK_LIBRARY' can designate a default location for
'awk' program files. For the '-f' option, if the specified file name
has no device or directory path information in it, 'gawk' looks in the
current directory first, then in the directory specified by the
translation of 'AWK_LIBRARY' if the file is not found. If, after
searching in both directories, the file still is not found, 'gawk'
appends the suffix '.awk' to the file name and retries the file search.
If 'AWK_LIBRARY' has no definition, a default value of 'SYS$LIBRARY:' is
used for it.
File: gawk.info, Node: VMS Running, Next: VMS GNV, Prev: VMS Installation Details, Up: VMS Installation
B.3.2.4 Running 'gawk' on VMS
.............................
Command-line parsing and quoting conventions are significantly different
on VMS, so examples in this Info file or from other sources often need
minor changes. They _are_ minor though, and all 'awk' programs should
run correctly.
Here are a couple of trivial tests:
$ gawk -- "BEGIN {print ""Hello, World!""}"
$ gawk -"W" version
! could also be -"W version" or "-W version"
Note that uppercase and mixed-case text must be quoted.
The VMS port of 'gawk' includes a 'DCL'-style interface in addition
to the original shell-style interface (see the help entry for details).
One side effect of dual command-line parsing is that if there is only a
single parameter (as in the quoted string program), the command becomes
ambiguous. To work around this, the normally optional '--' flag is
required to force Unix-style parsing rather than 'DCL' parsing. If any
other dash-type options (or multiple parameters such as data files to
process) are present, there is no ambiguity and '--' can be omitted.
The 'exit' value is a Unix-style value and is encoded into a VMS exit
status value when the program exits.
The VMS severity bits will be set based on the 'exit' value. A
failure is indicated by 1, and VMS sets the 'ERROR' status. A fatal
error is indicated by 2, and VMS sets the 'FATAL' status. All other
values will have the 'SUCCESS' status. The exit value is encoded to
comply with VMS coding standards and will have the 'C_FACILITY_NO' of
'0x350000' with the constant '0xA000' added to the number shifted over
by 3 bits to make room for the severity codes.
To extract the actual 'gawk' exit code from the VMS status, use:
unix_status = (vms_status .and. %x7f8) / 8
A C program that uses 'exec()' to call 'gawk' will get the original
Unix-style exit value.
Older versions of 'gawk' for VMS treated a Unix exit code 0 as 1, a
failure as 2, a fatal error as 4, and passed all the other numbers
through. This violated the VMS exit status coding requirements.
VAX/VMS floating point uses unbiased rounding. *Note Round
Function::.
VMS reports time values in GMT unless one of the 'SYS$TIMEZONE_RULE'
or 'TZ' logical names is set. Older versions of VMS, such as VAX/VMS
7.3, do not set these logical names.
The default search path, when looking for 'awk' program files
specified by the '-f' option, is '"SYS$DISK:[],AWK_LIBRARY:"'. The
logical name 'AWKPATH' can be used to override this default. The format
of 'AWKPATH' is a comma-separated list of directory specifications.
When defining it, the value should be quoted so that it retains a single
translation and not a multitranslation 'RMS' searchlist.
This restriction also applies to running 'gawk' under GNV, as
redirection is always to a DCL command.
If you are redirecting data to a VMS command or utility, the current
implementation requires that setting up a VMS foreign command that runs
a command file before invoking 'gawk'. (This restriction may be removed
in a future release of 'gawk' on VMS.)
Without this command file, the input data will also appear prepended
to the output data.
This also allows simulating POSIX commands that are not found on VMS
or the use of GNV utilities.
The example below is for 'gawk' redirecting data to the VMS 'sort'
command.
$ sort = "@device:[dir]vms_gawk_sort.com"
The command file needs to be of the format in the example below.
The first line inhibits the passed input data from also showing up in
the output. It must be in the format in the example.
The next line creates a foreign command that overrides the outer
foreign command which prevents an infinite recursion of command files.
The next to the last command redirects 'sys$input' to be
'sys$command', in order to pick up the data that is being redirected to
the command.
The last line runs the actual command. It must be the last command
as the data redirected from 'gawk' will be read when the command file
ends.
$!'f$verify(0,0)'
$ sort := sort
$ define/user sys$input sys$command:
$ sort sys$input: sys$output:
File: gawk.info, Node: VMS GNV, Next: VMS Old Gawk, Prev: VMS Running, Up: VMS Installation
B.3.2.5 The VMS GNV Project
...........................
The VMS GNV package provides a build environment similar to POSIX with
ports of a collection of open source tools. The 'gawk' found in the GNV
base kit is an older port. Currently, the GNV project is being
reorganized to supply individual PCSI packages for each component. See
<https://sourceforge.net/p/gnv/wiki/InstallingGNVPackages/>;.
The normal build procedure for 'gawk' produces a program that is
suitable for use with GNV.
The file 'vms/gawk_build_steps.txt' in the distribution documents the
procedure for building a VMS PCSI kit that is compatible with GNV.
File: gawk.info, Node: VMS Old Gawk, Prev: VMS GNV, Up: VMS Installation
B.3.2.6 Some VMS Systems Have An Old Version of 'gawk'
......................................................
Some versions of VMS have an old version of 'gawk'. To access it,
define a symbol, as follows:
$ gawk :== $sys$common:[syshlp.examples.tcpip.snmp]gawk.exe
This is apparently version 2.15.6, which is extremely old. We
recommend compiling and using the current version.
File: gawk.info, Node: Bugs, Next: Other Versions, Prev: Non-Unix Installation, Up: Installation
B.4 Reporting Problems and Bugs
===============================
There is nothing more dangerous than a bored archaeologist.
-- _Douglas Adams, 'The Hitchhiker's Guide to the Galaxy'_
If you have problems with 'gawk' or think that you have found a bug,
report it to the developers; we cannot promise to do anything, but we
might well want to fix it.
* Menu:
* Bug address:: Where to send reports to.
* Usenet:: Where not to send reports to.
* Maintainers:: Maintainers of non-*nix ports.
File: gawk.info, Node: Bug address, Next: Usenet, Up: Bugs
B.4.1 Submitting Bug Reports
----------------------------
Before reporting a bug, make sure you have really found a genuine bug.
First, verify that you have the latest version of 'gawk'. Many bugs
(usually subtle ones) are fixed at each release, and if yours is out of
date, the problem may already have been solved.
Second, please see if setting the environment variable 'LC_ALL' to
'LC_ALL=C' causes things to behave as you expect. If so, it's a locale
issue, and may or may not really be a bug.
Third, carefully reread the documentation and see if it says you can
do what you're trying to do. If it's not clear whether you should be
able to do something or not, report that too; it's a bug in the
documentation!
Finally, before reporting a bug or trying to fix it yourself, try to
isolate it to the smallest possible 'awk' program and input data file
that reproduce the problem. Then send us the program and data file,
some idea of what kind of Unix system you're using, the compiler you
used to compile 'gawk', and the exact results 'gawk' gave you. Also say
what you expected to occur; this helps us decide whether the problem is
really in the documentation.
Make sure to include the version number of 'gawk' you are using. You
can get this information with the command 'gawk --version'.
Once you have a precise problem description, send email to
<bug-gawk AT gnu.org>.
The 'gawk' maintainers subscribe to this address, and thus they will
receive your bug report. Although you can send mail to the maintainers
directly, the bug reporting address is preferred because the email list
is archived at the GNU Project. _All email must be in English. This is
the only language understood in common by all the maintainers._ In
addition, please be sure to send all mail in _plain text_, not (or not
exclusively) in HTML.
NOTE: Many distributions of GNU/Linux and the various BSD-based
operating systems have their own bug reporting systems. If you
report a bug using your distribution's bug reporting system, you
should also send a copy to <bug-gawk AT gnu.org>.
This is for two reasons. First, although some distributions
forward bug reports "upstream" to the GNU mailing list, many don't,
so there is a good chance that the 'gawk' maintainers won't even
see the bug report! Second, mail to the GNU list is archived, and
having everything at the GNU Project keeps things self-contained
and not dependent on other organizations.
Non-bug suggestions are always welcome as well. If you have
questions about things that are unclear in the documentation or are just
obscure features, ask on the bug list; we will try to help you out if we
can.
File: gawk.info, Node: Usenet, Next: Maintainers, Prev: Bug address, Up: Bugs
B.4.2 Please Don't Post Bug Reports to USENET
---------------------------------------------
I gave up on Usenet a couple of years ago and haven't really looked
back. It's like sports talk radio--you feel smarter for not having
read it.
-- _Chet Ramey_
Please do _not_ try to report bugs in 'gawk' by posting to the
Usenet/Internet newsgroup 'comp.lang.awk'. Although some of the 'gawk'
developers occasionally read this news group, the primary 'gawk'
maintainer no longer does. Thus it's virtually guaranteed that he will
_not_ see your posting. The steps described here are the only
officially recognized way for reporting bugs. Really.
File: gawk.info, Node: Maintainers, Prev: Usenet, Up: Bugs
B.4.3 Reporting Problems with Non-Unix Ports
--------------------------------------------
If you find bugs in one of the non-Unix ports of 'gawk', send an email
to the bug list, with a copy to the person who maintains that port. The
maintainers are named in the following list, as well as in the 'README'
file in the 'gawk' distribution. Information in the 'README' file
should be considered authoritative if it conflicts with this Info file.
The people maintaining the various 'gawk' ports are:
Unix and POSIX Arnold Robbins, <arnold AT skeeve.com>
systems
MS-DOS with DJGPP Juan Manuel Guerrero, <juan.guerrero AT gmx.de>
MS-Windows with MinGW Eli Zaretskii, <eliz AT gnu.org>
OS/2 Andreas Buening, <andreas.buening AT nexgo.de>
VMS John Malmberg, <wb8tyw AT qsl.net>
z/OS (OS/390) Daniel Richard G. <skunk AT iSKUNK.ORG>
Dave Pitts (Maintainer Emeritus), <dpitts AT cozx.com>
If your bug is also reproducible under Unix, send a copy of your
report to the <bug-gawk AT gnu.org> email list as well.
File: gawk.info, Node: Other Versions, Next: Installation summary, Prev: Bugs, Up: Installation
B.5 Other Freely Available 'awk' Implementations
================================================
It's kind of fun to put comments like this in your awk code:
'// Do C++ comments work? answer: yes! of course'
-- _Michael Brennan_
There are a number of other freely available 'awk' implementations.
This minor node briefly describes where to get them:
Unix 'awk'
Brian Kernighan, one of the original designers of Unix 'awk', has
made his implementation of 'awk' freely available. You can
retrieve this version via his home page
(http://www.cs.princeton.edu/~bwk). It is available in several
archive formats:
Shell archive
<http://www.cs.princeton.edu/~bwk/btl.mirror/awk.shar>;
Compressed 'tar' file
<http://www.cs.princeton.edu/~bwk/btl.mirror/awk.tar.gz>;
Zip file
<http://www.cs.princeton.edu/~bwk/btl.mirror/awk.zip>;
You can also retrieve it from GitHub:
git clone git://github.com/onetrueawk/awk bwkawk
This command creates a copy of the Git (https://git-scm.com)
repository in a directory named 'bwkawk'. If you leave that
argument off the 'git' command line, the repository copy is created
in a directory named 'awk'.
This version requires an ISO C (1990 standard) compiler; the C
compiler from GCC (the GNU Compiler Collection) works quite nicely.
*Note Common Extensions:: for a list of extensions in this 'awk'
that are not in POSIX 'awk'.
As a side note, Dan Bornstein has created a Git repository tracking
all the versions of BWK 'awk' that he could find. It's available
at <git://github.com/danfuzz/one-true-awk>;.
'mawk'
Michael Brennan wrote an independent implementation of 'awk',
called 'mawk'. It is available under the GPL (*note Copying::),
just as 'gawk' is.
The original distribution site for the 'mawk' source code no longer
has it. A copy is available at
<http://www.skeeve.com/gawk/mawk1.3.3.tar.gz>;.
In 2009, Thomas Dickey took on 'mawk' maintenance. Basic
information is available on the project's web page
(http://www.invisible-island.net/mawk). The download URL is
<http://invisible-island.net/datafiles/release/mawk.tar.gz>;.
Once you have it, 'gunzip' may be used to decompress this file.
Installation is similar to 'gawk''s (*note Unix Installation::).
*Note Common Extensions:: for a list of extensions in 'mawk' that
are not in POSIX 'awk'.
'mawk' 2.0
In 2016, Michael Brennan resumed 'mawk' development. His
development snapshots are available via Git from the project's
GitHub page (https://github.com/mikebrennan000/mawk-2).
'awka'
Written by Andrew Sumner, 'awka' translates 'awk' programs into C,
compiles them, and links them with a library of functions that
provide the core 'awk' functionality. It also has a number of
extensions.
The 'awk' translator is released under the GPL, and the library is
under the LGPL.
To get 'awka', go to <https://sourceforge.net/projects/awka>;.
The project seems to be frozen; no new code changes have been made
since approximately 2001.
'pawk'
Nelson H.F. Beebe at the University of Utah has modified BWK 'awk'
to provide timing and profiling information. It is different from
'gawk' with the '--profile' option (*note Profiling::) in that it
uses CPU-based profiling, not line-count profiling. You may find
it at either
<ftp://ftp.math.utah.edu/pub/pawk/pawk-20030606.tar.gz>; or
<http://www.math.utah.edu/pub/pawk/pawk-20030606.tar.gz>;.
BusyBox 'awk'
BusyBox is a GPL-licensed program providing small versions of many
applications within a single executable. It is aimed at embedded
systems. It includes a full implementation of POSIX 'awk'. When
building it, be careful not to do 'make install' as it will
overwrite copies of other applications in your '/usr/local/bin'.
For more information, see the project's home page
(https://busybox.net).
The OpenSolaris POSIX 'awk'
The versions of 'awk' in '/usr/xpg4/bin' and '/usr/xpg6/bin' on
Solaris are more or less POSIX-compliant. They are based on the
'awk' from Mortice Kern Systems for PCs. We were able to make this
code compile and work under GNU/Linux with 1-2 hours of work.
Making it more generally portable (using GNU Autoconf and/or
Automake) would take more work, and this has not been done, at
least to our knowledge.
The source code used to be available from the OpenSolaris website.
However, that project was ended and the website shut down.
Fortunately, the Illumos project
(https://wiki.illumos.org/display/illumos/illumos+Home) makes this
implementation available. You can view the files one at a time
from
<https://github.com/joyent/illumos-joyent/blob/master/usr/src/cmd/awk_xpg4>;.
'jawk'
This is an interpreter for 'awk' written in Java. It claims to be
a full interpreter, although because it uses Java facilities for
I/O and for regexp matching, the language it supports is different
from POSIX 'awk'. More information is available on the project's
home page (https://jawk.sourceforge.net).
Libmawk
This is an embeddable 'awk' interpreter derived from 'mawk'. For
more information, see <http://repo.hu/projects/libmawk/>;.
'pawk'
This is a Python module that claims to bring 'awk'-like features to
Python. See <https://github.com/alecthomas/pawk>; for more
information. (This is not related to Nelson Beebe's modified
version of BWK 'awk', described earlier.)
QSE 'awk'
This is an embeddable 'awk' interpreter. For more information, see
<https://code.google.com/p/qse/>;.
'QTawk'
This is an independent implementation of 'awk' distributed under
the GPL. It has a large number of extensions over standard 'awk'
and may not be 100% syntactically compatible with it. See
<http://www.quiktrim.org/QTawk.html>; for more information,
including the manual. The download link there is out of date; see
<http://www.quiktrim.org/#AdditionalResources>; for the latest
download link.
The project may also be frozen; no new code changes have been made
since approximately 2014.
Other versions
See also the "Versions and implementations" section of the
Wikipedia article
(https://en.wikipedia.org/wiki/Awk_language#Versions_and_implementations)
on 'awk' for information on additional versions.
File: gawk.info, Node: Installation summary, Prev: Other Versions, Up: Installation
B.6 Summary
===========
* The 'gawk' distribution is available from the GNU Project's main
distribution site, 'ftp.gnu.org'. The canonical build recipe is:
wget https://ftp.gnu.org/gnu/gawk/gawk-4.2.1.tar.gz
tar -xvpzf gawk-4.2.1.tar.gz
cd gawk-4.2.1
./configure && make && make check
NOTE: Because of the 'https://' URL, you may have to supply
the '--no-check-certificate' option to 'wget' to download the
file.
* 'gawk' may be built on non-POSIX systems as well. The currently
supported systems are MS-Windows using MSYS, DJGPP, MinGW, and
Cygwin, and both Vax/VMS and OpenVMS. Instructions for each system
are included in this major node.
* Bug reports should be sent via email to <bug-gawk AT gnu.org>. Bug
reports should be in English and should include the version of
'gawk', how it was compiled, and a short program and data file that
demonstrate the problem.
* There are a number of other freely available 'awk' implementations.
Many are POSIX-compliant; others are less so.
File: gawk.info, Node: Notes, Next: Basic Concepts, Prev: Installation, Up: Top
Appendix C Implementation Notes
*******************************
This appendix contains information mainly of interest to implementers
and maintainers of 'gawk'. Everything in it applies specifically to
'gawk' and not to other implementations.
* Menu:
* Compatibility Mode:: How to disable certain 'gawk'
extensions.
* Additions:: Making Additions To 'gawk'.
* Future Extensions:: New features that may be implemented one day.
* Implementation Limitations:: Some limitations of the implementation.
* Extension Design:: Design notes about the extension API.
* Old Extension Mechanism:: Some compatibility for old extensions.
* Notes summary:: Summary of implementation notes.
File: gawk.info, Node: Compatibility Mode, Next: Additions, Up: Notes
C.1 Downward Compatibility and Debugging
========================================
*Note POSIX/GNU::, for a summary of the GNU extensions to the 'awk'
language and program. All of these features can be turned off by
invoking 'gawk' with the '--traditional' option or with the '--posix'
option.
If 'gawk' is compiled for debugging with '-DDEBUG', then there is one
more option available on the command line:
'-Y'
'--parsedebug'
Print out the parse stack information as the program is being
parsed.
This option is intended only for serious 'gawk' developers and not
for the casual user. It probably has not even been compiled into your
version of 'gawk', since it slows down execution.
File: gawk.info, Node: Additions, Next: Future Extensions, Prev: Compatibility Mode, Up: Notes
C.2 Making Additions to 'gawk'
==============================
If you find that you want to enhance 'gawk' in a significant fashion,
you are perfectly free to do so. That is the point of having free
software; the source code is available and you are free to change it as
you want (*note Copying::).
This minor node discusses the ways you might want to change 'gawk' as
well as any considerations you should bear in mind.
* Menu:
* Accessing The Source:: Accessing the Git repository.
* Adding Code:: Adding code to the main body of
'gawk'.
* New Ports:: Porting 'gawk' to a new operating
system.
* Derived Files:: Why derived files are kept in the Git
repository.
File: gawk.info, Node: Accessing The Source, Next: Adding Code, Up: Additions
C.2.1 Accessing The 'gawk' Git Repository
-----------------------------------------
As 'gawk' is Free Software, the source code is always available. *note
Gawk Distribution:: describes how to get and build the formal, released
versions of 'gawk'.
However, if you want to modify 'gawk' and contribute back your
changes, you will probably wish to work with the development version.
To do so, you will need to access the 'gawk' source code repository.
The code is maintained using the Git distributed version control system
(https://git-scm.com). You will need to install it if your system
doesn't have it. Once you have done so, use the command:
git clone git://git.savannah.gnu.org/gawk.git
This clones the 'gawk' repository. If you are behind a firewall that
does not allow you to use the Git native protocol, you can still access
the repository using:
git clone https://git.savannah.gnu.org/r/gawk.git
Once you have made changes, you can use 'git diff' to produce a
patch, and send that to the 'gawk' maintainer; see *note Bugs::, for how
to do that.
Once upon a time there was Git-CVS gateway for use by people who
could not install Git. However, this gateway no longer works, so you
may have better luck using a more modern version control system like
Bazaar, that has a Git plug-in for working with Git repositories.
File: gawk.info, Node: Adding Code, Next: New Ports, Prev: Accessing The Source, Up: Additions
C.2.2 Adding New Features
-------------------------
You are free to add any new features you like to 'gawk'. However, if
you want your changes to be incorporated into the 'gawk' distribution,
there are several steps that you need to take in order to make it
possible to include them:
1. Before building the new feature into 'gawk' itself, consider
writing it as an extension (*note Dynamic Extensions::). If that's
not possible, continue with the rest of the steps in this list.
2. Be prepared to sign the appropriate paperwork. In order for the
FSF to distribute your changes, you must either place those changes
in the public domain and submit a signed statement to that effect,
or assign the copyright in your changes to the FSF. Both of these
actions are easy to do and _many_ people have done so already. If
you have questions, please contact me (*note Bugs::), or
<assign AT gnu.org>.
3. Get the latest version. It is much easier for me to integrate
changes if they are relative to the most recent distributed version
of 'gawk', or better yet, relative to the latest code in the Git
repository. If your version of 'gawk' is very old, I may not be
able to integrate your changes at all. (*Note Getting::, for
information on getting the latest version of 'gawk'.)
4. See *note (Version, standards, GNU Coding Standards)Top::. This
document describes how GNU software should be written. If you
haven't read it, please do so, preferably _before_ starting to
modify 'gawk'. (The 'GNU Coding Standards' are available from the
GNU Project's website (https://www.gnu.org/prep/standards/).
Texinfo, Info, and DVI versions are also available.)
5. Use the 'gawk' coding style. The C code for 'gawk' follows the
instructions in the 'GNU Coding Standards', with minor exceptions.
The code is formatted using the traditional "K&R" style,
particularly as regards to the placement of braces and the use of
TABs. In brief, the coding rules for 'gawk' are as follows:
* Use ANSI/ISO style (prototype) function headers when defining
functions.
* Put the name of the function at the beginning of its own line.
* Use '#elif' instead of nesting '#if' inside '#else'.
* Put the return type of the function, even if it is 'int', on
the line above the line with the name and arguments of the
function.
* Put spaces around parentheses used in control structures
('if', 'while', 'for', 'do', 'switch', and 'return').
* Do not put spaces in front of parentheses used in function
calls.
* Put spaces around all C operators and after commas in function
calls.
* Do not use the comma operator to produce multiple side
effects, except in 'for' loop initialization and increment
parts, and in macro bodies.
* Use real TABs for indenting, not spaces.
* Use the "K&R" brace layout style.
* Use comparisons against 'NULL' and ''\0'' in the conditions of
'if', 'while', and 'for' statements, as well as in the 'case's
of 'switch' statements, instead of just the plain pointer or
character value.
* Use 'true' and 'false' for 'bool' values, the 'NULL' symbolic
constant for pointer values, and the character constant ''\0''
where appropriate, instead of '1' and '0'.
* Provide one-line descriptive comments for each function.
* Do not use the 'alloca()' function for allocating memory off
the stack. Its use causes more portability trouble than is
worth the minor benefit of not having to free the storage.
Instead, use 'malloc()' and 'free()'.
* Do not use comparisons of the form '! strcmp(a, b)' or
similar. As Henry Spencer once said, "'strcmp()' is not a
boolean!" Instead, use 'strcmp(a, b) == 0'.
* If adding new bit flag values, use explicit hexadecimal
constants ('0x001', '0x002', '0x004', and so on) instead of
shifting one left by successive amounts ('(1<<0)', '(1<<1)',
and so on).
NOTE: If I have to reformat your code to follow the coding
style used in 'gawk', I may not bother to integrate your
changes at all.
6. Update the documentation. Along with your new code, please supply
new sections and/or chapters for this Info file. If at all
possible, please use real Texinfo, instead of just supplying
unformatted ASCII text (although even that is better than no
documentation at all). Conventions to be followed in 'GAWK:
Effective AWK Programming' are provided after the '@bye' at the end
of the Texinfo source file. If possible, please update the 'man'
page as well.
You will also have to sign paperwork for your documentation
changes.
7. Submit changes as unified diffs. Use 'diff -u -r -N' to compare
the original 'gawk' source tree with your version. I recommend
using the GNU version of 'diff', or best of all, 'git diff' or 'git
format-patch'. Send the output produced by 'diff' to me when you
submit your changes. (*Note Bugs::, for the electronic mail
information.)
Using this format makes it easy for me to apply your changes to the
master version of the 'gawk' source code (using 'patch'). If I
have to apply the changes manually, using a text editor, I may not
do so, particularly if there are lots of changes.
8. Include an entry for the 'ChangeLog' file with your submission.
This helps further minimize the amount of work I have to do, making
it easier for me to accept patches. It is simplest if you just
make this part of your diff.
Although this sounds like a lot of work, please remember that while
you may write the new code, I have to maintain it and support it. If it
isn't possible for me to do that with a minimum of extra work, then I
probably will not.
File: gawk.info, Node: New Ports, Next: Derived Files, Prev: Adding Code, Up: Additions
C.2.3 Porting 'gawk' to a New Operating System
----------------------------------------------
If you want to port 'gawk' to a new operating system, there are several
steps:
1. Follow the guidelines in *note Adding Code::, concerning coding
style, submission of diffs, and so on.
2. Be prepared to sign the appropriate paperwork. In order for the
FSF to distribute your code, you must either place your code in the
public domain and submit a signed statement to that effect, or
assign the copyright in your code to the FSF. Both of these actions
are easy to do and _many_ people have done so already. If you have
questions, please contact me, or <gnu AT gnu.org>.
3. When doing a port, bear in mind that your code must coexist
peacefully with the rest of 'gawk' and the other ports. Avoid
gratuitous changes to the system-independent parts of the code. If
at all possible, avoid sprinkling '#ifdef's just for your port
throughout the code.
If the changes needed for a particular system affect too much of
the code, I probably will not accept them. In such a case, you
can, of course, distribute your changes on your own, as long as you
comply with the GPL (*note Copying::).
4. A number of the files that come with 'gawk' are maintained by other
people. Thus, you should not change them unless it is for a very
good reason; i.e., changes are not out of the question, but changes
to these files are scrutinized extra carefully. The files are
'dfa.c', 'dfa.h', 'getopt.c', 'getopt.h', 'getopt1.c',
'getopt_int.h', 'gettext.h', 'regcomp.c', 'regex.c', 'regex.h',
'regex_internal.c', 'regex_internal.h', and 'regexec.c'.
5. A number of other files are provided by the GNU Autotools
(Autoconf, Automake, and GNU 'gettext'). You should not change
them either, unless it is for a very good reason. The files are
'ABOUT-NLS', 'config.guess', 'config.rpath', 'config.sub',
'depcomp', 'INSTALL', 'install-sh', 'missing', 'mkinstalldirs',
'xalloc.h', and 'ylwrap'.
6. Be willing to continue to maintain the port. Non-Unix operating
systems are supported by volunteers who maintain the code needed to
compile and run 'gawk' on their systems. If no-one volunteers to
maintain a port, it becomes unsupported and it may be necessary to
remove it from the distribution.
7. Supply an appropriate 'gawkmisc.???' file. Each port has its own
'gawkmisc.???' that implements certain operating system specific
functions. This is cleaner than a plethora of '#ifdef's scattered
throughout the code. The 'gawkmisc.c' in the main source directory
includes the appropriate 'gawkmisc.???' file from each
subdirectory. Be sure to update it as well.
Each port's 'gawkmisc.???' file has a suffix reminiscent of the
machine or operating system for the port--for example,
'pc/gawkmisc.pc' and 'vms/gawkmisc.vms'. The use of separate
suffixes, instead of plain 'gawkmisc.c', makes it possible to move
files from a port's subdirectory into the main subdirectory,
without accidentally destroying the real 'gawkmisc.c' file.
(Currently, this is only an issue for the PC operating system
ports.)
8. Supply a 'Makefile' as well as any other C source and header files
that are necessary for your operating system. All your code should
be in a separate subdirectory, with a name that is the same as, or
reminiscent of, either your operating system or the computer
system. If possible, try to structure things so that it is not
necessary to move files out of the subdirectory into the main
source directory. If that is not possible, then be sure to avoid
using names for your files that duplicate the names of files in the
main source directory.
9. Update the documentation. Please write a section (or sections) for
this Info file describing the installation and compilation steps
needed to compile and/or install 'gawk' for your system.
Following these steps makes it much easier to integrate your changes
into 'gawk' and have them coexist happily with other operating systems'
code that is already there.
In the code that you supply and maintain, feel free to use a coding
style and brace layout that suits your taste.
File: gawk.info, Node: Derived Files, Prev: New Ports, Up: Additions
C.2.4 Why Generated Files Are Kept In Git
-----------------------------------------
If you look at the 'gawk' source in the Git repository, you will notice
that it includes files that are automatically generated by GNU
infrastructure tools, such as 'Makefile.in' from Automake and even
'configure' from Autoconf.
This is different from many Free Software projects that do not store
the derived files, because that keeps the repository less cluttered, and
it is easier to see the substantive changes when comparing versions and
trying to understand what changed between commits.
However, there are several reasons why the 'gawk' maintainer likes to
have everything in the repository.
First, because it is then easy to reproduce any given version
completely, without relying upon the availability of (older, likely
obsolete, and maybe even impossible to find) other tools.
As an extreme example, if you ever even think about trying to
compile, oh, say, the V7 'awk', you will discover that not only do you
have to bootstrap the V7 'yacc' to do so, but you also need the V7
'lex'. And the latter is pretty much impossible to bring up on a modern
GNU/Linux system.(1)
(Or, let's say 'gawk' 1.2 required 'bison' whatever-it-was in 1989
and that there was no 'awkgram.c' file in the repository. Is there a
guarantee that we could find that 'bison' version? Or that _it_ would
build?)
If the repository has all the generated files, then it's easy to just
check them out and build. (Or _easier_, depending upon how far back we
go.)
And that brings us to the second (and stronger) reason why all the
files really need to be in Git. It boils down to who do you cater
to--the 'gawk' developer(s), or the user who just wants to check out a
version and try it out?
The 'gawk' maintainer wants it to be possible for any interested
'awk' user in the world to just clone the repository, check out the
branch of interest and build it. Without their having to have the
correct version(s) of the autotools.(2) That is the point of the
'bootstrap.sh' file. It touches the various other files in the right
order such that
# The canonical incantation for building GNU software:
./bootstrap.sh && ./configure && make
will _just work_.
This is extremely important for the 'master' and 'gawk-X.Y-stable'
branches.
Further, the 'gawk' maintainer would argue that it's also important
for the 'gawk' developers. When he tried to check out the 'xgawk'
branch(3) to build it, he couldn't. (No 'ltmain.sh' file, and he had no
idea how to create it, and that was not the only problem.)
He felt _extremely_ frustrated. With respect to that branch, the
maintainer is no different than Jane User who wants to try to build
'gawk-4.1-stable' or 'master' from the repository.
Thus, the maintainer thinks that it's not just important, but
critical, that for any given branch, the above incantation _just works_.
A third reason to have all the files is that without them, using 'git
bisect' to try to find the commit that introduced a bug is exceedingly
difficult. The maintainer tried to do that on another project that
requires running bootstrapping scripts just to create 'configure' and so
on; it was really painful. When the repository is self-contained, using
'git bisect' in it is very easy.
What are some of the consequences and/or actions to take?
1. We don't mind that there are differing files in the different
branches as a result of different versions of the autotools.
A. It's the maintainer's job to merge them and he will deal with
it.
B. He is really good at 'git diff x y > /tmp/diff1 ; gvim
/tmp/diff1' to remove the diffs that aren't of interest in
order to review code.
2. It would certainly help if everyone used the same versions of the
GNU tools as he does, which in general are the latest released
versions of Automake, Autoconf, 'bison', and GNU 'gettext'.
