awk(1p) - phpMan

AWK(1P)                    POSIX Programmer's Manual                   AWK(1P)

PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the  corresponding
       Linux  manual page for details of Linux behavior), or the interface may
       not be implemented on Linux.
NAME
       awk - pattern scanning and processing language
SYNOPSIS
       awk [-F ERE][-v assignment] ... program [argument ...]
       awk [-F ERE] -f progfile ...  [-v assignment] ...[argument ...]

DESCRIPTION
       The awk utility shall execute programs written in the  awk  programming
       language,  which  is  specialized for textual data manipulation. An awk
       program is a sequence of patterns and corresponding actions. When input
       is read that matches a pattern, the action associated with that pattern
       is carried out.
       Input shall be interpreted as a sequence  of  records.  By  default,  a
       record  is  a  line,  less  its  terminating <newline>, but this can be
       changed by using the RS built-in variable. Each record of  input  shall
       be  matched  in turn against each pattern in the program. For each pat-
       tern matched, the associated action shall be executed.
       The awk utility shall interpret each input  record  as  a  sequence  of
       fields  where,  by  default, a field is a string of non- <blank>s. This
       default white-space field delimiter can be  changed  by  using  the  FS
       built-in  variable  or  -F  ERE. The awk utility shall denote the first
       field in a record $1, the second $2, and so on.  The  symbol  $0  shall
       refer to the entire record; setting any other field causes the re-eval-
       uation of $0. Assigning to $0 shall  reset  the  values  of  all  other
       fields and the NF built-in variable.
OPTIONS
       The  awk  utility  shall  conform  to  the  Base  Definitions volume of
       IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines.
       The following options shall be supported:
       -F  ERE
              Define the input field separator  to  be  the  extended  regular
              expression  ERE,  before  any input is read; see Regular Expres-
              sions .
       -f  progfile
              Specify the pathname of the file progfile containing an awk pro-
              gram.  If  multiple  instances of this option are specified, the
              concatenation of the files specified as progfile  in  the  order
              specified shall be the awk program. The awk program can alterna-
              tively be specified in the command line as a single argument.
       -v  assignment
              The application shall ensure that the assignment argument is  in
              the  same  form as an assignment operand. The specified variable
              assignment shall occur  prior  to  executing  the  awk  program,
              including  the  actions associated with BEGIN patterns (if any).
              Multiple occurrences of this option can be specified.

OPERANDS
       The following operands shall be supported:
       program
              If no -f option is specified, the first operand to awk shall  be
              the  text  of  the awk program. The application shall supply the
              program operand as a single argument to awk. If  the  text  does
              not  end  in  a <newline>, awk shall interpret the text as if it
              did.
       argument
              Either of the following two types of argument can be intermixed:
       file
              A pathname of a file that contains the input to be  read,  which
              is  matched  against  the  set of patterns in the program. If no
              file operands are specified, or if a file operand  is  '-',  the
              standard input shall be used.
       assignment
              An  operand that begins with an underscore or alphabetic charac-
              ter from the portable character set (see the table in  the  Base
              Definitions  volume of IEEE Std 1003.1-2001, Section 6.1, Porta-
              ble Character Set), followed by a sequence of underscores,  dig-
              its,  and  alphabetics from the portable character set, followed
              by the '=' character, shall specify a variable assignment rather
              than  a  pathname.  The  characters before the '=' represent the
              name of an awk variable; if that name is an  awk  reserved  word
              (see Grammar ) the behavior is undefined. The characters follow-
              ing the equal sign shall be interpreted as if they  appeared  in
              the  awk  program preceded and followed by a double-quote ( ' )'
              character, as a STRING token (see Grammar ), except that if  the
              last  character  is  an  unescaped backslash, it shall be inter-
              preted as a literal backslash rather than as the first character
              of  the sequence "\"" . The variable shall be assigned the value
              of that STRING token and, if appropriate, shall be considered  a
              numeric  string  (see  Expressions  in awk ), the variable shall
              also be assigned its numeric value. Each such  variable  assign-
              ment  shall  occur just prior to the processing of the following
              file, if any. Thus, an assignment before the first file argument
              shall  be  executed  after  the BEGIN actions (if any), while an
              assignment after the last file argument shall occur  before  the
              END  actions  (if  any). If there are no file arguments, assign-
              ments shall be executed before processing the standard input.

STDIN
       The standard input shall be used only if no file  operands  are  speci-
       fied, or if a file operand is '-' ; see the INPUT FILES section. If the
       awk program contains no actions and no patterns,  but  is  otherwise  a
       valid  awk  program,  standard input and any file operands shall not be
       read and awk shall exit with a return status of zero.
INPUT FILES
       Input files to the awk program from any of the following sources  shall
       be text files:
        * Any  file  operands  or their equivalents, achieved by modifying the
          awk variables ARGV and ARGC
        * Standard input in the absence of any file operands
        * Arguments to the getline function
       Whether the variable RS is set to a value other  than  a  <newline>  or
       not,  for these files, implementations shall support records terminated
       with the specified separator up to {LINE_MAX}  bytes  and  may  support
       longer records.
       If  -f  progfile  is  specified,  the application shall ensure that the
       files named by each of the progfile option-arguments are text files and
       their concatenation, in the same order as they appear in the arguments,
       is an awk program.
ENVIRONMENT VARIABLES
       The following environment variables shall affect the execution of awk:
       LANG   Provide a default value for the  internationalization  variables
              that  are  unset  or  null.  (See the Base Definitions volume of
              IEEE Std 1003.1-2001, Section  8.2,  Internationalization  Vari-
              ables  for the precedence of internationalization variables used
              to determine the values of locale categories.)
       LC_ALL If set to a non-empty string value, override the values  of  all
              the other internationalization variables.
       LC_COLLATE
              Determine  the  locale  for  the behavior of ranges, equivalence
              classes, and multi-character collating elements  within  regular
              expressions and in comparisons of string values.
       LC_CTYPE
              Determine  the  locale  for  the  interpretation of sequences of
              bytes of text data as characters (for  example,  single-byte  as
              opposed  to multi-byte characters in arguments and input files),
              the behavior of character classes  within  regular  expressions,
              the  identification of characters as letters, and the mapping of
              uppercase and lowercase characters for the toupper  and  tolower
              functions.
       LC_MESSAGES
              Determine  the  locale  that should be used to affect the format
              and contents of diagnostic messages written to standard error.
       LC_NUMERIC
              Determine the radix character  used  when  interpreting  numeric
              input, performing conversions between numeric and string values,
              and formatting numeric output. Regardless of locale, the  period
              character  (the  decimal-point character of the POSIX locale) is
              the decimal-point character recognized in  processing  awk  pro-
              grams (including assignments in command line arguments).
       NLSPATH
              Determine the location of message catalogs for the processing of
              LC_MESSAGES .
       PATH   Determine the search path when looking for commands executed  by
              system(expr),  or  input  and output pipes; see the Base Defini-
              tions volume of  IEEE Std 1003.1-2001,  Chapter  8,  Environment
              Variables.

       In  addition,  all  environment  variables shall be visible via the awk
       variable ENVIRON.
ASYNCHRONOUS EVENTS
       Default.
STDOUT
       The nature of the output files depends on the awk program.
STDERR
       The standard error shall be used only for diagnostic messages.
OUTPUT FILES
       The nature of the output files depends on the awk program.
EXTENDED DESCRIPTION
   Overall Program Structure
       An awk program is composed of pairs of the form:

              pattern { action }
       Either the pattern or the action (including the enclosing brace charac-
       ters) can be omitted.
       A missing pattern shall match any record of input, and a missing action
       shall be equivalent to:

              { print }
       Execution of the awk program shall start by first executing the actions
       associated  with all BEGIN patterns in the order they occur in the pro-
       gram. Then each file operand (or standard input if no files were speci-
       fied)  shall be processed in turn by reading data from the file until a
       record separator is seen ( <newline> by default). Before the first ref-
       erence to a field in the record is evaluated, the record shall be split
       into fields, according to the rules in Regular Expressions,  using  the
       value of FS that was current at the time the record was read. Each pat-
       tern in the program then shall be evaluated in the order of occurrence,
       and  the  action  associated with each pattern that matches the current
       record executed. The action for a matching pattern  shall  be  executed
       before  evaluating subsequent patterns. Finally, the actions associated
       with all END patterns shall be executed in the order they occur in  the
       program.
   Expressions in awk
       Expressions describe computations used in patterns and actions.  In the
       following table, valid expression operations are given in  groups  from
       highest  precedence  first to lowest precedence last, with equal-prece-
       dence operators grouped between horizontal lines. In expression evalua-
       tion, where the grammar is formally ambiguous, higher precedence opera-
       tors shall be evaluated before lower precedence operators. In this  ta-
       ble  expr,  expr1,  expr2,  and  expr3  represent any expression, while
       lvalue represents any entity that can be assigned to (that is,  on  the
       left side of an assignment operator). The precise syntax of expressions
       is given in Grammar .
                 Table: Expressions in Decreasing Precedence in awk
    Syntax                Name                      Type of Result   Associativity
    ( expr )              Grouping                  Type of expr     N/A
    $expr                 Field reference           String           N/A
    ++ lvalue             Pre-increment             Numeric          N/A
    -- lvalue             Pre-decrement             Numeric          N/A
    lvalue ++             Post-increment            Numeric          N/A
    lvalue --             Post-decrement            Numeric          N/A
    expr ^ expr           Exponentiation            Numeric          Right
    ! expr                Logical not               Numeric          N/A
    + expr                Unary plus                Numeric          N/A
    - expr                Unary minus               Numeric          N/A
    expr * expr           Multiplication            Numeric          Left
    expr / expr           Division                  Numeric          Left
    expr % expr           Modulus                   Numeric          Left
    expr + expr           Addition                  Numeric          Left
    expr - expr           Subtraction               Numeric          Left
    expr expr             String concatenation      String           Left
    expr < expr           Less than                 Numeric          None
    expr <= expr          Less than or equal to     Numeric          None
    expr != expr          Not equal to              Numeric          None
    expr == expr          Equal to                  Numeric          None
    expr > expr           Greater than              Numeric          None
    expr >= expr          Greater than or equal to  Numeric          None
    expr ~ expr           ERE match                 Numeric          None
    expr !~ expr          ERE non-match             Numeric          None
    expr in array         Array membership          Numeric          Left
    ( index ) in array    Multi-dimension array     Numeric          Left
                          membership
    expr && expr          Logical AND               Numeric          Left
    expr || expr          Logical OR                Numeric          Left
    expr1 ? expr2 : expr3 Conditional expression    Type of selected Right
                                                    expr2 or expr3
    lvalue ^= expr        Exponentiation assignment Numeric          Right
    lvalue %= expr        Modulus assignment        Numeric          Right
    lvalue *= expr        Multiplication assignment Numeric          Right
    lvalue /= expr        Division assignment       Numeric          Right
    lvalue += expr        Addition assignment       Numeric          Right
    lvalue -= expr        Subtraction assignment    Numeric          Right
    lvalue = expr         Assignment                Type of expr     Right
       Each expression shall have either a string value, a numeric  value,  or
       both.  Except  as stated for specific contexts, the value of an expres-
       sion shall be implicitly converted to the type needed for  the  context
       in  which  it  is  used. A string value shall be converted to a numeric
       value by the equivalent of the following calls to functions defined  by
       the ISO C standard:

