PRINTF(1P) POSIX Programmer's Manual PRINTF(1P)
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implementation of this interface may differ (consult the corresponding
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NAME
printf -- write formatted output
SYNOPSIS
printf format [argument...]
DESCRIPTION
The printf utility shall write formatted operands to the standard out-
put. The argument operands shall be formatted under control of the for-
mat operand.
OPTIONS
None.
OPERANDS
The following operands shall be supported:
format A string describing the format to use to write the remaining
operands. See the EXTENDED DESCRIPTION section.
argument The strings to be written to standard output, under the con-
trol of format. See the EXTENDED DESCRIPTION section.
STDIN
Not used.
INPUT FILES
None.
ENVIRONMENT VARIABLES
The following environment variables shall affect the execution of
printf:
LANG Provide a default value for the internationalization vari-
ables that are unset or null. (See the Base Definitions vol-
ume of POSIX.1-2008, Section 8.2, Internationalization Vari-
ables 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_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).
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 locale for numeric formatting. It shall affect
the format of numbers written using the e, E, f, g, and G
conversion specifier characters (if supported).
NLSPATH Determine the location of message catalogs for the processing
of LC_MESSAGES.
ASYNCHRONOUS EVENTS
Default.
STDOUT
See the EXTENDED DESCRIPTION section.
STDERR
The standard error shall be used only for diagnostic messages.
OUTPUT FILES
None.
EXTENDED DESCRIPTION
The format operand shall be used as the format string described in the
Base Definitions volume of POSIX.1-2008, Chapter 5, File Format Nota-
tion with the following exceptions:
1. A <space> in the format string, in any context other than a flag of
a conversion specification, shall be treated as an ordinary charac-
ter that is copied to the output.
2. A '' character in the format string shall be treated as a '' char-
acter, not as a <space>.
3. In addition to the escape sequences shown in the Base Definitions
volume of POSIX.1-2008, Chapter 5, File Format Notation ('\\',
'\a', '\b', '\f', '\n', '\r', '\t', '\v'), "\ddd", where ddd is a
one, two, or three-digit octal number, shall be written as a byte
with the numeric value specified by the octal number.
4. The implementation shall not precede or follow output from the d or
u conversion specifiers with <blank> characters not specified by
the format operand.
5. The implementation shall not precede output from the o conversion
specifier with zeros not specified by the format operand.
6. The a, A, e, E, f, F, g, and G conversion specifiers need not be
supported.
7. An additional conversion specifier character, b, shall be supported
as follows. The argument shall be taken to be a string that may
contain <backslash>-escape sequences. The following <back-
slash>-escape sequences shall be supported:
-- The escape sequences listed in the Base Definitions volume of
POSIX.1-2008, Chapter 5, File Format Notation ('\\', '\a',
'\b', '\f', '\n', '\r', '\t', '\v'), which shall be converted
to the characters they represent
-- "\0ddd", where ddd is a zero, one, two, or three-digit octal
number that shall be converted to a byte with the numeric value
specified by the octal number
-- '\c', which shall not be written and shall cause printf to
ignore any remaining characters in the string operand contain-
ing it, any remaining string operands, and any additional char-
acters in the format operand
The interpretation of a <backslash> followed by any other sequence
of characters is unspecified.
Bytes from the converted string shall be written until the end of
the string or the number of bytes indicated by the precision speci-
fication is reached. If the precision is omitted, it shall be taken
to be infinite, so all bytes up to the end of the converted string
shall be written.
8. For each conversion specification that consumes an argument, the
next argument operand shall be evaluated and converted to the
appropriate type for the conversion as specified below.
9. The format operand shall be reused as often as necessary to satisfy
the argument operands. Any extra c or s conversion specifiers shall
be evaluated as if a null string argument were supplied; other
extra conversion specifications shall be evaluated as if a zero
argument were supplied. If the format operand contains no conver-
sion specifications and argument operands are present, the results
are unspecified.