Installing from source is quite easy. It's how the maintainer
worked for years (and still works). He had '/usr/local/bin' at the
front of his 'PATH' and just did:
wget https://ftp.gnu.org/gnu/PACKAGE/PACKAGE-X.Y.Z.tar.gz
tar -xpzvf PACKAGE-X.Y.Z.tar.gz
cd PACKAGE-X.Y.Z
./configure && make && make check
make install # as root
NOTE: Because of the 'https://' URL, you may have to supply
the '--no-check-certificate' option to 'wget' to download the
file.
Most of the above was originally written by the maintainer to other
'gawk' developers. It raised the objection from one of the developers
"... that anybody pulling down the source from Git is not an end user."
However, this is not true. There are "power 'awk' users" who can
build 'gawk' (using the magic incantation shown previously) but who
can't program in C. Thus, the major branches should be kept buildable
all the time.
It was then suggested that there be a 'cron' job to create nightly
tarballs of "the source." Here, the problem is that there are source
trees, corresponding to the various branches! So, nightly tarballs
aren't the answer, especially as the repository can go for weeks without
significant change being introduced.
Fortunately, the Git server can meet this need. For any given branch
named BRANCHNAME, use:
wget https://git.savannah.gnu.org/cgit/gawk.git/snapshot/gawk-BRANCHNAME.tar.gz
to retrieve a snapshot of the given branch.
---------- Footnotes ----------
(1) We tried. It was painful.
(2) There is one GNU program that is (in our opinion) severely
difficult to bootstrap from the Git repository. For example, on the
author's old (but still working) PowerPC Macintosh with Mac OS X 10.5,
it was necessary to bootstrap a ton of software, starting with Git
itself, in order to try to work with the latest code. It's not
pleasant, and especially on older systems, it's a big waste of time.
Starting with the latest tarball was no picnic either. The
maintainers had dropped '.gz' and '.bz2' files and only distribute
'.tar.xz' files. It was necessary to bootstrap 'xz' first!
(3) A branch (since removed) created by one of the other developers
that did not include the generated files.
File: gawk.info, Node: Future Extensions, Next: Implementation Limitations, Prev: Additions, Up: Notes
C.3 Probable Future Extensions
==============================
AWK is a language similar to PERL, only considerably more elegant.
-- _Arnold Robbins_
Hey!
-- _Larry Wall_
The 'TODO' file in the 'master' branch of the 'gawk' Git repository
lists possible future enhancements. Some of these relate to the source
code, and others to possible new features. Please see that file for the
list. *Note Additions::, if you are interested in tackling any of the
projects listed there.
File: gawk.info, Node: Implementation Limitations, Next: Extension Design, Prev: Future Extensions, Up: Notes
C.4 Some Limitations of the Implementation
==========================================
This following table describes limits of 'gawk' on a Unix-like system
(although it is variable even then). Other systems may have different
limits.
Item Limit
--------------------------------------------------------------------------
Characters in a character 2^(number of bits per byte)
class
Length of input record 'MAX_INT'
Length of output record Unlimited
Length of source line Unlimited
Number of fields in a 'MAX_LONG'
record
Number of file redirections Unlimited
Number of input records in 'MAX_LONG'
one file
Number of input records 'MAX_LONG'
total
Number of pipe redirections min(number of processes per user, number
of open files)
Numeric values Double-precision floating point (if not
using MPFR)
Size of a field 'MAX_INT'
Size of a literal string 'MAX_INT'
Size of a printf string 'MAX_INT'
File: gawk.info, Node: Extension Design, Next: Old Extension Mechanism, Prev: Implementation Limitations, Up: Notes
C.5 Extension API Design
========================
This minor node documents the design of the extension API, including a
discussion of some of the history and problems that needed to be solved.
The first version of extensions for 'gawk' was developed in the
mid-1990s and released with 'gawk' 3.1 in the late 1990s. The basic
mechanisms and design remained unchanged for close to 15 years, until
2012.
The old extension mechanism used data types and functions from 'gawk'
itself, with a "clever hack" to install extension functions.
'gawk' included some sample extensions, of which a few were really
useful. However, it was clear from the outset that the extension
mechanism was bolted onto the side and was not really well thought out.
* Menu:
* Old Extension Problems:: Problems with the old mechanism.
* Extension New Mechanism Goals:: Goals for the new mechanism.
* Extension Other Design Decisions:: Some other design decisions.
* Extension Future Growth:: Some room for future growth.
File: gawk.info, Node: Old Extension Problems, Next: Extension New Mechanism Goals, Up: Extension Design
C.5.1 Problems With The Old Mechanism
-------------------------------------
The old extension mechanism had several problems:
* It depended heavily upon 'gawk' internals. Any time the 'NODE'
structure(1) changed, an extension would have to be recompiled.
Furthermore, to really write extensions required understanding
something about 'gawk''s internal functions. There was some
documentation in this Info file, but it was quite minimal.
* Being able to call into 'gawk' from an extension required linker
facilities that are common on Unix-derived systems but that did not
work on MS-Windows systems; users wanting extensions on MS-Windows
had to statically link them into 'gawk', even though MS-Windows
supports dynamic loading of shared objects.
* The API would change occasionally as 'gawk' changed; no
compatibility between versions was ever offered or planned for.
Despite the drawbacks, the 'xgawk' project developers forked 'gawk'
and developed several significant extensions. They also enhanced
'gawk''s facilities relating to file inclusion and shared object access.
A new API was desired for a long time, but only in 2012 did the
'gawk' maintainer and the 'xgawk' developers finally start working on it
together. More information about the 'xgawk' project is provided in
*note gawkextlib::.
---------- Footnotes ----------
(1) A critical central data structure inside 'gawk'.
File: gawk.info, Node: Extension New Mechanism Goals, Next: Extension Other Design Decisions, Prev: Old Extension Problems, Up: Extension Design
C.5.2 Goals For A New Mechanism
-------------------------------
Some goals for the new API were:
* The API should be independent of 'gawk' internals. Changes in
'gawk' internals should not be visible to the writer of an
extension function.
* The API should provide _binary_ compatibility across 'gawk'
releases as long as the API itself does not change.
* The API should enable extensions written in C or C++ to have
roughly the same "appearance" to 'awk'-level code as 'awk'
functions do. This means that extensions should have:
- The ability to access function parameters.
- The ability to turn an undefined parameter into an array (call
by reference).
- The ability to create, access and update global variables.
- Easy access to all the elements of an array at once ("array
flattening") in order to loop over all the element in an easy
fashion for C code.
- The ability to create arrays (including 'gawk''s true arrays
of arrays).
Some additional important goals were:
* The API should use only features in ISO C 90, so that extensions
can be written using the widest range of C and C++ compilers. The
header should include the appropriate '#ifdef __cplusplus' and
'extern "C"' magic so that a C++ compiler could be used. (If using
C++, the runtime system has to be smart enough to call any
constructors and destructors, as 'gawk' is a C program. As of this
writing, this has not been tested.)
* The API mechanism should not require access to 'gawk''s symbols(1)
by the compile-time or dynamic linker, in order to enable creation
of extensions that also work on MS-Windows.
During development, it became clear that there were other features
that should be available to extensions, which were also subsequently
provided:
* Extensions should have the ability to hook into 'gawk''s I/O
redirection mechanism. In particular, the 'xgawk' developers
provided a so-called "open hook" to take over reading records.
During development, this was generalized to allow extensions to
hook into input processing, output processing, and two-way I/O.
* An extension should be able to provide a "call back" function to
perform cleanup actions when 'gawk' exits.
* An extension should be able to provide a version string so that
'gawk''s '--version' option can provide information about
extensions as well.
The requirement to avoid access to 'gawk''s symbols is, at first
glance, a difficult one to meet.
One design, apparently used by Perl and Ruby and maybe others, would
be to make the mainline 'gawk' code into a library, with the 'gawk'
utility a small C 'main()' function linked against the library.
This seemed like the tail wagging the dog, complicating build and
installation and making a simple copy of the 'gawk' executable from one
system to another (or one place to another on the same system!) into a
chancy operation.
Pat Rankin suggested the solution that was adopted. *Note Extension
Mechanism Outline::, for the details.
---------- Footnotes ----------
(1) The "symbols" are the variables and functions defined inside
'gawk'. Access to these symbols by code external to 'gawk' loaded
dynamically at runtime is problematic on MS-Windows.
File: gawk.info, Node: Extension Other Design Decisions, Next: Extension Future Growth, Prev: Extension New Mechanism Goals, Up: Extension Design
C.5.3 Other Design Decisions
----------------------------
As an arbitrary design decision, extensions can read the values of
predefined variables and arrays (such as 'ARGV' and 'FS'), but cannot
change them, with the exception of 'PROCINFO'.
The reason for this is to prevent an extension function from
affecting the flow of an 'awk' program outside its control. While a
real 'awk' function can do what it likes, that is at the discretion of
the programmer. An extension function should provide a service or make
a C API available for use within 'awk', and not mess with 'FS' or 'ARGC'
and 'ARGV'.
In addition, it becomes easy to start down a slippery slope. How
much access to 'gawk' facilities do extensions need? Do they need
'getline'? What about calling 'gsub()' or compiling regular
expressions? What about calling into 'awk' functions? (_That_ would be
messy.)
In order to avoid these issues, the 'gawk' developers chose to start
with the simplest, most basic features that are still truly useful.
Another decision is that although 'gawk' provides nice things like
MPFR, and arrays indexed internally by integers, these features are not
being brought out to the API in order to keep things simple and close to
traditional 'awk' semantics. (In fact, arrays indexed internally by
integers are so transparent that they aren't even documented!)
Additionally, all functions in the API check that their pointer input
parameters are not 'NULL'. If they are, they return an error. (It is a
good idea for extension code to verify that pointers received from
'gawk' are not 'NULL'. Such a thing should not happen, but the 'gawk'
developers are only human, and they have been known to occasionally make
mistakes.)
With time, the API will undoubtedly evolve; the 'gawk' developers
expect this to be driven by user needs. For now, the current API seems
to provide a minimal yet powerful set of features for creating
extensions.
File: gawk.info, Node: Extension Future Growth, Prev: Extension Other Design Decisions, Up: Extension Design
C.5.4 Room For Future Growth
----------------------------
The API can later be expanded, in two ways:
* 'gawk' passes an "extension id" into the extension when it first
loads the extension. The extension then passes this id back to
'gawk' with each function call. This mechanism allows 'gawk' to
identify the extension calling into it, should it need to know.
* Similarly, the extension passes a "name space" into 'gawk' when it
registers each extension function. This accommodates a possible
future mechanism for grouping extension functions and possibly
avoiding name conflicts.
Of course, as of this writing, no decisions have been made with
respect to any of the above.
File: gawk.info, Node: Old Extension Mechanism, Next: Notes summary, Prev: Extension Design, Up: Notes
C.6 Compatibility For Old Extensions
====================================
*note Dynamic Extensions::, describes the supported API and mechanisms
for writing extensions for 'gawk'. This API was introduced in version
4.1. However, for many years 'gawk' provided an extension mechanism
that required knowledge of 'gawk' internals and that was not as well
designed.
In order to provide a transition period, 'gawk' version 4.1 continues
to support the original extension mechanism. This will be true for the
life of exactly one major release. This support will be withdrawn, and
removed from the source code, at the next major release.
Briefly, original-style extensions should be compiled by including
the 'awk.h' header file in the extension source code. Additionally, you
must define the identifier 'GAWK' when building (use '-DGAWK' with
Unix-style compilers). Otherwise, the definitions in 'gawkapi.h' will
cause conflicts with those in 'awk.h' and your extension will not
compile.
Just as in previous versions, you load an old-style extension with
the 'extension()' built-in function (which is not otherwise documented).
This function in turn finds and loads the shared object file containing
the extension and calls its 'dl_load()' C routine.
Because original-style and new-style extensions use different
initialization routines ('dl_load()' versus 'dlload()'), they may safely
be installed in the same directory (to be found by 'AWKLIBPATH') without
conflict.
The 'gawk' development team strongly recommends that you convert any
old extensions that you may have to use the new API described in *note
Dynamic Extensions::.
File: gawk.info, Node: Notes summary, Prev: Old Extension Mechanism, Up: Notes
C.7 Summary
===========
* 'gawk''s extensions can be disabled with either the '--traditional'
option or with the '--posix' option. The '--parsedebug' option is
available if 'gawk' is compiled with '-DDEBUG'.
* The source code for 'gawk' is maintained in a publicly accessible
Git repository. Anyone may check it out and view the source.
* Contributions to 'gawk' are welcome. Following the steps outlined
in this major node will make it easier to integrate your
contributions into the code base. This applies both to new feature
contributions and to ports to additional operating systems.
* 'gawk' has some limits--generally those that are imposed by the
machine architecture.
* The extension API design was intended to solve a number of problems
with the previous extension mechanism, enable features needed by
the 'xgawk' project, and provide binary compatibility going
forward.
* The previous extension mechanism is still supported in version 4.1
of 'gawk', but it _will_ be removed in the next major release.
File: gawk.info, Node: Basic Concepts, Next: Glossary, Prev: Notes, Up: Top
Appendix D Basic Programming Concepts
*************************************
This major node attempts to define some of the basic concepts and terms
that are used throughout the rest of this Info file. As this Info file
is specifically about 'awk', and not about computer programming in
general, the coverage here is by necessity fairly cursory and
simplistic. (If you need more background, there are many other
introductory texts that you should refer to instead.)
* Menu:
* Basic High Level:: The high level view.
* Basic Data Typing:: A very quick intro to data types.
File: gawk.info, Node: Basic High Level, Next: Basic Data Typing, Up: Basic Concepts
D.1 What a Program Does
=======================
At the most basic level, the job of a program is to process some input
data and produce results. See *note Figure D.1: figure-general-flow.
_______
+------+ / \ +---------+
| Data | -----> < Program > -----> | Results |
+------+ \_______/ +---------+
Figure D.1: General Program Flow
The "program" in the figure can be either a compiled program(1) (such
as 'ls'), or it may be "interpreted". In the latter case, a
machine-executable program such as 'awk' reads your program, and then
uses the instructions in your program to process the data.
When you write a program, it usually consists of the following, very
basic set of steps, as shown in *note Figure D.2: figure-process-flow.:
______
+----------------+ / More \ No +----------+
| Initialization | -------> < Data > -------> | Clean Up |
+----------------+ ^ \ ? / +----------+
| +--+-+
| | Yes
| |
| V
| +---------+
+-----+ Process |
+---------+
Figure D.2: Basic Program Steps
Initialization
These are the things you do before actually starting to process
data, such as checking arguments, initializing any data you need to
work with, and so on. This step corresponds to 'awk''s 'BEGIN'
rule (*note BEGIN/END::).
If you were baking a cake, this might consist of laying out all the
mixing bowls and the baking pan, and making sure you have all the
ingredients that you need.
Processing
This is where the actual work is done. Your program reads data,
one logical chunk at a time, and processes it as appropriate.
In most programming languages, you have to manually manage the
reading of data, checking to see if there is more each time you
read a chunk. 'awk''s pattern-action paradigm (*note Getting
Started::) handles the mechanics of this for you.
In baking a cake, the processing corresponds to the actual labor:
breaking eggs, mixing the flour, water, and other ingredients, and
then putting the cake into the oven.
Clean Up
Once you've processed all the data, you may have things you need to
do before exiting. This step corresponds to 'awk''s 'END' rule
(*note BEGIN/END::).
After the cake comes out of the oven, you still have to wrap it in
plastic wrap to keep anyone from tasting it, as well as wash the
mixing bowls and utensils.
An "algorithm" is a detailed set of instructions necessary to
accomplish a task, or process data. It is much the same as a recipe for
baking a cake. Programs implement algorithms. Often, it is up to you
to design the algorithm and implement it, simultaneously.
The "logical chunks" we talked about previously are called "records",
similar to the records a company keeps on employees, a school keeps for
students, or a doctor keeps for patients. Each record has many
component parts, such as first and last names, date of birth, address,
and so on. The component parts are referred to as the "fields" of the
record.
The act of reading data is termed "input", and that of generating
results, not too surprisingly, is termed "output". They are often
referred to together as "input/output," and even more often, as "I/O"
for short. (You will also see "input" and "output" used as verbs.)
'awk' manages the reading of data for you, as well as the breaking it
up into records and fields. Your program's job is to tell 'awk' what to
do with the data. You do this by describing "patterns" in the data to
look for, and "actions" to execute when those patterns are seen. This
"data-driven" nature of 'awk' programs usually makes them both easier to
write and easier to read.
---------- Footnotes ----------
(1) Compiled programs are typically written in lower-level languages
such as C, C++, or Ada, and then translated, or "compiled", into a form
that the computer can execute directly.
File: gawk.info, Node: Basic Data Typing, Prev: Basic High Level, Up: Basic Concepts
D.2 Data Values in a Computer
=============================
In a program, you keep track of information and values in things called
"variables". A variable is just a name for a given value, such as
'first_name', 'last_name', 'address', and so on. 'awk' has several
predefined variables, and it has special names to refer to the current
input record and the fields of the record. You may also group multiple
associated values under one name, as an array.
Data, particularly in 'awk', consists of either numeric values, such
as 42 or 3.1415927, or string values. String values are essentially
anything that's not a number, such as a name. Strings are sometimes
referred to as "character data", since they store the individual
characters that comprise them. Individual variables, as well as numeric
and string variables, are referred to as "scalar" values. Groups of
values, such as arrays, are not scalars.
*note Computer Arithmetic::, provided a basic introduction to numeric
types (integer and floating-point) and how they are used in a computer.
Please review that information, including a number of caveats that were
presented.
While you are probably used to the idea of a number without a value
(i.e., zero), it takes a bit more getting used to the idea of
zero-length character data. Nevertheless, such a thing exists. It is
called the "null string". The null string is character data that has no
value. In other words, it is empty. It is written in 'awk' programs
like this: '""'.
Humans are used to working in decimal; i.e., base 10. In base 10,
numbers go from 0 to 9, and then "roll over" into the next column.
(Remember grade school? 42 = 4 x 10 + 2.)
There are other number bases though. Computers commonly use base 2
or "binary", base 8 or "octal", and base 16 or "hexadecimal". In
binary, each column represents two times the value in the column to its
right. Each column may contain either a 0 or a 1. Thus, binary 1010
represents (1 x 8) + (0 x 4) + (1 x 2) + (0 x 1), or decimal 10. Octal
and hexadecimal are discussed more in *note Nondecimal-numbers::.
At the very lowest level, computers store values as groups of binary
digits, or "bits". Modern computers group bits into groups of eight,
called "bytes". Advanced applications sometimes have to manipulate bits
directly, and 'gawk' provides functions for doing so.
Programs are written in programming languages. Hundreds, if not
thousands, of programming languages exist. One of the most popular is
the C programming language. The C language had a very strong influence
on the design of the 'awk' language.
There have been several versions of C. The first is often referred to
as "K&R" C, after the initials of Brian Kernighan and Dennis Ritchie,
the authors of the first book on C. (Dennis Ritchie created the
language, and Brian Kernighan was one of the creators of 'awk'.)
In the mid-1980s, an effort began to produce an international
standard for C. This work culminated in 1989, with the production of the
ANSI standard for C. This standard became an ISO standard in 1990. In
1999, a revised ISO C standard was approved and released. Where it
makes sense, POSIX 'awk' is compatible with 1999 ISO C.
File: gawk.info, Node: Glossary, Next: Copying, Prev: Basic Concepts, Up: Top
Glossary
********
Action
A series of 'awk' statements attached to a rule. If the rule's
pattern matches an input record, 'awk' executes the rule's action.
Actions are always enclosed in braces. (*Note Action Overview::.)
Ada
A programming language originally defined by the U.S. Department of
Defense for embedded programming. It was designed to enforce good
Software Engineering practices.
Amazing 'awk' Assembler
Henry Spencer at the University of Toronto wrote a retargetable
assembler completely as 'sed' and 'awk' scripts. It is thousands
of lines long, including machine descriptions for several eight-bit
microcomputers. It is a good example of a program that would have
been better written in another language.
Amazingly Workable Formatter ('awf')
Henry Spencer at the University of Toronto wrote a formatter that
accepts a large subset of the 'nroff -ms' and 'nroff -man'
formatting commands, using 'awk' and 'sh'.
Anchor
The regexp metacharacters '^' and '$', which force the match to the
beginning or end of the string, respectively.
ANSI
The American National Standards Institute. This organization
produces many standards, among them the standards for the C and C++
programming languages. These standards often become international
standards as well. See also "ISO."
Argument
An argument can be two different things. It can be an option or a
file name passed to a command while invoking it from the command
line, or it can be something passed to a "function" inside a
program, e.g. inside 'awk'.
In the latter case, an argument can be passed to a function in two
ways. Either it is given to the called function by value, i.e., a
copy of the value of the variable is made available to the called
function, but the original variable cannot be modified by the
function itself; or it is given by reference, i.e., a pointer to
the interested variable is passed to the function, which can then
directly modify it. In 'awk' scalars are passed by value, and
arrays are passed by reference. See "Pass By Value/Reference."
Array
A grouping of multiple values under the same name. Most languages
just provide sequential arrays. 'awk' provides associative arrays.
Assertion
A statement in a program that a condition is true at this point in
the program. Useful for reasoning about how a program is supposed
to behave.
Assignment
An 'awk' expression that changes the value of some 'awk' variable
or data object. An object that you can assign to is called an
"lvalue". The assigned values are called "rvalues". *Note
Assignment Ops::.
Associative Array
Arrays in which the indices may be numbers or strings, not just
sequential integers in a fixed range.
'awk' Language
The language in which 'awk' programs are written.
'awk' Program
An 'awk' program consists of a series of "patterns" and "actions",
collectively known as "rules". For each input record given to the
program, the program's rules are all processed in turn. 'awk'
programs may also contain function definitions.
'awk' Script
Another name for an 'awk' program.
Bash
The GNU version of the standard shell (the Bourne-Again SHell).
See also "Bourne Shell."
Binary
Base-two notation, where the digits are '0'-'1'. Since electronic
circuitry works "naturally" in base 2 (just think of Off/On),
everything inside a computer is calculated using base 2. Each
digit represents the presence (or absence) of a power of 2 and is
called a "bit". So, for example, the base-two number '10101' is
the same as decimal 21, ((1 x 16) + (1 x 4) + (1 x 1)).
Since base-two numbers quickly become very long to read and write,
they are usually grouped by 3 (i.e., they are read as octal
numbers), or by 4 (i.e., they are read as hexadecimal numbers).
There is no direct way to insert base 2 numbers in a C program. If
need arises, such numbers are usually inserted as octal or
hexadecimal numbers. The number of base-two digits that fit into
registers used for representing integer numbers in computers is a
rough indication of the computing power of the computer itself.
Most computers nowadays use 64 bits for representing integer
numbers in their registers, but 32-bit, 16-bit and 8-bit registers
have been widely used in the past. *Note Nondecimal-numbers::.
Bit
Short for "Binary Digit." All values in computer memory ultimately
reduce to binary digits: values that are either zero or one.
Groups of bits may be interpreted differently--as integers,
floating-point numbers, character data, addresses of other memory
objects, or other data. 'awk' lets you work with floating-point
numbers and strings. 'gawk' lets you manipulate bit values with
the built-in functions described in *note Bitwise Functions::.
Computers are often defined by how many bits they use to represent
integer values. Typical systems are 32-bit systems, but 64-bit
systems are becoming increasingly popular, and 16-bit systems have
essentially disappeared.
Boolean Expression
Named after the English mathematician Boole. See also "Logical
Expression."
Bourne Shell
The standard shell ('/bin/sh') on Unix and Unix-like systems,
originally written by Steven R. Bourne at Bell Laboratories. Many
shells (Bash, 'ksh', 'pdksh', 'zsh') are generally upwardly
compatible with the Bourne shell.
Braces
The characters '{' and '}'. Braces are used in 'awk' for
delimiting actions, compound statements, and function bodies.
Bracket Expression
Inside a "regular expression", an expression included in square
brackets, meant to designate a single character as belonging to a
specified character class. A bracket expression can contain a list
of one or more characters, like '[abc]', a range of characters,
like '[A-Z]', or a name, delimited by ':', that designates a known
set of characters, like '[:digit:]'. The form of bracket
expression enclosed between ':' is independent of the underlying
representation of the character themselves, which could utilize the
ASCII, EBCDIC, or Unicode codesets, depending on the architecture
of the computer system, and on localization. See also "Regular
Expression."
Built-in Function
The 'awk' language provides built-in functions that perform various
numerical, I/O-related, and string computations. Examples are
'sqrt()' (for the square root of a number) and 'substr()' (for a
substring of a string). 'gawk' provides functions for timestamp
management, bit manipulation, array sorting, type checking, and
runtime string translation. (*Note Built-in::.)
Built-in Variable
'ARGC', 'ARGV', 'CONVFMT', 'ENVIRON', 'FILENAME', 'FNR', 'FS',
'NF', 'NR', 'OFMT', 'OFS', 'ORS', 'RLENGTH', 'RSTART', 'RS', and
'SUBSEP' are the variables that have special meaning to 'awk'. In
addition, 'ARGIND', 'BINMODE', 'ERRNO', 'FIELDWIDTHS', 'FPAT',
'IGNORECASE', 'LINT', 'PROCINFO', 'RT', and 'TEXTDOMAIN' are the
variables that have special meaning to 'gawk'. Changing some of
them affects 'awk''s running environment. (*Note Built-in
Variables::.)
C
The system programming language that most GNU software is written
in. The 'awk' programming language has C-like syntax, and this
Info file points out similarities between 'awk' and C when
appropriate.
In general, 'gawk' attempts to be as similar to the 1990 version of
ISO C as makes sense.
C Shell
The C Shell ('csh' or its improved version, 'tcsh') is a Unix shell
that was created by Bill Joy in the late 1970s. The C shell was
differentiated from other shells by its interactive features and
overall style, which looks more like C. The C Shell is not backward
compatible with the Bourne Shell, so special attention is required
when converting scripts written for other Unix shells to the C
shell, especially with regard to the management of shell variables.
See also "Bourne Shell."
C++
A popular object-oriented programming language derived from C.
Character Class
See "Bracket Expression."
Character List
See "Bracket Expression."
Character Set
The set of numeric codes used by a computer system to represent the
characters (letters, numbers, punctuation, etc.) of a particular
country or place. The most common character set in use today is
ASCII (American Standard Code for Information Interchange). Many
European countries use an extension of ASCII known as ISO-8859-1
(ISO Latin-1). The Unicode character set (http://www.unicode.org)
is increasingly popular and standard, and is particularly widely
used on GNU/Linux systems.
CHEM
A preprocessor for 'pic' that reads descriptions of molecules and
produces 'pic' input for drawing them. It was written in 'awk' by
Brian Kernighan and Jon Bentley, and is available from
<http://netlib.org/typesetting/chem>;.
Comparison Expression
A relation that is either true or false, such as 'a < b'.
Comparison expressions are used in 'if', 'while', 'do', and 'for'
statements, and in patterns to select which input records to
process. (*Note Typing and Comparison::.)
Compiler
A program that translates human-readable source code into
machine-executable object code. The object code is then executed
directly by the computer. See also "Interpreter."
Complemented Bracket Expression
The negation of a "bracket expression". All that is _not_
described by a given bracket expression. The symbol '^' precedes
the negated bracket expression. E.g.: '[^[:digit:]]' designates
whatever character is not a digit. '[^bad]' designates whatever
character is not one of the letters 'b', 'a', or 'd'. See "Bracket
Expression."
Compound Statement
A series of 'awk' statements, enclosed in curly braces. Compound
statements may be nested. (*Note Statements::.)
Computed Regexps
See "Dynamic Regular Expressions."
Concatenation
Concatenating two strings means sticking them together, one after
another, producing a new string. For example, the string 'foo'
concatenated with the string 'bar' gives the string 'foobar'.
(*Note Concatenation::.)
Conditional Expression
An expression using the '?:' ternary operator, such as 'EXPR1 ?
EXPR2 : EXPR3'. The expression EXPR1 is evaluated; if the result
is true, the value of the whole expression is the value of EXPR2;
otherwise the value is EXPR3. In either case, only one of EXPR2
and EXPR3 is evaluated. (*Note Conditional Exp::.)
Control Statement
A control statement is an instruction to perform a given operation
or a set of operations inside an 'awk' program, if a given
condition is true. Control statements are: 'if', 'for', 'while',
and 'do' (*note Statements::).
Cookie
A peculiar goodie, token, saying or remembrance produced by or
presented to a program. (With thanks to Professor Doug McIlroy.)
Coprocess
A subordinate program with which two-way communications is
possible.
Curly Braces
See "Braces."
Dark Corner
An area in the language where specifications often were (or still
are) not clear, leading to unexpected or undesirable behavior.
Such areas are marked in this Info file with "(d.c.)" in the text
and are indexed under the heading "dark corner."
Data Driven
A description of 'awk' programs, where you specify the data you are
interested in processing, and what to do when that data is seen.
Data Objects
These are numbers and strings of characters. Numbers are converted
into strings and vice versa, as needed. (*Note Conversion::.)
Deadlock
The situation in which two communicating processes are each waiting
for the other to perform an action.
Debugger
A program used to help developers remove "bugs" from (de-bug) their
programs.
Double Precision
An internal representation of numbers that can have fractional
parts. Double precision numbers keep track of more digits than do
single precision numbers, but operations on them are sometimes more
expensive. This is the way 'awk' stores numeric values. It is the
C type 'double'.
Dynamic Regular Expression
A dynamic regular expression is a regular expression written as an
ordinary expression. It could be a string constant, such as
'"foo"', but it may also be an expression whose value can vary.
(*Note Computed Regexps::.)
Empty String
See "Null String."
Environment
A collection of strings, of the form 'NAME=VAL', that each program
has available to it. Users generally place values into the
environment in order to provide information to various programs.
Typical examples are the environment variables 'HOME' and 'PATH'.
Epoch
The date used as the "beginning of time" for timestamps. Time
values in most systems are represented as seconds since the epoch,
with library functions available for converting these values into
standard date and time formats.
The epoch on Unix and POSIX systems is 1970-01-01 00:00:00 UTC. See
also "GMT" and "UTC."
Escape Sequences
A special sequence of characters used for describing nonprinting
characters, such as '\n' for newline or '\033' for the ASCII ESC
(Escape) character. (*Note Escape Sequences::.)
Extension
An additional feature or change to a programming language or
utility not defined by that language's or utility's standard.
'gawk' has (too) many extensions over POSIX 'awk'.
FDL
See "Free Documentation License."
Field
When 'awk' reads an input record, it splits the record into pieces
separated by whitespace (or by a separator regexp that you can
change by setting the predefined variable 'FS'). Such pieces are
called fields. If the pieces are of fixed length, you can use the
built-in variable 'FIELDWIDTHS' to describe their lengths. If you
wish to specify the contents of fields instead of the field
separator, you can use the predefined variable 'FPAT' to do so.
(*Note Field Separators::, *note Constant Size::, and *note
Splitting By Content::.)
Flag
A variable whose truth value indicates the existence or
nonexistence of some condition.
Floating-Point Number
Often referred to in mathematical terms as a "rational" or real
number, this is just a number that can have a fractional part. See
also "Double Precision" and "Single Precision."
Format
Format strings control the appearance of output in the 'strftime()'
and 'sprintf()' functions, and in the 'printf' statement as well.
Also, data conversions from numbers to strings are controlled by
the format strings contained in the predefined variables 'CONVFMT'
and 'OFMT'. (*Note Control Letters::.)
Fortran
Shorthand for FORmula TRANslator, one of the first programming
languages available for scientific calculations. It was created by
John Backus, and has been available since 1957. It is still in use
today.
Free Documentation License
This document describes the terms under which this Info file is
published and may be copied. (*Note GNU Free Documentation
License::.)