              setlocale(LC_NUMERIC, "");
              numeric_value = atof(string_value);
       A  numeric  value that is exactly equal to the value of an integer (see
       Concepts Derived from the ISO C Standard )  shall  be  converted  to  a
       string  by the equivalent of a call to the sprintf function (see String
       Functions ) with the string "%d" as the fmt argument  and  the  numeric
       value  being  converted  as the first and only expr argument. Any other
       numeric value shall be converted to a string by  the  equivalent  of  a
       call  to the sprintf function with the value of the variable CONVFMT as
       the fmt argument and the numeric value being converted as the first and
       only  expr argument. The result of the conversion is unspecified if the
       value of CONVFMT is not a  floating-point  format  specification.  This
       volume   of  IEEE Std 1003.1-2001  specifies  no  explicit  conversions
       between numbers and strings. An application can force an expression  to
       be  treated  as  a  number  by adding zero to it, or can force it to be
       treated as a string by concatenating the null string ( "" ) to it.
       A string value shall be considered a numeric string if  it  comes  from
       one of the following:
        1. Field variables
        2. Input from the getline() function
        3. FILENAME
        4. ARGV array elements
        5. ENVIRON array elements
        6. Array elements created by the split() function
        7. A command line variable assignment
        8. Variable assignment from another numeric string variable
       and  after all the following conversions have been applied, the result-
       ing string would lexically be recognized as a NUMBER token as described
       by the lexical conventions in Grammar :
        * All leading and trailing <blank>s are discarded.
        * If the first non- <blank> is '+' or '-', it is discarded.
        * Changing  each  occurrence  of  the decimal point character from the
          current locale to a period.
       If a '-' character is ignored in the preceding description, the numeric
       value  of the numeric string shall be the negation of the numeric value
       of the recognized NUMBER token.  Otherwise, the numeric  value  of  the
       numeric  string  shall  be  the  numeric value of the recognized NUMBER
       token. Whether or not a string is a numeric string  shall  be  relevant
       only in contexts where that term is used in this section.
       When  an  expression  is used in a Boolean context, if it has a numeric
       value, a value of zero shall be treated as false and  any  other  value
       shall  be treated as true. Otherwise, a string value of the null string
       shall be treated as false and any other value shall be treated as true.
       A Boolean context shall be one of the following:
        * The first subexpression of a conditional expression
        * An expression operated on by logical NOT, logical AND, or logical OR
        * The second expression of a for statement
        * The expression of an if statement
        * The  expression of the while clause in either a while or do... while
          statement
        * An expression used as a pattern (as in Overall Program Structure)
       All arithmetic shall follow the semantics of floating-point  arithmetic
       as specified by the ISO C standard (see Concepts Derived from the ISO C
       Standard ).
       The value of the expression:

              expr1 ^ expr2
       shall be equivalent to the value returned by the ISO C  standard  func-
       tion call:

              pow(expr1, expr2)
       The expression:

              lvalue ^= expr
       shall be equivalent to the ISO C standard expression:

              lvalue = pow(lvalue, expr)
       except  that  lvalue  shall  be  evaluated  only once. The value of the
       expression:

              expr1 % expr2
       shall be equivalent to the value returned by the ISO C  standard  func-
       tion call:

              fmod(expr1, expr2)
       The expression:

              lvalue %= expr
       shall be equivalent to the ISO C standard expression:

              lvalue = fmod(lvalue, expr)
       except that lvalue shall be evaluated only once.
       Variables and fields shall be set by the assignment statement:

              lvalue = expression
       and the type of expression shall determine the resulting variable type.
       The assignment includes the arithmetic assignments ( "+=", "-=",  "*=",
       "/=",  "%=",  "^=",  "++",  "--" ) all of which shall produce a numeric
       result. The left-hand side of an assignment and the target of increment
       and  decrement operators can be one of a variable, an array with index,
       or a field selector.
       The awk language supplies arrays that are used for storing  numbers  or
       strings.  Arrays  need  not be declared. They shall initially be empty,
       and their sizes shall change dynamically. The  subscripts,  or  element
       identifiers,  are  strings, providing a type of associative array capa-
       bility. An array name followed by a subscript  within  square  brackets
       can be used as an lvalue and thus as an expression, as described in the
       grammar; see Grammar . Unsubscripted array names can be  used  in  only
       the following contexts:
        * A parameter in a function definition or function call
        * The  NAME  token following any use of the keyword in as specified in
          the grammar (see Grammar ); if the name used in this context is  not
          an array name, the behavior is undefined
       A  valid  array  index  shall  consist  of  one or more comma-separated
       expressions, similar to the way in which multi-dimensional  arrays  are
       indexed  in  some programming languages.  Because awk arrays are really
       one-dimensional, such a comma-separated list shall be  converted  to  a
       single  string  by  concatenating  the  string  values  of the separate
       expressions, each separated from the other by the value of  the  SUBSEP
       variable.   Thus,  the  following two index operations shall be equiva-
       lent:

              var[expr1, expr2, ... exprn]

              var[expr1 SUBSEP expr2 SUBSEP ... SUBSEP exprn]
       The application shall ensure that a multi-dimensioned index  used  with
       the  in operator is parenthesized. The in operator, which tests for the
       existence of a particular array element, shall not cause  that  element
       to  exist.  Any  other  reference  to a nonexistent array element shall
       automatically create it.
       Comparisons (with the '<', "<=", "!=", "==", '>', and  ">="  operators)
       shall  be  made  numerically  if  both  operands are numeric, if one is
       numeric and the other has a string value that is a numeric  string,  or
       if one is numeric and the other has the uninitialized value. Otherwise,
       operands shall be converted to strings as required and a string compar-
       ison  shall  be  made using the locale-specific collation sequence. The
       value of the comparison expression shall be 1 if the relation is  true,
       or 0 if the relation is false.
   Variables and Special Variables
       Variables  can be used in an awk program by referencing them.  With the
       exception of function parameters (see User-Defined  Functions  ),  they
       are not explicitly declared. Function parameter names shall be local to
       the function; all other variable names shall be global. The  same  name
       shall  not be used as both a function parameter name and as the name of
       a function or a special awk variable. The same name shall not  be  used
       both  as  a  variable name with global scope and as the name of a func-
       tion. The same name shall not be used within the same scope both  as  a
       scalar  variable  and  as an array.  Uninitialized variables, including
       scalar variables, array elements, and field variables,  shall  have  an
       uninitialized  value.  An uninitialized value shall have both a numeric
       value of zero and a string value of the  empty  string.  Evaluation  of
       variables  with  an  uninitialized  value, to either string or numeric,
       shall be determined by the context in which they are used.
       Field variables shall be designated by a '$' followed by  a  number  or
       numerical  expression. The effect of the field number expression evalu-
       ating to anything other than a  non-negative  integer  is  unspecified;
       uninitialized  variables  or  string  values  need  not be converted to
       numeric values in this context. New field variables can be  created  by
       assigning  a value to them.  References to nonexistent fields (that is,
       fields after $NF), shall evaluate to the uninitialized value. Such ref-
       erences  shall  not create new fields. However, assigning to a nonexis-
       tent field (for example, $(NF+2)=5) shall increase  the  value  of  NF;
       create  any  intervening fields with the uninitialized value; and cause
       the value of $0 to be recomputed, with the fields  being  separated  by
       the  value  of OFS. Each field variable shall have a string value or an
       uninitialized value when  created.   Field  variables  shall  have  the
       uninitialized value when created from $0 using FS and the variable does
       not contain any characters. If appropriate, the field variable shall be
       considered a numeric string (see Expressions in awk ).
       Implementations  shall  support  the  following other special variables
       that are set by awk:
       ARGC   The number of elements in the ARGV array.
       ARGV   An array of command line arguments, excluding  options  and  the
              program argument, numbered from zero to ARGC-1.
       The arguments in ARGV can be modified or added to; ARGC can be altered.
       As each input file ends, awk shall treat the next non-null  element  of
       ARGV,  up to the current value of ARGC-1, inclusive, as the name of the
       next input file. Thus, setting an element of ARGV to null means that it
       shall not be treated as an input file. The name '-' indicates the stan-
       dard input. If an argument matches the format of an assignment operand,
       this  argument  shall  be  treated  as an assignment rather than a file
       argument.
       CONVFMT
              The printf format for converting numbers to strings (except  for
              output statements, where OFMT is used); "%.6g" by default.
       ENVIRON
              An array representing the value of the environment, as described
              in the exec functions defined in the System Interfaces volume of
              IEEE Std 1003.1-2001.  The indices of the array shall be strings
              consisting of the names of the environment  variables,  and  the
              value  of each array element shall be a string consisting of the
              value of that variable. If appropriate, the environment variable
              shall  be considered a numeric string (see Expressions in awk );
              the array element shall also have its numeric value.
       In all cases where the behavior of awk is affected by environment vari-
       ables  (including the environment of any commands that awk executes via
       the system function or via pipeline redirections with the print  state-
       ment,  the  printf statement, or the getline function), the environment
       used shall be the environment at the time awk began  executing;  it  is
       implementation-defined whether any modification of ENVIRON affects this
       environment.
       FILENAME
              A pathname of the current input file. Inside a BEGIN action  the
              value  is undefined. Inside an END action the value shall be the
              name of the last input file processed.
       FNR    The ordinal number of the current record in  the  current  file.
              Inside  a  BEGIN  action  the value shall be zero. Inside an END
              action the value shall be the number of  the  last  record  pro-
              cessed in the last file processed.
       FS     Input field separator regular expression; a <space> by default.
       NF     The  number  of  fields  in  the  current record. Inside a BEGIN
              action, the use of NF is undefined  unless  a  getline  function
              without  a  var  argument is executed previously.  Inside an END
              action, NF shall retain the value it had  for  the  last  record
              read,  unless a subsequent, redirected, getline function without
              a var argument is performed prior to entering the END action.
       NR     The ordinal number of the  current  record  from  the  start  of
              input.  Inside a BEGIN action the value shall be zero. Inside an
              END action the value shall be the number of the last record pro-
              cessed.
       OFMT   The  printf  format  for converting numbers to strings in output
              statements (see Output Statements  );  "%.6g"  by  default.  The
              result  of the conversion is unspecified if the value of OFMT is
              not a floating-point format specification.
       OFS    The print statement output field separation; <space> by default.
       ORS    The print statement output  record  separator;  a  <newline>  by
              default.
       RLENGTH
              The length of the string matched by the match function.
       RS     The first character of the string value of RS shall be the input
              record separator; a <newline> by default. If  RS  contains  more
              than one character, the results are unspecified.  If RS is null,
              then records are separated by sequences consisting  of  a  <new-
              line>  plus  one  or more blank lines, leading or trailing blank
              lines shall not result in empty records at the beginning or  end
              of the input, and a <newline> shall always be a field separator,
              no matter what the value of FS is.
       RSTART The starting position of the string matched by the  match  func-
              tion,  numbering  from 1. This shall always be equivalent to the
              return value of the match function.
       SUBSEP The subscript separator string for multi-dimensional arrays; the
              default value is implementation-defined.

