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PROLOG | NAME | SYNOPSIS | DESCRIPTION | OPTIONS | OPERANDS | STDIN | INPUT FILES | ENVIRONMENT VARIABLES | ASYNCHRONOUS EVENTS | STDOUT | STDERR | OUTPUT FILES | EXTENDED DESCRIPTION | EXIT STATUS | CONSEQUENCES OF ERRORS | APPLICATION USAGE | EXAMPLES | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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PRINTF(1P) POSIX Programmer's Manual PRINTF(1P)
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.
printf — write formatted output
printf format [argument...]
The printf utility shall write formatted operands to the standard
output. The argument operands shall be formatted under control of
the format operand.
None.
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
control of format. See the EXTENDED DESCRIPTION
section.
Not used.
None.
The following environment variables shall affect the execution of
printf:
LANG Provide a default value for the internationalization
variables that are unset or null. (See the Base
Definitions volume of POSIX.1‐2017, Section 8.2,
Internationalization Variables 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.
Default.
See the EXTENDED DESCRIPTION section.
The standard error shall be used only for diagnostic messages.
None.
The format operand shall be used as the format string described
in the Base Definitions volume of POSIX.1‐2017, Chapter 5, File
Format Notation 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 character that is copied to the output.
2. A '' character in the format string shall be treated as a ''
character, not as a <space>.
3. In addition to the escape sequences shown in the Base
Definitions volume of POSIX.1‐2017, 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 can contain <backslash>-escape sequences. The
following <backslash>-escape sequences shall be supported:
-- The escape sequences listed in the Base Definitions
volume of POSIX.1‐2017, 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
containing it, any remaining string operands, and any
additional characters 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 specification 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 b, 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 conversion 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
containing 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 following extensions:
* A leading <plus-sign> or <hyphen-minus> 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 shall not be considered an error if an argument operand is not
completely used for a b, c, or s conversion.
The following exit values shall be returned:
0 Successful completion.
>0 An error occurred.
Default.
The following sections are informative.
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.1‐2017 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.1‐2017 which allows
field widths and precisions to be specified as '*' since the '*'
can be replaced directly in the format operand using shell
variable substitution. 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 limited to, at most, three octal digits, but
hexadecimal character constants 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(1p) 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
conversion 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.
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-complement 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 convey 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.1‐2017. 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
codeset, 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
standard codeset
43 Numeric value of the character '+' in the ISO/IEC 646:1991
standard codeset
45 Numeric value of the character '-' in the ISO/IEC 646:1991
standard codeset
Note that in a locale with multi-byte characters, the value of a
character is intended to be the value of the equivalent of the
wchar_t representation of the character as described in the
System Interfaces volume of POSIX.1‐2017.
The printf utility was added to provide functionality that has
historically been provided by echo. However, due to
irreconcilable differences in the various versions of echo
extant, the version has few special 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.1‐2017, Chapter 5, File Format Notation.
Earlier versions of this standard specified that arguments for
all conversions 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 conversions 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 distinguishing
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.
None.
awk(1p), bc(1p), echo(1p)
The Base Definitions volume of POSIX.1‐2017, Chapter 5, File
Format Notation, Chapter 8, Environment Variables
The System Interfaces volume of POSIX.1‐2017, fprintf(3p),
strtod(3p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 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 .
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.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 PRINTF(1P)
Pages that refer to this page: echo(1p), file(1p), pax(1p)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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