Free Software Foundation
A nonprofit organization dedicated to the production and
distribution of freely distributable software. It was founded by
Richard M. Stallman, the author of the original Emacs editor. GNU
Emacs is the most widely used version of Emacs today.
FSF
See "Free Software Foundation."
Function
A part of an 'awk' program that can be invoked from every point of
the program, to perform a task. 'awk' has several built-in
functions. Users can define their own functions in every part of
the program. Function can be recursive, i.e., they may invoke
themselves. *Note Functions::. In 'gawk' it is also possible to
have functions shared among different programs, and included where
required using the '@include' directive (*note Include Files::).
In 'gawk' the name of the function that should be invoked can be
generated at run time, i.e., dynamically. The 'gawk' extension API
provides constructor functions (*note Constructor Functions::).
'gawk'
The GNU implementation of 'awk'.
General Public License
This document describes the terms under which 'gawk' and its source
code may be distributed. (*Note Copying::.)
GMT
"Greenwich Mean Time." This is the old term for UTC. It is the
time of day used internally for Unix and POSIX systems. See also
"Epoch" and "UTC."
GNU
"GNU's not Unix". An on-going project of the Free Software
Foundation to create a complete, freely distributable,
POSIX-compliant computing environment.
GNU/Linux
A variant of the GNU system using the Linux kernel, instead of the
Free Software Foundation's Hurd kernel. The Linux kernel is a
stable, efficient, full-featured clone of Unix that has been ported
to a variety of architectures. It is most popular on PC-class
systems, but runs well on a variety of other systems too. The
Linux kernel source code is available under the terms of the GNU
General Public License, which is perhaps its most important aspect.
GPL
See "General Public License."
Hexadecimal
Base 16 notation, where the digits are '0'-'9' and 'A'-'F', with
'A' representing 10, 'B' representing 11, and so on, up to 'F' for
15. Hexadecimal numbers are written in C using a leading '0x', to
indicate their base. Thus, '0x12' is 18 ((1 x 16) + 2). *Note
Nondecimal-numbers::.
I/O
Abbreviation for "Input/Output," the act of moving data into and/or
out of a running program.
Input Record
A single chunk of data that is read in by 'awk'. Usually, an 'awk'
input record consists of one line of text. (*Note Records::.)
Integer
A whole number, i.e., a number that does not have a fractional
part.
Internationalization
The process of writing or modifying a program so that it can use
multiple languages without requiring further source code changes.
Interpreter
A program that reads human-readable source code directly, and uses
the instructions in it to process data and produce results. 'awk'
is typically (but not always) implemented as an interpreter. See
also "Compiler."
Interval Expression
A component of a regular expression that lets you specify repeated
matches of some part of the regexp. Interval expressions were not
originally available in 'awk' programs.
ISO
The International Organization for Standardization. This
organization produces international standards for many things,
including programming languages, such as C and C++. In the
computer arena, important standards like those for C, C++, and
POSIX become both American national and ISO international standards
simultaneously. This Info file refers to Standard C as "ISO C"
throughout. See the ISO website
(https://www.iso.org/iso/home/about.htm) for more information about
the name of the organization and its language-independent
three-letter acronym.
Java
A modern programming language originally developed by Sun
Microsystems (now Oracle) supporting Object-Oriented programming.
Although usually implemented by compiling to the instructions for a
standard virtual machine (the JVM), the language can be compiled to
native code.
Keyword
In the 'awk' language, a keyword is a word that has special
meaning. Keywords are reserved and may not be used as variable
names.
'gawk''s keywords are: 'BEGIN', 'BEGINFILE', 'END', 'ENDFILE',
'break', 'case', 'continue', 'default', 'delete', 'do...while',
'else', 'exit', 'for...in', 'for', 'function', 'func', 'if',
'next', 'nextfile', 'switch', and 'while'.
Korn Shell
The Korn Shell ('ksh') is a Unix shell which was developed by David
Korn at Bell Laboratories in the early 1980s. The Korn Shell is
backward-compatible with the Bourne shell and includes many
features of the C shell. See also "Bourne Shell."
Lesser General Public License
This document describes the terms under which binary library
archives or shared objects, and their source code may be
distributed.
LGPL
See "Lesser General Public License."
Linux
See "GNU/Linux."
Localization
The process of providing the data necessary for an
internationalized program to work in a particular language.
Logical Expression
An expression using the operators for logic, AND, OR, and NOT,
written '&&', '||', and '!' in 'awk'. Often called Boolean
expressions, after the mathematician who pioneered this kind of
mathematical logic.
Lvalue
An expression that can appear on the left side of an assignment
operator. In most languages, lvalues can be variables or array
elements. In 'awk', a field designator can also be used as an
lvalue.
Matching
The act of testing a string against a regular expression. If the
regexp describes the contents of the string, it is said to "match"
it.
Metacharacters
Characters used within a regexp that do not stand for themselves.
Instead, they denote regular expression operations, such as
repetition, grouping, or alternation.
Nesting
Nesting is where information is organized in layers, or where
objects contain other similar objects. In 'gawk' the '@include'
directive can be nested. The "natural" nesting of arithmetic and
logical operations can be changed using parentheses (*note
Precedence::).
No-op
An operation that does nothing.
Null String
A string with no characters in it. It is represented explicitly in
'awk' programs by placing two double quote characters next to each
other ('""'). It can appear in input data by having two successive
occurrences of the field separator appear next to each other.
Number
A numeric-valued data object. Modern 'awk' implementations use
double precision floating-point to represent numbers. Ancient
'awk' implementations used single precision floating-point.
Octal
Base-eight notation, where the digits are '0'-'7'. Octal numbers
are written in C using a leading '0', to indicate their base.
Thus, '013' is 11 ((1 x 8) + 3). *Note Nondecimal-numbers::.
Output Record
A single chunk of data that is written out by 'awk'. Usually, an
'awk' output record consists of one or more lines of text. *Note
Records::.
Pattern
Patterns tell 'awk' which input records are interesting to which
rules.
A pattern is an arbitrary conditional expression against which
input is tested. If the condition is satisfied, the pattern is
said to "match" the input record. A typical pattern might compare
the input record against a regular expression. (*Note Pattern
Overview::.)
PEBKAC
An acronym describing what is possibly the most frequent source of
computer usage problems. (Problem Exists Between Keyboard And
Chair.)
Plug-in
See "Extensions."
POSIX
The name for a series of standards that specify a Portable
Operating System interface. The "IX" denotes the Unix heritage of
these standards. The main standard of interest for 'awk' users is
'IEEE Standard for Information Technology, Standard 1003.1-2008'.
The 2008 POSIX standard can be found online at
<http://pubs.opengroup.org/onlinepubs/9699919799/>;.
Precedence
The order in which operations are performed when operators are used
without explicit parentheses.
Private
Variables and/or functions that are meant for use exclusively by
library functions and not for the main 'awk' program. Special care
must be taken when naming such variables and functions. (*Note
Library Names::.)
Range (of input lines)
A sequence of consecutive lines from the input file(s). A pattern
can specify ranges of input lines for 'awk' to process or it can
specify single lines. (*Note Pattern Overview::.)
Record
See "Input record" and "Output record."
Recursion
When a function calls itself, either directly or indirectly. If
this is clear, stop, and proceed to the next entry. Otherwise,
refer to the entry for "recursion."
Redirection
Redirection means performing input from something other than the
standard input stream, or performing output to something other than
the standard output stream.
You can redirect input to the 'getline' statement using the '<',
'|', and '|&' operators. You can redirect the output of the
'print' and 'printf' statements to a file or a system command,
using the '>', '>>', '|', and '|&' operators. (*Note Getline::,
and *note Redirection::.)
Reference Counts
An internal mechanism in 'gawk' to minimize the amount of memory
needed to store the value of string variables. If the value
assumed by a variable is used in more than one place, only one copy
of the value itself is kept, and the associated reference count is
increased when the same value is used by an additional variable,
and decreased when the related variable is no longer in use. When
the reference count goes to zero, the memory space used to store
the value of the variable is freed.
Regexp
See "Regular Expression."
Regular Expression
A regular expression ("regexp" for short) is a pattern that denotes
a set of strings, possibly an infinite set. For example, the
regular expression 'R.*xp' matches any string starting with the
letter 'R' and ending with the letters 'xp'. In 'awk', regular
expressions are used in patterns and in conditional expressions.
Regular expressions may contain escape sequences. (*Note
Regexp::.)
Regular Expression Constant
A regular expression constant is a regular expression written
within slashes, such as '/foo/'. This regular expression is chosen
when you write the 'awk' program and cannot be changed during its
execution. (*Note Regexp Usage::.)
Regular Expression Operators
See "Metacharacters."
Rounding
Rounding the result of an arithmetic operation can be tricky. More
than one way of rounding exists, and in 'gawk' it is possible to
choose which method should be used in a program. *Note Setting the
rounding mode::.
Rule
A segment of an 'awk' program that specifies how to process single
input records. A rule consists of a "pattern" and an "action".
'awk' reads an input record; then, for each rule, if the input
record satisfies the rule's pattern, 'awk' executes the rule's
action. Otherwise, the rule does nothing for that input record.
Rvalue
A value that can appear on the right side of an assignment
operator. In 'awk', essentially every expression has a value.
These values are rvalues.
Scalar
A single value, be it a number or a string. Regular variables are
scalars; arrays and functions are not.
Search Path
In 'gawk', a list of directories to search for 'awk' program source
files. In the shell, a list of directories to search for
executable programs.
'sed'
See "Stream Editor."
Seed
The initial value, or starting point, for a sequence of random
numbers.
Shell
The command interpreter for Unix and POSIX-compliant systems. The
shell works both interactively, and as a programming language for
batch files, or shell scripts.
Short-Circuit
The nature of the 'awk' logical operators '&&' and '||'. If the
value of the entire expression is determinable from evaluating just
the lefthand side of these operators, the righthand side is not
evaluated. (*Note Boolean Ops::.)
Side Effect
A side effect occurs when an expression has an effect aside from
merely producing a value. Assignment expressions, increment and
decrement expressions, and function calls have side effects.
(*Note Assignment Ops::.)
Single Precision
An internal representation of numbers that can have fractional
parts. Single precision numbers keep track of fewer digits than do
double precision numbers, but operations on them are sometimes less
expensive in terms of CPU time. This is the type used by some
ancient versions of 'awk' to store numeric values. It is the C
type 'float'.
Space
The character generated by hitting the space bar on the keyboard.
Special File
A file name interpreted internally by 'gawk', instead of being
handed directly to the underlying operating system--for example,
'/dev/stderr'. (*Note Special Files::.)
Statement
An expression inside an 'awk' program in the action part of a
pattern-action rule, or inside an 'awk' function. A statement can
be a variable assignment, an array operation, a loop, etc.
Stream Editor
A program that reads records from an input stream and processes
them one or more at a time. This is in contrast with batch
programs, which may expect to read their input files in entirety
before starting to do anything, as well as with interactive
programs which require input from the user.
String
A datum consisting of a sequence of characters, such as 'I am a
string'. Constant strings are written with double quotes in the
'awk' language and may contain escape sequences. (*Note Escape
Sequences::.)
Tab
The character generated by hitting the 'TAB' key on the keyboard.
It usually expands to up to eight spaces upon output.
Text Domain
A unique name that identifies an application. Used for grouping
messages that are translated at runtime into the local language.
Timestamp
A value in the "seconds since the epoch" format used by Unix and
POSIX systems. Used for the 'gawk' functions 'mktime()',
'strftime()', and 'systime()'. See also "Epoch," "GMT," and "UTC."
Unix
A computer operating system originally developed in the early
1970's at AT&T Bell Laboratories. It initially became popular in
universities around the world and later moved into commercial
environments as a software development system and network server
system. There are many commercial versions of Unix, as well as
several work-alike systems whose source code is freely available
(such as GNU/Linux, NetBSD (http://www.netbsd.org), FreeBSD
(https://www.freebsd.org), and OpenBSD (http://www.openbsd.org)).
UTC
The accepted abbreviation for "Universal Coordinated Time." This
is standard time in Greenwich, England, which is used as a
reference time for day and date calculations. See also "Epoch" and
"GMT."
Variable
A name for a value. In 'awk', variables may be either scalars or
arrays.
Whitespace
A sequence of space, TAB, or newline characters occurring inside an
input record or a string.
File: gawk.info, Node: Copying, Next: GNU Free Documentation License, Prev: Glossary, Up: Top
GNU General Public License
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country that you have reason to believe are valid.
If, pursuant to or in connection with a single transaction or
arrangement, you convey, or propagate by procuring conveyance of, a
covered work, and grant a patent license to some of the parties
receiving the covered work authorizing them to use, propagate,
modify or convey a specific copy of the covered work, then the
patent license you grant is automatically extended to all
recipients of the covered work and works based on it.
A patent license is "discriminatory" if it does not include within
the scope of its coverage, prohibits the exercise of, or is
conditioned on the non-exercise of one or more of the rights that
are specifically granted under this License. You may not convey a
covered work if you are a party to an arrangement with a third
party that is in the business of distributing software, under which
you make payment to the third party based on the extent of your
activity of conveying the work, and under which the third party
grants, to any of the parties who would receive the covered work
from you, a discriminatory patent license (a) in connection with
copies of the covered work conveyed by you (or copies made from
those copies), or (b) primarily for and in connection with specific
products or compilations that contain the covered work, unless you
entered into that arrangement, or that patent license was granted,
prior to 28 March 2007.
Nothing in this License shall be construed as excluding or limiting
any implied license or other defenses to infringement that may
otherwise be available to you under applicable patent law.
12. No Surrender of Others' Freedom.
If conditions are imposed on you (whether by court order, agreement
or otherwise) that contradict the conditions of this License, they
do not excuse you from the conditions of this License. If you
cannot convey a covered work so as to satisfy simultaneously your
obligations under this License and any other pertinent obligations,
then as a consequence you may not convey it at all. For example,
if you agree to terms that obligate you to collect a royalty for
further conveying from those to whom you convey the Program, the
only way you could satisfy both those terms and this License would
be to refrain entirely from conveying the Program.
13. Use with the GNU Affero General Public License.
Notwithstanding any other provision of this License, you have
permission to link or combine any covered work with a work licensed
under version 3 of the GNU Affero General Public License into a
single combined work, and to convey the resulting work. The terms
of this License will continue to apply to the part which is the
covered work, but the special requirements of the GNU Affero
General Public License, section 13, concerning interaction through
a network will apply to the combination as such.
14. Revised Versions of this License.
The Free Software Foundation may publish revised and/or new
versions of the GNU General Public License from time to time. Such
new versions will be similar in spirit to the present version, but
may differ in detail to address new problems or concerns.
Each version is given a distinguishing version number. If the
Program specifies that a certain numbered version of the GNU
General Public License "or any later version" applies to it, you
have the option of following the terms and conditions either of
that numbered version or of any later version published by the Free
Software Foundation. If the Program does not specify a version
number of the GNU General Public License, you may choose any
version ever published by the Free Software Foundation.
If the Program specifies that a proxy can decide which future
versions of the GNU General Public License can be used, that
proxy's public statement of acceptance of a version permanently
authorizes you to choose that version for the Program.
Later license versions may give you additional or different
permissions. However, no additional obligations are imposed on any
author or copyright holder as a result of your choosing to follow a
later version.
15. Disclaimer of Warranty.
THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY
APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE
COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS"
WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED,
INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE
RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU.
SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL
NECESSARY SERVICING, REPAIR OR CORRECTION.
16. Limitation of Liability.
IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN
WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES
AND/OR CONVEYS THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR
DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR
CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE
THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA
BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD
PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER
PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF
THE POSSIBILITY OF SUCH DAMAGES.
17. Interpretation of Sections 15 and 16.
If the disclaimer of warranty and limitation of liability provided
above cannot be given local legal effect according to their terms,
reviewing courts shall apply local law that most closely
approximates an absolute waiver of all civil liability in
connection with the Program, unless a warranty or assumption of
liability accompanies a copy of the Program in return for a fee.
END OF TERMS AND CONDITIONS
===========================
How to Apply These Terms to Your New Programs
=============================================
If you develop a new program, and you want it to be of the greatest
possible use to the public, the best way to achieve this is to make it
free software which everyone can redistribute and change under these
terms.
To do so, attach the following notices to the program. It is safest
to attach them to the start of each source file to most effectively
state the exclusion of warranty; and each file should have at least the
"copyright" line and a pointer to where the full notice is found.
ONE LINE TO GIVE THE PROGRAM'S NAME AND A BRIEF IDEA OF WHAT IT DOES.
Copyright (C) YEAR NAME OF AUTHOR
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or (at
your option) any later version.
This program is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <https://www.gnu.org/licenses/>;.
Also add information on how to contact you by electronic and paper
mail.
If the program does terminal interaction, make it output a short
notice like this when it starts in an interactive mode:
PROGRAM Copyright (C) YEAR NAME OF AUTHOR
This program comes with ABSOLUTELY NO WARRANTY; for details type 'show w'.
This is free software, and you are welcome to redistribute it
under certain conditions; type 'show c' for details.
The hypothetical commands 'show w' and 'show c' should show the
appropriate parts of the General Public License. Of course, your
program's commands might be different; for a GUI interface, you would
use an "about box".
You should also get your employer (if you work as a programmer) or
school, if any, to sign a "copyright disclaimer" for the program, if
necessary. For more information on this, and how to apply and follow
the GNU GPL, see <https://www.gnu.org/licenses/>;.
The GNU General Public License does not permit incorporating your
program into proprietary programs. If your program is a subroutine
library, you may consider it more useful to permit linking proprietary
applications with the library. If this is what you want to do, use the
GNU Lesser General Public License instead of this License. But first,
please read <https://www.gnu.org/philosophy/why-not-lgpl.html>;.
File: gawk.info, Node: GNU Free Documentation License, Next: Index, Prev: Copying, Up: Top
GNU Free Documentation License
******************************
Version 1.3, 3 November 2008
Copyright (C) 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
<https://fsf.org/>;
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
0. PREAMBLE
The purpose of this License is to make a manual, textbook, or other
functional and useful document "free" in the sense of freedom: to
assure everyone the effective freedom to copy and redistribute it,
with or without modifying it, either commercially or
noncommercially. Secondarily, this License preserves for the
author and publisher a way to get credit for their work, while not
being considered responsible for modifications made by others.
This License is a kind of "copyleft", which means that derivative
works of the document must themselves be free in the same sense.
It complements the GNU General Public License, which is a copyleft
license designed for free software.
We have designed this License in order to use it for manuals for
free software, because free software needs free documentation: a
free program should come with manuals providing the same freedoms
that the software does. But this License is not limited to
software manuals; it can be used for any textual work, regardless
of subject matter or whether it is published as a printed book. We
recommend this License principally for works whose purpose is
instruction or reference.
1. APPLICABILITY AND DEFINITIONS
This License applies to any manual or other work, in any medium,
that contains a notice placed by the copyright holder saying it can
be distributed under the terms of this License. Such a notice
grants a world-wide, royalty-free license, unlimited in duration,
to use that work under the conditions stated herein. The
"Document", below, refers to any such manual or work. Any member
of the public is a licensee, and is addressed as "you". You accept
the license if you copy, modify or distribute the work in a way
requiring permission under copyright law.
A "Modified Version" of the Document means any work containing the
Document or a portion of it, either copied verbatim, or with
modifications and/or translated into another language.
A "Secondary Section" is a named appendix or a front-matter section
of the Document that deals exclusively with the relationship of the
publishers or authors of the Document to the Document's overall
subject (or to related matters) and contains nothing that could
fall directly within that overall subject. (Thus, if the Document
is in part a textbook of mathematics, a Secondary Section may not
explain any mathematics.) The relationship could be a matter of
historical connection with the subject or with related matters, or
of legal, commercial, philosophical, ethical or political position
regarding them.
The "Invariant Sections" are certain Secondary Sections whose
titles are designated, as being those of Invariant Sections, in the
notice that says that the Document is released under this License.
If a section does not fit the above definition of Secondary then it
is not allowed to be designated as Invariant. The Document may
contain zero Invariant Sections. If the Document does not identify
any Invariant Sections then there are none.
The "Cover Texts" are certain short passages of text that are
listed, as Front-Cover Texts or Back-Cover Texts, in the notice
that says that the Document is released under this License. A
Front-Cover Text may be at most 5 words, and a Back-Cover Text may
be at most 25 words.
A "Transparent" copy of the Document means a machine-readable copy,
represented in a format whose specification is available to the
general public, that is suitable for revising the document
straightforwardly with generic text editors or (for images composed
of pixels) generic paint programs or (for drawings) some widely
available drawing editor, and that is suitable for input to text
formatters or for automatic translation to a variety of formats
suitable for input to text formatters. A copy made in an otherwise
Transparent file format whose markup, or absence of markup, has
been arranged to thwart or discourage subsequent modification by
readers is not Transparent. An image format is not Transparent if
used for any substantial amount of text. A copy that is not
"Transparent" is called "Opaque".
Examples of suitable formats for Transparent copies include plain
ASCII without markup, Texinfo input format, LaTeX input format,
SGML or XML using a publicly available DTD, and standard-conforming
simple HTML, PostScript or PDF designed for human modification.
Examples of transparent image formats include PNG, XCF and JPG.
Opaque formats include proprietary formats that can be read and
edited only by proprietary word processors, SGML or XML for which
the DTD and/or processing tools are not generally available, and
the machine-generated HTML, PostScript or PDF produced by some word
processors for output purposes only.
The "Title Page" means, for a printed book, the title page itself,
plus such following pages as are needed to hold, legibly, the
material this License requires to appear in the title page. For
works in formats which do not have any title page as such, "Title
Page" means the text near the most prominent appearance of the
work's title, preceding the beginning of the body of the text.
The "publisher" means any person or entity that distributes copies
of the Document to the public.
A section "Entitled XYZ" means a named subunit of the Document
whose title either is precisely XYZ or contains XYZ in parentheses
following text that translates XYZ in another language. (Here XYZ
stands for a specific section name mentioned below, such as
"Acknowledgements", "Dedications", "Endorsements", or "History".)
To "Preserve the Title" of such a section when you modify the
Document means that it remains a section "Entitled XYZ" according
to this definition.
The Document may include Warranty Disclaimers next to the notice
which states that this License applies to the Document. These
Warranty Disclaimers are considered to be included by reference in
this License, but only as regards disclaiming warranties: any other
implication that these Warranty Disclaimers may have is void and
has no effect on the meaning of this License.
2. VERBATIM COPYING
You may copy and distribute the Document in any medium, either
commercially or noncommercially, provided that this License, the
copyright notices, and the license notice saying this License
applies to the Document are reproduced in all copies, and that you
add no other conditions whatsoever to those of this License. You
may not use technical measures to obstruct or control the reading
or further copying of the copies you make or distribute. However,
you may accept compensation in exchange for copies. If you
distribute a large enough number of copies you must also follow the
conditions in section 3.
You may also lend copies, under the same conditions stated above,
and you may publicly display copies.
3. COPYING IN QUANTITY
If you publish printed copies (or copies in media that commonly
have printed covers) of the Document, numbering more than 100, and
the Document's license notice requires Cover Texts, you must
enclose the copies in covers that carry, clearly and legibly, all
these Cover Texts: Front-Cover Texts on the front cover, and
Back-Cover Texts on the back cover. Both covers must also clearly
and legibly identify you as the publisher of these copies. The
front cover must present the full title with all words of the title
equally prominent and visible. You may add other material on the
covers in addition. Copying with changes limited to the covers, as
long as they preserve the title of the Document and satisfy these
conditions, can be treated as verbatim copying in other respects.
If the required texts for either cover are too voluminous to fit
legibly, you should put the first ones listed (as many as fit
reasonably) on the actual cover, and continue the rest onto
adjacent pages.
If you publish or distribute Opaque copies of the Document
numbering more than 100, you must either include a machine-readable
Transparent copy along with each Opaque copy, or state in or with
each Opaque copy a computer-network location from which the general
network-using public has access to download using public-standard
network protocols a complete Transparent copy of the Document, free
of added material. If you use the latter option, you must take
reasonably prudent steps, when you begin distribution of Opaque
copies in quantity, to ensure that this Transparent copy will
remain thus accessible at the stated location until at least one
year after the last time you distribute an Opaque copy (directly or
through your agents or retailers) of that edition to the public.
It is requested, but not required, that you contact the authors of
the Document well before redistributing any large number of copies,
to give them a chance to provide you with an updated version of the
Document.
4. MODIFICATIONS
You may copy and distribute a Modified Version of the Document
under the conditions of sections 2 and 3 above, provided that you
release the Modified Version under precisely this License, with the
Modified Version filling the role of the Document, thus licensing
distribution and modification of the Modified Version to whoever
possesses a copy of it. In addition, you must do these things in
the Modified Version:
A. Use in the Title Page (and on the covers, if any) a title
distinct from that of the Document, and from those of previous
versions (which should, if there were any, be listed in the
History section of the Document). You may use the same title
as a previous version if the original publisher of that
version gives permission.
B. List on the Title Page, as authors, one or more persons or
entities responsible for authorship of the modifications in
the Modified Version, together with at least five of the
principal authors of the Document (all of its principal
authors, if it has fewer than five), unless they release you
from this requirement.
C. State on the Title page the name of the publisher of the
Modified Version, as the publisher.
D. Preserve all the copyright notices of the Document.
E. Add an appropriate copyright notice for your modifications
adjacent to the other copyright notices.
F. Include, immediately after the copyright notices, a license
notice giving the public permission to use the Modified
Version under the terms of this License, in the form shown in
the Addendum below.
G. Preserve in that license notice the full lists of Invariant
Sections and required Cover Texts given in the Document's
license notice.
H. Include an unaltered copy of this License.
I. Preserve the section Entitled "History", Preserve its Title,
and add to it an item stating at least the title, year, new
authors, and publisher of the Modified Version as given on the
Title Page. If there is no section Entitled "History" in the
Document, create one stating the title, year, authors, and
publisher of the Document as given on its Title Page, then add
an item describing the Modified Version as stated in the
previous sentence.
J. Preserve the network location, if any, given in the Document
for public access to a Transparent copy of the Document, and
likewise the network locations given in the Document for
previous versions it was based on. These may be placed in the
"History" section. You may omit a network location for a work
that was published at least four years before the Document
itself, or if the original publisher of the version it refers
to gives permission.
K. For any section Entitled "Acknowledgements" or "Dedications",
Preserve the Title of the section, and preserve in the section
all the substance and tone of each of the contributor
acknowledgements and/or dedications given therein.
L. Preserve all the Invariant Sections of the Document, unaltered
in their text and in their titles. Section numbers or the
equivalent are not considered part of the section titles.
M. Delete any section Entitled "Endorsements". Such a section
may not be included in the Modified Version.
N. Do not retitle any existing section to be Entitled
"Endorsements" or to conflict in title with any Invariant
Section.
O. Preserve any Warranty Disclaimers.
If the Modified Version includes new front-matter sections or
appendices that qualify as Secondary Sections and contain no
material copied from the Document, you may at your option designate
some or all of these sections as invariant. To do this, add their
titles to the list of Invariant Sections in the Modified Version's
license notice. These titles must be distinct from any other
section titles.
You may add a section Entitled "Endorsements", provided it contains
nothing but endorsements of your Modified Version by various
parties--for example, statements of peer review or that the text
has been approved by an organization as the authoritative
definition of a standard.
You may add a passage of up to five words as a Front-Cover Text,
and a passage of up to 25 words as a Back-Cover Text, to the end of
the list of Cover Texts in the Modified Version. Only one passage
of Front-Cover Text and one of Back-Cover Text may be added by (or
through arrangements made by) any one entity. If the Document
already includes a cover text for the same cover, previously added
by you or by arrangement made by the same entity you are acting on
behalf of, you may not add another; but you may replace the old
one, on explicit permission from the previous publisher that added
the old one.
The author(s) and publisher(s) of the Document do not by this
License give permission to use their names for publicity for or to
assert or imply endorsement of any Modified Version.
5. COMBINING DOCUMENTS
You may combine the Document with other documents released under
this License, under the terms defined in section 4 above for
modified versions, provided that you include in the combination all
of the Invariant Sections of all of the original documents,
unmodified, and list them all as Invariant Sections of your
combined work in its license notice, and that you preserve all
their Warranty Disclaimers.
The combined work need only contain one copy of this License, and
multiple identical Invariant Sections may be replaced with a single
copy. If there are multiple Invariant Sections with the same name
but different contents, make the title of each such section unique
by adding at the end of it, in parentheses, the name of the
original author or publisher of that section if known, or else a
unique number. Make the same adjustment to the section titles in
the list of Invariant Sections in the license notice of the
combined work.
In the combination, you must combine any sections Entitled
"History" in the various original documents, forming one section
Entitled "History"; likewise combine any sections Entitled
"Acknowledgements", and any sections Entitled "Dedications". You
must delete all sections Entitled "Endorsements."
6. COLLECTIONS OF DOCUMENTS
You may make a collection consisting of the Document and other
documents released under this License, and replace the individual
copies of this License in the various documents with a single copy
that is included in the collection, provided that you follow the
rules of this License for verbatim copying of each of the documents
in all other respects.
You may extract a single document from such a collection, and
distribute it individually under this License, provided you insert
a copy of this License into the extracted document, and follow this
License in all other respects regarding verbatim copying of that
document.
7. AGGREGATION WITH INDEPENDENT WORKS
A compilation of the Document or its derivatives with other
separate and independent documents or works, in or on a volume of a
storage or distribution medium, is called an "aggregate" if the
copyright resulting from the compilation is not used to limit the
legal rights of the compilation's users beyond what the individual
works permit. When the Document is included in an aggregate, this
License does not apply to the other works in the aggregate which
are not themselves derivative works of the Document.
If the Cover Text requirement of section 3 is applicable to these
copies of the Document, then if the Document is less than one half
of the entire aggregate, the Document's Cover Texts may be placed
on covers that bracket the Document within the aggregate, or the
electronic equivalent of covers if the Document is in electronic
form. Otherwise they must appear on printed covers that bracket
the whole aggregate.
8. TRANSLATION
Translation is considered a kind of modification, so you may
distribute translations of the Document under the terms of section
4. Replacing Invariant Sections with translations requires special
permission from their copyright holders, but you may include
translations of some or all Invariant Sections in addition to the
original versions of these Invariant Sections. You may include a
translation of this License, and all the license notices in the
Document, and any Warranty Disclaimers, provided that you also
include the original English version of this License and the
original versions of those notices and disclaimers. In case of a
disagreement between the translation and the original version of
this License or a notice or disclaimer, the original version will
prevail.
If a section in the Document is Entitled "Acknowledgements",
"Dedications", or "History", the requirement (section 4) to
Preserve its Title (section 1) will typically require changing the
actual title.
9. TERMINATION
You may not copy, modify, sublicense, or distribute the Document
except as expressly provided under this License. Any attempt
otherwise to copy, modify, sublicense, or distribute it is void,
and will automatically terminate your rights under this License.
However, if you cease all violation of this License, then your
license from a particular copyright holder is reinstated (a)
provisionally, unless and until the copyright holder explicitly and
finally terminates your license, and (b) permanently, if the
copyright holder fails to notify you of the violation by some
reasonable means prior to 60 days after the cessation.
Moreover, your license from a particular copyright holder is
reinstated permanently if the copyright holder notifies you of the
violation by some reasonable means, this is the first time you have
received notice of violation of this License (for any work) from
that copyright holder, and you cure the violation prior to 30 days
after your receipt of the notice.
Termination of your rights under this section does not terminate
the licenses of parties who have received copies or rights from you
under this License. If your rights have been terminated and not
permanently reinstated, receipt of a copy of some or all of the
same material does not give you any rights to use it.