   Regular Expressions
       The awk utility shall make use of the extended regular expression nota-
       tion (see the Base Definitions volume of IEEE Std 1003.1-2001,  Section
       9.4,  Extended  Regular Expressions) except that it shall allow the use
       of C-language conventions for escaping special  characters  within  the
       EREs,  as  specified  in  the  table  in the Base Definitions volume of
       IEEE Std 1003.1-2001, Chapter 5, File Format  Notation  (  '\\',  '\a',
       '\b',  '\f',  '\n',  '\r',  '\t', '\v' ) and the following table; these
       escape sequences shall be recognized both inside  and  outside  bracket
       expressions.  Note that records need not be separated by <newline>s and
       string constants can contain <newline>s, so even the "\n"  sequence  is
       valid  in  awk EREs. Using a slash character within an ERE requires the
       escaping shown in the following table.
                           Table: Escape Sequences in awk
       Escape
       Sequence Description                    Meaning
       \"       Backslash quotation-mark       Quotation-mark character
       \/       Backslash slash                Slash character
       \ddd     A backslash character followed The character whose encoding
                by the longest sequence of     is represented by the one,
                one, two, or three octal-digit two, or three-digit octal
                characters (01234567). If all  integer. Multi-byte characters
                of the digits are 0 (that is,  require multiple, concatenated
                representation of the NUL      escape sequences of this type,
                character), the behavior is    including the leading '\' for
                undefined.                     each byte.
       \c       A backslash character followed Undefined
                by any character not described
                in this table or in the table
                in the Base Definitions volume
                of IEEE Std 1003.1-2001, Chap-
                ter 5, File Format Notation (
                '\\', '\a', '\b', '\f', '\n',
                '\r', '\t', '\v' ).
       A  regular expression can be matched against a specific field or string
       by using one of the two regular expression matching operators, '~'  and
       "!~"  .  These  operators shall interpret their right-hand operand as a
       regular expression and their left-hand operand as a string. If the reg-
       ular  expression  matches the string, the '~' expression shall evaluate
       to a value of 1, and the "!~" expression shall evaluate to a  value  of
       0. (The regular expression matching operation is as defined by the term
       matched in the Base Definitions volume of IEEE Std 1003.1-2001, Section
       9.1,  Regular  Expression Definitions, where a match occurs on any part
       of the string unless the regular expression is limited with the circum-
       flex or dollar sign special characters.) If the regular expression does
       not match the string, the '~' expression shall evaluate to a  value  of
       0,  and  the  "!~"  expression  shall  evaluate to a value of 1. If the
       right-hand operand is any expression other than the lexical token  ERE,
       the  string value of the expression shall be interpreted as an extended
       regular expression, including the escape conventions  described  above.
       Note that these same escape conventions shall also be applied in deter-
       mining the value of a string literal (the lexical  token  STRING),  and
       thus  shall  be  applied a second time when a string literal is used in
       this context.
       When an ERE token appears as an expression in any context other than as
       the  right-hand  of  the '~' or "!~" operator or as one of the built-in
       function arguments described below, the value of the resulting  expres-
       sion shall be the equivalent of:

              $0 ~ /ere/
       The ere argument to the gsub, match, sub functions, and the fs argument
       to the split function (see String Functions ) shall be  interpreted  as
       extended  regular  expressions. These can be either ERE tokens or arbi-
       trary expressions, and shall be interpreted in the same manner  as  the
       right-hand side of the '~' or "!~" operator.
       An  extended regular expression can be used to separate fields by using
       the -F ERE option or by assigning a string containing the expression to
       the built-in variable FS. The default value of the FS variable shall be
       a single <space>. The following describes FS behavior:
        1. If FS is a null string, the behavior is unspecified.
        2. If FS is a single character:
            a. If FS is <space>, skip leading and  trailing  <blank>s;  fields
               shall be delimited by sets of one or more <blank>s.
            b. Otherwise,  if  FS  is  any  other character c, fields shall be
               delimited by each single occurrence of c.
        3. Otherwise, the string value of FS shall  be  considered  to  be  an
           extended regular expression. Each occurrence of a sequence matching
           the extended regular expression shall delimit fields.
       Except for the '~' and "!~" operators, and in the gsub,  match,  split,
       and  sub  built-in  functions,  ERE  matching  shall  be based on input
       records; that is, record separator characters (the first  character  of
       the  value of the variable RS, <newline> by default) cannot be embedded
       in the expression, and no expression shall match the  record  separator
       character.  If the record separator is not <newline>, <newline>s embed-
       ded in the expression can be matched. For the '~' and  "!~"  operators,
       and  in  those  four built-in functions, ERE matching shall be based on
       text strings; that is,  any  character  (including  <newline>  and  the
       record  separator)  can  be embedded in the pattern, and an appropriate
       pattern shall match any character. However, in all  awk  ERE  matching,
       the  use of one or more NUL characters in the pattern, input record, or
       text string produces undefined results.
   Patterns
       A pattern is any valid expression, a range specified by two expressions
       separated by a comma, or one of the two special patterns BEGIN or END.
   Special Patterns
       The  awk  utility  shall recognize two special patterns, BEGIN and END.
       Each BEGIN pattern shall be matched once and its associated action exe-
       cuted  before the first record of input is read (except possibly by use
       of the getline function-see Input/Output and General Functions -  in  a
       prior  BEGIN  action)  and before command line assignment is done. Each
       END pattern shall be matched once and its  associated  action  executed
       after  the last record of input has been read. These two patterns shall
       have associated actions.
       BEGIN and END shall not combine with other patterns. Multiple BEGIN and
       END  patterns  shall  be allowed. The actions associated with the BEGIN
       patterns shall be executed in the order specified in  the  program,  as
       are  the  END  actions. An END pattern can precede a BEGIN pattern in a
       program.
       If an awk program consists of only actions with the pattern BEGIN,  and
       the  BEGIN  action contains no getline function, awk shall exit without
       reading its input when the last statement in the last BEGIN  action  is
       executed.  If  an awk program consists of only actions with the pattern
       END or only actions with the patterns BEGIN and END, the input shall be
       read before the statements in the END actions are executed.
   Expression Patterns
       An expression pattern shall be evaluated as if it were an expression in
       a Boolean context. If the result is true, the pattern shall be  consid-
       ered to match, and the associated action (if any) shall be executed. If
       the result is false, the action shall not be executed.
   Pattern Ranges
       A pattern range consists of two expressions separated by  a  comma;  in
       this  case,  the  action  shall  be performed for all records between a
       match of the first expression and the following  match  of  the  second
       expression, inclusive. At this point, the pattern range can be repeated
       starting at input records subsequent to the end of the matched range.
   Actions
       An action is a sequence of statements as shown in the grammar in  Gram-
       mar . Any single statement can be replaced by a statement list enclosed
       in braces. The application shall ensure that statements in a  statement
       list  are separated by <newline>s or semicolons. Statements in a state-
       ment list shall be executed sequentially in the order that they appear.
       The expression acting as the conditional in an if  statement  shall  be
       evaluated  and  if  it is non-zero or non-null, the following statement
       shall be executed; otherwise, if else is present, the statement follow-
       ing the else shall be executed.
       The  if,  while,  do...  while, for, break, and continue statements are
       based on the ISO C standard (see Concepts Derived from the ISO C  Stan-
       dard  ),  except  that  the  Boolean  expressions  shall  be treated as
       described in Expressions in awk , and except in the case of:

              for (variable in array)
       which shall iterate, assigning each index of array to  variable  in  an
       unspecified  order.  The results of adding new elements to array within
       such a for loop are undefined. If a break or continue statement  occurs
       outside of a loop, the behavior is undefined.
       The  delete  statement shall remove an individual array element.  Thus,
       the following code deletes an entire array:

              for (index in array)
                  delete array[index]
       The next statement shall cause all further processing  of  the  current
       input  record  to  be  abandoned.  The  behavior is undefined if a next
       statement appears or is invoked in a BEGIN or END action.
       The exit statement shall invoke all END actions in the order  in  which
       they occur in the program source and then terminate the program without
       reading further input. An exit statement inside  an  END  action  shall
       terminate  the  program without further execution of END actions. If an
       expression is specified in an exit statement, its numeric  value  shall
       be  the exit status of awk, unless subsequent errors are encountered or
       a subsequent exit statement with an expression is executed.
   Output Statements
       Both print and printf statements shall  write  to  standard  output  by
       default.  The output shall be written to the location specified by out-
       put_redirection if one is supplied, as follows:

              > expression>> expression| expression
       In all cases, the expression shall be evaluated  to  produce  a  string
       that is used as a pathname into which to write (for '>' or ">>" ) or as
       a command to be executed (for '|' ). Using the first two forms, if  the
       file  of  that name is not currently open, it shall be opened, creating
       it if necessary and using the first form, truncating the file. The out-
       put  then  shall  be  appended to the file. As long as the file remains
       open, subsequent calls in which expression evaluates to the same string
       value  shall  simply  append  output to the file. The file remains open
       until the close function (see Input/Output and General Functions  )  is
       called with an expression that evaluates to the same string value.
       The third form shall write output onto a stream piped to the input of a
       command. The stream shall be created if no  stream  is  currently  open
       with  the  value of expression as its command name.  The stream created
       shall be equivalent to one created by a call to  the  popen()  function
       defined  in  the  System Interfaces volume of IEEE Std 1003.1-2001 with
       the value of expression as the command argument and a value of w as the
       mode  argument. As long as the stream remains open, subsequent calls in
       which expression evaluates to the same string value shall write  output
       to  the  existing  stream. The stream shall remain open until the close
       function (see Input/Output and General Functions ) is  called  with  an
       expression  that evaluates to the same string value.  At that time, the
       stream shall be closed as if by a call to the pclose() function defined
       in the System Interfaces volume of IEEE Std 1003.1-2001.
       As  described in detail by the grammar in Grammar , these output state-
       ments shall take a comma-separated list of expressions referred  to  in
       the  grammar by the non-terminal symbols expr_list, print_expr_list, or
       print_expr_list_opt. This list is referred to here  as  the  expression
       list, and each member is referred to as an expression argument.
       The  print  statement shall write the value of each expression argument
       onto the indicated output stream separated by the current output  field
       separator (see variable OFS above), and terminated by the output record
       separator (see variable ORS above). All expression arguments  shall  be
       taken  as  strings, being converted if necessary; this conversion shall
       be as described in Expressions in awk , with  the  exception  that  the
       printf format in OFMT shall be used instead of the value in CONVFMT. An
       empty expression list shall stand for the whole input record ($0).
       The printf statement shall produce output based on a  notation  similar
       to  the File Format Notation used to describe file formats in this vol-
       ume  of  IEEE Std 1003.1-2001  (see  the  Base  Definitions  volume  of
       IEEE Std 1003.1-2001,  Chapter  5, File Format Notation).  Output shall
       be produced as specified with the  first  expression  argument  as  the
       string  format  and subsequent expression arguments as the strings arg1
       to argn, inclusive, with the following exceptions:
        1. The format shall be an actual character string rather than a graph-
           ical  representation.  Therefore, it cannot contain empty character
           positions. The <space> in the format string, in any  context  other
           than  a  flag of a conversion specification, shall be treated as an
           ordinary character that is copied to the output.
        2. If the character set contains a ' ' character  and  that  character
           appears  in  the  format string, it shall be treated as an ordinary
           character that is copied to the output.
        3. The escape sequences beginning with a backslash character shall  be
           treated  as sequences of ordinary characters that are copied to the
           output. Note that these same sequences shall be  interpreted  lexi-
           cally  by  awk  when they appear in literal strings, but they shall
           not be treated specially by the printf statement.
        4. A field width or precision can be specified as  the  '*'  character
           instead  of a digit string. In this case the next argument from the
           expression list shall be fetched and its numeric value taken as the
           field width or precision.
        5. The implementation shall not precede or follow output from the d or
           u conversion specifier characters with <blank>s  not  specified  by
           the format string.
        6. The  implementation  shall not precede output from the o conversion
           specifier character with leading zeros not specified by the  format
           string.
        7. For  the  c  conversion  specifier character: if the argument has a
           numeric value, the character whose encoding is that value shall  be
           output.  If the value is zero or is not the encoding of any charac-
           ter in the character set, the behavior is undefined. If  the  argu-
           ment  does  not  have  a  numeric value, the first character of the
           string value shall be output; if the string does  not  contain  any
           characters, the behavior is undefined.
        8. For  each  conversion  specification that consumes an argument, the
           next expression argument shall be evaluated. With the exception  of
           the  c conversion specifier character, the value shall be converted
           (according to the rules specified in Expressions in awk  )  to  the
           appropriate type for the conversion specification.
        9. If  there  are insufficient expression arguments to satisfy all the
           conversion specifications in the format  string,  the  behavior  is
           undefined.
       10. If  any  character  sequence in the format string begins with a '%'
           character, but does not form a valid conversion specification,  the
           behavior is unspecified.
       Both print and printf can output at least {LINE_MAX} bytes.
   Functions
       The  awk  language  has  a  variety  of built-in functions: arithmetic,
       string, input/output, and general.
   Arithmetic Functions
       The arithmetic functions, except for int, shall be based on  the  ISO C
       standard  (see Concepts Derived from the ISO C Standard ). The behavior
       is undefined in cases where the ISO C standard specifies that an  error
       be  returned  or  that  the behavior is undefined. Although the grammar
       (see Grammar ) permits built-in functions to appear with  no  arguments
       or  parentheses,  unless  the  argument or parentheses are indicated as
       optional in the following list (by  displaying  them  within  the  "[]"
       brackets), such use is undefined.
       atan2(y,x)
              Return arctangent of y/x in radians in the range [-pi,pi].
       cos(x) Return cosine of x, where x is in radians.
       sin(x) Return sine of x, where x is in radians.
       exp(x) Return the exponential function of x.
       log(x) Return the natural logarithm of x.
       sqrt(x)
              Return the square root of x.
       int(x) Return the argument truncated to an integer. Truncation shall be
              toward 0 when x>0.
       rand() Return a random number n, such that 0<=n<1.
       srand([expr])
              Set the seed value for rand to expr or use the time  of  day  if
              expr is omitted. The previous seed value shall be returned.

   String Functions
       The string functions in the following list shall be supported. Although
       the grammar (see Grammar ) permits built-in functions to appear with no
       arguments  or parentheses, unless the argument or parentheses are indi-
       cated as optional in the following list (by displaying them within  the
       "[]" brackets), such use is undefined.
       gsub(ere, repl[, in])
              Behave  like  sub  (see below), except that it shall replace all
              occurrences of the  regular  expression  (like  the  ed  utility
              global substitute) in $0 or in the in argument, when specified.
       index(s, t)
              Return  the position, in characters, numbering from 1, in string
              s where string t first occurs, or zero if it does not  occur  at
              all.
       length[([s])]
              Return  the  length,  in  characters, of its argument taken as a
              string, or of the whole record, $0, if there is no argument.
       match(s, ere)
              Return the position, in characters, numbering from 1, in  string
              s  where  the extended regular expression ere occurs, or zero if
              it does not occur at all. RSTART shall be set  to  the  starting
              position  (which  is the same as the returned value), zero if no
              match is found; RLENGTH shall  be  set  to  the  length  of  the
              matched string, -1 if no match is found.
       split(s, a[, fs  ])
              Split  the  string  s into array elements a[1], a[2], ..., a[n],
              and return n. All elements of the array shall be deleted  before
              the  split  is  performed. The separation shall be done with the
              ERE fs or with the field separator FS if fs is not  given.  Each
              array  element  shall  have  a string value when created and, if
              appropriate, the array element shall  be  considered  a  numeric
              string (see Expressions in awk ). The effect of a null string as
              the value of fs is unspecified.
       sprintf(fmt, expr, expr, ...)
              Format the expressions according to the printf format  given  by
              fmt and return the resulting string.
       sub(ere, repl[, in  ])
              Substitute the string repl in place of the first instance of the
              extended regular expression ERE in string in and return the num-
              ber  of  substitutions.  An  ampersand  ( '&' ) appearing in the
              string repl shall be replaced by the string from in that matches
              the ERE. An ampersand preceded with a backslash ( '\' ) shall be
              interpreted as the literal ampersand character. An occurrence of
              two  consecutive backslashes shall be interpreted as just a sin-
              gle literal backslash character. Any other occurrence of a back-
              slash  (for  example,  preceding  any  other character) shall be
              treated as a literal backslash character. Note that if repl is a
              string  literal  (the  lexical  token STRING; see Grammar ), the
              handling of the ampersand character  occurs  after  any  lexical
              processing, including any lexical backslash escape sequence pro-
              cessing. If in is specified and it is not an lvalue (see Expres-
              sions in awk ), the behavior is undefined. If in is omitted, awk
              shall use the current record ($0) in its place.
       substr(s, m[, n  ])
              Return the at most n-character substring of  s  that  begins  at
              position m, numbering from 1. If n is omitted, or if n specifies
              more characters than are left in the string, the length  of  the
              substring shall be limited by the length of the string s.
       tolower(s)
              Return  a string based on the string s. Each character in s that
              is an uppercase letter specified to have a  tolower  mapping  by
              the LC_CTYPE category of the current locale shall be replaced in
              the returned string by the lowercase  letter  specified  by  the
              mapping.  Other  characters  in  s  shall  be  unchanged  in the
              returned string.
       toupper(s)
              Return a string based on the string s. Each character in s  that
              is a lowercase letter specified to have a toupper mapping by the
              LC_CTYPE category of the  current  locale  is  replaced  in  the
              returned  string  by  the uppercase letter specified by the map-
              ping. Other characters  in  s  are  unchanged  in  the  returned
              string.