10. If a character sequence in the format operand begins with a '%'
character, but does not form a valid conversion specification, the
behavior is unspecified.
11. The argument to the c conversion specifier can be a string contain-
ing zero or more bytes. If it contains one or more bytes, the first
byte shall be written and any additional bytes shall be ignored. If
the argument is an empty string, it is unspecified whether nothing
is written or a null byte is written.
The argument operands shall be treated as strings if the corresponding
conversion specifier is b, c, or s, and shall be evaluated as if by the
strtod() function if the corresponding conversion specifier is a, A, e,
E, f, F, g, or G. Otherwise, they shall be evaluated as unsuffixed C
integer constants, as described by the ISO C standard, with the follow-
ing extensions:
* A leading <plus-sign> or minus-sign shall be allowed.
* If the leading character is a single-quote or double-quote, the
value shall be the numeric value in the underlying codeset of the
character following the single-quote or double-quote.
* Suffixed integer constants may be allowed.
If an argument operand cannot be completely converted into an internal
value appropriate to the corresponding conversion specification, a
diagnostic message shall be written to standard error and the utility
shall not exit with a zero exit status, but shall continue processing
any remaining operands and shall write the value accumulated at the
time the error was detected to standard output.
It is not considered an error if an argument operand is not completely
used for a c or s conversion.
EXIT STATUS
The following exit values shall be returned:
0 Successful completion.
>0 An error occurred.
CONSEQUENCES OF ERRORS
Default.
The following sections are informative.
APPLICATION USAGE
The floating-point formatting conversion specifications of printf() are
not required because all arithmetic in the shell is integer arithmetic.
The awk utility performs floating-point calculations and provides its
own printf function. The bc utility can perform arbitrary-precision
floating-point arithmetic, but does not provide extensive formatting
capabilities. (This printf utility cannot really be used to format bc
output; it does not support arbitrary precision.) Implementations are
encouraged to support the floating-point conversions as an extension.
Note that this printf utility, like the printf() function defined in
the System Interfaces volume of POSIX.1-2008 on which it is based,
makes no special provision for dealing with multi-byte characters when
using the %c conversion specification or when a precision is specified
in a %b or %s conversion specification. Applications should be
extremely cautious using either of these features when there are multi-
byte characters in the character set.
No provision is made in this volume of POSIX.1-2008 which allows field
widths and precisions to be specified as '*' since the '*' can be
replaced directly in the format operand using shell variable substitu-
tion. Implementations can also provide this feature as an extension if
they so choose.
Hexadecimal character constants as defined in the ISO C standard are
not recognized in the format operand because there is no consistent way
to detect the end of the constant. Octal character constants are lim-
ited to, at most, three octal digits, but hexadecimal character con-
stants are only terminated by a non-hex-digit character. In the ISO C
standard, the "##" concatenation operator can be used to terminate a
constant and follow it with a hexadecimal character to be written. In
the shell, concatenation occurs before the printf utility has a chance
to parse the end of the hexadecimal constant.
The %b conversion specification is not part of the ISO C standard; it
has been added here as a portable way to process <backslash>-escapes
expanded in string operands as provided by the echo utility. See also
the APPLICATION USAGE section of echo for ways to use printf as a
replacement for all of the traditional versions of the echo utility.
If an argument cannot be parsed correctly for the corresponding conver-
sion specification, the printf utility is required to report an error.
Thus, overflow and extraneous characters at the end of an argument
being used for a numeric conversion shall be reported as errors.