10. FUTURE REVISIONS OF THIS LICENSE
The Free Software Foundation may publish new, revised versions of
the GNU Free Documentation License from time to time. Such new
versions will be similar in spirit to the present version, but may
differ in detail to address new problems or concerns. See
<https://www.gnu.org/copyleft/>;.
Each version of the License is given a distinguishing version
number. If the Document specifies that a particular numbered
version of this License "or any later version" applies to it, you
have the option of following the terms and conditions either of
that specified version or of any later version that has been
published (not as a draft) by the Free Software Foundation. If the
Document does not specify a version number of this License, you may
choose any version ever published (not as a draft) by the Free
Software Foundation. If the Document specifies that a proxy can
decide which future versions of this License can be used, that
proxy's public statement of acceptance of a version permanently
authorizes you to choose that version for the Document.
11. RELICENSING
"Massive Multiauthor Collaboration Site" (or "MMC Site") means any
World Wide Web server that publishes copyrightable works and also
provides prominent facilities for anybody to edit those works. A
public wiki that anybody can edit is an example of such a server.
A "Massive Multiauthor Collaboration" (or "MMC") contained in the
site means any set of copyrightable works thus published on the MMC
site.
"CC-BY-SA" means the Creative Commons Attribution-Share Alike 3.0
license published by Creative Commons Corporation, a not-for-profit
corporation with a principal place of business in San Francisco,
California, as well as future copyleft versions of that license
published by that same organization.
"Incorporate" means to publish or republish a Document, in whole or
in part, as part of another Document.
An MMC is "eligible for relicensing" if it is licensed under this
License, and if all works that were first published under this
License somewhere other than this MMC, and subsequently
incorporated in whole or in part into the MMC, (1) had no cover
texts or invariant sections, and (2) were thus incorporated prior
to November 1, 2008.
The operator of an MMC Site may republish an MMC contained in the
site under CC-BY-SA on the same site at any time before August 1,
2009, provided the MMC is eligible for relicensing.
ADDENDUM: How to use this License for your documents
====================================================
To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and license
notices just after the title page:
Copyright (C) YEAR YOUR NAME.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
Texts. A copy of the license is included in the section entitled ``GNU
Free Documentation License''.
If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the "with...Texts." line with this:
with the Invariant Sections being LIST THEIR TITLES, with
the Front-Cover Texts being LIST, and with the Back-Cover Texts
being LIST.
If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.
If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of free
software license, such as the GNU General Public License, to permit
their use in free software.
File: gawk.info, Node: Index, Prev: GNU Free Documentation License, Up: Top
Index
*****
* Menu:
* ! (exclamation point), ! operator: Boolean Ops. (line 69)
* ! (exclamation point), ! operator <1>: Precedence. (line 51)
* ! (exclamation point), ! operator <2>: Ranges. (line 47)
* ! (exclamation point), ! operator <3>: Egrep Program. (line 174)
* ! (exclamation point), != operator: Comparison Operators.
(line 11)
* ! (exclamation point), != operator <1>: Precedence. (line 64)
* ! (exclamation point), !~ operator: Regexp Usage. (line 19)
* ! (exclamation point), !~ operator <1>: Computed Regexps. (line 6)
* ! (exclamation point), !~ operator <2>: Case-sensitivity. (line 26)
* ! (exclamation point), !~ operator <3>: Regexp Constants. (line 6)
* ! (exclamation point), !~ operator <4>: Comparison Operators.
(line 11)
* ! (exclamation point), !~ operator <5>: Comparison Operators.
(line 98)
* ! (exclamation point), !~ operator <6>: Precedence. (line 79)
* ! (exclamation point), !~ operator <7>: Expression Patterns.
(line 24)
* " (double quote), in regexp constants: Computed Regexps. (line 30)
* " (double quote), in shell commands: Quoting. (line 54)
* # (number sign), #! (executable scripts): Executable Scripts.
(line 6)
* # (number sign), commenting: Comments. (line 6)
* $ (dollar sign), $ field operator: Fields. (line 19)
* $ (dollar sign), $ field operator <1>: Precedence. (line 42)
* $ (dollar sign), incrementing fields and arrays: Increment Ops.
(line 30)
* $ (dollar sign), regexp operator: Regexp Operators. (line 35)
* % (percent sign), % operator: Precedence. (line 54)
* % (percent sign), %= operator: Assignment Ops. (line 129)
* % (percent sign), %= operator <1>: Precedence. (line 94)
* & (ampersand), && operator: Boolean Ops. (line 59)
* & (ampersand), && operator <1>: Precedence. (line 85)
* & (ampersand), gsub()/gensub()/sub() functions and: Gory Details.
(line 6)
* ' (single quote): One-shot. (line 15)
* ' (single quote) in gawk command lines: Long. (line 35)
* ' (single quote), in shell commands: Quoting. (line 48)
* ' (single quote), vs. apostrophe: Comments. (line 27)
* ' (single quote), with double quotes: Quoting. (line 73)
* () (parentheses), in a profile: Profiling. (line 146)
* () (parentheses), regexp operator: Regexp Operators. (line 81)
* * (asterisk), * operator, as multiplication operator: Precedence.
(line 54)
* * (asterisk), * operator, as regexp operator: Regexp Operators.
(line 89)
* * (asterisk), * operator, null strings, matching: String Functions.
(line 539)
* * (asterisk), ** operator: Arithmetic Ops. (line 81)
* * (asterisk), ** operator <1>: Precedence. (line 48)
* * (asterisk), **= operator: Assignment Ops. (line 129)
* * (asterisk), **= operator <1>: Precedence. (line 94)
* * (asterisk), *= operator: Assignment Ops. (line 129)
* * (asterisk), *= operator <1>: Precedence. (line 94)
* + (plus sign), + operator: Precedence. (line 51)
* + (plus sign), + operator <1>: Precedence. (line 57)
* + (plus sign), ++ operator: Increment Ops. (line 11)
* + (plus sign), ++ operator <1>: Increment Ops. (line 40)
* + (plus sign), ++ operator <2>: Precedence. (line 45)
* + (plus sign), += operator: Assignment Ops. (line 81)
* + (plus sign), += operator <1>: Precedence. (line 94)
* + (plus sign), regexp operator: Regexp Operators. (line 105)
* , (comma), in range patterns: Ranges. (line 6)
* - (hyphen), - operator: Precedence. (line 51)
* - (hyphen), - operator <1>: Precedence. (line 57)
* - (hyphen), -- operator: Increment Ops. (line 48)
* - (hyphen), -- operator <1>: Precedence. (line 45)
* - (hyphen), -= operator: Assignment Ops. (line 129)
* - (hyphen), -= operator <1>: Precedence. (line 94)
* - (hyphen), filenames beginning with: Options. (line 60)
* - (hyphen), in bracket expressions: Bracket Expressions. (line 25)
* --assign option: Options. (line 32)
* --bignum option: Options. (line 217)
* --characters-as-bytes option: Options. (line 69)
* --copyright option: Options. (line 89)
* --debug option: Options. (line 108)
* --disable-extensions configuration option: Additional Configuration Options.
(line 9)
* --disable-lint configuration option: Additional Configuration Options.
(line 15)
* --disable-mpfr configuration option: Additional Configuration Options.
(line 32)
* --disable-nls configuration option: Additional Configuration Options.
(line 37)
* --dump-variables option: Options. (line 94)
* --dump-variables option, using for library functions: Library Names.
(line 45)
* --enable-versioned-extension-dir configuration option: Additional Configuration Options.
(line 42)
* --exec option: Options. (line 139)
* --field-separator option: Options. (line 21)
* --file option: Options. (line 25)
* --gen-pot option: Options. (line 161)
* --gen-pot option <1>: String Extraction. (line 6)
* --gen-pot option <2>: String Extraction. (line 6)
* --help option: Options. (line 168)
* --include option: Options. (line 173)
* --lint option: Command Line. (line 20)
* --lint option <1>: Options. (line 198)
* --lint-old option: Options. (line 313)
* --load option: Options. (line 186)
* --no-optimize option: Options. (line 299)
* --non-decimal-data option: Options. (line 223)
* --non-decimal-data option <1>: Nondecimal Data. (line 6)
* --non-decimal-data option, strtonum() function and: Nondecimal Data.
(line 35)
* --optimize option: Options. (line 248)
* --posix option: Options. (line 271)
* --posix option, --traditional option and: Options. (line 286)
* --pretty-print option: Options. (line 237)
* --profile option: Options. (line 259)
* --profile option <1>: Profiling. (line 12)
* --re-interval option: Options. (line 292)
* --sandbox option: Options. (line 304)
* --sandbox option, disabling system() function: I/O Functions.
(line 128)
* --sandbox option, input redirection with getline: Getline. (line 19)
* --sandbox option, output redirection with print, printf: Redirection.
(line 6)
* --source option: Options. (line 117)
* --traditional option: Options. (line 82)
* --traditional option, --posix option and: Options. (line 286)
* --use-lc-numeric option: Options. (line 232)
* --version option: Options. (line 318)
* --with-whiny-user-strftime configuration option: Additional Configuration Options.
(line 48)
* -b option: Options. (line 69)
* -c option: Options. (line 82)
* -C option: Options. (line 89)
* -d option: Options. (line 94)
* -D option: Options. (line 108)
* -e option: Options. (line 117)
* -E option: Options. (line 139)
* -e option <1>: Options. (line 354)
* -f option: Long. (line 12)
* -F option: Options. (line 21)
* -f option <1>: Options. (line 25)
* -F option, -Ft sets FS to TAB: Options. (line 326)
* -F option, command-line: Command Line Field Separator.
(line 6)
* -f option, multiple uses: Options. (line 331)
* -g option: Options. (line 161)
* -h option: Options. (line 168)
* -i option: Options. (line 173)
* -l option: Options. (line 186)
* -l option <1>: Options. (line 198)
* -L option: Options. (line 313)
* -M option: Options. (line 217)
* -n option: Options. (line 223)
* -N option: Options. (line 232)
* -o option: Options. (line 237)
* -O option: Options. (line 248)
* -p option: Options. (line 259)
* -P option: Options. (line 271)
* -r option: Options. (line 292)
* -s option: Options. (line 299)
* -S option: Options. (line 304)
* -v option: Options. (line 32)
* -V option: Options. (line 318)
* -v option <1>: Assignment Options. (line 12)
* -W option: Options. (line 47)
* . (period), regexp operator: Regexp Operators. (line 44)
* .gmo files: Explaining gettext. (line 42)
* .gmo files, specifying directory of: Explaining gettext. (line 54)
* .gmo files, specifying directory of <1>: Programmer i18n. (line 48)
* .mo files, converting from .po: I18N Example. (line 66)
* .po files: Explaining gettext. (line 37)
* .po files <1>: Translator i18n. (line 6)
* .po files, converting to .mo: I18N Example. (line 66)
* .pot files: Explaining gettext. (line 31)
* / (forward slash) to enclose regular expressions: Regexp. (line 10)
* / (forward slash), / operator: Precedence. (line 54)
* / (forward slash), /= operator: Assignment Ops. (line 129)
* / (forward slash), /= operator <1>: Precedence. (line 94)
* / (forward slash), /= operator, vs. /=.../ regexp constant: Assignment Ops.
(line 149)
* / (forward slash), patterns and: Expression Patterns. (line 24)
* /= operator vs. /=.../ regexp constant: Assignment Ops. (line 149)
* /dev/... special files: Special FD. (line 48)
* /dev/fd/N special files (gawk): Special FD. (line 48)
* /inet/... special files (gawk): TCP/IP Networking. (line 6)
* /inet4/... special files (gawk): TCP/IP Networking. (line 6)
* /inet6/... special files (gawk): TCP/IP Networking. (line 6)
* ; (semicolon), AWKPATH variable and: PC Using. (line 13)
* ; (semicolon), separating statements in actions: Statements/Lines.
(line 90)
* ; (semicolon), separating statements in actions <1>: Action Overview.
(line 19)
* ; (semicolon), separating statements in actions <2>: Statements.
(line 10)
* < (left angle bracket), < operator: Comparison Operators.
(line 11)
* < (left angle bracket), < operator <1>: Precedence. (line 64)
* < (left angle bracket), < operator (I/O): Getline/File. (line 6)
* < (left angle bracket), <= operator: Comparison Operators.
(line 11)
* < (left angle bracket), <= operator <1>: Precedence. (line 64)
* = (equals sign), = operator: Assignment Ops. (line 6)
* = (equals sign), == operator: Comparison Operators.
(line 11)
* = (equals sign), == operator <1>: Precedence. (line 64)
* > (right angle bracket), > operator: Comparison Operators.
(line 11)
* > (right angle bracket), > operator <1>: Precedence. (line 64)
* > (right angle bracket), > operator (I/O): Redirection. (line 22)
* > (right angle bracket), >= operator: Comparison Operators.
(line 11)
* > (right angle bracket), >= operator <1>: Precedence. (line 64)
* > (right angle bracket), >> operator (I/O): Redirection. (line 50)
* > (right angle bracket), >> operator (I/O) <1>: Precedence. (line 64)
* ? (question mark), ?: operator: Precedence. (line 91)
* ? (question mark), regexp operator: Regexp Operators. (line 111)
* ? (question mark), regexp operator <1>: GNU Regexp Operators.
(line 62)
* @-notation for indirect function calls: Indirect Calls. (line 47)
* @include directive: Include Files. (line 8)
* @load directive: Loading Shared Libraries.
(line 8)
* [] (square brackets), regexp operator: Regexp Operators. (line 56)
* \ (backslash): Comments. (line 50)
* \ (backslash), as field separator: Command Line Field Separator.
(line 24)
* \ (backslash), continuing lines and: Statements/Lines. (line 19)
* \ (backslash), continuing lines and, comments and: Statements/Lines.
(line 75)
* \ (backslash), continuing lines and, in csh: Statements/Lines.
(line 43)
* \ (backslash), gsub()/gensub()/sub() functions and: Gory Details.
(line 6)
* \ (backslash), in bracket expressions: Bracket Expressions. (line 25)
* \ (backslash), in escape sequences: Escape Sequences. (line 6)
* \ (backslash), in escape sequences <1>: Escape Sequences. (line 103)
* \ (backslash), in escape sequences, POSIX and: Escape Sequences.
(line 108)
* \ (backslash), in regexp constants: Computed Regexps. (line 30)
* \ (backslash), in shell commands: Quoting. (line 48)
* \ (backslash), regexp operator: Regexp Operators. (line 18)
* \ (backslash), \" escape sequence: Escape Sequences. (line 85)
* \ (backslash), \' operator (gawk): GNU Regexp Operators.
(line 59)
* \ (backslash), \/ escape sequence: Escape Sequences. (line 76)
* \ (backslash), \< operator (gawk): GNU Regexp Operators.
(line 33)
* \ (backslash), \> operator (gawk): GNU Regexp Operators.
(line 37)
* \ (backslash), \a escape sequence: Escape Sequences. (line 34)
* \ (backslash), \b escape sequence: Escape Sequences. (line 38)
* \ (backslash), \B operator (gawk): GNU Regexp Operators.
(line 46)
* \ (backslash), \f escape sequence: Escape Sequences. (line 41)
* \ (backslash), \n escape sequence: Escape Sequences. (line 44)
* \ (backslash), \NNN escape sequence: Escape Sequences. (line 56)
* \ (backslash), \r escape sequence: Escape Sequences. (line 47)
* \ (backslash), \s operator (gawk): GNU Regexp Operators.
(line 13)
* \ (backslash), \S operator (gawk): GNU Regexp Operators.
(line 17)
* \ (backslash), \t escape sequence: Escape Sequences. (line 50)
* \ (backslash), \v escape sequence: Escape Sequences. (line 53)
* \ (backslash), \w operator (gawk): GNU Regexp Operators.
(line 22)
* \ (backslash), \W operator (gawk): GNU Regexp Operators.
(line 28)
* \ (backslash), \x escape sequence: Escape Sequences. (line 61)
* \ (backslash), \y operator (gawk): GNU Regexp Operators.
(line 41)
* \ (backslash), \` operator (gawk): GNU Regexp Operators.
(line 57)
* ^ (caret), in bracket expressions: Bracket Expressions. (line 25)
* ^ (caret), in FS: Regexp Field Splitting.
(line 59)
* ^ (caret), regexp operator: Regexp Operators. (line 22)
* ^ (caret), regexp operator <1>: GNU Regexp Operators.
(line 62)
* ^ (caret), ^ operator: Precedence. (line 48)
* ^ (caret), ^= operator: Assignment Ops. (line 129)
* ^ (caret), ^= operator <1>: Precedence. (line 94)
* _ (underscore), C macro: Explaining gettext. (line 71)
* _ (underscore), in names of private variables: Library Names.
(line 29)
* _ (underscore), translatable string: Programmer i18n. (line 69)
* _gr_init() user-defined function: Group Functions. (line 83)
* _ord_init() user-defined function: Ordinal Functions. (line 16)
* _pw_init() user-defined function: Passwd Functions. (line 105)
* {} (braces): Profiling. (line 142)
* {} (braces), actions and: Action Overview. (line 19)
* {} (braces), statements, grouping: Statements. (line 10)
* | (vertical bar): Regexp Operators. (line 70)
* | (vertical bar), | operator (I/O): Getline/Pipe. (line 10)
* | (vertical bar), | operator (I/O) <1>: Redirection. (line 57)
* | (vertical bar), | operator (I/O) <2>: Precedence. (line 64)
* | (vertical bar), |& operator (I/O): Getline/Coprocess. (line 6)
* | (vertical bar), |& operator (I/O) <1>: Redirection. (line 96)
* | (vertical bar), |& operator (I/O) <2>: Precedence. (line 64)
* | (vertical bar), |& operator (I/O) <3>: Two-way I/O. (line 27)
* | (vertical bar), |& operator (I/O), pipes, closing: Close Files And Pipes.
(line 120)
* | (vertical bar), || operator: Boolean Ops. (line 59)
* | (vertical bar), || operator <1>: Precedence. (line 88)
* ~ (tilde), ~ operator: Regexp Usage. (line 19)
* ~ (tilde), ~ operator <1>: Computed Regexps. (line 6)
* ~ (tilde), ~ operator <2>: Case-sensitivity. (line 26)
* ~ (tilde), ~ operator <3>: Regexp Constants. (line 6)
* ~ (tilde), ~ operator <4>: Comparison Operators.
(line 11)
* ~ (tilde), ~ operator <5>: Comparison Operators.
(line 98)
* ~ (tilde), ~ operator <6>: Precedence. (line 79)
* ~ (tilde), ~ operator <7>: Expression Patterns. (line 24)
* accessing fields: Fields. (line 6)
* accessing global variables from extensions: Symbol Table Access.
(line 6)
* account information: Passwd Functions. (line 16)
* account information <1>: Group Functions. (line 6)
* actions: Action Overview. (line 6)
* actions, control statements in: Statements. (line 6)
* actions, default: Very Simple. (line 35)
* actions, empty: Very Simple. (line 40)
* Ada programming language: Glossary. (line 11)
* adding, features to gawk: Adding Code. (line 6)
* adding, fields: Changing Fields. (line 53)
* advanced features, fixed-width data: Constant Size. (line 6)
* advanced features, gawk: Advanced Features. (line 6)
* advanced features, network programming: TCP/IP Networking. (line 6)
* advanced features, nondecimal input data: Nondecimal Data. (line 6)
* advanced features, processes, communicating with: Two-way I/O.
(line 6)
* advanced features, specifying field content: Splitting By Content.
(line 9)
* Aho, Alfred: History. (line 17)
* Aho, Alfred <1>: Contributors. (line 12)
* alarm clock example program: Alarm Program. (line 11)
* alarm.awk program: Alarm Program. (line 31)
* algorithms: Basic High Level. (line 57)
* allocating memory for extensions: Memory Allocation Functions.
(line 6)
* amazing awk assembler (aaa): Glossary. (line 16)
* amazingly workable formatter (awf): Glossary. (line 23)
* ambiguity, syntactic: /= operator vs. /=.../ regexp constant: Assignment Ops.
(line 149)
* ampersand (&), && operator: Boolean Ops. (line 59)
* ampersand (&), && operator <1>: Precedence. (line 85)
* ampersand (&), gsub()/gensub()/sub() functions and: Gory Details.
(line 6)
* anagram.awk program: Anagram Program. (line 21)
* anagrams, finding: Anagram Program. (line 6)
* and: Bitwise Functions. (line 40)
* AND bitwise operation: Bitwise Functions. (line 6)
* and Boolean-logic operator: Boolean Ops. (line 6)
* ANSI: Glossary. (line 32)
* API informational variables: Extension API Informational Variables.
(line 6)
* API version: Extension Versioning.
(line 6)
* arbitrary precision: Arbitrary Precision Arithmetic.
(line 6)
* arbitrary precision integers: Arbitrary Precision Integers.
(line 6)
* arbitrary-precision: Computer Arithmetic. (line 61)
* archaeologists: Bugs. (line 6)
* arctangent: Numeric Functions. (line 12)
* ARGC/ARGV variables: Auto-set. (line 15)
* ARGC/ARGV variables, command-line arguments: Other Arguments.
(line 15)
* ARGC/ARGV variables, how to use: ARGC and ARGV. (line 6)
* ARGC/ARGV variables, portability and: Executable Scripts. (line 59)
* ARGIND variable: Auto-set. (line 44)
* ARGIND variable, command-line arguments: Other Arguments. (line 15)
* arguments, command-line: Other Arguments. (line 6)
* arguments, command-line <1>: Auto-set. (line 15)
* arguments, command-line <2>: ARGC and ARGV. (line 6)
* arguments, command-line, invoking awk: Command Line. (line 6)
* arguments, in function calls: Function Calls. (line 18)
* arguments, processing: Getopt Function. (line 6)
* ARGV array, indexing into: Other Arguments. (line 15)
* arithmetic operators: Arithmetic Ops. (line 6)
* array manipulation in extensions: Array Manipulation. (line 6)
* array members: Reference to Elements.
(line 6)
* array scanning order, controlling: Controlling Scanning.
(line 14)
* array, number of elements: String Functions. (line 200)
* arrays: Arrays. (line 6)
* arrays of arrays: Arrays of Arrays. (line 6)
* arrays, an example of using: Array Example. (line 6)
* arrays, and IGNORECASE variable: Array Intro. (line 100)
* arrays, as parameters to functions: Pass By Value/Reference.
(line 44)
* arrays, associative: Array Intro. (line 48)
* arrays, associative, library functions and: Library Names. (line 58)
* arrays, deleting entire contents: Delete. (line 39)
* arrays, elements that don't exist: Reference to Elements.
(line 23)
* arrays, elements, assigning values: Assigning Elements. (line 6)
* arrays, elements, deleting: Delete. (line 6)
* arrays, elements, order of access by in operator: Scanning an Array.
(line 48)
* arrays, elements, retrieving number of: String Functions. (line 42)
* arrays, for statement and: Scanning an Array. (line 20)
* arrays, indexing: Array Intro. (line 48)
* arrays, merging into strings: Join Function. (line 6)
* arrays, multidimensional: Multidimensional. (line 10)
* arrays, multidimensional, scanning: Multiscanning. (line 11)
* arrays, numeric subscripts: Numeric Array Subscripts.
(line 6)
* arrays, referencing elements: Reference to Elements.
(line 6)
* arrays, scanning: Scanning an Array. (line 6)
* arrays, sorting: Array Sorting Functions.
(line 6)
* arrays, sorting, and IGNORECASE variable: Array Sorting Functions.
(line 83)
* arrays, sparse: Array Intro. (line 76)
* arrays, subscripts, uninitialized variables as: Uninitialized Subscripts.
(line 6)
* arrays, unassigned elements: Reference to Elements.
(line 18)
* artificial intelligence, gawk and: Distribution contents.
(line 58)
* ASCII: Ordinal Functions. (line 45)
* ASCII <1>: Glossary. (line 194)
* asort: String Functions. (line 42)
* asort <1>: Array Sorting Functions.
(line 6)
* asort() function (gawk), arrays, sorting: Array Sorting Functions.
(line 6)
* asorti: String Functions. (line 42)
* asorti <1>: Array Sorting Functions.
(line 6)
* asorti() function (gawk), arrays, sorting: Array Sorting Functions.
(line 6)
* assert() function (C library): Assert Function. (line 6)
* assert() user-defined function: Assert Function. (line 28)
* assertions: Assert Function. (line 6)
* assign values to variables, in debugger: Viewing And Changing Data.
(line 58)
* assignment operators: Assignment Ops. (line 6)
* assignment operators, evaluation order: Assignment Ops. (line 110)
* assignment operators, lvalues/rvalues: Assignment Ops. (line 31)
* assignments as filenames: Ignoring Assigns. (line 6)
* associative arrays: Array Intro. (line 48)
* asterisk (*), * operator, as multiplication operator: Precedence.
(line 54)
* asterisk (*), * operator, as regexp operator: Regexp Operators.
(line 89)
* asterisk (*), * operator, null strings, matching: String Functions.
(line 539)
* asterisk (*), ** operator: Arithmetic Ops. (line 81)
* asterisk (*), ** operator <1>: Precedence. (line 48)
* asterisk (*), **= operator: Assignment Ops. (line 129)
* asterisk (*), **= operator <1>: Precedence. (line 94)
* asterisk (*), *= operator: Assignment Ops. (line 129)
* asterisk (*), *= operator <1>: Precedence. (line 94)
* atan2: Numeric Functions. (line 12)
* automatic displays, in debugger: Debugger Info. (line 24)
* awf (amazingly workable formatter) program: Glossary. (line 23)
* awk debugging, enabling: Options. (line 108)
* awk language, POSIX version: Assignment Ops. (line 138)
* awk profiling, enabling: Options. (line 259)
* awk programs: Getting Started. (line 12)
* awk programs <1>: Executable Scripts. (line 6)
* awk programs <2>: Two Rules. (line 6)
* awk programs, complex: When. (line 27)
* awk programs, documenting: Comments. (line 6)
* awk programs, documenting <1>: Library Names. (line 6)
* awk programs, examples of: Sample Programs. (line 6)
* awk programs, execution of: Next Statement. (line 16)
* awk programs, internationalizing: I18N Functions. (line 6)
* awk programs, internationalizing <1>: Programmer i18n. (line 6)
* awk programs, lengthy: Long. (line 6)
* awk programs, lengthy, assertions: Assert Function. (line 6)
* awk programs, location of: Options. (line 25)
* awk programs, location of <1>: Options. (line 139)
* awk programs, location of <2>: Options. (line 173)
* awk programs, one-line examples: Very Simple. (line 46)
* awk programs, profiling: Profiling. (line 6)
* awk programs, running: Running gawk. (line 6)
* awk programs, running <1>: Long. (line 6)
* awk programs, running, from shell scripts: One-shot. (line 22)
* awk programs, running, without input files: Read Terminal. (line 16)
* awk programs, shell variables in: Using Shell Variables.
(line 6)
* awk, function of: Getting Started. (line 6)
* awk, gawk and: Preface. (line 21)
* awk, gawk and <1>: This Manual. (line 14)
* awk, history of: History. (line 17)
* awk, implementation issues, pipes: Redirection. (line 129)
* awk, implementations: Other Versions. (line 6)
* awk, implementations, limits: Getline Notes. (line 14)
* awk, invoking: Command Line. (line 6)
* awk, new vs. old: Names. (line 6)
* awk, new vs. old, OFMT variable: Strings And Numbers. (line 56)
* awk, POSIX and: Preface. (line 21)
* awk, POSIX and, See Also POSIX awk: Preface. (line 21)
* awk, regexp constants and: Comparison Operators.
(line 103)
* awk, See Also gawk: Preface. (line 34)
* awk, terms describing: This Manual. (line 6)
* awk, uses for: Preface. (line 21)
* awk, uses for <1>: Getting Started. (line 12)
* awk, uses for <2>: When. (line 6)
* awk, versions of: V7/SVR3.1. (line 6)
* awk, versions of, changes between SVR3.1 and SVR4: SVR4. (line 6)
* awk, versions of, changes between SVR4 and POSIX awk: POSIX.
(line 6)
* awk, versions of, changes between V7 and SVR3.1: V7/SVR3.1. (line 6)
* awk, versions of, See Also Brian Kernighan's awk: BTL. (line 6)
* awk, versions of, See Also Brian Kernighan's awk <1>: Other Versions.
(line 13)
* awka compiler for awk: Other Versions. (line 73)
* AWKLIBPATH environment variable: AWKLIBPATH Variable. (line 6)
* AWKPATH environment variable: AWKPATH Variable. (line 6)
* AWKPATH environment variable <1>: PC Using. (line 13)
* awkprof.out file: Profiling. (line 6)
* awksed.awk program: Simple Sed. (line 25)
* awkvars.out file: Options. (line 94)
* b debugger command (alias for break): Breakpoint Control. (line 11)
* backslash (\): Comments. (line 50)
* backslash (\), as field separator: Command Line Field Separator.
(line 24)
* backslash (\), continuing lines and: Statements/Lines. (line 19)
* backslash (\), continuing lines and, comments and: Statements/Lines.
(line 75)
* backslash (\), continuing lines and, in csh: Statements/Lines.
(line 43)
* backslash (\), gsub()/gensub()/sub() functions and: Gory Details.
(line 6)
* backslash (\), in bracket expressions: Bracket Expressions. (line 25)
* backslash (\), in escape sequences: Escape Sequences. (line 6)
* backslash (\), in escape sequences <1>: Escape Sequences. (line 103)
* backslash (\), in escape sequences, POSIX and: Escape Sequences.
(line 108)
* backslash (\), in regexp constants: Computed Regexps. (line 30)
* backslash (\), in shell commands: Quoting. (line 48)
* backslash (\), regexp operator: Regexp Operators. (line 18)
* backslash (\), \" escape sequence: Escape Sequences. (line 85)
* backslash (\), \' operator (gawk): GNU Regexp Operators.
(line 59)
* backslash (\), \/ escape sequence: Escape Sequences. (line 76)
* backslash (\), \< operator (gawk): GNU Regexp Operators.
(line 33)
* backslash (\), \> operator (gawk): GNU Regexp Operators.
(line 37)
* backslash (\), \a escape sequence: Escape Sequences. (line 34)
* backslash (\), \b escape sequence: Escape Sequences. (line 38)
* backslash (\), \B operator (gawk): GNU Regexp Operators.
(line 46)
* backslash (\), \f escape sequence: Escape Sequences. (line 41)
* backslash (\), \n escape sequence: Escape Sequences. (line 44)
* backslash (\), \NNN escape sequence: Escape Sequences. (line 56)
* backslash (\), \r escape sequence: Escape Sequences. (line 47)
* backslash (\), \s operator (gawk): GNU Regexp Operators.
(line 13)
* backslash (\), \S operator (gawk): GNU Regexp Operators.
(line 17)
* backslash (\), \t escape sequence: Escape Sequences. (line 50)
* backslash (\), \v escape sequence: Escape Sequences. (line 53)
* backslash (\), \w operator (gawk): GNU Regexp Operators.
(line 22)
* backslash (\), \W operator (gawk): GNU Regexp Operators.
(line 28)
* backslash (\), \x escape sequence: Escape Sequences. (line 61)
* backslash (\), \y operator (gawk): GNU Regexp Operators.
(line 41)
* backslash (\), \` operator (gawk): GNU Regexp Operators.
(line 57)
* backtrace debugger command: Execution Stack. (line 13)
* Beebe, Nelson H.F.: Acknowledgments. (line 60)
* Beebe, Nelson H.F. <1>: Other Versions. (line 87)
* BEGIN pattern: Field Separators. (line 44)
* BEGIN pattern <1>: BEGIN/END. (line 6)
* BEGIN pattern <2>: Using BEGIN/END. (line 6)
* BEGIN pattern, and profiling: Profiling. (line 62)
* BEGIN pattern, assert() user-defined function and: Assert Function.