       All  of  the  preceding functions that take ERE as a parameter expect a
       pattern or a string valued expression that is a regular  expression  as
       defined in Regular Expressions .
   Input/Output and General Functions
       The input/output and general functions are:
       close(expression)
              Close  the file or pipe opened by a print or printf statement or
              a call to getline with the same  string-valued  expression.  The
              limit  on the number of open expression arguments is implementa-
              tion-defined. If the close was successful,  the  function  shall
              return zero; otherwise, it shall return non-zero.
       expression |  getline [var]
              Read  a record of input from a stream piped from the output of a
              command.  The stream shall be created if no stream is  currently
              open  with  the  value  of  expression  as its command name. The
              stream created shall be equivalent to one created by a  call  to
              the popen() function with the value of expression as the command
              argument and a value of r as the mode argument. As long  as  the
              stream remains open, subsequent calls in which expression evalu-
              ates to the same string value shall read subsequent records from
              the  stream.  The stream shall remain open until the close func-
              tion is called with an expression that  evaluates  to  the  same
              string  value. At that time, the stream shall be closed as if by
              a call to the pclose() function. If var is omitted,  $0  and  NF
              shall  be  set; otherwise, var shall be set and, if appropriate,
              it shall be considered a numeric string (see Expressions in  awk
              ).
       The  getline  operator  can  form  ambiguous  constructs when there are
       unparenthesized operators (including concatenate) to the  left  of  the
       '|'  (to  the  beginning  of the expression containing getline). In the
       context of the '$' operator, '|' shall behave as  if  it  had  a  lower
       precedence  than  '$'  .  The  result  of evaluating other operators is
       unspecified, and conforming applications  shall  parenthesize  properly
       all such usages.
       getline
              Set  $0  to  the  next input record from the current input file.
              This form of getline shall set the NF, NR, and FNR variables.
       getline  var
              Set variable var to the next input record from the current input
              file  and,  if  appropriate,  var  shall be considered a numeric
              string (see Expressions in awk ). This form of getline shall set
              the FNR and NR variables.
       getline [var]  < expression
              Read  the next record of input from a named file. The expression
              shall be evaluated to produce a string that is used as  a  path-
              name.  If  the file of that name is not currently open, it shall
              be opened. As long as the stream remains open, subsequent  calls
              in  which  expression  evaluates  to the same string value shall
              read subsequent records from the file.  The  file  shall  remain
              open  until the close function is called with an expression that
              evaluates to the same string value. If var is omitted, $0 and NF
              shall  be  set; otherwise, var shall be set and, if appropriate,
              it shall be considered a numeric string (see Expressions in  awk
              ).
       The  getline  operator  can  form  ambiguous  constructs when there are
       unparenthesized binary operators (including concatenate) to  the  right
       of  the  '<'  (up to the end of the expression containing the getline).
       The result of evaluating such a construct is unspecified, and  conform-
       ing applications shall parenthesize properly all such usages.
       system(expression)
              Execute  the  command given by expression in a manner equivalent
              to the system() function defined in the System Interfaces volume
              of  IEEE Std 1003.1-2001  and return the exit status of the com-
              mand.

       All forms of getline shall return 1 for successful input, zero for end-
       of-file, and -1 for an error.
       Where  strings are used as the name of a file or pipeline, the applica-
       tion shall ensure that the strings are textually identical.  The termi-
       nology  "same  string  value"  implies  that "equivalent strings", even
       those that differ only by <space>s, represent different files.
   User-Defined Functions
       The awk language also provides user-defined functions.  Such  functions
       can be defined as:

              function name([parameter, ...]) { statements }
       A  function  can be referred to anywhere in an awk program; in particu-
       lar, its use can precede its definition. The scope  of  a  function  is
       global.
       Function  parameters,  if present, can be either scalars or arrays; the
       behavior is undefined if an array name is passed as  a  parameter  that
       the function uses as a scalar, or if a scalar expression is passed as a
       parameter that the function uses as an array. Function parameters shall
       be passed by value if scalar and by reference if array name.
       The  number of parameters in the function definition need not match the
       number of parameters in the function call. Excess formal parameters can
       be  used as local variables. If fewer arguments are supplied in a func-
       tion call than are in the function  definition,  the  extra  parameters
       that  are  used  in  the function body as scalars shall evaluate to the
       uninitialized value until they are otherwise initialized, and the extra
       parameters  that  are  used  in  the  function  body as arrays shall be
       treated as uninitialized arrays where each  element  evaluates  to  the
       uninitialized value until otherwise initialized.
       When  invoking  a  function,  no  white space can be placed between the
       function name and the opening parenthesis. Function calls can be nested
       and  recursive  calls  can be made upon functions. Upon return from any
       nested or recursive function call, the values of  all  of  the  calling
       function's  parameters  shall be unchanged, except for array parameters
       passed by reference. The return statement  can  be  used  to  return  a
       value.  If a return statement appears outside of a function definition,
       the behavior is undefined.
       In the function definition, <newline>s shall  be  optional  before  the
       opening  brace  and  after  the closing brace. Function definitions can
       appear anywhere in the program where a pattern-action pair is allowed.
   Grammar
       The grammar in this section and the lexical conventions in the  follow-
       ing  section  shall  together describe the syntax for awk programs. The
       general conventions for this style of grammar are described in  Grammar
       Conventions  .  A  valid program can be represented as the non-terminal
       symbol program in the grammar. This formal syntax shall take precedence
       over the preceding text syntax description.

              %token NAME NUMBER STRING ERE
              %token FUNC_NAME   /* Name followed by '(' without white space. */

              /* Keywords  */
              %token       Begin   End
              /*          'BEGIN' 'END'                            */

              %token       Break   Continue   Delete   Do   Else
              /*          'break' 'continue' 'delete' 'do' 'else'  */

              %token       Exit   For   Function   If   In
              /*          'exit' 'for' 'function' 'if' 'in'        */

              %token       Next   Print   Printf   Return   While
              /*          'next' 'print' 'printf' 'return' 'while' */

              /* Reserved function names */
              %token BUILTIN_FUNC_NAME
                          /* One token for the following:
                           * atan2 cos sin exp log sqrt int rand srand
                           * gsub index length match split sprintf sub
                           * substr tolower toupper close system
                           */
              %token GETLINE
                          /* Syntactically different from other built-ins. */

              /* Two-character tokens. */
              %token ADD_ASSIGN SUB_ASSIGN MUL_ASSIGN DIV_ASSIGN MOD_ASSIGN POW_ASSIGN
              /*     '+='       '-='       '*='       '/='       '%='       '^=' */

              %token OR   AND  NO_MATCH   EQ   LE   GE   NE   INCR  DECR  APPEND
              /*     '||' '&&' '!~' '==' '<=' '>=' '!=' '++'  '--'  '>>'   */

              /* One-character tokens. */
              %token '{' '}' '(' ')' '[' ']' ',' ';' NEWLINE
              %token '+' '-' '*' '%' '^' '!' '>' '<' '|' '?' ':' '~' '$' '='

              %start program
              %%

              program          : item_list
                               | actionless_item_list
                               ;

              item_list        : newline_opt
                               | actionless_item_list item terminator
                               | item_list            item terminator
                               | item_list          action terminator
                               ;

              actionless_item_list : item_list            pattern terminator
                               | actionless_item_list pattern terminator
                               ;

              item             : pattern action
                               | Function NAME      '(' param_list_opt ')'
                                     newline_opt action
                               | Function FUNC_NAME '(' param_list_opt ')'
                                     newline_opt action
                               ;

              param_list_opt   : /* empty */
                               | param_list
                               ;

              param_list       : NAME
                               | param_list ',' NAME
                               ;

              pattern          : Begin
                               | End
                               | expr
                               | expr ',' newline_opt expr
                               ;

              action           : '{' newline_opt                             '}'
                               | '{' newline_opt terminated_statement_list   '}'
                               | '{' newline_opt unterminated_statement_list '}'
                               ;

              terminator       : terminator ';'
                               | terminator NEWLINE
                               |            ';'
                               |            NEWLINE
                               ;

              terminated_statement_list : terminated_statement
                               | terminated_statement_list terminated_statement
                               ;

              unterminated_statement_list : unterminated_statement
                               | terminated_statement_list unterminated_statement
                               ;

              terminated_statement : action newline_opt
                               | If '(' expr ')' newline_opt terminated_statement
                               | If '(' expr ')' newline_opt terminated_statement
                                     Else newline_opt terminated_statement
                               | While '(' expr ')' newline_opt terminated_statement
                               | For '(' simple_statement_opt ';'
                                    expr_opt ';' simple_statement_opt ')' newline_opt
                                    terminated_statement
                               | For '(' NAME In NAME ')' newline_opt
                                    terminated_statement
                               | ';' newline_opt
                               | terminatable_statement NEWLINE newline_opt
                               | terminatable_statement ';'     newline_opt
                               ;

              unterminated_statement : terminatable_statement
                               | If '(' expr ')' newline_opt unterminated_statement
                               | If '(' expr ')' newline_opt terminated_statement
                                    Else newline_opt unterminated_statement
                               | While '(' expr ')' newline_opt unterminated_statement
                               | For '(' simple_statement_opt ';'
                                expr_opt ';' simple_statement_opt ')' newline_opt
                                    unterminated_statement
                               | For '(' NAME In NAME ')' newline_opt
                                    unterminated_statement
                               ;

              terminatable_statement : simple_statement
                               | Break
                               | Continue
                               | Next
                               | Exit expr_opt
                               | Return expr_opt
                               | Do newline_opt terminated_statement While '(' expr ')'
                               ;

              simple_statement_opt : /* empty */
                               | simple_statement
                               ;

              simple_statement : Delete NAME '[' expr_list ']'
                               | expr
                               | print_statement
                               ;

              print_statement  : simple_print_statement
                               | simple_print_statement output_redirection
                               ;

              simple_print_statement : Print  print_expr_list_opt
                               | Print  '(' multiple_expr_list ')'
                               | Printf print_expr_list
                               | Printf '(' multiple_expr_list ')'
                               ;

              output_redirection : '>'    expr
                               | APPEND expr
                               | '|'    expr
                               ;

              expr_list_opt    : /* empty */
                               | expr_list
                               ;

              expr_list        : expr
                               | multiple_expr_list
                               ;

              multiple_expr_list : expr ',' newline_opt expr
                               | multiple_expr_list ',' newline_opt expr
                               ;

              expr_opt         : /* empty */
                               | expr
                               ;

              expr             : unary_expr
                               | non_unary_expr
                               ;

              unary_expr       : '+' expr
                               | '-' expr
                               | unary_expr '^'      expr
                               | unary_expr '*'      expr
                               | unary_expr '/'      expr
                               | unary_expr '%'      expr
                               | unary_expr '+'      expr
                               | unary_expr '-'      expr
                               | unary_expr          non_unary_expr
                               | unary_expr '<'      expr
                               | unary_expr LE       expr
                               | unary_expr NE       expr
                               | unary_expr EQ       expr
                               | unary_expr '>'      expr
                               | unary_expr GE       expr
                               | unary_expr '~'      expr
                               | unary_expr NO_MATCH expr
                               | unary_expr In NAME
                               | unary_expr AND newline_opt expr
                               | unary_expr OR  newline_opt expr
                               | unary_expr '?' expr ':' expr
                               | unary_input_function
                               ;