EXAMPLES
To alert the user and then print and read a series of prompts:
printf "\aPlease fill in the following: \nName: "
read name
printf "Phone number: "
read phone
To read out a list of right and wrong answers from a file, calculate
the percentage correctly, and print them out. The numbers are right-
justified and separated by a single <tab>. The percentage is written
to one decimal place of accuracy:
while read right wrong ; do
percent=$(echo "scale=1;($right*100)/($right+$wrong)" | bc)
printf "%2d right\t%2d wrong\t(%s%%)\n" \
$right $wrong $percent
done < database_file
The command:
printf "%5d%4d\n" 1 21 321 4321 54321
produces:
1 21
3214321
54321 0
Note that the format operand is used three times to print all of the
given strings and that a '0' was supplied by printf to satisfy the last
%4d conversion specification.
The printf utility is required to notify the user when conversion
errors are detected while producing numeric output; thus, the following
results would be expected on an implementation with 32-bit twos-comple-
ment integers when %d is specified as the format operand:
+------------+-------------+-------------------------------------------+
| | Standard | |
| Argument | Output | Diagnostic Output |
+------------+-------------+-------------------------------------------+
|5a | 5 | printf: "5a" not completely converted |
|9999999999 | 2147483647 | printf: "9999999999" arithmetic overflow |
|-9999999999 | -2147483648 | printf: "-9999999999" arithmetic overflow |
|ABC | 0 | printf: "ABC" expected numeric value |
+------------+-------------+-------------------------------------------+
The diagnostic message format is not specified, but these examples con-
vey the type of information that should be reported. Note that the
value shown on standard output is what would be expected as the return
value from the strtol() function as defined in the System Interfaces
volume of POSIX.1-2008. A similar correspondence exists between %u and
strtoul() and %e, %f, and %g (if the implementation supports floating-
point conversions) and strtod().
In a locale using the ISO/IEC 646:1991 standard as the underlying code-
set, the command:
printf "%d\n" 3 +3 -3 \'3 \"+3 "'-3"
produces:
3 Numeric value of constant 3
3 Numeric value of constant 3
-3 Numeric value of constant -3
51 Numeric value of the character '3' in the ISO/IEC 646:1991 stan-
dard codeset
43 Numeric value of the character '+' in the ISO/IEC 646:1991 stan-
dard codeset
45 Numeric value of the character '-' in the ISO/IEC 646:1991 stan-
dard codeset
Note that in a locale with multi-byte characters, the value of a char-
acter is intended to be the value of the equivalent of the wchar_t rep-
resentation of the character as described in the System Interfaces vol-
ume of POSIX.1-2008.
RATIONALE
The printf utility was added to provide functionality that has histori-
cally been provided by echo. However, due to irreconcilable differ-
ences in the various versions of echo extant, the version has few spe-
cial features, leaving those to this new printf utility, which is based
on one in the Ninth Edition system.
The EXTENDED DESCRIPTION section almost exactly matches the printf()
function in the ISO C standard, although it is described in terms of
the file format notation in the Base Definitions volume of
POSIX.1-2008, Chapter 5, File Format Notation.
Earlier versions of this standard specified that arguments for all con-
versions other than b, c, and s were evaluated in the same way (as C
constants, but with stated exceptions). For implementations supporting
the floating-point conversions it was not clear whether integer conver-
sions need only accept integer constants and floating-point conversions
need only accept floating-point constants, or whether both types of
conversions should accept both types of constants. Also by not distin-
guishing between them, the requirement relating to a leading single-
quote or double-quote applied to floating-point conversions even though
this provided no useful functionality to applications that was not
already available through the integer conversions. The current standard
clarifies the situation by specifying that the arguments for floating-
point conversions are evaluated as if by strtod(), and the arguments
for integer conversions are evaluated as C integer constants, with the
special treatment of leading single-quote and double-quote applying
only to integer conversions.
FUTURE DIRECTIONS
None.
SEE ALSO
awk, bc, echo
The Base Definitions volume of POSIX.1-2008, Chapter 5, File Format
Notation, Chapter 8, Environment Variables
The System Interfaces volume of POSIX.1-2008, fprintf(), strtod()
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the source
files to man page format. To report such errors, see https://www.ker-
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 PRINTF(1P)
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