(line 83)
* BEGIN pattern, Boolean patterns and: Expression Patterns. (line 70)
* BEGIN pattern, exit statement and: Exit Statement. (line 12)
* BEGIN pattern, getline and: Getline Notes. (line 19)
* BEGIN pattern, headings, adding: Print Examples. (line 42)
* BEGIN pattern, next/nextfile statements and: I/O And BEGIN/END.
(line 36)
* BEGIN pattern, next/nextfile statements and <1>: Next Statement.
(line 44)
* BEGIN pattern, OFS/ORS variables, assigning values to: Output Separators.
(line 20)
* BEGIN pattern, operators and: Using BEGIN/END. (line 17)
* BEGIN pattern, print statement and: I/O And BEGIN/END. (line 15)
* BEGIN pattern, pwcat program: Passwd Functions. (line 143)
* BEGIN pattern, running awk programs and: Cut Program. (line 63)
* BEGIN pattern, TEXTDOMAIN variable and: Programmer i18n. (line 60)
* BEGINFILE pattern: BEGINFILE/ENDFILE. (line 6)
* BEGINFILE pattern, Boolean patterns and: Expression Patterns.
(line 70)
* beginfile() user-defined function: Filetrans Function. (line 62)
* Bentley, Jon: Glossary. (line 204)
* Benzinger, Michael: Contributors. (line 100)
* Berry, Karl: Acknowledgments. (line 33)
* Berry, Karl <1>: Acknowledgments. (line 75)
* Berry, Karl <2>: Ranges and Locales. (line 74)
* binary input/output: User-modified. (line 15)
* bindtextdomain: I18N Functions. (line 11)
* bindtextdomain <1>: Programmer i18n. (line 48)
* bindtextdomain() function (C library): Explaining gettext. (line 50)
* bindtextdomain() function (gawk), portability and: I18N Portability.
(line 33)
* BINMODE variable: User-modified. (line 15)
* BINMODE variable <1>: PC Using. (line 20)
* bit-manipulation functions: Bitwise Functions. (line 6)
* bits2str() user-defined function: Bitwise Functions. (line 69)
* bitwise AND: Bitwise Functions. (line 40)
* bitwise complement: Bitwise Functions. (line 44)
* bitwise OR: Bitwise Functions. (line 50)
* bitwise XOR: Bitwise Functions. (line 57)
* bitwise, complement: Bitwise Functions. (line 25)
* bitwise, operations: Bitwise Functions. (line 6)
* bitwise, shift: Bitwise Functions. (line 32)
* body, in actions: Statements. (line 10)
* body, in loops: While Statement. (line 14)
* Boolean expressions: Boolean Ops. (line 6)
* Boolean expressions, as patterns: Expression Patterns. (line 39)
* Boolean operators, See Boolean expressions: Boolean Ops. (line 6)
* Bourne shell, quoting rules for: Quoting. (line 18)
* braces ({}): Profiling. (line 142)
* braces ({}), actions and: Action Overview. (line 19)
* braces ({}), statements, grouping: Statements. (line 10)
* bracket expressions: Regexp Operators. (line 56)
* bracket expressions <1>: Bracket Expressions. (line 6)
* bracket expressions, character classes: Bracket Expressions.
(line 40)
* bracket expressions, collating elements: Bracket Expressions.
(line 86)
* bracket expressions, collating symbols: Bracket Expressions.
(line 93)
* bracket expressions, complemented: Regexp Operators. (line 64)
* bracket expressions, equivalence classes: Bracket Expressions.
(line 99)
* bracket expressions, non-ASCII: Bracket Expressions. (line 86)
* bracket expressions, range expressions: Bracket Expressions.
(line 6)
* break debugger command: Breakpoint Control. (line 11)
* break statement: Break Statement. (line 6)
* breakpoint: Debugging Terms. (line 33)
* breakpoint at location, how to delete: Breakpoint Control. (line 36)
* breakpoint commands: Debugger Execution Control.
(line 10)
* breakpoint condition: Breakpoint Control. (line 54)
* breakpoint, delete by number: Breakpoint Control. (line 64)
* breakpoint, how to disable or enable: Breakpoint Control. (line 69)
* breakpoint, setting: Breakpoint Control. (line 11)
* Brennan, Michael: Foreword3. (line 84)
* Brennan, Michael <1>: Foreword4. (line 33)
* Brennan, Michael <2>: Acknowledgments. (line 79)
* Brennan, Michael <3>: Delete. (line 56)
* Brennan, Michael <4>: Simple Sed. (line 25)
* Brennan, Michael <5>: Other Versions. (line 6)
* Brennan, Michael <6>: Other Versions. (line 48)
* Brian Kernighan's awk: When. (line 21)
* Brian Kernighan's awk <1>: Escape Sequences. (line 112)
* Brian Kernighan's awk <2>: GNU Regexp Operators.
(line 85)
* Brian Kernighan's awk <3>: Regexp Field Splitting.
(line 67)
* Brian Kernighan's awk <4>: Getline/Pipe. (line 62)
* Brian Kernighan's awk <5>: Concatenation. (line 36)
* Brian Kernighan's awk <6>: I/O And BEGIN/END. (line 15)
* Brian Kernighan's awk <7>: Break Statement. (line 51)
* Brian Kernighan's awk <8>: Continue Statement. (line 44)
* Brian Kernighan's awk <9>: Nextfile Statement. (line 47)
* Brian Kernighan's awk <10>: Delete. (line 51)
* Brian Kernighan's awk <11>: String Functions. (line 495)
* Brian Kernighan's awk <12>: Gory Details. (line 19)
* Brian Kernighan's awk <13>: I/O Functions. (line 43)
* Brian Kernighan's awk, extensions: BTL. (line 6)
* Brian Kernighan's awk, source code: Other Versions. (line 13)
* Brini, Davide: Signature Program. (line 6)
* Brink, Jeroen: DOS Quoting. (line 10)
* Broder, Alan J.: Contributors. (line 91)
* Brown, Martin: Contributors. (line 85)
* BSD-based operating systems: Glossary. (line 746)
* bt debugger command (alias for backtrace): Execution Stack. (line 13)
* Buening, Andreas: Acknowledgments. (line 60)
* Buening, Andreas <1>: Contributors. (line 95)
* Buening, Andreas <2>: Maintainers. (line 14)
* buffering, input/output: I/O Functions. (line 165)
* buffering, input/output <1>: Two-way I/O. (line 53)
* buffering, interactive vs. noninteractive: I/O Functions. (line 76)
* buffers, flushing: I/O Functions. (line 32)
* buffers, flushing <1>: I/O Functions. (line 165)
* buffers, operators for: GNU Regexp Operators.
(line 51)
* bug reports, email address, bug-gawk AT gnu.org: Bug address. (line 31)
* bug-gawk AT gnu.org bug reporting address: Bug address. (line 31)
* built-in functions: Functions. (line 6)
* built-in functions, evaluation order: Calling Built-in. (line 30)
* BusyBox Awk: Other Versions. (line 97)
* c.e., See common extensions: Conventions. (line 51)
* call by reference: Pass By Value/Reference.
(line 44)
* call by value: Pass By Value/Reference.
(line 15)
* call stack, display in debugger: Execution Stack. (line 13)
* caret (^), in bracket expressions: Bracket Expressions. (line 25)
* caret (^), regexp operator: Regexp Operators. (line 22)
* caret (^), regexp operator <1>: GNU Regexp Operators.
(line 62)
* caret (^), ^ operator: Precedence. (line 48)
* caret (^), ^= operator: Assignment Ops. (line 129)
* caret (^), ^= operator <1>: Precedence. (line 94)
* case keyword: Switch Statement. (line 6)
* case sensitivity, and regexps: User-modified. (line 79)
* case sensitivity, and string comparisons: User-modified. (line 79)
* case sensitivity, array indices and: Array Intro. (line 100)
* case sensitivity, converting case: String Functions. (line 525)
* case sensitivity, example programs: Library Functions. (line 53)
* case sensitivity, gawk: Case-sensitivity. (line 26)
* case sensitivity, regexps and: Case-sensitivity. (line 6)
* CGI, awk scripts for: Options. (line 139)
* character classes, See bracket expressions: Regexp Operators.
(line 56)
* character lists in regular expression: Bracket Expressions. (line 6)
* character lists, See bracket expressions: Regexp Operators. (line 56)
* character sets (machine character encodings): Ordinal Functions.
(line 45)
* character sets (machine character encodings) <1>: Glossary. (line 194)
* character sets, See Also bracket expressions: Regexp Operators.
(line 56)
* characters, counting: Wc Program. (line 6)
* characters, transliterating: Translate Program. (line 6)
* characters, values of as numbers: Ordinal Functions. (line 6)
* Chassell, Robert J.: Acknowledgments. (line 33)
* chdir() extension function: Extension Sample File Functions.
(line 12)
* checking for MPFR: Checking for MPFR. (line 6)
* chem utility: Glossary. (line 204)
* chr() extension function: Extension Sample Ord.
(line 15)
* chr() user-defined function: Ordinal Functions. (line 16)
* clear debugger command: Breakpoint Control. (line 36)
* Cliff random numbers: Cliff Random Function.
(line 6)
* cliff_rand() user-defined function: Cliff Random Function.
(line 12)
* close: Close Files And Pipes.
(line 18)
* close <1>: I/O Functions. (line 10)
* close file or coprocess: I/O Functions. (line 10)
* close() function, portability: Close Files And Pipes.
(line 81)
* close() function, return value: Close Files And Pipes.
(line 132)
* close() function, two-way pipes and: Two-way I/O. (line 60)
* Close, Diane: Manual History. (line 34)
* Close, Diane <1>: Contributors. (line 21)
* Collado, Manuel: Acknowledgments. (line 60)
* collating elements: Bracket Expressions. (line 86)
* collating symbols: Bracket Expressions. (line 93)
* Colombo, Antonio: Acknowledgments. (line 60)
* Colombo, Antonio <1>: Contributors. (line 143)
* columns, aligning: Print Examples. (line 69)
* columns, cutting: Cut Program. (line 6)
* comma (,), in range patterns: Ranges. (line 6)
* command completion, in debugger: Readline Support. (line 6)
* command line arguments, PROCINFO["argv": Auto-set. (line 154)
* command line, arguments: Other Arguments. (line 6)
* command line, arguments <1>: Auto-set. (line 15)
* command line, arguments <2>: ARGC and ARGV. (line 6)
* command line, directories on: Command-line directories.
(line 6)
* command line, formats: Running gawk. (line 12)
* command line, FS on, setting: Command Line Field Separator.
(line 6)
* command line, invoking awk from: Command Line. (line 6)
* command line, option -f: Long. (line 12)
* command line, options: Options. (line 6)
* command line, options, end of: Options. (line 55)
* command line, variables, assigning on: Assignment Options. (line 6)
* command-line options, processing: Getopt Function. (line 6)
* command-line options, string extraction: String Extraction. (line 6)
* commands debugger command: Debugger Execution Control.
(line 10)
* commands to execute at breakpoint: Debugger Execution Control.
(line 10)
* commenting: Comments. (line 6)
* commenting, backslash continuation and: Statements/Lines. (line 75)
* common extensions, ** operator: Arithmetic Ops. (line 30)
* common extensions, **= operator: Assignment Ops. (line 138)
* common extensions, /dev/stderr special file: Special FD. (line 48)
* common extensions, /dev/stdin special file: Special FD. (line 48)
* common extensions, /dev/stdout special file: Special FD. (line 48)
* common extensions, BINMODE variable: PC Using. (line 20)
* common extensions, delete to delete entire arrays: Delete. (line 39)
* common extensions, func keyword: Definition Syntax. (line 99)
* common extensions, length() applied to an array: String Functions.
(line 200)
* common extensions, RS as a regexp: gawk split records. (line 6)
* common extensions, single character fields: Single Character Fields.
(line 6)
* common extensions, \x escape sequence: Escape Sequences. (line 61)
* comp.lang.awk newsgroup: Usenet. (line 11)
* comparison expressions: Typing and Comparison.
(line 9)
* comparison expressions, as patterns: Expression Patterns. (line 14)
* comparison expressions, string vs. regexp: Comparison Operators.
(line 79)
* compatibility mode (gawk), extensions: POSIX/GNU. (line 6)
* compatibility mode (gawk), file names: Special Caveats. (line 9)
* compatibility mode (gawk), hexadecimal numbers: Nondecimal-numbers.
(line 59)
* compatibility mode (gawk), octal numbers: Nondecimal-numbers.
(line 59)
* compatibility mode (gawk), specifying: Options. (line 82)
* compiled programs: Basic High Level. (line 13)
* compiled programs <1>: Glossary. (line 216)
* compiling gawk for Cygwin: Cygwin. (line 6)
* compiling gawk for MS-Windows: PC Compiling. (line 11)
* compiling gawk for VMS: VMS Compilation. (line 6)
* compl: Bitwise Functions. (line 44)
* complement, bitwise: Bitwise Functions. (line 25)
* compound statements, control statements and: Statements. (line 10)
* concatenating: Concatenation. (line 9)
* condition debugger command: Breakpoint Control. (line 54)
* conditional expressions: Conditional Exp. (line 6)
* configuration option, --disable-extensions: Additional Configuration Options.
(line 9)
* configuration option, --disable-lint: Additional Configuration Options.
(line 15)
* configuration option, --disable-mpfr: Additional Configuration Options.
(line 32)
* configuration option, --disable-nls: Additional Configuration Options.
(line 37)
* configuration option, --enable-versioned-extension-dir: Additional Configuration Options.
(line 42)
* configuration option, --with-whiny-user-strftime: Additional Configuration Options.
(line 48)
* configuration options, gawk: Additional Configuration Options.
(line 6)
* constant regexps: Regexp Usage. (line 57)
* constants, nondecimal: Nondecimal Data. (line 6)
* constants, numeric: Scalar Constants. (line 6)
* constants, types of: Constants. (line 6)
* continue program, in debugger: Debugger Execution Control.
(line 33)
* continue statement: Continue Statement. (line 6)
* control statements: Statements. (line 6)
* controlling array scanning order: Controlling Scanning.
(line 14)
* convert string to lower case: String Functions. (line 526)
* convert string to number: String Functions. (line 393)
* convert string to upper case: String Functions. (line 532)
* converting integer array subscripts: Numeric Array Subscripts.
(line 31)
* converting, dates to timestamps: Time Functions. (line 78)
* converting, numbers to strings: Strings And Numbers. (line 6)
* converting, numbers to strings <1>: Bitwise Functions. (line 109)
* converting, strings to numbers: Strings And Numbers. (line 6)
* converting, strings to numbers <1>: Bitwise Functions. (line 109)
* CONVFMT variable: Strings And Numbers. (line 29)
* CONVFMT variable <1>: User-modified. (line 30)
* CONVFMT variable, and array subscripts: Numeric Array Subscripts.
(line 6)
* cookie: Glossary. (line 255)
* coprocesses: Redirection. (line 96)
* coprocesses <1>: Two-way I/O. (line 27)
* coprocesses, closing: Close Files And Pipes.
(line 6)
* coprocesses, getline from: Getline/Coprocess. (line 6)
* cos: Numeric Functions. (line 16)
* cosine: Numeric Functions. (line 16)
* counting: Wc Program. (line 6)
* csh utility: Statements/Lines. (line 43)
* csh utility, POSIXLY_CORRECT environment variable: Options. (line 372)
* csh utility, |& operator, comparison with: Two-way I/O. (line 27)
* ctime() user-defined function: Function Example. (line 74)
* Curreli, Marco: Contributors. (line 147)
* currency symbols, localization: Explaining gettext. (line 104)
* current system time: Time Functions. (line 68)
* custom.h file: Configuration Philosophy.
(line 30)
* customized input parser: Input Parsers. (line 6)
* customized output wrapper: Output Wrappers. (line 6)
* customized two-way processor: Two-way processors. (line 6)
* cut utility: Cut Program. (line 6)
* cut utility <1>: Cut Program. (line 6)
* cut.awk program: Cut Program. (line 45)
* d debugger command (alias for delete): Breakpoint Control. (line 64)
* d.c., See dark corner: Conventions. (line 42)
* dark corner: Conventions. (line 42)
* dark corner <1>: Glossary. (line 266)
* dark corner, "0" is actually true: Truth Values. (line 24)
* dark corner, /= operator vs. /=.../ regexp constant: Assignment Ops.
(line 149)
* dark corner, ARGV variable, value of: Executable Scripts. (line 59)
* dark corner, array subscripts: Uninitialized Subscripts.
(line 43)
* dark corner, break statement: Break Statement. (line 51)
* dark corner, close() function: Close Files And Pipes.
(line 132)
* dark corner, command-line arguments: Assignment Options. (line 43)
* dark corner, continue statement: Continue Statement. (line 44)
* dark corner, CONVFMT variable: Strings And Numbers. (line 39)
* dark corner, empty programs: Command Line. (line 20)
* dark corner, escape sequences: Other Arguments. (line 38)
* dark corner, escape sequences, for metacharacters: Escape Sequences.
(line 144)
* dark corner, exit statement: Exit Statement. (line 30)
* dark corner, field separators: Full Line Fields. (line 22)
* dark corner, FILENAME variable: Getline Notes. (line 19)
* dark corner, FILENAME variable <1>: Auto-set. (line 108)
* dark corner, FNR/NR variables: Auto-set. (line 389)
* dark corner, format-control characters: Control Letters. (line 18)
* dark corner, format-control characters <1>: Control Letters.
(line 93)
* dark corner, FS as null string: Single Character Fields.
(line 20)
* dark corner, input files: awk split records. (line 110)
* dark corner, invoking awk: Command Line. (line 16)
* dark corner, length() function: String Functions. (line 186)
* dark corner, locale's decimal point character: Locale influences conversions.
(line 17)
* dark corner, multiline records: Multiple Line. (line 35)
* dark corner, NF variable, decrementing: Changing Fields. (line 107)
* dark corner, OFMT variable: OFMT. (line 27)
* dark corner, range patterns, line continuation and: Ranges. (line 64)
* dark corner, regexp as second argument to index(): String Functions.
(line 164)
* dark corner, regexp constants: Standard Regexp Constants.
(line 6)
* dark corner, regexp constants, /= operator and: Assignment Ops.
(line 149)
* dark corner, regexp constants, as arguments to user-defined functions: Standard Regexp Constants.
(line 43)
* dark corner, split() function: String Functions. (line 363)
* dark corner, strings, storing: gawk split records. (line 82)
* dark corner, value of ARGV[0]: Auto-set. (line 39)
* dark corner, ^, in FS: Regexp Field Splitting.
(line 59)
* data, fixed-width: Constant Size. (line 6)
* data-driven languages: Basic High Level. (line 74)
* database, group, reading: Group Functions. (line 6)
* database, users, reading: Passwd Functions. (line 6)
* date utility, GNU: Time Functions. (line 17)
* date utility, POSIX: Time Functions. (line 255)
* dates, converting to timestamps: Time Functions. (line 78)
* dates, information related to, localization: Explaining gettext.
(line 112)
* Davies, Stephen: Acknowledgments. (line 60)
* Davies, Stephen <1>: Contributors. (line 75)
* Day, Robert P.J.: Acknowledgments. (line 79)
* dcgettext: I18N Functions. (line 21)
* dcgettext <1>: Programmer i18n. (line 20)
* dcgettext() function (gawk), portability and: I18N Portability.
(line 33)
* dcngettext: I18N Functions. (line 27)
* dcngettext <1>: Programmer i18n. (line 37)
* dcngettext() function (gawk), portability and: I18N Portability.
(line 33)
* deadlocks: Two-way I/O. (line 53)
* debugger commands, b (break): Breakpoint Control. (line 11)
* debugger commands, backtrace: Execution Stack. (line 13)
* debugger commands, break: Breakpoint Control. (line 11)
* debugger commands, bt (backtrace): Execution Stack. (line 13)
* debugger commands, c (continue): Debugger Execution Control.
(line 33)
* debugger commands, clear: Breakpoint Control. (line 36)
* debugger commands, commands: Debugger Execution Control.
(line 10)
* debugger commands, condition: Breakpoint Control. (line 54)
* debugger commands, continue: Debugger Execution Control.
(line 33)
* debugger commands, d (delete): Breakpoint Control. (line 64)
* debugger commands, delete: Breakpoint Control. (line 64)
* debugger commands, disable: Breakpoint Control. (line 69)
* debugger commands, display: Viewing And Changing Data.
(line 8)
* debugger commands, down: Execution Stack. (line 23)
* debugger commands, dump: Miscellaneous Debugger Commands.
(line 9)
* debugger commands, e (enable): Breakpoint Control. (line 73)
* debugger commands, enable: Breakpoint Control. (line 73)
* debugger commands, end: Debugger Execution Control.
(line 10)
* debugger commands, eval: Viewing And Changing Data.
(line 23)
* debugger commands, f (frame): Execution Stack. (line 27)
* debugger commands, finish: Debugger Execution Control.
(line 39)
* debugger commands, frame: Execution Stack. (line 27)
* debugger commands, h (help): Miscellaneous Debugger Commands.
(line 67)
* debugger commands, help: Miscellaneous Debugger Commands.
(line 67)
* debugger commands, i (info): Debugger Info. (line 13)
* debugger commands, ignore: Breakpoint Control. (line 87)
* debugger commands, info: Debugger Info. (line 13)
* debugger commands, l (list): Miscellaneous Debugger Commands.
(line 73)
* debugger commands, list: Miscellaneous Debugger Commands.
(line 73)
* debugger commands, n (next): Debugger Execution Control.
(line 43)
* debugger commands, next: Debugger Execution Control.
(line 43)
* debugger commands, nexti: Debugger Execution Control.
(line 49)
* debugger commands, ni (nexti): Debugger Execution Control.
(line 49)
* debugger commands, o (option): Debugger Info. (line 57)
* debugger commands, option: Debugger Info. (line 57)
* debugger commands, p (print): Viewing And Changing Data.
(line 35)
* debugger commands, print: Viewing And Changing Data.
(line 35)
* debugger commands, printf: Viewing And Changing Data.
(line 53)
* debugger commands, q (quit): Miscellaneous Debugger Commands.
(line 100)
* debugger commands, quit: Miscellaneous Debugger Commands.
(line 100)
* debugger commands, r (run): Debugger Execution Control.
(line 62)
* debugger commands, return: Debugger Execution Control.
(line 54)
* debugger commands, run: Debugger Execution Control.
(line 62)
* debugger commands, s (step): Debugger Execution Control.
(line 68)
* debugger commands, set: Viewing And Changing Data.
(line 58)
* debugger commands, si (stepi): Debugger Execution Control.
(line 75)
* debugger commands, silent: Debugger Execution Control.
(line 10)
* debugger commands, step: Debugger Execution Control.
(line 68)
* debugger commands, stepi: Debugger Execution Control.
(line 75)
* debugger commands, t (tbreak): Breakpoint Control. (line 90)
* debugger commands, tbreak: Breakpoint Control. (line 90)
* debugger commands, trace: Miscellaneous Debugger Commands.
(line 108)
* debugger commands, u (until): Debugger Execution Control.
(line 82)
* debugger commands, undisplay: Viewing And Changing Data.
(line 79)
* debugger commands, until: Debugger Execution Control.
(line 82)
* debugger commands, unwatch: Viewing And Changing Data.
(line 83)
* debugger commands, up: Execution Stack. (line 36)
* debugger commands, w (watch): Viewing And Changing Data.
(line 66)
* debugger commands, watch: Viewing And Changing Data.
(line 66)
* debugger commands, where (backtrace): Execution Stack. (line 13)
* debugger default list amount: Debugger Info. (line 69)
* debugger history file: Debugger Info. (line 81)
* debugger history size: Debugger Info. (line 65)
* debugger options: Debugger Info. (line 57)
* debugger prompt: Debugger Info. (line 78)
* debugger, how to start: Debugger Invocation. (line 6)
* debugger, read commands from a file: Debugger Info. (line 97)
* debugging awk programs: Debugger. (line 6)
* debugging gawk, bug reports: Bugs. (line 9)
* decimal point character, locale specific: Options. (line 283)
* decrement operators: Increment Ops. (line 35)
* default keyword: Switch Statement. (line 6)
* Deifik, Scott: Acknowledgments. (line 60)
* Deifik, Scott <1>: Contributors. (line 54)
* delete ARRAY: Delete. (line 39)
* delete breakpoint at location: Breakpoint Control. (line 36)
* delete breakpoint by number: Breakpoint Control. (line 64)
* delete debugger command: Breakpoint Control. (line 64)
* delete statement: Delete. (line 6)
* delete watchpoint: Viewing And Changing Data.
(line 83)
* deleting elements in arrays: Delete. (line 6)
* deleting entire arrays: Delete. (line 39)
* Demaille, Akim: Acknowledgments. (line 60)
* describe call stack frame, in debugger: Debugger Info. (line 27)
* differences between gawk and awk: String Functions. (line 200)
* differences in awk and gawk, ARGC/ARGV variables: ARGC and ARGV.
(line 89)
* differences in awk and gawk, ARGIND variable: Auto-set. (line 44)
* differences in awk and gawk, array elements, deleting: Delete.
(line 39)
* differences in awk and gawk, AWKLIBPATH environment variable: AWKLIBPATH Variable.
(line 6)
* differences in awk and gawk, AWKPATH environment variable: AWKPATH Variable.
(line 6)
* differences in awk and gawk, BEGIN/END patterns: I/O And BEGIN/END.
(line 15)
* differences in awk and gawk, BEGINFILE/ENDFILE patterns: BEGINFILE/ENDFILE.
(line 6)
* differences in awk and gawk, BINMODE variable: User-modified.
(line 15)
* differences in awk and gawk, BINMODE variable <1>: PC Using.
(line 20)
* differences in awk and gawk, close() function: Close Files And Pipes.
(line 81)
* differences in awk and gawk, close() function <1>: Close Files And Pipes.
(line 132)
* differences in awk and gawk, command-line directories: Command-line directories.
(line 6)
* differences in awk and gawk, ERRNO variable: Auto-set. (line 87)
* differences in awk and gawk, error messages: Special FD. (line 19)
* differences in awk and gawk, FIELDWIDTHS variable: User-modified.
(line 37)
* differences in awk and gawk, FPAT variable: User-modified. (line 46)
* differences in awk and gawk, FUNCTAB variable: Auto-set. (line 134)
* differences in awk and gawk, function arguments (gawk): Calling Built-in.
(line 16)
* differences in awk and gawk, getline command: Getline. (line 19)
* differences in awk and gawk, IGNORECASE variable: User-modified.
(line 79)
* differences in awk and gawk, implementation limitations: Getline Notes.
(line 14)
* differences in awk and gawk, implementation limitations <1>: Redirection.
(line 129)
* differences in awk and gawk, indirect function calls: Indirect Calls.
(line 6)
* differences in awk and gawk, input/output operators: Getline/Coprocess.
(line 6)
* differences in awk and gawk, input/output operators <1>: Redirection.
(line 96)
* differences in awk and gawk, line continuations: Conditional Exp.
(line 34)
* differences in awk and gawk, LINT variable: User-modified. (line 90)
* differences in awk and gawk, match() function: String Functions.
(line 262)
* differences in awk and gawk, print/printf statements: Format Modifiers.
(line 13)
* differences in awk and gawk, PROCINFO array: Auto-set. (line 148)
* differences in awk and gawk, read timeouts: Read Timeout. (line 6)
* differences in awk and gawk, record separators: awk split records.
(line 124)
* differences in awk and gawk, regexp constants: Standard Regexp Constants.
(line 43)
* differences in awk and gawk, regular expressions: Case-sensitivity.
(line 26)
* differences in awk and gawk, retrying input: Retrying Input.
(line 6)
* differences in awk and gawk, RS/RT variables: gawk split records.
(line 58)
* differences in awk and gawk, RT variable: Auto-set. (line 327)
* differences in awk and gawk, single-character fields: Single Character Fields.
(line 6)
* differences in awk and gawk, split() function: String Functions.
(line 350)
* differences in awk and gawk, strings: Scalar Constants. (line 20)
* differences in awk and gawk, strings, storing: gawk split records.
(line 76)
* differences in awk and gawk, SYMTAB variable: Auto-set. (line 331)
* differences in awk and gawk, TEXTDOMAIN variable: User-modified.
(line 155)
* differences in awk and gawk, trunc-mod operation: Arithmetic Ops.
(line 66)
* directories, command-line: Command-line directories.
(line 6)
* directories, searching: Programs Exercises. (line 70)
* directories, searching for loadable extensions: AWKLIBPATH Variable.
(line 6)
* directories, searching for source files: AWKPATH Variable. (line 6)
* disable breakpoint: Breakpoint Control. (line 69)
* disable debugger command: Breakpoint Control. (line 69)
* display debugger command: Viewing And Changing Data.
(line 8)
* display debugger options: Debugger Info. (line 57)
* division: Arithmetic Ops. (line 44)
* do-while statement: Do Statement. (line 6)
* do-while statement, use of regexps in: Regexp Usage. (line 19)
* documentation, of awk programs: Library Names. (line 6)
* documentation, online: Manual History. (line 11)
* documents, searching: Dupword Program. (line 6)
* dollar sign ($), $ field operator: Fields. (line 19)
* dollar sign ($), $ field operator <1>: Precedence. (line 42)
* dollar sign ($), incrementing fields and arrays: Increment Ops.
(line 30)
* dollar sign ($), regexp operator: Regexp Operators. (line 35)
* double quote ("), in regexp constants: Computed Regexps. (line 30)
* double quote ("), in shell commands: Quoting. (line 54)
* double-precision: Computer Arithmetic. (line 61)
* down debugger command: Execution Stack. (line 23)
* Drepper, Ulrich: Acknowledgments. (line 52)
* Duman, Patrice: Acknowledgments. (line 75)
* dump all variables of a program: Options. (line 94)
* dump debugger command: Miscellaneous Debugger Commands.
(line 9)
* dupword.awk program: Dupword Program. (line 31)
* dynamic profiling: Profiling. (line 177)
* dynamically loaded extensions: Dynamic Extensions. (line 6)
* e debugger command (alias for enable): Breakpoint Control. (line 73)
* EBCDIC: Ordinal Functions. (line 45)
* effective group ID of gawk user: Auto-set. (line 172)
* effective user ID of gawk user: Auto-set. (line 180)
* egrep utility: Bracket Expressions. (line 34)
* egrep utility <1>: Egrep Program. (line 6)
* egrep.awk program: Egrep Program. (line 53)
* elements in arrays, assigning values: Assigning Elements. (line 6)
* elements in arrays, deleting: Delete. (line 6)
* elements in arrays, order of access by in operator: Scanning an Array.
(line 48)
* elements in arrays, scanning: Scanning an Array. (line 6)
* elements of arrays: Reference to Elements.
(line 6)
* email address for bug reports, bug-gawk AT gnu.org: Bug address.
(line 31)
* empty array elements: Reference to Elements.
(line 18)
* empty pattern: Empty. (line 6)
* empty strings: awk split records. (line 114)
* empty strings, See null strings: Regexp Field Splitting.
(line 43)
* EMRED: TCP/IP Networking. (line 6)
* enable breakpoint: Breakpoint Control. (line 73)
* enable debugger command: Breakpoint Control. (line 73)
* end debugger command: Debugger Execution Control.
(line 10)
* END pattern: BEGIN/END. (line 6)
* END pattern <1>: Using BEGIN/END. (line 6)
* END pattern, and profiling: Profiling. (line 62)
* END pattern, assert() user-defined function and: Assert Function.
(line 75)
* END pattern, Boolean patterns and: Expression Patterns. (line 70)
* END pattern, exit statement and: Exit Statement. (line 12)
* END pattern, next/nextfile statements and: I/O And BEGIN/END.
