usr.bin/printf/printf.1

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.\"	from: @(#)printf.1	8.1 (Berkeley) 6/6/93
.\"
.Dd May 19, 2021
.Dt PRINTF 1
.Os
.Sh NAME
.Nm printf
.Nd formatted output
.Sh SYNOPSIS
.Nm
.Ar format
.Op Ar arguments  ...
.Sh DESCRIPTION
.Nm
formats and prints its arguments, after the first, under control
of the
.Ar format  .
The
.Ar format
is a character string which contains three types of objects: plain characters,
which are simply copied to standard output, character escape sequences which
are converted and copied to the standard output, and format specifications,
each of which causes printing of the next successive
.Ar argument  .
.Pp
The
.Ar arguments
after the first are treated as strings if the corresponding format is
either
.Cm b ,
.Cm B ,
.Cm c ,
or
.Cm s ;
otherwise it is evaluated as a C\~constant, with the following extensions:
.Bl -bullet -offset indent
.It
A leading plus or minus sign is allowed.
.It
If the leading character is a single or double quote, the value is the ASCII
code of the next character.
.El
.Pp
The format string is reused as often as necessary to satisfy the
.Ar arguments  .
Any extra format specifications are evaluated with zero or the null
string.
.Pp
Character escape sequences are in backslash notation as defined in
.St -ansiC .
The characters and their meanings are as follows:
.Bl -tag -offset indent -width Cm
.It Cm \ee
Write an
.Aq escape
character.
.It Cm \ea
Write a
.Aq bell
character.
.It Cm \eb
Write a
.Aq backspace
character.
.It Cm \ef
Write a
.Aq form-feed
character.
.It Cm \en
Write a
.Aq new-line
character.
.It Cm \er
Write a
.Aq carriage return
character.
.It Cm \et
Write a
.Aq tab
character.
.It Cm \ev
Write a
.Aq vertical tab
character.
.It Cm \e\(aq
Write a
.Aq single quote
character.
.It Cm \e\*q
Write a
.Aq double quote
character.
.It Cm \e\e
Write a backslash character.
.It Cm \e Ns Ar num
Write an 8-bit character whose ASCII
value is the 1-, 2-, or 3-digit octal number
.Ar num .
.It Cm \ex Ns Ar xx
Write an 8-bit character whose ASCII
value is the 1- or 2-digit hexadecimal number
.Ar xx .
.El
.Pp
Each format specification is introduced by the percent character
.Pq Ql \&% .
To produce a literal percent
.Pq Ql \&%
in the output, write the percent character twice:
.Pq Ql \&%% .
This is not a format conversion.
The remainder of the format specification includes,
in the following order:
.Bl -tag -width 5n
.It Zero or more of the following flags :
.Bl -tag -width Cm
.It Cm #
A
.Ql \&#
character specifying that the value should be printed in an
.Dq alternative form .
For
.Cm b ,
.Cm c ,
.Cm d ,
and
.Cm s
formats, this option has no effect.
For the
.Cm o
format the precision of the number is increased to force the first
character of the output string to a zero.
For the
.Cm x
.Pq Cm X
format, a non-zero result has the string
.Ql 0x
.Pq Ql 0X
prepended to it.
For
.Cm e ,
.Cm E ,
.Cm f ,
.Cm F ,
.Cm g ,
and
.Cm G
formats, the result will always contain a decimal point, even if no
digits follow the point (normally, a decimal point only appears in the
results of those formats if a digit follows the decimal point).
For
.Cm g
and
.Cm G
formats, trailing zeros are not removed from the result as they
would otherwise be.
.\" I turned this off - decided it isn't a valid use of '#'
.\" For the
.\" .Cm B
.\" format, backslash-escape sequences are expanded first;
.It Cm \&\-
A minus sign which specifies
.Em left adjustment
of the output in the indicated field;
.It Cm \&+
A plus sign which specifies that there should always be
a sign placed before the number when using signed formats.
.It Sq Cm \&\ \&
A
.Aq space
character which specifies that a space should be left before
a positive number for a signed format.
A
.Ql \&+
overrides a
.Aq space
if both are used;
.It Cm \&0
A digit zero character which specifies that zero-padding should be used
rather than space-padding.
A
.Ql \-
overrides a
.Ql \&0
if both are used;
.El
.It Field Width :
An optional digit string specifying a
.Em field width ;
if the output string has fewer characters than the field width it will
be space-padded on the left (or right, if the left-adjustment indicator
has been given) to make up the field width (note that a leading zero
is a flag, but an embedded zero is part of a field width);
.It Precision :
An optional period
.Pq Ql \&. ,
followed by an optional digit string giving a
.Em precision
which specifies the number of digits to appear after the decimal point,
for
.Cm e
and
.Cm f
formats, or the maximum number of characters to be printed
from a string
.Sm off
.Pf ( Cm b ,
.Sm on
.Cm B ,
and
.Cm s
formats); if the digit string is missing, the precision is treated
as zero;
.It Format :
A character which indicates the type of format to use (one of
.Cm diouxXfFeEgGaAbBcs ) .
.El
.Pp
A field width or precision may be
.Sq Cm \&*
instead of a digit string.
In this case an
.Ar argument
supplies the field width or precision.
.Pp
The format characters and their meanings are:
.Bl -tag -width Fl
.It Cm diouXx
The
.Ar argument ,
which must represent an integer constant,
with an optional leading plus or minus sign,
is printed as a signed decimal
.Cm ( d
or
.Cm i ) ,
unsigned octal
.Cm ( o ) ,
unsigned decimal
.Cm ( u ) ,
or unsigned hexadecimal
.Cm ( X
or
.Cm x ) .
