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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
printf — write formatted output
SYNOPSIS
printf format [argument...]
DESCRIPTION
The printf utility shall write formatted operands to the standard output. The argument
operands shall be formatted under control of the format 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 control 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 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.
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.1‐2017, 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.
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.1‐2017 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 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.
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-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.
RATIONALE
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.
FUTURE DIRECTIONS
None.
SEE ALSO
awk, bc, echo
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(), strtod()
COPYRIGHT
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 .