(line 36)
* END pattern, next/nextfile statements and <1>: Next Statement.
(line 44)
* END pattern, operators and: Using BEGIN/END. (line 17)
* END pattern, print statement and: I/O And BEGIN/END. (line 15)
* ENDFILE pattern: BEGINFILE/ENDFILE. (line 6)
* ENDFILE pattern, Boolean patterns and: Expression Patterns. (line 70)
* endfile() user-defined function: Filetrans Function. (line 62)
* endgrent() function (C library): Group Functions. (line 213)
* endgrent() user-defined function: Group Functions. (line 216)
* endpwent() function (C library): Passwd Functions. (line 208)
* endpwent() user-defined function: Passwd Functions. (line 211)
* English, Steve: Advanced Features. (line 6)
* ENVIRON array: Auto-set. (line 59)
* environment variables used by gawk: Environment Variables.
(line 6)
* environment variables, in ENVIRON array: Auto-set. (line 59)
* epoch, definition of: Glossary. (line 310)
* equals sign (=), = operator: Assignment Ops. (line 6)
* equals sign (=), == operator: Comparison Operators.
(line 11)
* equals sign (=), == operator <1>: Precedence. (line 64)
* EREs (Extended Regular Expressions): Bracket Expressions. (line 34)
* ERRNO variable: Auto-set. (line 87)
* ERRNO variable <1>: TCP/IP Networking. (line 54)
* ERRNO variable, with BEGINFILE pattern: BEGINFILE/ENDFILE. (line 26)
* ERRNO variable, with close() function: Close Files And Pipes.
(line 140)
* ERRNO variable, with getline command: Getline. (line 19)
* error handling: Special FD. (line 19)
* error handling, ERRNO variable and: Auto-set. (line 87)
* error output: Special FD. (line 6)
* escape processing, gsub()/gensub()/sub() functions: Gory Details.
(line 6)
* escape sequences, in strings: Escape Sequences. (line 6)
* eval debugger command: Viewing And Changing Data.
(line 23)
* evaluate expressions, in debugger: Viewing And Changing Data.
(line 23)
* evaluation order: Increment Ops. (line 60)
* evaluation order, concatenation: Concatenation. (line 41)
* evaluation order, functions: Calling Built-in. (line 30)
* examining fields: Fields. (line 6)
* exclamation point (!), ! operator: Boolean Ops. (line 69)
* exclamation point (!), ! operator <1>: Precedence. (line 51)
* exclamation point (!), ! operator <2>: Egrep Program. (line 174)
* exclamation point (!), != operator: Comparison Operators.
(line 11)
* exclamation point (!), != operator <1>: Precedence. (line 64)
* exclamation point (!), !~ operator: Regexp Usage. (line 19)
* exclamation point (!), !~ operator <1>: Computed Regexps. (line 6)
* exclamation point (!), !~ operator <2>: Case-sensitivity. (line 26)
* exclamation point (!), !~ operator <3>: Regexp Constants. (line 6)
* exclamation point (!), !~ operator <4>: Comparison Operators.
(line 11)
* exclamation point (!), !~ operator <5>: Comparison Operators.
(line 98)
* exclamation point (!), !~ operator <6>: Precedence. (line 79)
* exclamation point (!), !~ operator <7>: Expression Patterns.
(line 24)
* exit debugger command: Miscellaneous Debugger Commands.
(line 64)
* exit statement: Exit Statement. (line 6)
* exit status, of gawk: Exit Status. (line 6)
* exit status, of VMS: VMS Running. (line 28)
* exit the debugger: Miscellaneous Debugger Commands.
(line 64)
* exit the debugger <1>: Miscellaneous Debugger Commands.
(line 100)
* exp: Numeric Functions. (line 19)
* expand utility: Very Simple. (line 73)
* Expat XML parser library: gawkextlib. (line 40)
* exponent: Numeric Functions. (line 19)
* expressions: Expressions. (line 6)
* expressions, as patterns: Expression Patterns. (line 6)
* expressions, assignment: Assignment Ops. (line 6)
* expressions, Boolean: Boolean Ops. (line 6)
* expressions, comparison: Typing and Comparison.
(line 9)
* expressions, conditional: Conditional Exp. (line 6)
* expressions, matching, See comparison expressions: Typing and Comparison.
(line 9)
* expressions, selecting: Conditional Exp. (line 6)
* Extended Regular Expressions (EREs): Bracket Expressions. (line 34)
* extension API: Extension API Description.
(line 6)
* extension API informational variables: Extension API Informational Variables.
(line 6)
* extension API version: Extension Versioning.
(line 6)
* extension API, version number: Auto-set. (line 266)
* extension example: Extension Example. (line 6)
* extension registration: Registration Functions.
(line 6)
* extension search path: Finding Extensions. (line 6)
* extensions distributed with gawk: Extension Samples. (line 6)
* extensions, allocating memory: Memory Allocation Functions.
(line 6)
* extensions, Brian Kernighan's awk: BTL. (line 6)
* extensions, Brian Kernighan's awk <1>: Common Extensions. (line 6)
* extensions, common, ** operator: Arithmetic Ops. (line 30)
* extensions, common, **= operator: Assignment Ops. (line 138)
* extensions, common, /dev/stderr special file: Special FD. (line 48)
* extensions, common, /dev/stdin special file: Special FD. (line 48)
* extensions, common, /dev/stdout special file: Special FD. (line 48)
* extensions, common, BINMODE variable: PC Using. (line 20)
* extensions, common, delete to delete entire arrays: Delete. (line 39)
* extensions, common, fflush() function: I/O Functions. (line 43)
* extensions, common, func keyword: Definition Syntax. (line 99)
* extensions, common, length() applied to an array: String Functions.
(line 200)
* extensions, common, RS as a regexp: gawk split records. (line 6)
* extensions, common, single character fields: Single Character Fields.
(line 6)
* extensions, common, \x escape sequence: Escape Sequences. (line 61)
* extensions, in gawk, not in POSIX awk: POSIX/GNU. (line 6)
* extensions, loading, @load directive: Loading Shared Libraries.
(line 8)
* extensions, mawk: Common Extensions. (line 6)
* extensions, where to find: gawkextlib. (line 6)
* extract.awk program: Extract Program. (line 79)
* extraction, of marked strings (internationalization): String Extraction.
(line 6)
* f debugger command (alias for frame): Execution Stack. (line 27)
* false, logical: Truth Values. (line 6)
* FDL (Free Documentation License): GNU Free Documentation License.
(line 8)
* features, adding to gawk: Adding Code. (line 6)
* features, deprecated: Obsolete. (line 6)
* features, undocumented: Undocumented. (line 6)
* Fenlason, Jay: History. (line 30)
* Fenlason, Jay <1>: Contributors. (line 19)
* fflush: I/O Functions. (line 28)
* field numbers: Nonconstant Fields. (line 6)
* field operator $: Fields. (line 19)
* field operators, dollar sign as: Fields. (line 19)
* field separator, in multiline records: Multiple Line. (line 41)
* field separator, on command line: Command Line Field Separator.
(line 6)
* field separator, POSIX and: Full Line Fields. (line 16)
* field separators: Field Separators. (line 15)
* field separators <1>: User-modified. (line 53)
* field separators <2>: User-modified. (line 116)
* field separators, choice of: Field Separators. (line 50)
* field separators, FIELDWIDTHS variable and: User-modified. (line 37)
* field separators, FPAT variable and: User-modified. (line 46)
* field separators, regular expressions as: Field Separators. (line 50)
* field separators, regular expressions as <1>: Regexp Field Splitting.
(line 6)
* field separators, See Also OFS: Changing Fields. (line 64)
* field separators, spaces as: Cut Program. (line 103)
* fields: Reading Files. (line 14)
* fields <1>: Fields. (line 6)
* fields <2>: Basic High Level. (line 62)
* fields, adding: Changing Fields. (line 53)
* fields, changing contents of: Changing Fields. (line 6)
* fields, cutting: Cut Program. (line 6)
* fields, examining: Fields. (line 6)
* fields, number of: Fields. (line 33)
* fields, numbers: Nonconstant Fields. (line 6)
* fields, printing: Print Examples. (line 20)
* fields, separating: Field Separators. (line 15)
* fields, separating <1>: Field Separators. (line 15)
* fields, single-character: Single Character Fields.
(line 6)
* FIELDWIDTHS variable: Fixed width data. (line 17)
* FIELDWIDTHS variable <1>: User-modified. (line 37)
* file descriptors: Special FD. (line 6)
* file inclusion, @include directive: Include Files. (line 8)
* file names, distinguishing: Auto-set. (line 55)
* file names, in compatibility mode: Special Caveats. (line 9)
* file names, standard streams in gawk: Special FD. (line 48)
* FILENAME variable: Reading Files. (line 6)
* FILENAME variable <1>: Auto-set. (line 108)
* FILENAME variable, getline, setting with: Getline Notes. (line 19)
* filenames, assignments as: Ignoring Assigns. (line 6)
* files, .gmo: Explaining gettext. (line 42)
* files, .gmo, specifying directory of: Explaining gettext. (line 54)
* files, .gmo, specifying directory of <1>: Programmer i18n. (line 48)
* files, .mo, converting from .po: I18N Example. (line 66)
* files, .po: Explaining gettext. (line 37)
* files, .po <1>: Translator i18n. (line 6)
* files, .po, converting to .mo: I18N Example. (line 66)
* files, .pot: Explaining gettext. (line 31)
* files, /dev/... special files: Special FD. (line 48)
* files, /inet/... (gawk): TCP/IP Networking. (line 6)
* files, /inet4/... (gawk): TCP/IP Networking. (line 6)
* files, /inet6/... (gawk): TCP/IP Networking. (line 6)
* files, awk programs in: Long. (line 6)
* files, awkprof.out: Profiling. (line 6)
* files, awkvars.out: Options. (line 94)
* files, closing: I/O Functions. (line 10)
* files, descriptors, See file descriptors: Special FD. (line 6)
* files, group: Group Functions. (line 6)
* files, initialization and cleanup: Filetrans Function. (line 6)
* files, input, See input files: Read Terminal. (line 16)
* files, log, timestamps in: Time Functions. (line 6)
* files, managing: Data File Management.
(line 6)
* files, managing, data file boundaries: Filetrans Function. (line 6)
* files, message object: Explaining gettext. (line 42)
* files, message object, converting from portable object files: I18N Example.
(line 66)
* files, message object, specifying directory of: Explaining gettext.
(line 54)
* files, message object, specifying directory of <1>: Programmer i18n.
(line 48)
* files, multiple passes over: Other Arguments. (line 56)
* files, multiple, duplicating output into: Tee Program. (line 6)
* files, output, See output files: Close Files And Pipes.
(line 6)
* files, password: Passwd Functions. (line 16)
* files, portable object: Explaining gettext. (line 37)
* files, portable object <1>: Translator i18n. (line 6)
* files, portable object template: Explaining gettext. (line 31)
* files, portable object, converting to message object files: I18N Example.
(line 66)
* files, portable object, generating: Options. (line 161)
* files, processing, ARGIND variable and: Auto-set. (line 50)
* files, reading: Rewind Function. (line 6)
* files, reading, multiline records: Multiple Line. (line 6)
* files, searching for regular expressions: Egrep Program. (line 6)
* files, skipping: File Checking. (line 6)
* files, source, search path for: Programs Exercises. (line 70)
* files, splitting: Split Program. (line 6)
* files, Texinfo, extracting programs from: Extract Program. (line 6)
* find substring in string: String Functions. (line 155)
* finding extensions: Finding Extensions. (line 6)
* finish debugger command: Debugger Execution Control.
(line 39)
* Fish, Fred: Contributors. (line 51)
* fixed-width data: Constant Size. (line 6)
* flag variables: Boolean Ops. (line 69)
* flag variables <1>: Tee Program. (line 20)
* floating-point, numbers: Computer Arithmetic. (line 49)
* floating-point, numbers, arbitrary precision: Arbitrary Precision Arithmetic.
(line 6)
* floating-point, numbers, arbitrary-precision: Computer Arithmetic.
(line 61)
* floating-point, numbers, double-precision: Computer Arithmetic.
(line 61)
* floating-point, numbers, single-precision: Computer Arithmetic.
(line 61)
* floating-point, VAX/VMS: VMS Running. (line 50)
* flush buffered output: I/O Functions. (line 28)
* fnmatch() extension function: Extension Sample Fnmatch.
(line 12)
* FNR variable: Records. (line 6)
* FNR variable <1>: Auto-set. (line 118)
* FNR variable, changing: Auto-set. (line 389)
* for statement: For Statement. (line 6)
* for statement, looping over arrays: Scanning an Array. (line 20)
* fork() extension function: Extension Sample Fork.
(line 11)
* format specifiers: Basic Printf. (line 15)
* format specifiers, mixing regular with positional specifiers: Printf Ordering.
(line 57)
* format specifiers, printf statement: Control Letters. (line 6)
* format specifiers, strftime() function (gawk): Time Functions.
(line 91)
* format time string: Time Functions. (line 50)
* formats, numeric output: OFMT. (line 6)
* formatting output: Printf. (line 6)
* formatting strings: String Functions. (line 386)
* forward slash (/) to enclose regular expressions: Regexp. (line 10)
* forward slash (/), / operator: Precedence. (line 54)
* forward slash (/), /= operator: Assignment Ops. (line 129)
* forward slash (/), /= operator <1>: Precedence. (line 94)
* forward slash (/), /= operator, vs. /=.../ regexp constant: Assignment Ops.
(line 149)
* forward slash (/), patterns and: Expression Patterns. (line 24)
* FPAT variable: Splitting By Content.
(line 25)
* FPAT variable <1>: User-modified. (line 46)
* frame debugger command: Execution Stack. (line 27)
* Free Documentation License (FDL): GNU Free Documentation License.
(line 8)
* Free Software Foundation (FSF): Manual History. (line 6)
* Free Software Foundation (FSF) <1>: Getting. (line 10)
* Free Software Foundation (FSF) <2>: Glossary. (line 370)
* Free Software Foundation (FSF) <3>: Glossary. (line 403)
* FreeBSD: Glossary. (line 746)
* FS variable: Field Separators. (line 15)
* FS variable <1>: User-modified. (line 53)
* FS variable, --field-separator option and: Options. (line 21)
* FS variable, as null string: Single Character Fields.
(line 20)
* FS variable, as TAB character: Options. (line 280)
* FS variable, changing value of: Field Separators. (line 34)
* FS variable, running awk programs and: Cut Program. (line 63)
* FS variable, setting from command line: Command Line Field Separator.
(line 6)
* FS, containing ^: Regexp Field Splitting.
(line 59)
* FS, in multiline records: Multiple Line. (line 41)
* FSF (Free Software Foundation): Manual History. (line 6)
* FSF (Free Software Foundation) <1>: Getting. (line 10)
* FSF (Free Software Foundation) <2>: Glossary. (line 370)
* FSF (Free Software Foundation) <3>: Glossary. (line 403)
* fts() extension function: Extension Sample File Functions.
(line 60)
* FUNCTAB array: Auto-set. (line 134)
* function calls: Function Calls. (line 6)
* function calls, indirect: Indirect Calls. (line 6)
* function calls, indirect, @-notation for: Indirect Calls. (line 47)
* function definition example: Function Example. (line 6)
* function pointers: Indirect Calls. (line 6)
* functions, arrays as parameters to: Pass By Value/Reference.
(line 44)
* functions, built-in: Function Calls. (line 10)
* functions, built-in <1>: Functions. (line 6)
* functions, built-in, evaluation order: Calling Built-in. (line 30)
* functions, defining: Definition Syntax. (line 10)
* functions, library: Library Functions. (line 6)
* functions, library, assertions: Assert Function. (line 6)
* functions, library, associative arrays and: Library Names. (line 58)
* functions, library, C library: Getopt Function. (line 6)
* functions, library, character values as numbers: Ordinal Functions.
(line 6)
* functions, library, Cliff random numbers: Cliff Random Function.
(line 6)
* functions, library, command-line options: Getopt Function. (line 6)
* functions, library, example program for using: Igawk Program.
(line 6)
* functions, library, group database, reading: Group Functions.
(line 6)
* functions, library, managing data files: Data File Management.
(line 6)
* functions, library, managing time: Getlocaltime Function.
(line 6)
* functions, library, merging arrays into strings: Join Function.
(line 6)
* functions, library, rounding numbers: Round Function. (line 6)
* functions, library, user database, reading: Passwd Functions.
(line 6)
* functions, names of: Definition Syntax. (line 24)
* functions, recursive: Definition Syntax. (line 89)
* functions, string-translation: I18N Functions. (line 6)
* functions, undefined: Pass By Value/Reference.
(line 68)
* functions, user-defined: User-defined. (line 6)
* functions, user-defined, calling: Function Caveats. (line 6)
* functions, user-defined, counts, in a profile: Profiling. (line 137)
* functions, user-defined, library of: Library Functions. (line 6)
* functions, user-defined, next/nextfile statements and: Next Statement.
(line 44)
* functions, user-defined, next/nextfile statements and <1>: Nextfile Statement.
(line 47)
* G-d: Acknowledgments. (line 94)
* G., Daniel Richard: Acknowledgments. (line 60)
* G., Daniel Richard <1>: Maintainers. (line 14)
* Garfinkle, Scott: Contributors. (line 35)
* gawk program, dynamic profiling: Profiling. (line 177)
* gawk version: Auto-set. (line 241)
* gawk, ARGIND variable in: Other Arguments. (line 15)
* gawk, awk and: Preface. (line 21)
* gawk, awk and <1>: This Manual. (line 14)
* gawk, bitwise operations in: Bitwise Functions. (line 40)
* gawk, break statement in: Break Statement. (line 51)
* gawk, character classes and: Bracket Expressions. (line 108)
* gawk, coding style in: Adding Code. (line 37)
* gawk, command-line options, and regular expressions: GNU Regexp Operators.
(line 73)
* gawk, configuring: Configuration Philosophy.
(line 6)
* gawk, configuring, options: Additional Configuration Options.
(line 6)
* gawk, continue statement in: Continue Statement. (line 44)
* gawk, distribution: Distribution contents.
(line 6)
* gawk, ERRNO variable in: Getline. (line 19)
* gawk, ERRNO variable in <1>: Close Files And Pipes.
(line 140)
* gawk, ERRNO variable in <2>: BEGINFILE/ENDFILE. (line 26)
* gawk, ERRNO variable in <3>: Auto-set. (line 87)
* gawk, ERRNO variable in <4>: TCP/IP Networking. (line 54)
* gawk, escape sequences: Escape Sequences. (line 121)
* gawk, extensions, disabling: Options. (line 271)
* gawk, features, adding: Adding Code. (line 6)
* gawk, features, advanced: Advanced Features. (line 6)
* gawk, field separators and: User-modified. (line 74)
* gawk, FIELDWIDTHS variable in: Fixed width data. (line 17)
* gawk, FIELDWIDTHS variable in <1>: User-modified. (line 37)
* gawk, file names in: Special Files. (line 6)
* gawk, format-control characters: Control Letters. (line 18)
* gawk, format-control characters <1>: Control Letters. (line 93)
* gawk, FPAT variable in: Splitting By Content.
(line 25)
* gawk, FPAT variable in <1>: User-modified. (line 46)
* gawk, FUNCTAB array in: Auto-set. (line 134)
* gawk, function arguments and: Calling Built-in. (line 16)
* gawk, hexadecimal numbers and: Nondecimal-numbers. (line 41)
* gawk, IGNORECASE variable in: Case-sensitivity. (line 26)
* gawk, IGNORECASE variable in <1>: User-modified. (line 79)
* gawk, IGNORECASE variable in <2>: Array Intro. (line 100)
* gawk, IGNORECASE variable in <3>: String Functions. (line 58)
* gawk, IGNORECASE variable in <4>: Array Sorting Functions.
(line 83)
* gawk, implementation issues: Notes. (line 6)
* gawk, implementation issues, debugging: Compatibility Mode. (line 6)
* gawk, implementation issues, downward compatibility: Compatibility Mode.
(line 6)
* gawk, implementation issues, limits: Getline Notes. (line 14)
* gawk, implementation issues, pipes: Redirection. (line 129)
* gawk, installing: Installation. (line 6)
* gawk, internationalization and, See internationalization: Internationalization.
(line 13)
* gawk, interpreter, adding code to: Using Internal File Ops.
(line 6)
* gawk, interval expressions and: Regexp Operators. (line 139)
* gawk, line continuation in: Conditional Exp. (line 34)
* gawk, LINT variable in: User-modified. (line 90)
* gawk, list of contributors to: Contributors. (line 6)
* gawk, MS-Windows version of: PC Using. (line 13)
* gawk, newlines in: Statements/Lines. (line 12)
* gawk, octal numbers and: Nondecimal-numbers. (line 41)
* gawk, predefined variables and: Built-in Variables. (line 14)
* gawk, PROCINFO array in: Auto-set. (line 148)
* gawk, PROCINFO array in <1>: Time Functions. (line 49)
* gawk, PROCINFO array in <2>: Two-way I/O. (line 114)
* gawk, regexp constants and: Standard Regexp Constants.
(line 28)
* gawk, regular expressions, case sensitivity: Case-sensitivity.
(line 26)
* gawk, regular expressions, operators: GNU Regexp Operators.
(line 6)
* gawk, regular expressions, precedence: Regexp Operators. (line 161)
* gawk, RT variable in: awk split records. (line 124)
* gawk, RT variable in <1>: Multiple Line. (line 130)
* gawk, RT variable in <2>: Auto-set. (line 327)
* gawk, See Also awk: Preface. (line 34)
* gawk, source code, obtaining: Getting. (line 6)
* gawk, splitting fields and: Testing field creation.
(line 6)
* gawk, string-translation functions: I18N Functions. (line 6)
* gawk, SYMTAB array in: Auto-set. (line 331)
* gawk, TEXTDOMAIN variable in: User-modified. (line 155)
* gawk, timestamps: Time Functions. (line 6)
* gawk, uses for: Preface. (line 34)
* gawk, versions of, information about, printing: Options. (line 318)
* gawk, VMS version of: VMS Installation. (line 6)
* gawk, word-boundary operator: GNU Regexp Operators.
(line 66)
* gawkextlib: gawkextlib. (line 6)
* gawkextlib project: gawkextlib. (line 6)
* gawklibpath_append shell function: Shell Startup Files. (line 29)
* gawklibpath_default shell function: Shell Startup Files. (line 22)
* gawklibpath_prepend shell function: Shell Startup Files. (line 25)
* gawkpath_append shell function: Shell Startup Files. (line 19)
* gawkpath_default shell function: Shell Startup Files. (line 12)
* gawkpath_prepend shell function: Shell Startup Files. (line 15)
* General Public License (GPL): Glossary. (line 394)
* General Public License, See GPL: Manual History. (line 11)
* generate time values: Time Functions. (line 25)
* gensub: Standard Regexp Constants.
(line 43)
* gensub <1>: String Functions. (line 89)
* gensub() function (gawk), escape processing: Gory Details. (line 6)
* getaddrinfo() function (C library): TCP/IP Networking. (line 39)
* getgrent() function (C library): Group Functions. (line 6)
* getgrent() function (C library) <1>: Group Functions. (line 202)
* getgrent() user-defined function: Group Functions. (line 6)
* getgrent() user-defined function <1>: Group Functions. (line 205)
* getgrgid() function (C library): Group Functions. (line 184)
* getgrgid() user-defined function: Group Functions. (line 187)
* getgrnam() function (C library): Group Functions. (line 173)
* getgrnam() user-defined function: Group Functions. (line 178)
* getgruser() function (C library): Group Functions. (line 193)
* getgruser() function, user-defined: Group Functions. (line 196)
* getline command: Reading Files. (line 20)
* getline command, coprocesses, using from: Getline/Coprocess.
(line 6)
* getline command, coprocesses, using from <1>: Close Files And Pipes.
(line 6)
* getline command, deadlock and: Two-way I/O. (line 53)
* getline command, explicit input with: Getline. (line 6)
* getline command, FILENAME variable and: Getline Notes. (line 19)
* getline command, return values: Getline. (line 19)
* getline command, variants: Getline Summary. (line 6)
* getline command, _gr_init() user-defined function: Group Functions.
(line 83)
* getline command, _pw_init() function: Passwd Functions. (line 154)
* getline from a file: Getline/File. (line 6)
* getline into a variable: Getline/Variable. (line 6)
* getline statement, BEGINFILE/ENDFILE patterns and: BEGINFILE/ENDFILE.
(line 54)
* getlocaltime() user-defined function: Getlocaltime Function.
(line 16)
* getopt() function (C library): Getopt Function. (line 15)
* getopt() user-defined function: Getopt Function. (line 108)
* getopt() user-defined function <1>: Getopt Function. (line 134)
* getpwent() function (C library): Passwd Functions. (line 16)
* getpwent() function (C library) <1>: Passwd Functions. (line 196)
* getpwent() user-defined function: Passwd Functions. (line 16)
* getpwent() user-defined function <1>: Passwd Functions. (line 200)
* getpwnam() function (C library): Passwd Functions. (line 175)
* getpwnam() user-defined function: Passwd Functions. (line 180)
* getpwuid() function (C library): Passwd Functions. (line 186)
* getpwuid() user-defined function: Passwd Functions. (line 190)
* gettext library: Explaining gettext. (line 6)
* gettext library, locale categories: Explaining gettext. (line 81)
* gettext() function (C library): Explaining gettext. (line 63)
* gettimeofday() extension function: Extension Sample Time.
(line 12)
* git utility: gawkextlib. (line 31)
* git utility <1>: Other Versions. (line 29)
* git utility <2>: Accessing The Source.
(line 10)
* git utility <3>: Adding Code. (line 112)
* Git, use of for gawk source code: Derived Files. (line 6)
* GNITS mailing list: Acknowledgments. (line 52)
* GNU awk, See gawk: Preface. (line 51)
* GNU Free Documentation License: GNU Free Documentation License.
(line 8)
* GNU General Public License: Glossary. (line 394)
* GNU Lesser General Public License: Glossary. (line 489)
* GNU long options: Command Line. (line 13)
* GNU long options <1>: Options. (line 6)
* GNU long options, printing list of: Options. (line 168)
* GNU Project: Manual History. (line 11)
* GNU Project <1>: Glossary. (line 403)
* GNU/Linux: Manual History. (line 28)
* GNU/Linux <1>: I18N Example. (line 57)
* GNU/Linux <2>: Glossary. (line 746)
* Gordon, Assaf: Contributors. (line 108)
* GPL (General Public License): Manual History. (line 11)
* GPL (General Public License) <1>: Glossary. (line 394)
* GPL (General Public License), printing: Options. (line 89)
* grcat program: Group Functions. (line 16)
* Grigera, Juan: Contributors. (line 58)
* group database, reading: Group Functions. (line 6)
* group file: Group Functions. (line 6)
* group ID of gawk user: Auto-set. (line 190)
* groups, information about: Group Functions. (line 6)
* gsub: Standard Regexp Constants.
(line 43)
* gsub <1>: String Functions. (line 139)
* gsub() function, arguments of: String Functions. (line 465)
* gsub() function, escape processing: Gory Details. (line 6)
* Guerrero, Juan Manuel: Acknowledgments. (line 60)
* Guerrero, Juan Manuel <1>: Contributors. (line 150)
* Guerrero, Juan Manuel <2>: Maintainers. (line 14)
* h debugger command (alias for help): Miscellaneous Debugger Commands.
(line 67)
* Hankerson, Darrel: Acknowledgments. (line 60)
* Hankerson, Darrel <1>: Contributors. (line 61)
* Haque, John: Contributors. (line 111)
* Hartholz, Elaine: Acknowledgments. (line 38)
* Hartholz, Marshall: Acknowledgments. (line 38)
* Hasegawa, Isamu: Contributors. (line 97)
* help debugger command: Miscellaneous Debugger Commands.
(line 67)
* hexadecimal numbers: Nondecimal-numbers. (line 6)
* hexadecimal values, enabling interpretation of: Options. (line 223)
* history expansion, in debugger: Readline Support. (line 6)
* histsort.awk program: History Sorting. (line 25)
* Hughes, Phil: Acknowledgments. (line 43)
* HUP signal, for dynamic profiling: Profiling. (line 209)
* hyphen (-), - operator: Precedence. (line 51)
* hyphen (-), - operator <1>: Precedence. (line 57)
* hyphen (-), -- operator: Increment Ops. (line 48)
* hyphen (-), -- operator <1>: Precedence. (line 45)
* hyphen (-), -= operator: Assignment Ops. (line 129)
* hyphen (-), -= operator <1>: Precedence. (line 94)
* hyphen (-), filenames beginning with: Options. (line 60)
* hyphen (-), in bracket expressions: Bracket Expressions. (line 25)
* i debugger command (alias for info): Debugger Info. (line 13)
* id utility: Id Program. (line 6)
* id.awk program: Id Program. (line 31)
* if statement: If Statement. (line 6)
* if statement, actions, changing: Ranges. (line 25)
* if statement, use of regexps in: Regexp Usage. (line 19)
* igawk.sh program: Igawk Program. (line 124)
* ignore breakpoint: Breakpoint Control. (line 87)
* ignore debugger command: Breakpoint Control. (line 87)
* IGNORECASE variable: User-modified. (line 79)
* IGNORECASE variable, and array indices: Array Intro. (line 100)
* IGNORECASE variable, and array sorting functions: Array Sorting Functions.
(line 83)
* IGNORECASE variable, in example programs: Library Functions.
(line 53)
* IGNORECASE variable, with ~ and !~ operators: Case-sensitivity.
(line 26)
* Illumos: Other Versions. (line 114)
* Illumos, POSIX-compliant awk: Other Versions. (line 114)
* implementation issues, gawk: Notes. (line 6)
* implementation issues, gawk, debugging: Compatibility Mode. (line 6)
* implementation issues, gawk, limits: Getline Notes. (line 14)
* implementation issues, gawk, limits <1>: Redirection. (line 129)
* in operator: Comparison Operators.
(line 11)
* in operator <1>: Precedence. (line 82)
* in operator <2>: For Statement. (line 75)
* in operator, index existence in multidimensional arrays: Multidimensional.
(line 41)
* in operator, order of array access: Scanning an Array. (line 48)
* in operator, testing if array element exists: Reference to Elements.
(line 38)
* in operator, use in loops: Scanning an Array. (line 17)
* including files, @include directive: Include Files. (line 8)
* increment operators: Increment Ops. (line 6)
* index: String Functions. (line 155)
* indexing arrays: Array Intro. (line 48)
* indirect function calls: Indirect Calls. (line 6)
* indirect function calls, @-notation: Indirect Calls. (line 47)
* infinite precision: Arbitrary Precision Arithmetic.
(line 6)
* info debugger command: Debugger Info. (line 13)
* initialization, automatic: More Complex. (line 39)
* inplace extension: Extension Sample Inplace.
(line 6)
* input files: Reading Files. (line 6)
* input files, closing: Close Files And Pipes.
(line 6)
* input files, counting elements in: Wc Program. (line 6)
* input files, examples: Sample Data Files. (line 6)
* input files, reading: Reading Files. (line 6)
* input files, running awk without: Read Terminal. (line 6)
* input files, running awk without <1>: Read Terminal. (line 16)
* input files, variable assignments and: Other Arguments. (line 26)
* input pipeline: Getline/Pipe. (line 10)
* input record, length of: String Functions. (line 177)
* input redirection: Getline/File. (line 6)
* input, data, nondecimal: Nondecimal Data. (line 6)
* input, explicit: Getline. (line 6)
* input, files, See input files: Multiple Line. (line 6)
* input, multiline records: Multiple Line. (line 6)
* input, splitting into records: Records. (line 6)
* input, standard: Read Terminal. (line 6)
* input, standard <1>: Special FD. (line 6)
* input/output functions: I/O Functions. (line 6)
* input/output, binary: User-modified. (line 15)
* input/output, from BEGIN and END: I/O And BEGIN/END. (line 6)
* input/output, two-way: Two-way I/O. (line 27)
* insomnia, cure for: Alarm Program. (line 6)
* installation, VMS: VMS Installation. (line 6)
* installing gawk: Installation. (line 6)
* instruction tracing, in debugger: Debugger Info. (line 90)
* int: Numeric Functions. (line 24)
* INT signal (MS-Windows): Profiling. (line 212)
* integer array indices: Numeric Array Subscripts.