              non_unary_expr   : '(' expr ')'
                               | '!' expr
                               | non_unary_expr '^'      expr
                               | non_unary_expr '*'      expr
                               | non_unary_expr '/'      expr
                               | non_unary_expr '%'      expr
                               | non_unary_expr '+'      expr
                               | non_unary_expr '-'      expr
                               | non_unary_expr          non_unary_expr
                               | non_unary_expr '<'      expr
                               | non_unary_expr LE       expr
                               | non_unary_expr NE       expr
                               | non_unary_expr EQ       expr
                               | non_unary_expr '>'      expr
                               | non_unary_expr GE       expr
                               | non_unary_expr '~'      expr
                               | non_unary_expr NO_MATCH expr
                               | non_unary_expr In NAME
                               | '(' multiple_expr_list ')' In NAME
                               | non_unary_expr AND newline_opt expr
                               | non_unary_expr OR  newline_opt expr
                               | non_unary_expr '?' expr ':' expr
                               | NUMBER
                               | STRING
                               | lvalue
                               | ERE
                               | lvalue INCR
                               | lvalue DECR
                               | INCR lvalue
                               | DECR lvalue
                               | lvalue POW_ASSIGN expr
                               | lvalue MOD_ASSIGN expr
                               | lvalue MUL_ASSIGN expr
                               | lvalue DIV_ASSIGN expr
                               | lvalue ADD_ASSIGN expr
                               | lvalue SUB_ASSIGN expr
                               | lvalue '=' expr
                               | FUNC_NAME '(' expr_list_opt ')'
                                    /* no white space allowed before '(' */
                               | BUILTIN_FUNC_NAME '(' expr_list_opt ')'
                               | BUILTIN_FUNC_NAME
                               | non_unary_input_function
                               ;

              print_expr_list_opt : /* empty */
                               | print_expr_list
                               ;

              print_expr_list  : print_expr
                               | print_expr_list ',' newline_opt print_expr
                               ;

              print_expr       : unary_print_expr
                               | non_unary_print_expr
                               ;

              unary_print_expr : '+' print_expr
                               | '-' print_expr
                               | unary_print_expr '^'      print_expr
                               | unary_print_expr '*'      print_expr
                               | unary_print_expr '/'      print_expr
                               | unary_print_expr '%'      print_expr
                               | unary_print_expr '+'      print_expr
                               | unary_print_expr '-'      print_expr
                               | unary_print_expr          non_unary_print_expr
                               | unary_print_expr '~'      print_expr
                               | unary_print_expr NO_MATCH print_expr
                               | unary_print_expr In NAME
                               | unary_print_expr AND newline_opt print_expr
                               | unary_print_expr OR  newline_opt print_expr
                               | unary_print_expr '?' print_expr ':' print_expr
                               ;

              non_unary_print_expr : '(' expr ')'
                               | '!' print_expr
                               | non_unary_print_expr '^'      print_expr
                               | non_unary_print_expr '*'      print_expr
                               | non_unary_print_expr '/'      print_expr
                               | non_unary_print_expr '%'      print_expr
                               | non_unary_print_expr '+'      print_expr
                               | non_unary_print_expr '-'      print_expr
                               | non_unary_print_expr          non_unary_print_expr
                               | non_unary_print_expr '~'      print_expr
                               | non_unary_print_expr NO_MATCH print_expr
                               | non_unary_print_expr In NAME
                               | '(' multiple_expr_list ')' In NAME
                               | non_unary_print_expr AND newline_opt print_expr
                               | non_unary_print_expr OR  newline_opt print_expr
                               | non_unary_print_expr '?' print_expr ':' print_expr
                               | NUMBER
                               | STRING
                               | lvalue
                               | ERE
                               | lvalue INCR
                               | lvalue DECR
                               | INCR lvalue
                               | DECR lvalue
                               | lvalue POW_ASSIGN print_expr
                               | lvalue MOD_ASSIGN print_expr
                               | lvalue MUL_ASSIGN print_expr
                               | lvalue DIV_ASSIGN print_expr
                               | lvalue ADD_ASSIGN print_expr
                               | lvalue SUB_ASSIGN print_expr
                               | lvalue '=' print_expr
                               | FUNC_NAME '(' expr_list_opt ')'
                                   /* no white space allowed before '(' */
                               | BUILTIN_FUNC_NAME '(' expr_list_opt ')'
                               | BUILTIN_FUNC_NAME
                               ;

              lvalue           : NAME
                               | NAME '[' expr_list ']'
                               | '$' expr
                               ;

              non_unary_input_function : simple_get
                               | simple_get '<' expr
                               | non_unary_expr '|' simple_get
                               ;

              unary_input_function : unary_expr '|' simple_get
                               ;

              simple_get       : GETLINE
                               | GETLINE lvalue
                               ;

              newline_opt      : /* empty */
                               | newline_opt NEWLINE
                               ;
       This grammar has several ambiguities that shall be resolved as follows:
        * Operator  precedence  and  associativity  shall  be  as described in
          Expressions in Decreasing Precedence in awk .
        * In case of ambiguity, an else shall  be  associated  with  the  most
          immediately preceding if that would satisfy the grammar.
        * In  some  contexts,  a slash ( '/' ) that is used to surround an ERE
          could also be the division operator. This shall be resolved in  such
          a  way  that wherever the division operator could appear, a slash is
          assumed to be the division operator. (There  is  no  unary  division
          operator.)
       One  convention  that  might  not be obvious from the formal grammar is
       where <newline>s are acceptable. There are several  obvious  placements
       such  as terminating a statement, and a backslash can be used to escape
       <newline>s between any lexical tokens. In addition, <newline>s  without
       backslashes  can  follow a comma, an open brace, logical AND operator (
       "&&" ), logical OR operator ( "||" ), the do keyword, the else keyword,
       and  the  closing  parenthesis  of  an if, for, or while statement. For
       example:

              { print  $1,
                       $2 }
   Lexical Conventions
       The lexical conventions for awk programs, with respect to the preceding
       grammar, shall be as follows:
        1. Except  as noted, awk shall recognize the longest possible token or
           delimiter beginning at a given point.
        2. A comment shall consist of any characters beginning with the number
           sign character and terminated by, but excluding the next occurrence
           of, a <newline>. Comments shall have no effect, except  to  delimit
           lexical tokens.
        3. The <newline> shall be recognized as the token NEWLINE.
        4. A  backslash  character  immediately  followed by a <newline> shall
           have no effect.
        5. The token STRING shall represent a string constant. A  string  con-
           stant shall begin with the character ' .' Within a string constant,
           a backslash character  shall  be  considered  to  begin  an  escape
           sequence  as  specified in the table in the Base Definitions volume
           of IEEE Std 1003.1-2001, Chapter 5, File Format  Notation  (  '\\',
           '\a', '\b', '\f', '\n', '\r', '\t', '\v' ). In addition, the escape
           sequences in Expressions in Decreasing Precedence in awk  shall  be
           recognized. A <newline> shall not occur within a string constant. A
           string constant shall be terminated by the first  unescaped  occur-
           rence of the character '' after the one that begins the string con-
           stant. The value of  the  string  shall  be  the  sequence  of  all
           unescaped  characters  and  values of escape sequences between, but
           not including, the two delimiting '' characters.
        6. The token ERE represents an extended regular  expression  constant.
           An  ERE  constant  shall begin with the slash character.  Within an
           ERE constant, a backslash character shall be considered to begin an
           escape  sequence  as specified in the table in the Base Definitions
           volume of IEEE Std 1003.1-2001, Chapter 5, File Format Notation. In
           addition,  the escape sequences in Expressions in Decreasing Prece-
           dence in awk shall be recognized. The application shall ensure that
           a  <newline> does not occur within an ERE constant. An ERE constant
           shall be terminated by the first unescaped occurrence of the  slash
           character  after the one that begins the ERE constant. The extended
           regular expression represented by the ERE  constant  shall  be  the
           sequence of all unescaped characters and values of escape sequences
           between, but not including, the two delimiting slash characters.
        7. A <blank> shall have no effect, except to delimit lexical tokens or
           within STRING or ERE tokens.
        8. The  token  NUMBER shall represent a numeric constant. Its form and
           numeric value shall be equivalent to either of the tokens floating-
           constant  or  integer-constant  as specified by the ISO C standard,
           with the following exceptions:
            a. An integer constant cannot begin with 0x or include  the  hexa-
               decimal  digits  'a',  'b',  'c', 'd', 'e', 'f', 'A', 'B', 'C',
               'D', 'E', or 'F' .
            b. The value of an integer constant  beginning  with  0  shall  be
               taken in decimal rather than octal.
            c. An integer constant cannot include a suffix ( 'u', 'U', 'l', or
               'L' ).
            d. A floating constant cannot include a suffix ( 'f', 'F', 'l', or
               'L' ).
       If  the  value  is too large or too small to be representable (see Con-
       cepts Derived from the ISO C Standard ), the behavior is undefined.
        9. A sequence of underscores, digits, and alphabetics from the  porta-
           ble   character   set   (see   the   Base   Definitions  volume  of
           IEEE Std 1003.1-2001, Section 6.1, Portable Character Set),  begin-
           ning with an underscore or alphabetic, shall be considered a word.
       10. The  following words are keywords that shall be recognized as indi-
           vidual tokens; the name of the token is the same as the keyword:

BEGIN           delete          END             function        in              printf
break           do              exit            getline         next            return
continue        else            for             if              print           while

       11. The following words are names of built-in functions  and  shall  be
           recognized as the token BUILTIN_FUNC_NAME:

atan2           gsub            log             split           sub             toupper
close           index           match           sprintf         substr
cos             int             rand            sqrt            system
exp             length          sin             srand           tolower

       The  above-listed  keywords and names of built-in functions are consid-
       ered reserved words.
       12. The token NAME shall consist of a word that is not a keyword  or  a
           name  of a built-in function and is not followed immediately (with-
           out any delimiters) by the '(' character.
       13. The token FUNC_NAME shall consist of a word that is not  a  keyword
           or a name of a built-in function, followed immediately (without any
           delimiters) by the '(' character. The '(' character  shall  not  be
           included as part of the token.
       14. The  following  two-character  sequences shall be recognized as the
           named tokens:
                      Token Name   Sequence   Token Name   Sequence
                      ADD_ASSIGN   +=         NO_MATCH     !~
                      SUB_ASSIGN   -=         EQ           ==
                      MUL_ASSIGN   *=         LE           <=
                      DIV_ASSIGN   /=         GE           >=
                      MOD_ASSIGN   %=         NE           !=
                      POW_ASSIGN   ^=         INCR         ++
                      OR           ||         DECR         --
                      AND          &&         APPEND       >>
       15. The following single characters shall be recognized as tokens whose
           names are the character:

           <newline> { } ( ) [ ] , ; + - * % ^ ! > < | ? : ~ $ =
       There  is  a lexical ambiguity between the token ERE and the tokens '/'
       and DIV_ASSIGN. When an input sequence begins with a slash character in
       any syntactic context where the token '/' or DIV_ASSIGN could appear as
       the next token in a valid program, the longer of those two tokens  that
       can  be  recognized shall be recognized. In any other syntactic context
       where the token ERE could appear as the next token in a valid  program,
       the token ERE shall be recognized.
EXIT STATUS
       The following exit values shall be returned:
        0     All input files were processed successfully.
       >0     An error occurred.