.It Cm fF
The
.Ar argument
is printed in the style
.Oo Li \&\- Oc Ns Ar \&\^ddd Ns Li \&\^\&. Ns Ar ddd
where the number of
.Ar d Ns \|'s
after the decimal point is equal to the precision specification for
the argument.
If the precision is missing, 6 digits are given; if the precision
is explicitly 0, no digits and no decimal point are printed.
If the number is Infinity, or Not-a-Number, then
.Ql inf
or
.Ql nan
is printed for
.Cm f
format, and
.Ql INF
or
.Ql NAN
for
.Cm F
format.
.It Cm eE
The
.Ar argument
is printed in the style
.Oo Li \&\- Oc Ns Ar \&\^d Ns Li \&. Ns Ar ddd Ns Li \&\|e\*(Pm Ns Ar \&\|dd
where there
is one digit before the decimal point and the number after is equal to
the precision specification for the argument; when the precision is
missing, 6 digits are produced.
An upper-case
.Ql E
is used for an
.Cm E
format, and upper-case for Infinity and NaN as for
.Cm F
format.
.It Cm gG
The
.Ar argument
is printed in style
.Cm f
.Pq Cm F
or in style
.Cm e
.Pq Cm E
whichever gives full precision in minimum space.
.It Cm aA
The
.Ar argument
is treated as a floating point number,
for which the underlying hexadecimal representation is
printed.
See
.Xr printf 3
for the details.
.It Cm b
Characters from the string
.Ar argument
are printed with backslash-escape sequences expanded.
.Pp
The following additional backslash-escape sequences are supported:
.Bl -tag -width Cm
.It Cm \ec
Causes
.Nm
to ignore any remaining characters in the string operand containing it,
any remaining string operands, and any additional characters in
the format operand.
.It Cm \e0 Ns Ar num
Write an 8-bit character whose ASCII value is the 1-, 2-, or
3-digit octal number
.Ar num .
.It Cm \e^ Ns Ar c
Write the control character
.Ar c .
Generates characters
.Sq \e000
through
.Sq \e037 ,
and
.Sq \e177
(from
.Ql \e^\&? ) .
.It Cm \eM^ Ns Ar c
Write the control character
.Ar c
with the 8th bit set.
Generates characters
.Sq \e200
through
.Sq \e237 ,
and
.Sq \e377
(from
.Ql \eM^\&? ) .
.It Cm \eM\- Ns Ar c
Write the character
.Ar c
with the 8th bit set.
Generates characters
.Sq \e241
through
.Sq \e376 .
.El
.It Cm B
Characters from the string
.Ar argument
are printed with unprintable characters backslash-escaped using the
.Sq Cm \e Ns Ar c ,
.Sq Cm \e^ Ns Ar c ,
.Sq Cm \eM^ Ns Ar c ,
or
.Sq Cm \eM\- Ns Ar c
formats described above.
.It Cm c
The first character of
.Ar argument
is printed.
.It Cm s
Characters from the string
.Ar argument
are printed until the end is reached or until the number of characters
indicated by the precision specification is reached; if the
precision is omitted, all characters in the string are printed.
.El
.Pp
In no case does a non-existent or small field width cause truncation of
a field; padding takes place only if the specified field width exceeds
the actual width.
.Pp
If the first character of
.Ar format
is a dash,
.Ar format
must be preceded by a word consisting of two dashes
.Pq Sq Fl Fl
to prevent it
from being interpreted as an option string.
.Sh EXIT STATUS
.Ex -std
.Sh SEE ALSO
.Xr echo 1 ,
.Xr printf 3 ,
.Xr vis 3 ,
.Xr printf 9
.Sh STANDARDS
The
.Nm
utility conforms to
.St -p1003.1-2001 .
.Pp
Support for the floating point formats and
.Sq Cm \&*
as a field width and precision
are optional in POSIX.
.Pp
The behaviour of the
.Cm \&%B
format and the
.Cm \e\(aq ,
.Cm \e\*q ,
.Cm \ee ,
.Cm \e Ns Ar num ,
and
.Cm \e Ns Oo Cm M Oc Ns Oo Cm \- Ns Li \&\(or Ns Cm ^ Oc Ns Ar c
escape sequences are undefined in POSIX.
.Sh BUGS
Since the floating point numbers are translated from ASCII to
floating-point and then back again, floating-point precision may be lost.
.Pp
Hexadecimal character constants are restricted to, and should be specified
as, two character constants.
This is contrary to the ISO C standard but
does guarantee detection of the end of the constant.
.Sh NOTES
All formats which treat the
.Ar argument
as a number first convert the
.Ar argument
from its external representation as a character string
to an internal numeric representation, and then apply the
format to the internal numeric representation, producing
another external character string representation.
One might expect the
.Cm \&%c
format to do likewise, but in fact it does not.
.Pp
To convert a string representation of a decimal, octal, or hexadecimal
number into the corresponding character, two nested
.Nm
invocations may be used, in which the inner invocation
converts the input to an octal string, and the outer
invocation uses the octal string as part of a format.
For example, the following command outputs the character whose code
is 0x0a, which is a newline in ASCII:
.Pp
.Dl printf \*q$(printf \(aq\e\e%o\(aq 0x0a)\*q