(line 31)
* integers, arbitrary precision: Arbitrary Precision Integers.
(line 6)
* integers, unsigned: Computer Arithmetic. (line 41)
* interacting with other programs: I/O Functions. (line 106)
* internationalization: I18N Functions. (line 6)
* internationalization <1>: I18N and L10N. (line 6)
* internationalization, localization: User-modified. (line 155)
* internationalization, localization <1>: Internationalization.
(line 13)
* internationalization, localization, character classes: Bracket Expressions.
(line 108)
* internationalization, localization, gawk and: Internationalization.
(line 13)
* internationalization, localization, locale categories: Explaining gettext.
(line 81)
* internationalization, localization, marked strings: Programmer i18n.
(line 13)
* internationalization, localization, portability and: I18N Portability.
(line 6)
* internationalizing a program: Explaining gettext. (line 6)
* interpreted programs: Basic High Level. (line 13)
* interpreted programs <1>: Glossary. (line 443)
* interval expressions, regexp operator: Regexp Operators. (line 116)
* inventory-shipped file: Sample Data Files. (line 32)
* invoke shell command: I/O Functions. (line 106)
* isarray: Type Functions. (line 11)
* ISO: Glossary. (line 454)
* ISO 8859-1: Glossary. (line 194)
* ISO Latin-1: Glossary. (line 194)
* Jacobs, Andrew: Passwd Functions. (line 90)
* Jaegermann, Michal: Acknowledgments. (line 60)
* Jaegermann, Michal <1>: Contributors. (line 46)
* Java implementation of awk: Other Versions. (line 122)
* Java programming language: Glossary. (line 466)
* jawk: Other Versions. (line 122)
* Jedi knights: Undocumented. (line 6)
* Johansen, Chris: Signature Program. (line 25)
* join() user-defined function: Join Function. (line 18)
* Kahrs, Ju"rgen: Acknowledgments. (line 60)
* Kahrs, Ju"rgen <1>: Contributors. (line 71)
* Kasal, Stepan: Acknowledgments. (line 60)
* Kenobi, Obi-Wan: Undocumented. (line 6)
* Kernighan, Brian: History. (line 17)
* Kernighan, Brian <1>: Conventions. (line 38)
* Kernighan, Brian <2>: Acknowledgments. (line 79)
* Kernighan, Brian <3>: Getline/Pipe. (line 6)
* Kernighan, Brian <4>: Concatenation. (line 6)
* Kernighan, Brian <5>: Library Functions. (line 12)
* Kernighan, Brian <6>: BTL. (line 6)
* Kernighan, Brian <7>: Contributors. (line 12)
* Kernighan, Brian <8>: Other Versions. (line 13)
* Kernighan, Brian <9>: Basic Data Typing. (line 54)
* Kernighan, Brian <10>: Glossary. (line 204)
* kill command, dynamic profiling: Profiling. (line 186)
* Knights, jedi: Undocumented. (line 6)
* Kwok, Conrad: Contributors. (line 35)
* l debugger command (alias for list): Miscellaneous Debugger Commands.
(line 73)
* labels.awk program: Labels Program. (line 51)
* Langston, Peter: Advanced Features. (line 6)
* LANGUAGE environment variable: Explaining gettext. (line 120)
* languages, data-driven: Basic High Level. (line 74)
* LC_ALL locale category: Explaining gettext. (line 117)
* LC_COLLATE locale category: Explaining gettext. (line 94)
* LC_CTYPE locale category: Explaining gettext. (line 98)
* LC_MESSAGES locale category: Explaining gettext. (line 88)
* LC_MESSAGES locale category, bindtextdomain() function (gawk): Programmer i18n.
(line 103)
* LC_MONETARY locale category: Explaining gettext. (line 104)
* LC_NUMERIC locale category: Explaining gettext. (line 108)
* LC_TIME locale category: Explaining gettext. (line 112)
* left angle bracket (<), < operator: Comparison Operators.
(line 11)
* left angle bracket (<), < operator <1>: Precedence. (line 64)
* left angle bracket (<), < operator (I/O): Getline/File. (line 6)
* left angle bracket (<), <= operator: Comparison Operators.
(line 11)
* left angle bracket (<), <= operator <1>: Precedence. (line 64)
* left shift: Bitwise Functions. (line 47)
* left shift, bitwise: Bitwise Functions. (line 32)
* leftmost longest match: Multiple Line. (line 26)
* length: String Functions. (line 170)
* length of input record: String Functions. (line 177)
* length of string: String Functions. (line 170)
* Lesser General Public License (LGPL): Glossary. (line 489)
* LGPL (Lesser General Public License): Glossary. (line 489)
* libmawk: Other Versions. (line 130)
* libraries of awk functions: Library Functions. (line 6)
* libraries of awk functions, assertions: Assert Function. (line 6)
* libraries of awk functions, associative arrays and: Library Names.
(line 58)
* libraries of awk functions, character values as numbers: Ordinal Functions.
(line 6)
* libraries of awk functions, command-line options: Getopt Function.
(line 6)
* libraries of awk functions, example program for using: Igawk Program.
(line 6)
* libraries of awk functions, group database, reading: Group Functions.
(line 6)
* libraries of awk functions, managing, data files: Data File Management.
(line 6)
* libraries of awk functions, managing, time: Getlocaltime Function.
(line 6)
* libraries of awk functions, merging arrays into strings: Join Function.
(line 6)
* libraries of awk functions, rounding numbers: Round Function.
(line 6)
* libraries of awk functions, user database, reading: Passwd Functions.
(line 6)
* line breaks: Statements/Lines. (line 6)
* line continuations: Boolean Ops. (line 64)
* line continuations, gawk: Conditional Exp. (line 34)
* line continuations, in print statement: Print Examples. (line 75)
* line continuations, with C shell: More Complex. (line 31)
* lines, blank, printing: Print. (line 22)
* lines, counting: Wc Program. (line 6)
* lines, duplicate, removing: History Sorting. (line 6)
* lines, matching ranges of: Ranges. (line 6)
* lines, skipping between markers: Ranges. (line 43)
* lint checking: User-modified. (line 90)
* lint checking, array elements: Delete. (line 34)
* lint checking, array subscripts: Uninitialized Subscripts.
(line 43)
* lint checking, empty programs: Command Line. (line 16)
* lint checking, issuing warnings: Options. (line 198)
* lint checking, POSIXLY_CORRECT environment variable: Options.
(line 357)
* lint checking, undefined functions: Pass By Value/Reference.
(line 85)
* LINT variable: User-modified. (line 90)
* Linux: Manual History. (line 28)
* Linux <1>: I18N Example. (line 57)
* Linux <2>: Glossary. (line 746)
* list all global variables, in debugger: Debugger Info. (line 48)
* list debugger command: Miscellaneous Debugger Commands.
(line 73)
* list function definitions, in debugger: Debugger Info. (line 30)
* loading extensions, @load directive: Loading Shared Libraries.
(line 8)
* loading, extensions: Options. (line 186)
* local variables, in a function: Variable Scope. (line 6)
* locale categories: Explaining gettext. (line 81)
* locale decimal point character: Options. (line 283)
* locale, definition of: Locales. (line 6)
* localization: I18N and L10N. (line 6)
* localization, See internationalization, localization: I18N and L10N.
(line 6)
* log: Numeric Functions. (line 29)
* log files, timestamps in: Time Functions. (line 6)
* logarithm: Numeric Functions. (line 29)
* logical false/true: Truth Values. (line 6)
* logical operators, See Boolean expressions: Boolean Ops. (line 6)
* login information: Passwd Functions. (line 16)
* long options: Command Line. (line 13)
* loops: While Statement. (line 6)
* loops, break statement and: Break Statement. (line 6)
* loops, continue statements and: For Statement. (line 64)
* loops, count for header, in a profile: Profiling. (line 131)
* loops, do-while: Do Statement. (line 6)
* loops, exiting: Break Statement. (line 6)
* loops, for, array scanning: Scanning an Array. (line 6)
* loops, for, iterative: For Statement. (line 6)
* loops, See Also while statement: While Statement. (line 6)
* loops, while: While Statement. (line 6)
* ls utility: More Complex. (line 15)
* lshift: Bitwise Functions. (line 47)
* lvalues/rvalues: Assignment Ops. (line 31)
* mail-list file: Sample Data Files. (line 6)
* mailing labels, printing: Labels Program. (line 6)
* mailing list, GNITS: Acknowledgments. (line 52)
* Malmberg, John: Acknowledgments. (line 60)
* Malmberg, John <1>: Maintainers. (line 14)
* Malmberg, John E.: Contributors. (line 140)
* mark parity: Ordinal Functions. (line 45)
* marked string extraction (internationalization): String Extraction.
(line 6)
* marked strings, extracting: String Extraction. (line 6)
* Marx, Groucho: Increment Ops. (line 60)
* match: String Functions. (line 210)
* match regexp in string: String Functions. (line 210)
* match() function, RSTART/RLENGTH variables: String Functions.
(line 227)
* matching, expressions, See comparison expressions: Typing and Comparison.
(line 9)
* matching, leftmost longest: Multiple Line. (line 26)
* matching, null strings: String Functions. (line 539)
* mawk utility: Escape Sequences. (line 121)
* mawk utility <1>: Getline/Pipe. (line 62)
* mawk utility <2>: Concatenation. (line 36)
* mawk utility <3>: Nextfile Statement. (line 47)
* mawk utility <4>: Other Versions. (line 48)
* maximum precision supported by MPFR library: Auto-set. (line 255)
* McIlroy, Doug: Glossary. (line 255)
* McPhee, Patrick: Contributors. (line 103)
* message object files: Explaining gettext. (line 42)
* message object files, converting from portable object files: I18N Example.
(line 66)
* message object files, specifying directory of: Explaining gettext.
(line 54)
* message object files, specifying directory of <1>: Programmer i18n.
(line 48)
* messages from extensions: Printing Messages. (line 6)
* metacharacters in regular expressions: Regexp Operators. (line 6)
* metacharacters, escape sequences for: Escape Sequences. (line 140)
* minimum precision required by MPFR library: Auto-set. (line 258)
* mktime: Time Functions. (line 25)
* modifiers, in format specifiers: Format Modifiers. (line 6)
* monetary information, localization: Explaining gettext. (line 104)
* Moore, Duncan: Getline Notes. (line 40)
* MPFR, checking availability of: Checking for MPFR. (line 6)
* msgfmt utility: I18N Example. (line 66)
* multiple precision: Arbitrary Precision Arithmetic.
(line 6)
* multiple-line records: Multiple Line. (line 6)
* n debugger command (alias for next): Debugger Execution Control.
(line 43)
* names, arrays/variables: Library Names. (line 6)
* names, functions: Definition Syntax. (line 24)
* names, functions <1>: Library Names. (line 6)
* namespace issues: Library Names. (line 6)
* namespace issues, functions: Definition Syntax. (line 24)
* NetBSD: Glossary. (line 746)
* networks, programming: TCP/IP Networking. (line 6)
* networks, support for: Special Network. (line 6)
* newlines: Statements/Lines. (line 6)
* newlines <1>: Options. (line 277)
* newlines <2>: Boolean Ops. (line 69)
* newlines, as record separators: awk split records. (line 12)
* newlines, in dynamic regexps: Computed Regexps. (line 60)
* newlines, in regexp constants: Computed Regexps. (line 70)
* newlines, printing: Print Examples. (line 11)
* newlines, separating statements in actions: Action Overview.
(line 19)
* newlines, separating statements in actions <1>: Statements. (line 10)
* next debugger command: Debugger Execution Control.
(line 43)
* next file statement: Feature History. (line 168)
* next statement: Boolean Ops. (line 95)
* next statement <1>: Next Statement. (line 6)
* next statement, BEGIN/END patterns and: I/O And BEGIN/END. (line 36)
* next statement, BEGINFILE/ENDFILE patterns and: BEGINFILE/ENDFILE.
(line 50)
* next statement, user-defined functions and: Next Statement. (line 44)
* nextfile statement: Nextfile Statement. (line 6)
* nextfile statement, BEGIN/END patterns and: I/O And BEGIN/END.
(line 36)
* nextfile statement, BEGINFILE/ENDFILE patterns and: BEGINFILE/ENDFILE.
(line 26)
* nextfile statement, user-defined functions and: Nextfile Statement.
(line 47)
* nexti debugger command: Debugger Execution Control.
(line 49)
* NF variable: Fields. (line 33)
* NF variable <1>: Auto-set. (line 123)
* NF variable, decrementing: Changing Fields. (line 107)
* ni debugger command (alias for nexti): Debugger Execution Control.
(line 49)
* noassign.awk program: Ignoring Assigns. (line 15)
* non-existent array elements: Reference to Elements.
(line 23)
* not Boolean-logic operator: Boolean Ops. (line 6)
* NR variable: Records. (line 6)
* NR variable <1>: Auto-set. (line 143)
* NR variable, changing: Auto-set. (line 389)
* null strings: awk split records. (line 114)
* null strings <1>: Regexp Field Splitting.
(line 43)
* null strings <2>: Truth Values. (line 6)
* null strings <3>: Basic Data Typing. (line 26)
* null strings in gawk arguments, quoting and: Quoting. (line 82)
* null strings, and deleting array elements: Delete. (line 27)
* null strings, as array subscripts: Uninitialized Subscripts.
(line 43)
* null strings, converting numbers to strings: Strings And Numbers.
(line 21)
* null strings, matching: String Functions. (line 539)
* number as string of bits: Bitwise Functions. (line 109)
* number of array elements: String Functions. (line 200)
* number sign (#), #! (executable scripts): Executable Scripts.
(line 6)
* number sign (#), commenting: Comments. (line 6)
* numbers, as array subscripts: Numeric Array Subscripts.
(line 6)
* numbers, as values of characters: Ordinal Functions. (line 6)
* numbers, Cliff random: Cliff Random Function.
(line 6)
* numbers, converting: Strings And Numbers. (line 6)
* numbers, converting <1>: Bitwise Functions. (line 109)
* numbers, converting, to strings: User-modified. (line 30)
* numbers, converting, to strings <1>: User-modified. (line 107)
* numbers, hexadecimal: Nondecimal-numbers. (line 6)
* numbers, octal: Nondecimal-numbers. (line 6)
* numbers, rounding: Round Function. (line 6)
* numeric constants: Scalar Constants. (line 6)
* numeric functions: Numeric Functions. (line 6)
* numeric, output format: OFMT. (line 6)
* numeric, strings: Variable Typing. (line 67)
* o debugger command (alias for option): Debugger Info. (line 57)
* obsolete features: Obsolete. (line 6)
* octal numbers: Nondecimal-numbers. (line 6)
* octal values, enabling interpretation of: Options. (line 223)
* OFMT variable: OFMT. (line 15)
* OFMT variable <1>: Strings And Numbers. (line 56)
* OFMT variable <2>: User-modified. (line 107)
* OFMT variable, POSIX awk and: OFMT. (line 27)
* OFS variable: Changing Fields. (line 64)
* OFS variable <1>: Output Separators. (line 6)
* OFS variable <2>: User-modified. (line 116)
* OpenBSD: Glossary. (line 746)
* OpenSolaris: Other Versions. (line 105)
* operating systems, BSD-based: Manual History. (line 28)
* operating systems, PC, gawk on: PC Using. (line 6)
* operating systems, PC, gawk on, installing: PC Installation.
(line 6)
* operating systems, porting gawk to: New Ports. (line 6)
* operating systems, See Also GNU/Linux, PC operating systems, Unix: Installation.
(line 6)
* operations, bitwise: Bitwise Functions. (line 6)
* operators, arithmetic: Arithmetic Ops. (line 6)
* operators, assignment: Assignment Ops. (line 6)
* operators, assignment <1>: Assignment Ops. (line 31)
* operators, assignment, evaluation order: Assignment Ops. (line 110)
* operators, Boolean, See Boolean expressions: Boolean Ops. (line 6)
* operators, decrement/increment: Increment Ops. (line 6)
* operators, GNU-specific: GNU Regexp Operators.
(line 6)
* operators, input/output: Getline/File. (line 6)
* operators, input/output <1>: Getline/Pipe. (line 10)
* operators, input/output <2>: Getline/Coprocess. (line 6)
* operators, input/output <3>: Redirection. (line 22)
* operators, input/output <4>: Redirection. (line 96)
* operators, input/output <5>: Precedence. (line 64)
* operators, input/output <6>: Precedence. (line 64)
* operators, input/output <7>: Precedence. (line 64)
* operators, logical, See Boolean expressions: Boolean Ops. (line 6)
* operators, precedence: Increment Ops. (line 60)
* operators, precedence <1>: Precedence. (line 6)
* operators, relational, See operators, comparison: Typing and Comparison.
(line 9)
* operators, short-circuit: Boolean Ops. (line 59)
* operators, string: Concatenation. (line 9)
* operators, string-matching: Regexp Usage. (line 19)
* operators, string-matching, for buffers: GNU Regexp Operators.
(line 51)
* operators, word-boundary (gawk): GNU Regexp Operators.
(line 66)
* option debugger command: Debugger Info. (line 57)
* options, command-line: Options. (line 6)
* options, command-line, end of: Options. (line 55)
* options, command-line, invoking awk: Command Line. (line 6)
* options, command-line, processing: Getopt Function. (line 6)
* options, deprecated: Obsolete. (line 6)
* options, long: Command Line. (line 13)
* options, long <1>: Options. (line 6)
* options, printing list of: Options. (line 168)
* or: Bitwise Functions. (line 50)
* OR bitwise operation: Bitwise Functions. (line 6)
* or Boolean-logic operator: Boolean Ops. (line 6)
* ord() extension function: Extension Sample Ord.
(line 12)
* ord() user-defined function: Ordinal Functions. (line 16)
* order of evaluation, concatenation: Concatenation. (line 41)
* ORS variable: Output Separators. (line 20)
* ORS variable <1>: User-modified. (line 122)
* output field separator, See OFS variable: Changing Fields. (line 64)
* output record separator, See ORS variable: Output Separators.
(line 20)
* output redirection: Redirection. (line 6)
* output wrapper: Output Wrappers. (line 6)
* output, buffering: I/O Functions. (line 32)
* output, buffering <1>: I/O Functions. (line 165)
* output, duplicating into files: Tee Program. (line 6)
* output, files, closing: Close Files And Pipes.
(line 6)
* output, format specifier, OFMT: OFMT. (line 15)
* output, formatted: Printf. (line 6)
* output, pipes: Redirection. (line 57)
* output, printing, See printing: Printing. (line 6)
* output, records: Output Separators. (line 20)
* output, standard: Special FD. (line 6)
* p debugger command (alias for print): Viewing And Changing Data.
(line 35)
* Papadopoulos, Panos: Contributors. (line 131)
* parent process ID of gawk process: Auto-set. (line 230)
* parentheses (), in a profile: Profiling. (line 146)
* parentheses (), regexp operator: Regexp Operators. (line 81)
* password file: Passwd Functions. (line 16)
* patsplit: String Functions. (line 296)
* patterns: Patterns and Actions.
(line 6)
* patterns, comparison expressions as: Expression Patterns. (line 14)
* patterns, counts, in a profile: Profiling. (line 118)
* patterns, default: Very Simple. (line 35)
* patterns, empty: Empty. (line 6)
* patterns, expressions as: Regexp Patterns. (line 6)
* patterns, ranges in: Ranges. (line 6)
* patterns, regexp constants as: Expression Patterns. (line 34)
* patterns, types of: Pattern Overview. (line 15)
* pawk (profiling version of Brian Kernighan's awk): Other Versions.
(line 87)
* pawk, awk-like facilities for Python: Other Versions. (line 134)
* PC operating systems, gawk on: PC Using. (line 6)
* PC operating systems, gawk on, installing: PC Installation. (line 6)
* percent sign (%), % operator: Precedence. (line 54)
* percent sign (%), %= operator: Assignment Ops. (line 129)
* percent sign (%), %= operator <1>: Precedence. (line 94)
* period (.), regexp operator: Regexp Operators. (line 44)
* Perl: Future Extensions. (line 6)
* Peters, Arno: Contributors. (line 88)
* Peterson, Hal: Contributors. (line 40)
* pipe, closing: Close Files And Pipes.
(line 6)
* pipe, input: Getline/Pipe. (line 10)
* pipe, output: Redirection. (line 57)
* Pitts, Dave: Acknowledgments. (line 60)
* Pitts, Dave <1>: Maintainers. (line 14)
* Plauger, P.J.: Library Functions. (line 12)
* plug-in: Extension Intro. (line 6)
* plus sign (+), + operator: Precedence. (line 51)
* plus sign (+), + operator <1>: Precedence. (line 57)
* plus sign (+), ++ operator: Increment Ops. (line 11)
* plus sign (+), ++ operator <1>: Increment Ops. (line 40)
* plus sign (+), ++ operator <2>: Precedence. (line 45)
* plus sign (+), += operator: Assignment Ops. (line 81)
* plus sign (+), += operator <1>: Precedence. (line 94)
* plus sign (+), regexp operator: Regexp Operators. (line 105)
* pointers to functions: Indirect Calls. (line 6)
* portability: Escape Sequences. (line 103)
* portability, #! (executable scripts): Executable Scripts. (line 33)
* portability, ** operator and: Arithmetic Ops. (line 81)
* portability, **= operator and: Assignment Ops. (line 144)
* portability, ARGV variable: Executable Scripts. (line 59)
* portability, backslash continuation and: Statements/Lines. (line 30)
* portability, backslash in escape sequences: Escape Sequences.
(line 108)
* portability, close() function and: Close Files And Pipes.
(line 81)
* portability, data files as single record: gawk split records.
(line 65)
* portability, deleting array elements: Delete. (line 56)
* portability, example programs: Library Functions. (line 42)
* portability, functions, defining: Definition Syntax. (line 114)
* portability, gawk: New Ports. (line 6)
* portability, gettext library and: Explaining gettext. (line 11)
* portability, internationalization and: I18N Portability. (line 6)
* portability, length() function: String Functions. (line 179)
* portability, new awk vs. old awk: Strings And Numbers. (line 56)
* portability, next statement in user-defined functions: Pass By Value/Reference.
(line 88)
* portability, NF variable, decrementing: Changing Fields. (line 115)
* portability, operators: Increment Ops. (line 60)
* portability, operators, not in POSIX awk: Precedence. (line 97)
* portability, POSIXLY_CORRECT environment variable: Options. (line 377)
* portability, substr() function: String Functions. (line 515)
* portable object files: Explaining gettext. (line 37)
* portable object files <1>: Translator i18n. (line 6)
* portable object files, converting to message object files: I18N Example.
(line 66)
* portable object files, generating: Options. (line 161)
* portable object template files: Explaining gettext. (line 31)
* porting gawk: New Ports. (line 6)
* positional specifiers, printf statement: Format Modifiers. (line 13)
* positional specifiers, printf statement <1>: Printf Ordering.
(line 6)
* positional specifiers, printf statement, mixing with regular formats: Printf Ordering.
(line 57)
* POSIX awk: This Manual. (line 14)
* POSIX awk <1>: Assignment Ops. (line 138)
* POSIX awk, ** operator and: Precedence. (line 97)
* POSIX awk, **= operator and: Assignment Ops. (line 144)
* POSIX awk, < operator and: Getline/File. (line 26)
* POSIX awk, arithmetic operators and: Arithmetic Ops. (line 30)
* POSIX awk, backslashes in string constants: Escape Sequences.
(line 108)
* POSIX awk, BEGIN/END patterns: I/O And BEGIN/END. (line 15)
* POSIX awk, bracket expressions and: Bracket Expressions. (line 34)
* POSIX awk, bracket expressions and, character classes: Bracket Expressions.
(line 40)
* POSIX awk, bracket expressions and, character classes <1>: Bracket Expressions.
(line 108)
* POSIX awk, break statement and: Break Statement. (line 51)
* POSIX awk, changes in awk versions: POSIX. (line 6)
* POSIX awk, continue statement and: Continue Statement. (line 44)
* POSIX awk, CONVFMT variable and: User-modified. (line 30)
* POSIX awk, date utility and: Time Functions. (line 255)
* POSIX awk, field separators and: Full Line Fields. (line 16)
* POSIX awk, function keyword in: Definition Syntax. (line 99)
* POSIX awk, functions and, gsub()/sub(): Gory Details. (line 90)
* POSIX awk, functions and, length(): String Functions. (line 179)
* POSIX awk, GNU long options and: Options. (line 15)
* POSIX awk, interval expressions in: Regexp Operators. (line 135)
* POSIX awk, next/nextfile statements and: Next Statement. (line 44)
* POSIX awk, numeric strings and: Variable Typing. (line 67)
* POSIX awk, OFMT variable and: OFMT. (line 27)
* POSIX awk, OFMT variable and <1>: Strings And Numbers. (line 56)
* POSIX awk, period (.), using: Regexp Operators. (line 51)
* POSIX awk, printf format strings and: Format Modifiers. (line 157)
* POSIX awk, regular expressions and: Regexp Operators. (line 161)
* POSIX awk, timestamps and: Time Functions. (line 6)
* POSIX awk, | I/O operator and: Getline/Pipe. (line 56)
* POSIX mode: Options. (line 271)
* POSIX mode <1>: Options. (line 357)
* POSIX, awk and: Preface. (line 21)
* POSIX, gawk extensions not included in: POSIX/GNU. (line 6)
* POSIX, programs, implementing in awk: Clones. (line 6)
* POSIXLY_CORRECT environment variable: Options. (line 357)
* PREC variable: User-modified. (line 127)
* precedence: Increment Ops. (line 60)
* precedence <1>: Precedence. (line 6)
* precedence, regexp operators: Regexp Operators. (line 156)
* predefined variables: Built-in Variables. (line 6)
* predefined variables, -v option, setting with: Options. (line 41)
* predefined variables, conveying information: Auto-set. (line 6)
* predefined variables, user-modifiable: User-modified. (line 6)
* print debugger command: Viewing And Changing Data.
(line 35)
* print statement: Printing. (line 16)
* print statement, BEGIN/END patterns and: I/O And BEGIN/END. (line 15)
* print statement, commas, omitting: Print Examples. (line 30)
* print statement, I/O operators in: Precedence. (line 70)
* print statement, line continuations and: Print Examples. (line 75)
* print statement, OFMT variable and: User-modified. (line 116)
* print statement, See Also redirection, of output: Redirection.
(line 17)
* print statement, sprintf() function and: Round Function. (line 6)
* print variables, in debugger: Viewing And Changing Data.
(line 35)
* printf debugger command: Viewing And Changing Data.
(line 53)
* printf statement: Printing. (line 16)
* printf statement <1>: Printf. (line 6)
* printf statement, columns, aligning: Print Examples. (line 69)
* printf statement, format-control characters: Control Letters.
(line 6)
* printf statement, I/O operators in: Precedence. (line 70)
* printf statement, modifiers: Format Modifiers. (line 6)
* printf statement, positional specifiers: Format Modifiers. (line 13)
* printf statement, positional specifiers <1>: Printf Ordering.
(line 6)
* printf statement, positional specifiers, mixing with regular formats: Printf Ordering.
(line 57)
* printf statement, See Also redirection, of output: Redirection.
(line 17)
* printf statement, sprintf() function and: Round Function. (line 6)
* printf statement, syntax of: Basic Printf. (line 6)
* printing: Printing. (line 6)
* printing messages from extensions: Printing Messages. (line 6)
* printing, list of options: Options. (line 168)
* printing, mailing labels: Labels Program. (line 6)
* printing, unduplicated lines of text: Uniq Program. (line 6)
* printing, user information: Id Program. (line 6)
* private variables: Library Names. (line 11)
* process group ID of gawk process: Auto-set. (line 224)
* process ID of gawk process: Auto-set. (line 227)
* processes, two-way communications with: Two-way I/O. (line 6)
* processing data: Basic High Level. (line 6)
* PROCINFO array: Auto-set. (line 148)
* PROCINFO array <1>: Time Functions. (line 49)
* PROCINFO array <2>: Passwd Functions. (line 6)
* PROCINFO array, and communications via ptys: Two-way I/O. (line 114)
* PROCINFO array, and group membership: Group Functions. (line 6)
* PROCINFO array, and user and group ID numbers: Id Program. (line 15)
* PROCINFO array, testing the field splitting: Passwd Functions.
(line 154)
* PROCINFO, values of sorted_in: Controlling Scanning.
(line 26)
* profiling awk programs: Profiling. (line 6)
* profiling awk programs, dynamically: Profiling. (line 177)
* program identifiers: Auto-set. (line 193)
* program, definition of: Getting Started. (line 21)
* programming conventions, --non-decimal-data option: Nondecimal Data.
(line 35)
* programming conventions, ARGC/ARGV variables: Auto-set. (line 35)
* programming conventions, exit statement: Exit Statement. (line 38)
* programming conventions, function parameters: Return Statement.
(line 44)
* programming conventions, functions, calling: Calling Built-in.
(line 10)
* programming conventions, functions, writing: Definition Syntax.
(line 71)
* programming conventions, gawk extensions: Internal File Ops.
(line 45)
* programming conventions, private variable names: Library Names.
(line 23)
* programming language, recipe for: History. (line 6)
* programming languages, Ada: Glossary. (line 11)
* programming languages, data-driven vs. procedural: Getting Started.
(line 12)
* programming languages, Java: Glossary. (line 466)
* programming, basic steps: Basic High Level. (line 18)
* programming, concepts: Basic Concepts. (line 6)
* programming, concepts <1>: Basic Concepts. (line 6)
* pwcat program: Passwd Functions. (line 23)
* q debugger command (alias for quit): Miscellaneous Debugger Commands.
(line 100)
* QSE awk: Other Versions. (line 140)
* Quanstrom, Erik: Alarm Program. (line 8)
* question mark (?), ?: operator: Precedence. (line 91)
* question mark (?), regexp operator: Regexp Operators. (line 111)
* question mark (?), regexp operator <1>: GNU Regexp Operators.
(line 62)
* QuikTrim Awk: Other Versions. (line 144)
* quit debugger command: Miscellaneous Debugger Commands.
(line 100)
* QUIT signal (MS-Windows): Profiling. (line 212)
* quoting in gawk command lines: Long. (line 26)
* quoting in gawk command lines, tricks for: Quoting. (line 91)
* quoting, for small awk programs: Comments. (line 27)
* r debugger command (alias for run): Debugger Execution Control.
(line 62)
* Rakitzis, Byron: History Sorting. (line 25)
* Ramey, Chet: Acknowledgments. (line 60)
* Ramey, Chet <1>: General Data Types. (line 6)
* rand: Numeric Functions. (line 34)
* random numbers, Cliff: Cliff Random Function.
(line 6)
* random numbers, rand()/srand() functions: Numeric Functions.
(line 34)
* random numbers, seed of: Numeric Functions. (line 64)
* range expressions (regexps): Bracket Expressions. (line 6)
* range patterns: Ranges. (line 6)
* range patterns, line continuation and: Ranges. (line 64)
* Rankin, Pat: Acknowledgments. (line 60)
* Rankin, Pat <1>: Assignment Ops. (line 99)
* Rankin, Pat <2>: Contributors. (line 38)
* RapidJson JSON parser library: gawkextlib. (line 37)
* reada() extension function: Extension Sample Read write array.