       The  exit  status  can  be  altered within the program by using an exit
       expression.
CONSEQUENCES OF ERRORS
       If any file operand is specified and the named file cannot be accessed,
       awk  shall  write  a diagnostic message to standard error and terminate
       without any further action.
       If the program specified by either the program operand  or  a  progfile
       operand  is  not  a  valid  awk  program  (as specified in the EXTENDED
       DESCRIPTION section), the behavior is undefined.
       The following sections are informative.
APPLICATION USAGE
       The index, length, match, and substr functions should not  be  confused
       with  similar  functions  in  the ISO C standard; the awk versions deal
       with characters, while the ISO C standard deals with bytes.
       Because the concatenation operation is represented by adjacent  expres-
       sions  rather  than  an explicit operator, it is often necessary to use
       parentheses to enforce the proper evaluation precedence.
EXAMPLES
       The awk program specified in the command line is most easily  specified
       within  single-quotes  (for  example, 'program') for applications using
       sh, because awk programs commonly contain characters that  are  special
       to  the shell, including double-quotes.  In the cases where an awk pro-
       gram contains single-quote characters, it is usually easiest to specify
       most of the program as strings within single-quotes concatenated by the
       shell with quoted single-quote characters. For example:

              awk '/'\''/ { print "quote:", $0 }'
       prints all lines from the  standard  input  containing  a  single-quote
       character, prefixed with quote:.
       The following are examples of simple awk programs:
        1. Write  to  the standard output all input lines for which field 3 is
           greater than 5:

           $3 > 5
        2. Write every tenth line:

           (NR % 10) == 0
        3. Write any line with a substring matching the regular expression:

           /(G|D)(2[0-9][[:alpha:]]*)/
        4. Print any line with a substring containing a 'G' or  'D',  followed
           by  a sequence of digits and characters.  This example uses charac-
           ter classes digit and alpha to match language-independent digit and
           alphabetic characters respectively:

           /(G|D)([[:digit:][:alpha:]]*)/
        5. Write  any  line  in  which  the  second  field matches the regular
           expression and the fourth field does not:

           $2 ~ /xyz/ && $4 !~ /xyz/
        6. Write any line in which the second field contains a backslash:

           $2 ~ /\\/
        7. Write any line in which the second field contains a backslash. Note
           that  backslash escapes are interpreted twice; once in lexical pro-
           cessing of the string and once in processing  the  regular  expres-
           sion:

           $2 ~ "\\\\"
        8. Write the second to the last and the last field in each line. Sepa-
           rate the fields by a colon:

           {OFS=":";print $(NF-1), $NF}
        9. Write the line number and number of fields in each line. The  three
           strings  representing the line number, the colon, and the number of
           fields are concatenated and that string is written to standard out-
           put:

           {print NR ":" NF}
       10. Write lines longer than 72 characters:

           length($0) > 72
       11. Write the first two fields in opposite order separated by OFS:

           { print $2, $1 }
       12. Same,  with  input  fields  separated  by  a  comma or <space>s and
           <tab>s, or both:

           BEGIN { FS = ",[ \t]*|[ \t]+" }
                 { print $2, $1 }
       13. Add up the first column, print sum, and average:

                {s += $1 }
           END   {print "sum is ", s, " average is", s/NR}
       14. Write fields in reverse order, one per line  (many  lines  out  for
           each line in):

           { for (i = NF; i > 0; --i) print $i }
       15. Write all lines between occurrences of the strings start and stop:

           /start/, /stop/
       16. Write  all  lines  whose first field is different from the previous
           one:

           $1 != prev { print; prev = $1 }
       17. Simulate echo:

           BEGIN  {
                   for (i = 1; i < ARGC; ++i)
                   printf("%s%s", ARGV[i], i==ARGC-1?"\n":" ")
           }
       18. Write the path prefixes contained in the PATH environment variable,
           one per line:

           BEGIN  {
                   n = split (ENVIRON["PATH"], path, ":")
                   for (i = 1; i <= n; ++i)
                   print path[i]
           }
       19. If there is a file named input containing page headers of the form:

           Page #
       and a file named program that contains:

              /Page/   { $2 = n++; }
                       { print }
       then the command line:

              awk -f program n=5 input
       prints the file input, filling in page numbers starting at 5.
RATIONALE
       This  description  is based on the new awk, "nawk", (see the referenced
       The AWK Programming Language), which introduced a number  of  new  fea-
       tures to the historical awk:
        1. New keywords: delete, do, function, return
        2. New  built-in functions: atan2, close, cos, gsub, match, rand, sin,
           srand, sub, system
        3. New predefined variables: FNR, ARGC, ARGV, RSTART, RLENGTH, SUBSEP
        4. New expression operators: ?, :, ,, ^
        5. The FS variable and the third argument to  split,  now  treated  as
           extended regular expressions.
        6. The  operator  precedence, changed to more closely match the C lan-
           guage.  Two examples of code that operate differently are:

           while ( n /= 10 > 1) ...
           if (!"wk" ~ /bwk/) ...
       Several features have been added based on newer implementations of awk:
        * Multiple instances of -f progfile are permitted.
        * The new option -v assignment.
        * The new predefined variable ENVIRON.
        * New built-in functions toupper and tolower.
        * More formatting capabilities are added to printf to match the  ISO C
          standard.
       The  overall awk syntax has always been based on the C language, with a
       few features from the shell command language and other sources. Because
       of this, it is not completely compatible with any other language, which
       has caused confusion for some users.  It is not the intent of the stan-
       dard developers to address such issues.  A few relatively minor changes
       toward making the language more compatible with the ISO C standard were
       made;  most  of  these  changes  are based on similar changes in recent
       implementations, as described above. There  remain  several  C-language
       conventions  that  are not in awk. One of the notable ones is the comma
       operator, which is commonly used to specify multiple expressions in the
       C  language  for statement. Also, there are various places where awk is
       more restrictive than the C language regarding the type  of  expression
       that  can  be used in a given context. These limitations are due to the
       different features that the awk language does provide.
       Regular expressions in awk have been extended somewhat from  historical
       implementations  to  make  them  a  pure  superset  of extended regular
       expressions, as defined by IEEE Std 1003.1-2001 (see the  Base  Defini-
       tions  volume  of  IEEE Std 1003.1-2001,  Section 9.4, Extended Regular
       Expressions).  The main extensions  are  internationalization  features
       and  interval expressions.  Historical implementations of awk have long
       supported backslash escape sequences as an extension to extended  regu-
       lar expressions, and this extension has been retained despite inconsis-
       tency with other utilities. The number of escape  sequences  recognized
       in  both extended regular expressions and strings has varied (generally
       increasing with time)  among  implementations.  The  set  specified  by
       IEEE Std 1003.1-2001  includes  most sequences known to be supported by
       popular implementations and by the ISO C standard. One sequence that is
       not  supported  is hexadecimal value escapes beginning with '\x' . This
       would allow values expressed in more than 9 bits to be used within  awk
       as in the ISO C standard. However, because this syntax has a non-deter-
       ministic length, it does not permit the subsequent character  to  be  a
       hexadecimal  digit. This limitation can be dealt with in the C language
       by the use of lexical string concatenation. In the awk  language,  con-
       catenation  could  also be a solution for strings, but not for extended
       regular expressions (either lexical ERE tokens or strings used  dynami-
       cally  as regular expressions). Because of this limitation, the feature
       has not been added to IEEE Std 1003.1-2001.
       When a string variable is used in a context where an  extended  regular
       expression normally appears (where the lexical token ERE is used in the
       grammar) the string does not contain the literal slashes.
       Some versions of awk allow the form:

              func name(args, ... ) { statements }
       This has been deprecated by the authors of the language, who asked that
       it not be specified.
       Historical  implementations of awk produce an error if a next statement
       is executed in a BEGIN action, and cause awk to  terminate  if  a  next
       statement is executed in an END action. This behavior has not been doc-
       umented, and it was not believed that it was necessary  to  standardize
       it.
       The  specification  of conversions between string and numeric values is
       much more detailed than in the documentation of historical  implementa-
       tions or in the referenced The AWK Programming Language.  Although most
       of the behavior is designed to be intuitive, the details are  necessary
       to  ensure  compatible behavior from different implementations. This is
       especially important in relational expressions since the types  of  the
       operands determine whether a string or numeric comparison is performed.
       From the perspective of an application writer, it is usually sufficient
       to  expect  intuitive behavior and to force conversions (by adding zero
       or concatenating a null string) when the type of an expression does not
       obviously match what is needed. The intent has been to specify histori-
       cal practice in almost all cases. The one exception is that, in histor-
       ical  implementations, variables and constants maintain both string and
       numeric values after their original value is converted by any use. This
       means  that referencing a variable or constant can have unexpected side
       effects. For example, with  historical  implementations  the  following
       program:

              {
                  a = "+2"
                  b = 2
                  if (NR % 2)
                      c = a + b
                  if (a == b)
                      print "numeric comparison"
                  else
                      print "string comparison"
              }
       would  perform a numeric comparison (and output numeric comparison) for
       each odd-numbered line, but perform a  string  comparison  (and  output
       string  comparison)  for  each even-numbered line. IEEE Std 1003.1-2001
       ensures that comparisons will be numeric if necessary. With  historical
       implementations, the following program:

              BEGIN {
                  OFMT = "%e"
                  print 3.14
                  OFMT = "%f"
                  print 3.14
              }
       would  output  "3.140000e+00" twice, because in the second print state-
       ment the constant "3.14" would have a string value  from  the  previous
       conversion. IEEE Std 1003.1-2001 requires that the output of the second
       print statement be "3.140000" . The behavior of historical  implementa-
       tions was seen as too unintuitive and unpredictable.
       It  was  pointed out that with the rules contained in early drafts, the
       following script would print nothing:

              BEGIN {
                  y[1.5] = 1
                  OFMT = "%e"
                  print y[1.5]
              }
       Therefore, a new variable, CONVFMT, was introduced. The  OFMT  variable
       is now restricted to affecting output conversions of numbers to strings
       and CONVFMT is used for internal conversions, such  as  comparisons  or
       array  indexing.  The  default  value  is the same as that for OFMT, so
       unless a program changes CONVFMT (which  no  historical  program  would
       do),  it  will receive the historical behavior associated with internal
       string conversions.
       The POSIX awk lexical and syntactic conventions are specified more for-
       mally  than in other sources. Again the intent has been to specify his-
       torical practice. One convention that may not be obvious from the  for-
       mal  grammar  as  in  other verbal descriptions is where <newline>s are
       acceptable. There are several obvious placements such as terminating  a
       statement, and a backslash can be used to escape <newline>s between any
       lexical tokens. In addition, <newline>s without backslashes can  follow
       a  comma,  an open brace, a logical AND operator ( "&&" ), a logical OR
       operator ( "||" ), the do keyword, the else keyword,  and  the  closing
       parenthesis of an if, for, or while statement. For example:

              { print $1,
                      $2 }
       The  requirement that awk add a trailing <newline> to the program argu-
       ment text is to simplify the grammar, making it match a  text  file  in
       form.  There  is  no  way for an application or test suite to determine
       whether a literal <newline> is added or whether awk simply acts  as  if
       it did.
       IEEE Std 1003.1-2001 requires several changes from historical implemen-
       tations in order to support  internationalization.  Probably  the  most
       subtle  of  these is the use of the decimal-point character, defined by
       the LC_NUMERIC category of the locale, in representations of  floating-
       point  numbers.   This locale-specific character is used in recognizing
       numeric input, in converting between strings and numeric values, and in
       formatting  output. However, regardless of locale, the period character
       (the decimal-point character of the POSIX locale) is the  decimal-point
       character  recognized in processing awk programs (including assignments
       in command line arguments). This is essentially the same convention  as
       the  one  used in the ISO C standard. The difference is that the C lan-
       guage includes the setlocale() function, which permits  an  application
       to  modify  its  locale.  Because  of  this capability, a C application
       begins executing with its locale set to the C locale, and only executes
       in  the  environment-specified  locale after an explicit call to setlo-
       cale(). However, adding such an elaborate new feature to the  awk  lan-
       guage  was seen as inappropriate for IEEE Std 1003.1-2001. It is possi-
       ble to execute an awk program explicitly in any desired locale by  set-
       ting the environment in the shell.
       The  undefined behavior resulting from NULs in extended regular expres-
       sions allows future extensions for the  GNU  gawk  program  to  process
       binary data.
       The  behavior  in  the case of invalid awk programs (including lexical,
       syntactic, and semantic errors) is undefined because it was  considered
       overly  limiting  on  implementations  to  specify.  In most cases such
       errors can be expected to produce a diagnostic and a non-zero exit sta-
       tus. However, some implementations may choose to extend the language in
       ways that make use of certain invalid constructs.  Other  invalid  con-
       structs  might  be deemed worthy of a warning, but otherwise cause some
       reasonable behavior.  Still other constructs may be very  difficult  to
       detect  in some implementations.  Also, different implementations might
       detect a given error during an initial parsing of the  program  (before
       reading  any  input  files) while others might detect it when executing
       the program after reading some input. Implementors should be aware that
       diagnosing errors as early as possible and producing useful diagnostics
       can ease debugging of applications, and  thus  make  an  implementation
       more usable.
       The  unspecified  behavior  from  using multi-character RS values is to
       allow possible future extensions based on extended regular  expressions
       used  for  record separators. Historical implementations take the first
       character of the string and ignore the others.
       Unspecified behavior when split( string, array, <null>) is used  is  to
       allow  a proposed future extension that would split up a string into an
       array of individual characters.
       In the context of the getline function, equally good arguments for dif-
       ferent  precedences  of  the  | and < operators can be made. Historical
       practice has been that:

              getline < "a" "b"
       is parsed as:

              ( getline < "a" ) "b"
       although many would argue that the intent was that the file  ab  should
       be read. However:

              getline < "x" + 1
       parses as:

              getline < ( "x" + 1 )
       Similar  problems  occur with the | version of getline, particularly in
       combination with $. For example:

              $"echo hi" | getline
       (This situation is particularly problematic when used in a print state-
       ment, where the |getline part might be a redirection of the print.)
       Since in most cases such constructs are not (or at least should not) be
       used (because they have a natural ambiguity for which there is no  con-
       ventional  parsing),  the  meaning  of  these  constructs has been made
       explicitly unspecified. (The effect is that  a  conforming  application
       that runs into the problem must parenthesize to resolve the ambiguity.)
       There appeared to be few if any actual uses of such constructs.
       Grammars can be written that would cause an error under  these  circum-
       stances.   Where  backwards-compatibility is not a large consideration,
       implementors may wish to use such grammars.
       Some historical implementations have allowed some built-in functions to
       be called without an argument list, the result being a default argument
       list chosen in some "reasonable" way. Use of length as  a  synonym  for
       length($0)  is the only one of these forms that is thought to be widely
       known or widely used; this particular form  is  documented  in  various
       places  (for example, most historical awk reference pages, although not
       in the referenced The AWK Programming Language) as legitimate practice.
       With  this  exception,  default argument lists have always been undocu-
       mented and vaguely defined, and it is not at all clear how (or if) they
       should  be  generalized  to user-defined functions.  They add no useful
       functionality and preclude possible future extensions that  might  need
       to  name  functions without calling them.  Not standardizing them seems
       the simplest course. The standard  developers  considered  that  length
       merited special treatment, however, since it has been documented in the
       past and sees possibly substantial use in historical programs.  Accord-
       ingly,  this  usage  has  been made legitimate, but Issue 5 removed the
       obsolescent marking for XSI-conforming implementations and many  other-
       wise conforming applications depend on this feature.
       In  sub  and  gsub,  if  repl  is  a  string literal (the lexical token
       STRING), then two consecutive backslash characters should  be  used  in
       the string to ensure a single backslash will precede the ampersand when
       the resultant string is passed to the function. (For example, to  spec-
       ify  one  literal  ampersand  in the replacement string, use gsub( ERE,
       "\\&" ).)
       Historically the only special character in the repl argument of sub and
       gsub string functions was the ampersand ( '&' ) character and preceding
       it with the backslash character was used to turn off its special  mean-
       ing.
       The  description  in  the ISO POSIX-2:1993 standard introduced behavior
       such that the backslash character was another special character and  it
       was  unspecified  whether there were any other special characters. This
       description introduced several portability problems, some of which  are
       described  below,  and so it has been replaced with the more historical
       description. Some of the problems include:
        * Historically, to create the replacement string, a script  could  use
          gsub(  ERE, "\\&" ), but with the ISO POSIX-2:1993 standard wording,
          it was necessary to use gsub( ERE, "\\\\&" ).  Backslash  characters
          are  doubled here because all string literals are subject to lexical
          analysis, which would reduce each pair of backslash characters to  a
          single backslash before being passed to gsub.
        * Since  it was unspecified what the special characters were, for por-
          table scripts to guarantee that characters  are  printed  literally,
          each  character had to be preceded with a backslash. (For example, a
          portable script had to use  gsub(  ERE,  "\\h\\i"  )  to  produce  a
          replacement string of "hi" .)
       The  description  for  comparisons in the ISO POSIX-2:1993 standard did
       not properly describe historical practice because of  the  way  numeric
       strings  are compared as numbers. The current rules cause the following
       code:

              if (0 == "000")
                  print "strange, but true"
              else
                  print "not true"
       to do a numeric comparison, causing the if to  succeed.  It  should  be
       intuitively  obvious  that  this  is incorrect behavior, and indeed, no
       historical implementation of awk actually behaves this way.
       To fix this problem, the definition of numeric string was  enhanced  to
       include  only those values obtained from specific circumstances (mostly
       external sources) where it is not possible to  determine  unambiguously
       whether the value is intended to be a string or a numeric.
       Variables  that  are assigned to a numeric string shall also be treated
       as a numeric string. (For example, the notion of a numeric  string  can
       be propagated across assignments.) In comparisons, all variables having
       the uninitialized value are to be treated as a numeric operand evaluat-
       ing to the numeric value zero.
       Uninitialized  variables  include  all  types  of  variables  including
       scalars, array elements, and fields. The definition of an uninitialized
       value  in  Variables and Special Variables is necessary to describe the
       value placed on uninitialized variables and on fields  that  are  valid
       (for example, < $NF) but have no characters in them and to describe how
       these variables are to be used in comparisons. A valid field,  such  as
       $1,  that has no characters in it can be obtained from an input line of
       "\t\t" when FS= '\t' . Historically, the comparison ( $1<10)  was  done
       numerically after evaluating $1 to the value zero.
       The  phrase  "...  also  shall  have  the  numeric value of the numeric
       string" was removed from several sections of the ISO POSIX-2:1993 stan-
       dard  because  is specifies an unnecessary implementation detail. It is
       not necessary for IEEE Std 1003.1-2001 to specify that these objects be
       assigned  two  different  values.  It is only necessary to specify that
       these objects may evaluate to two different values  depending  on  con-
       text.
       The  description  of numeric string processing is based on the behavior
       of the atof() function in  the  ISO C  standard.  While  it  is  not  a
       requirement for an implementation to use this function, many historical
       implementations of awk do. In the ISO C standard,  floating-point  con-
       stants  use  a  period  as  a  decimal point character for the language
       itself, independent of the current locale, but the atof() function  and
       the associated strtod() function use the decimal point character of the
       current locale when converting strings to numeric values. Similarly  in
       awk, floating-point constants in an awk script use a period independent
       of the locale, but input strings use the decimal point character of the
       locale.
FUTURE DIRECTIONS
       None.
SEE ALSO
       Grammar  Conventions,  grep,  lex, sed, the System Interfaces volume of
       IEEE Std 1003.1-2001, atof(), exec, popen(), setlocale(), strtod()
COPYRIGHT
       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
       Specifications  Issue  6,  Copyright  (C) 2001-2003 by the Institute of
       Electrical and Electronics Engineers, Inc and The Open  Group.  In  the
       event of any discrepancy between this version and the original IEEE and
       The Open Group Standard, the original IEEE and The Open Group  Standard
       is  the  referee document. The original Standard can be obtained online
       at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group                  2003                              AWK(1P)