(line 18)
* readable data files, checking: File Checking. (line 6)
* readable.awk program: File Checking. (line 11)
* readdir extension: Extension Sample Readdir.
(line 9)
* readfile() extension function: Extension Sample Readfile.
(line 12)
* readfile() user-defined function: Readfile Function. (line 30)
* reading input files: Reading Files. (line 6)
* recipe for a programming language: History. (line 6)
* record separators: awk split records. (line 6)
* record separators <1>: User-modified. (line 136)
* record separators, changing: awk split records. (line 85)
* record separators, regular expressions as: awk split records.
(line 124)
* record separators, with multiline records: Multiple Line. (line 10)
* records: Reading Files. (line 14)
* records <1>: Basic High Level. (line 62)
* records, multiline: Multiple Line. (line 6)
* records, printing: Print. (line 22)
* records, splitting input into: Records. (line 6)
* records, terminating: awk split records. (line 124)
* records, treating files as: gawk split records. (line 92)
* recursive functions: Definition Syntax. (line 89)
* redirect gawk output, in debugger: Debugger Info. (line 73)
* redirection of input: Getline/File. (line 6)
* redirection of output: Redirection. (line 6)
* redirection on VMS: VMS Running. (line 64)
* reference counting, sorting arrays: Array Sorting Functions.
(line 77)
* regexp: Regexp. (line 6)
* regexp constants: Regexp Usage. (line 57)
* regexp constants <1>: Regexp Constants. (line 6)
* regexp constants <2>: Comparison Operators.
(line 103)
* regexp constants, /=.../, /= operator and: Assignment Ops. (line 149)
* regexp constants, as patterns: Expression Patterns. (line 34)
* regexp constants, in gawk: Standard Regexp Constants.
(line 28)
* regexp constants, slashes vs. quotes: Computed Regexps. (line 30)
* regexp constants, vs. string constants: Computed Regexps. (line 40)
* register extension: Registration Functions.
(line 6)
* regular expressions: Regexp. (line 6)
* regular expressions as field separators: Field Separators. (line 50)
* regular expressions, anchors in: Regexp Operators. (line 22)
* regular expressions, as field separators: Regexp Field Splitting.
(line 6)
* regular expressions, as patterns: Regexp Usage. (line 6)
* regular expressions, as patterns <1>: Regexp Patterns. (line 6)
* regular expressions, as record separators: awk split records.
(line 124)
* regular expressions, case sensitivity: Case-sensitivity. (line 6)
* regular expressions, case sensitivity <1>: User-modified. (line 79)
* regular expressions, computed: Computed Regexps. (line 6)
* regular expressions, constants, See regexp constants: Regexp Usage.
(line 57)
* regular expressions, dynamic: Computed Regexps. (line 6)
* regular expressions, dynamic, with embedded newlines: Computed Regexps.
(line 60)
* regular expressions, gawk, command-line options: GNU Regexp Operators.
(line 73)
* regular expressions, interval expressions and: Options. (line 292)
* regular expressions, leftmost longest match: Leftmost Longest.
(line 6)
* regular expressions, operators: Regexp Usage. (line 19)
* regular expressions, operators <1>: Regexp Operators. (line 6)
* regular expressions, operators, for buffers: GNU Regexp Operators.
(line 51)
* regular expressions, operators, for words: GNU Regexp Operators.
(line 6)
* regular expressions, operators, gawk: GNU Regexp Operators.
(line 6)
* regular expressions, operators, precedence of: Regexp Operators.
(line 156)
* regular expressions, searching for: Egrep Program. (line 6)
* relational operators, See comparison operators: Typing and Comparison.
(line 9)
* replace in string: String Functions. (line 411)
* retrying input: Retrying Input. (line 6)
* return debugger command: Debugger Execution Control.
(line 54)
* return statement, user-defined functions: Return Statement. (line 6)
* return value, close() function: Close Files And Pipes.
(line 132)
* rev() user-defined function: Function Example. (line 54)
* revoutput extension: Extension Sample Revout.
(line 11)
* revtwoway extension: Extension Sample Rev2way.
(line 12)
* rewind() user-defined function: Rewind Function. (line 15)
* right angle bracket (>), > operator: Comparison Operators.
(line 11)
* right angle bracket (>), > operator <1>: Precedence. (line 64)
* right angle bracket (>), > operator (I/O): Redirection. (line 22)
* right angle bracket (>), >= operator: Comparison Operators.
(line 11)
* right angle bracket (>), >= operator <1>: Precedence. (line 64)
* right angle bracket (>), >> operator (I/O): Redirection. (line 50)
* right angle bracket (>), >> operator (I/O) <1>: Precedence. (line 64)
* right shift: Bitwise Functions. (line 54)
* right shift, bitwise: Bitwise Functions. (line 32)
* Ritchie, Dennis: Basic Data Typing. (line 54)
* RLENGTH variable: Auto-set. (line 314)
* RLENGTH variable, match() function and: String Functions. (line 227)
* Robbins, Arnold: Command Line Field Separator.
(line 71)
* Robbins, Arnold <1>: Getline/Pipe. (line 40)
* Robbins, Arnold <2>: Passwd Functions. (line 90)
* Robbins, Arnold <3>: Alarm Program. (line 6)
* Robbins, Arnold <4>: General Data Types. (line 6)
* Robbins, Arnold <5>: Contributors. (line 152)
* Robbins, Arnold <6>: Maintainers. (line 14)
* Robbins, Arnold <7>: Future Extensions. (line 6)
* Robbins, Bill: Getline/Pipe. (line 40)
* Robbins, Harry: Acknowledgments. (line 94)
* Robbins, Jean: Acknowledgments. (line 94)
* Robbins, Miriam: Acknowledgments. (line 94)
* Robbins, Miriam <1>: Getline/Pipe. (line 40)
* Robbins, Miriam <2>: Passwd Functions. (line 90)
* Rommel, Kai Uwe: Contributors. (line 43)
* round to nearest integer: Numeric Functions. (line 24)
* round() user-defined function: Round Function. (line 16)
* rounding numbers: Round Function. (line 6)
* ROUNDMODE variable: User-modified. (line 131)
* ROUNDMODE variable <1>: Setting the rounding mode.
(line 6)
* RS variable: awk split records. (line 12)
* RS variable <1>: User-modified. (line 136)
* RS variable, multiline records and: Multiple Line. (line 17)
* rshift: Bitwise Functions. (line 54)
* RSTART variable: Auto-set. (line 320)
* RSTART variable, match() function and: String Functions. (line 227)
* RT variable: awk split records. (line 124)
* RT variable <1>: Multiple Line. (line 130)
* RT variable <2>: Auto-set. (line 327)
* Rubin, Paul: History. (line 30)
* Rubin, Paul <1>: Contributors. (line 16)
* rule, definition of: Getting Started. (line 21)
* run debugger command: Debugger Execution Control.
(line 62)
* rvalues/lvalues: Assignment Ops. (line 31)
* s debugger command (alias for step): Debugger Execution Control.
(line 68)
* sample debugging session: Sample Debugging Session.
(line 6)
* sandbox mode: Options. (line 304)
* save debugger options: Debugger Info. (line 85)
* scalar or array: Type Functions. (line 11)
* scalar values: Basic Data Typing. (line 13)
* scanning arrays: Scanning an Array. (line 6)
* scanning multidimensional arrays: Multiscanning. (line 11)
* Schorr, Andrew: Acknowledgments. (line 60)
* Schorr, Andrew <1>: Auto-set. (line 359)
* Schorr, Andrew <2>: Contributors. (line 136)
* Schreiber, Bert: Acknowledgments. (line 38)
* Schreiber, Rita: Acknowledgments. (line 38)
* search and replace in strings: String Functions. (line 89)
* search in string: String Functions. (line 155)
* search paths: Programs Exercises. (line 70)
* search paths <1>: PC Using. (line 13)
* search paths <2>: VMS Running. (line 57)
* search paths, for loadable extensions: AWKLIBPATH Variable. (line 6)
* search paths, for source files: AWKPATH Variable. (line 6)
* search paths, for source files <1>: Programs Exercises. (line 70)
* search paths, for source files <2>: PC Using. (line 13)
* search paths, for source files <3>: VMS Running. (line 57)
* searching, files for regular expressions: Egrep Program. (line 6)
* searching, for words: Dupword Program. (line 6)
* sed utility: Full Line Fields. (line 22)
* sed utility <1>: Simple Sed. (line 6)
* sed utility <2>: Glossary. (line 16)
* seeding random number generator: Numeric Functions. (line 64)
* semicolon (;), AWKPATH variable and: PC Using. (line 13)
* semicolon (;), separating statements in actions: Statements/Lines.
(line 90)
* semicolon (;), separating statements in actions <1>: Action Overview.
(line 19)
* semicolon (;), separating statements in actions <2>: Statements.
(line 10)
* separators, field: User-modified. (line 53)
* separators, field <1>: User-modified. (line 116)
* separators, field, FIELDWIDTHS variable and: User-modified. (line 37)
* separators, field, FPAT variable and: User-modified. (line 46)
* separators, for records: awk split records. (line 6)
* separators, for records <1>: awk split records. (line 85)
* separators, for records <2>: User-modified. (line 136)
* separators, for records, regular expressions as: awk split records.
(line 124)
* separators, for statements in actions: Action Overview. (line 19)
* separators, subscript: User-modified. (line 149)
* set breakpoint: Breakpoint Control. (line 11)
* set debugger command: Viewing And Changing Data.
(line 58)
* set directory of message catalogs: I18N Functions. (line 11)
* set watchpoint: Viewing And Changing Data.
(line 66)
* shadowing of variable values: Definition Syntax. (line 77)
* shell quoting, rules for: Quoting. (line 6)
* shells, piping commands into: Redirection. (line 136)
* shells, quoting: Using Shell Variables.
(line 12)
* shells, quoting, rules for: Quoting. (line 18)
* shells, scripts: One-shot. (line 22)
* shells, sea: Undocumented. (line 9)
* shells, variables: Using Shell Variables.
(line 6)
* shift, bitwise: Bitwise Functions. (line 32)
* short-circuit operators: Boolean Ops. (line 59)
* show all source files, in debugger: Debugger Info. (line 45)
* show breakpoints: Debugger Info. (line 21)
* show function arguments, in debugger: Debugger Info. (line 18)
* show local variables, in debugger: Debugger Info. (line 34)
* show name of current source file, in debugger: Debugger Info.
(line 37)
* show watchpoints: Debugger Info. (line 51)
* si debugger command (alias for stepi): Debugger Execution Control.
(line 75)
* side effects: Concatenation. (line 41)
* side effects <1>: Increment Ops. (line 11)
* side effects <2>: Increment Ops. (line 75)
* side effects, array indexing: Reference to Elements.
(line 43)
* side effects, asort() function: Array Sorting Functions.
(line 24)
* side effects, assignment expressions: Assignment Ops. (line 22)
* side effects, Boolean operators: Boolean Ops. (line 30)
* side effects, conditional expressions: Conditional Exp. (line 22)
* side effects, decrement/increment operators: Increment Ops. (line 11)
* side effects, FILENAME variable: Getline Notes. (line 19)
* side effects, function calls: Function Calls. (line 57)
* side effects, statements: Action Overview. (line 32)
* sidebar, A Constant's Base Does Not Affect Its Value: Nondecimal-numbers.
(line 63)
* sidebar, Backslash Before Regular Characters: Escape Sequences.
(line 106)
* sidebar, Beware The Smoke and Mirrors!: Bitwise Functions. (line 127)
* sidebar, Changing FS Does Not Affect the Fields: Full Line Fields.
(line 14)
* sidebar, Changing NR and FNR: Auto-set. (line 387)
* sidebar, Controlling Output Buffering with system(): I/O Functions.
(line 163)
* sidebar, Escape Sequences for Metacharacters: Escape Sequences.
(line 138)
* sidebar, FS and IGNORECASE: Field Splitting Summary.
(line 37)
* sidebar, Interactive Versus Noninteractive Buffering: I/O Functions.
(line 74)
* sidebar, Matching the Null String: String Functions. (line 537)
* sidebar, Operator Evaluation Order: Increment Ops. (line 58)
* sidebar, Piping into sh: Redirection. (line 134)
* sidebar, Pre-POSIX awk Used OFMT for String Conversion: Strings And Numbers.
(line 54)
* sidebar, Recipe for a Programming Language: History. (line 6)
* sidebar, Rounding Modes and Conversion: Setting the rounding mode.
(line 65)
* sidebar, RS = "\0" Is Not Portable: gawk split records. (line 63)
* sidebar, So Why Does gawk Have BEGINFILE and ENDFILE?: Filetrans Function.
(line 83)
* sidebar, Syntactic Ambiguities Between /= and Regular Expressions: Assignment Ops.
(line 147)
* sidebar, Understanding #!: Executable Scripts. (line 31)
* sidebar, Understanding $0: Changing Fields. (line 134)
* sidebar, Using close()'s Return Value: Close Files And Pipes.
(line 130)
* sidebar, Using \n in Bracket Expressions of Dynamic Regexps: Computed Regexps.
(line 58)
* SIGHUP signal, for dynamic profiling: Profiling. (line 209)
* SIGINT signal (MS-Windows): Profiling. (line 212)
* signals, HUP/SIGHUP, for profiling: Profiling. (line 209)
* signals, INT/SIGINT (MS-Windows): Profiling. (line 212)
* signals, QUIT/SIGQUIT (MS-Windows): Profiling. (line 212)
* signals, USR1/SIGUSR1, for profiling: Profiling. (line 186)
* signature program: Signature Program. (line 6)
* SIGQUIT signal (MS-Windows): Profiling. (line 212)
* SIGUSR1 signal, for dynamic profiling: Profiling. (line 186)
* silent debugger command: Debugger Execution Control.
(line 10)
* sin: Numeric Functions. (line 75)
* sine: Numeric Functions. (line 75)
* single quote ('): One-shot. (line 15)
* single quote (') in gawk command lines: Long. (line 35)
* single quote ('), in shell commands: Quoting. (line 48)
* single quote ('), vs. apostrophe: Comments. (line 27)
* single quote ('), with double quotes: Quoting. (line 73)
* single-character fields: Single Character Fields.
(line 6)
* single-precision: Computer Arithmetic. (line 61)
* single-step execution, in the debugger: Debugger Execution Control.
(line 43)
* Skywalker, Luke: Undocumented. (line 6)
* sleep utility: Alarm Program. (line 109)
* sleep() extension function: Extension Sample Time.
(line 22)
* Solaris, POSIX-compliant awk: Other Versions. (line 105)
* sort array: String Functions. (line 42)
* sort array indices: String Functions. (line 42)
* sort function, arrays, sorting: Array Sorting Functions.
(line 6)
* sort utility: Word Sorting. (line 50)
* sort utility, coprocesses and: Two-way I/O. (line 66)
* sorting characters in different languages: Explaining gettext.
(line 94)
* source code, awka: Other Versions. (line 73)
* source code, Brian Kernighan's awk: Other Versions. (line 13)
* source code, BusyBox Awk: Other Versions. (line 97)
* source code, gawk: Gawk Distribution. (line 6)
* source code, Illumos awk: Other Versions. (line 114)
* source code, jawk: Other Versions. (line 122)
* source code, libmawk: Other Versions. (line 130)
* source code, mawk: Other Versions. (line 48)
* source code, mixing: Options. (line 117)
* source code, pawk: Other Versions. (line 87)
* source code, pawk (Python version): Other Versions. (line 134)
* source code, QSE awk: Other Versions. (line 140)
* source code, QuikTrim Awk: Other Versions. (line 144)
* source code, Solaris awk: Other Versions. (line 105)
* source files, search path for: Programs Exercises. (line 70)
* sparse arrays: Array Intro. (line 76)
* Spencer, Henry: Glossary. (line 16)
* split: String Functions. (line 317)
* split string into array: String Functions. (line 296)
* split utility: Split Program. (line 6)
* split() function, array elements, deleting: Delete. (line 61)
* split.awk program: Split Program. (line 30)
* sprintf: OFMT. (line 15)
* sprintf <1>: String Functions. (line 386)
* sprintf() function, OFMT variable and: User-modified. (line 116)
* sprintf() function, print/printf statements and: Round Function.
(line 6)
* sqrt: Numeric Functions. (line 78)
* square brackets ([]), regexp operator: Regexp Operators. (line 56)
* square root: Numeric Functions. (line 78)
* srand: Numeric Functions. (line 82)
* stack frame: Debugging Terms. (line 10)
* Stallman, Richard: Manual History. (line 6)
* Stallman, Richard <1>: Acknowledgments. (line 18)
* Stallman, Richard <2>: Contributors. (line 24)
* Stallman, Richard <3>: Glossary. (line 370)
* standard error: Special FD. (line 6)
* standard input: Read Terminal. (line 6)
* standard input <1>: Special FD. (line 6)
* standard output: Special FD. (line 6)
* starting the debugger: Debugger Invocation. (line 6)
* stat() extension function: Extension Sample File Functions.
(line 18)
* statements, compound, control statements and: Statements. (line 10)
* statements, control, in actions: Statements. (line 6)
* statements, multiple: Statements/Lines. (line 90)
* step debugger command: Debugger Execution Control.
(line 68)
* stepi debugger command: Debugger Execution Control.
(line 75)
* stop automatic display, in debugger: Viewing And Changing Data.
(line 79)
* stream editors: Full Line Fields. (line 22)
* stream editors <1>: Simple Sed. (line 6)
* strftime: Time Functions. (line 50)
* string constants: Scalar Constants. (line 15)
* string constants, vs. regexp constants: Computed Regexps. (line 40)
* string extraction (internationalization): String Extraction.
(line 6)
* string length: String Functions. (line 170)
* string operators: Concatenation. (line 9)
* string, regular expression match: String Functions. (line 210)
* string-manipulation functions: String Functions. (line 6)
* string-matching operators: Regexp Usage. (line 19)
* string-translation functions: I18N Functions. (line 6)
* strings splitting, example: String Functions. (line 336)
* strings, converting: Strings And Numbers. (line 6)
* strings, converting <1>: Bitwise Functions. (line 109)
* strings, converting letter case: String Functions. (line 525)
* strings, converting, numbers to: User-modified. (line 30)
* strings, converting, numbers to <1>: User-modified. (line 107)
* strings, empty, See null strings: awk split records. (line 114)
* strings, extracting: String Extraction. (line 6)
* strings, for localization: Programmer i18n. (line 13)
* strings, length limitations: Scalar Constants. (line 20)
* strings, merging arrays into: Join Function. (line 6)
* strings, null: Regexp Field Splitting.
(line 43)
* strings, numeric: Variable Typing. (line 67)
* strtonum: String Functions. (line 393)
* strtonum() function (gawk), --non-decimal-data option and: Nondecimal Data.
(line 35)
* sub: Standard Regexp Constants.
(line 43)
* sub <1>: String Functions. (line 411)
* sub() function, arguments of: String Functions. (line 465)
* sub() function, escape processing: Gory Details. (line 6)
* subscript separators: User-modified. (line 149)
* subscripts in arrays, multidimensional: Multidimensional. (line 10)
* subscripts in arrays, multidimensional, scanning: Multiscanning.
(line 11)
* subscripts in arrays, numbers as: Numeric Array Subscripts.
(line 6)
* subscripts in arrays, uninitialized variables as: Uninitialized Subscripts.
(line 6)
* SUBSEP variable: User-modified. (line 149)
* SUBSEP variable, and multidimensional arrays: Multidimensional.
(line 16)
* substitute in string: String Functions. (line 89)
* substr: String Functions. (line 484)
* substring: String Functions. (line 484)
* Sumner, Andrew: Other Versions. (line 73)
* supplementary groups of gawk process: Auto-set. (line 271)
* switch statement: Switch Statement. (line 6)
* SYMTAB array: Auto-set. (line 331)
* syntactic ambiguity: /= operator vs. /=.../ regexp constant: Assignment Ops.
(line 149)
* system: I/O Functions. (line 106)
* systime: Time Functions. (line 68)
* t debugger command (alias for tbreak): Breakpoint Control. (line 90)
* tbreak debugger command: Breakpoint Control. (line 90)
* Tcl: Library Names. (line 58)
* TCP/IP: TCP/IP Networking. (line 6)
* TCP/IP, support for: Special Network. (line 6)
* tee utility: Tee Program. (line 6)
* tee.awk program: Tee Program. (line 26)
* temporary breakpoint: Breakpoint Control. (line 90)
* terminating records: awk split records. (line 124)
* testbits.awk program: Bitwise Functions. (line 69)
* testext extension: Extension Sample API Tests.
(line 6)
* Texinfo: Conventions. (line 6)
* Texinfo <1>: Library Functions. (line 33)
* Texinfo <2>: Dupword Program. (line 17)
* Texinfo <3>: Extract Program. (line 12)
* Texinfo <4>: Distribution contents.
(line 83)
* Texinfo <5>: Adding Code. (line 100)
* Texinfo, chapter beginnings in files: Regexp Operators. (line 22)
* Texinfo, extracting programs from source files: Extract Program.
(line 6)
* text, printing: Print. (line 22)
* text, printing, unduplicated lines of: Uniq Program. (line 6)
* TEXTDOMAIN variable: User-modified. (line 155)
* TEXTDOMAIN variable <1>: Programmer i18n. (line 8)
* TEXTDOMAIN variable, BEGIN pattern and: Programmer i18n. (line 60)
* TEXTDOMAIN variable, portability and: I18N Portability. (line 20)
* textdomain() function (C library): Explaining gettext. (line 28)
* tilde (~), ~ operator: Regexp Usage. (line 19)
* tilde (~), ~ operator <1>: Computed Regexps. (line 6)
* tilde (~), ~ operator <2>: Case-sensitivity. (line 26)
* tilde (~), ~ operator <3>: Regexp Constants. (line 6)
* tilde (~), ~ operator <4>: Comparison Operators.
(line 11)
* tilde (~), ~ operator <5>: Comparison Operators.
(line 98)
* tilde (~), ~ operator <6>: Precedence. (line 79)
* tilde (~), ~ operator <7>: Expression Patterns. (line 24)
* time functions: Time Functions. (line 6)
* time, alarm clock example program: Alarm Program. (line 11)
* time, localization and: Explaining gettext. (line 112)
* time, managing: Getlocaltime Function.
(line 6)
* time, retrieving: Time Functions. (line 17)
* timeout, reading input: Read Timeout. (line 6)
* timestamps: Time Functions. (line 6)
* timestamps <1>: Time Functions. (line 68)
* timestamps, converting dates to: Time Functions. (line 78)
* timestamps, formatted: Getlocaltime Function.
(line 6)
* tolower: String Functions. (line 526)
* toupper: String Functions. (line 532)
* tr utility: Translate Program. (line 6)
* trace debugger command: Miscellaneous Debugger Commands.
(line 108)
* traceback, display in debugger: Execution Stack. (line 13)
* translate string: I18N Functions. (line 21)
* translate.awk program: Translate Program. (line 55)
* treating files, as single records: gawk split records. (line 92)
* troubleshooting, --non-decimal-data option: Options. (line 223)
* troubleshooting, == operator: Comparison Operators.
(line 37)
* troubleshooting, awk uses FS not IFS: Field Separators. (line 29)
* troubleshooting, backslash before nonspecial character: Escape Sequences.
(line 108)
* troubleshooting, division: Arithmetic Ops. (line 44)
* troubleshooting, fatal errors, field widths, specifying: Fixed width data.
(line 17)
* troubleshooting, fatal errors, printf format strings: Format Modifiers.
(line 157)
* troubleshooting, fflush() function: I/O Functions. (line 63)
* troubleshooting, function call syntax: Function Calls. (line 30)
* troubleshooting, gawk: Compatibility Mode. (line 6)
* troubleshooting, gawk, bug reports: Bugs. (line 9)
* troubleshooting, gawk, fatal errors, function arguments: Calling Built-in.
(line 16)
* troubleshooting, getline function: File Checking. (line 25)
* troubleshooting, gsub()/sub() functions: String Functions. (line 475)
* troubleshooting, match() function: String Functions. (line 291)
* troubleshooting, print statement, omitting commas: Print Examples.
(line 30)
* troubleshooting, printing: Redirection. (line 112)
* troubleshooting, quotes with file names: Special FD. (line 62)
* troubleshooting, readable data files: File Checking. (line 6)
* troubleshooting, regexp constants vs. string constants: Computed Regexps.
(line 40)
* troubleshooting, string concatenation: Concatenation. (line 27)
* troubleshooting, substr() function: String Functions. (line 502)
* troubleshooting, system() function: I/O Functions. (line 128)
* troubleshooting, typographical errors, global variables: Options.
(line 99)
* true, logical: Truth Values. (line 6)
* Trueman, David: History. (line 30)
* Trueman, David <1>: Acknowledgments. (line 47)
* Trueman, David <2>: Contributors. (line 31)
* trunc-mod operation: Arithmetic Ops. (line 66)
* truth values: Truth Values. (line 6)
* type conversion: Strings And Numbers. (line 21)
* type, of variable: Type Functions. (line 14)
* typeof: Type Functions. (line 14)
* u debugger command (alias for until): Debugger Execution Control.
(line 82)
* unassigned array elements: Reference to Elements.
(line 18)
* undefined functions: Pass By Value/Reference.
(line 68)
* underscore (_), C macro: Explaining gettext. (line 71)
* underscore (_), in names of private variables: Library Names.
(line 29)
* underscore (_), translatable string: Programmer i18n. (line 69)
* undisplay debugger command: Viewing And Changing Data.
(line 79)
* undocumented features: Undocumented. (line 6)
* Unicode: Ordinal Functions. (line 45)
* Unicode <1>: Ranges and Locales. (line 61)
* Unicode <2>: Glossary. (line 194)
* uninitialized variables, as array subscripts: Uninitialized Subscripts.
(line 6)
* uniq utility: Uniq Program. (line 6)
* uniq.awk program: Uniq Program. (line 65)
* Unix: Glossary. (line 746)
* Unix awk, backslashes in escape sequences: Escape Sequences.
(line 121)
* Unix awk, close() function and: Close Files And Pipes.
(line 132)
* Unix awk, password files, field separators and: Command Line Field Separator.
(line 62)
* Unix, awk scripts and: Executable Scripts. (line 6)
* unsigned integers: Computer Arithmetic. (line 41)
* until debugger command: Debugger Execution Control.
(line 82)
* unwatch debugger command: Viewing And Changing Data.
(line 83)
* up debugger command: Execution Stack. (line 36)
* user database, reading: Passwd Functions. (line 6)
* user-defined functions: User-defined. (line 6)
* user-defined, functions, counts, in a profile: Profiling. (line 137)
* user-defined, variables: Variables. (line 6)
* user-modifiable variables: User-modified. (line 6)
* users, information about, printing: Id Program. (line 6)
* users, information about, retrieving: Passwd Functions. (line 16)
* USR1 signal, for dynamic profiling: Profiling. (line 186)
* values, numeric: Basic Data Typing. (line 13)
* values, string: Basic Data Typing. (line 13)
* variable assignments and input files: Other Arguments. (line 26)
* variable type: Type Functions. (line 14)
* variable typing: Typing and Comparison.
(line 9)
* variables: Other Features. (line 6)
* variables <1>: Basic Data Typing. (line 6)
* variables, assigning on command line: Assignment Options. (line 6)
* variables, built-in: Using Variables. (line 23)
* variables, flag: Boolean Ops. (line 69)
* variables, getline command into, using: Getline/Variable. (line 6)
* variables, getline command into, using <1>: Getline/Variable/File.
(line 6)
* variables, getline command into, using <2>: Getline/Variable/Pipe.
(line 6)
* variables, getline command into, using <3>: Getline/Variable/Coprocess.
(line 6)
* variables, global, for library functions: Library Names. (line 11)
* variables, global, printing list of: Options. (line 94)
* variables, initializing: Using Variables. (line 23)
* variables, local to a function: Variable Scope. (line 6)
* variables, predefined: Built-in Variables. (line 6)
* variables, predefined -v option, setting with: Options. (line 41)
* variables, predefined conveying information: Auto-set. (line 6)
* variables, private: Library Names. (line 11)
* variables, setting: Options. (line 32)
* variables, shadowing: Definition Syntax. (line 77)
* variables, types of: Assignment Ops. (line 39)
* variables, types of, comparison expressions and: Typing and Comparison.
(line 9)
* variables, uninitialized, as array subscripts: Uninitialized Subscripts.
(line 6)
* variables, user-defined: Variables. (line 6)
* version of gawk: Auto-set. (line 241)
* version of gawk extension API: Auto-set. (line 266)
* version of GNU MP library: Auto-set. (line 249)
* version of GNU MPFR library: Auto-set. (line 251)
* vertical bar (|): Regexp Operators. (line 70)
* vertical bar (|), | operator (I/O): Getline/Pipe. (line 10)
* vertical bar (|), | operator (I/O) <1>: Precedence. (line 64)
* vertical bar (|), |& operator (I/O): Getline/Coprocess. (line 6)
* vertical bar (|), |& operator (I/O) <1>: Precedence. (line 64)
* vertical bar (|), |& operator (I/O) <2>: Two-way I/O. (line 27)
* vertical bar (|), || operator: Boolean Ops. (line 59)
* vertical bar (|), || operator <1>: Precedence. (line 88)
* Vinschen, Corinna: Acknowledgments. (line 60)
* w debugger command (alias for watch): Viewing And Changing Data.
(line 66)
* w utility: Fixed width data. (line 17)
* wait() extension function: Extension Sample Fork.
(line 22)
* waitpid() extension function: Extension Sample Fork.
(line 18)
* walk_array() user-defined function: Walking Arrays. (line 14)
* Wall, Larry: Array Intro. (line 6)
* Wall, Larry <1>: Future Extensions. (line 6)
* Wallin, Anders: Contributors. (line 106)
* warnings, issuing: Options. (line 198)
* watch debugger command: Viewing And Changing Data.
(line 66)
* watchpoint: Debugging Terms. (line 42)
* wc utility: Wc Program. (line 6)
* wc.awk program: Wc Program. (line 46)
* Weinberger, Peter: History. (line 17)
* Weinberger, Peter <1>: Contributors. (line 12)
* where debugger command: Execution Stack. (line 13)
* where debugger command (alias for backtrace): Execution Stack.
(line 13)
* while statement: While Statement. (line 6)
* while statement, use of regexps in: Regexp Usage. (line 19)
* whitespace, as field separators: Default Field Splitting.
(line 6)
* whitespace, functions, calling: Calling Built-in. (line 10)
* whitespace, newlines as: Options. (line 277)
* Williams, Kent: Contributors. (line 35)
* Woehlke, Matthew: Contributors. (line 82)
* Woods, John: Contributors. (line 28)
* word boundaries, matching: GNU Regexp Operators.
(line 41)
* word, regexp definition of: GNU Regexp Operators.
(line 6)
* word-boundary operator (gawk): GNU Regexp Operators.
(line 66)
* wordfreq.awk program: Word Sorting. (line 56)
* words, counting: Wc Program. (line 6)
* words, duplicate, searching for: Dupword Program. (line 6)
* words, usage counts, generating: Word Sorting. (line 6)
* writea() extension function: Extension Sample Read write array.
(line 12)
* xgettext utility: String Extraction. (line 13)
* xor: Bitwise Functions. (line 57)
* XOR bitwise operation: Bitwise Functions. (line 6)
* Yawitz, Efraim: Contributors. (line 134)
* Zaretskii, Eli: Acknowledgments. (line 60)
* Zaretskii, Eli <1>: Contributors. (line 56)
* Zaretskii, Eli <2>: Maintainers. (line 14)
* zerofile.awk program: Empty Files. (line 20)
* Zoulas, Christos: Contributors. (line 67)
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