utils-std

printf.1 (11442B)

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.Dd July 1, 2020
.Dt PRINTF 1
.Os
.Sh NAME
.Nm printf
.Nd formatted output
.Sh SYNOPSIS
.Nm
.Ar format Op Ar argument...
.Sh DESCRIPTION
The
.Nm
utility 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 argument
after the first are treated as strings if the corresponding format is
either
.Cm c , b
or
.Cm s ;
otherwise it is evaluated as a C constant, with the following extensions:
.Pp
.Bl -bullet -offset indent -compact
.It
A leading plus or minus sign is allowed.
.It
If the leading character is a single or double quote, the value is the
character code of the next character.
.El
.Pp
The format string is reused as often as necessary to satisfy the
.Ar argument .
Any extra format specifications are evaluated with zero or the null
string.
.Pp
Character escape sequences are in backslash notation as defined in the
.St -ansiC ,
with extensions.
The characters and their meanings
are as follows:
.Pp
.Bl -tag -width Ds -offset indent -compact
.It Cm \ea
Write a <bell> character.
.It Cm \eb
Write a <backspace> character.
.It Cm \ef
Write a <form-feed> character.
.It Cm \en
Write a <new-line> character.
.It Cm \er
Write a <carriage return> character.
.It Cm \et
Write a <tab> character.
.It Cm \ev
Write a <vertical tab> character.
.It Cm \e\'
Write a <single quote> character.
.It Cm \e\e
Write a backslash character.
.It Cm \ex Ns Ar hex-num
Write a byte whose
value is the 1- or 2-digits
hexadecimal number
.Ar hex-num .
.It Cm \e Ns Ar num
Write a byte whose
value is the 1-, 2-, or 3-digit
octal number
.Ar num .
.El
Multibyte characters can be constructed using multiple
.Cm \ex Ns Ar hex-num
or
.Cm \e Ns Ar num
sequences.
.Pp
Each format specification is introduced by the percent character
(``%'').
The remainder of the format specification includes,
in the following order:
.Bl -tag -width Ds
.It "Zero or more of the following flags:"
.Bl -tag -width Ds
.It Cm #
A `#' character
specifying that the value should be printed in an ``alternate form''.
For
.Cm b , c , d , s
and
.Cm u
formats, this option has no effect.
For the
.Cm o
formats 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
.Li 0x
.Pq Li 0X
prepended to it.
For
.Cm a , A , e , E , f , F , 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;
.It Cm \&\-
A minus sign `\-' which specifies
.Em left adjustment
of the output in the indicated field;
.It Cm \&+
A `+' character specifying that there should always be
a sign placed before the number when using signed formats.
.It Sq \&\ \&
A space specifying that a blank should be left before a positive number
for a signed format.
A `+' overrides a space if both are used;
.It Cm \&0
A zero `0' character indicating that zero-padding should be used
rather than blank-padding.
A `\-' overrides a `0' if both are used;
.El
.It "Field Width:"
An optional digit string specifying a
.Em field width ;
if the output string has fewer bytes than the field width it will
be blank-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,
.Sq Cm \&.\& ,
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 bytes to be printed
from a string; 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 diouxXfFeEgGaAcsb ) .
The uppercase formats differ from their lowercase counterparts only in
that the output of the former is entirely in uppercase.
The floating-point format specifiers
.Pq Cm fFeEgGaA
may be prefixed by an
.Cm L
to request that additional precision be used, if available.
.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
is printed as a signed decimal (d or i), unsigned octal, unsigned decimal,
or unsigned hexadecimal (X or x), respectively.
.It Cm fF
The
.Ar argument
is printed in the style `[\-]ddd.ddd' where the number of d'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.
The values \*[If] and \*[Na] are printed as
.Ql inf
and
.Ql nan ,
respectively.
.It Cm eE
The
.Ar argument
is printed in the style
.Cm e
.Sm off
.Sq Op - Ar d.ddd No \(+- Ar dd
.Sm on
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.
The values \*[If] and \*[Na] are printed as
.Ql inf
and
.Ql nan ,
respectively.
.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 printed in style
.Sm off
.Sq Op - Ar h.hhh No \(+- Li p Ar d
.Sm on
where there is one digit before the hexadecimal point and the number
after is equal to the precision specification for the argument;
when the precision is missing, enough digits are produced to convey
the argument's exact double-precision floating-point representation.
The values \*[If] and \*[Na] are printed as
.Ql inf
and
.Ql nan ,
respectively.
.It Cm c
The first byte of
.Ar argument
is printed.
.It Cm s
Bytes from the string
.Ar argument
are printed until the end is reached or until the number of bytes
indicated by the precision specification is reached; however if the
precision is 0 or missing, the string is printed entirely.
.It Cm b
As for
.Cm s ,
but interpret character escapes in backslash notation in the string
.Ar argument .
The permitted escape sequences are slightly different in that
octal escapes are
.Cm \e0 Ns Ar num
instead of
.Cm \e Ns Ar num
and that an additional escape sequence
.Cm \ec
stops further output from this
.Nm
invocation.
.It Cm n$
Allows reordering of the output according to
.Ar argument .
.It Cm \&%
Print a `%'; no argument is used.
.El
.Pp
The decimal point
character is defined in the program's locale (category
.Dv LC_NUMERIC ) .
.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
Some shells may provide a builtin
.Nm
command which is similar or identical to this utility.
Consult the
.Xr builtin 1
manual page.
.Sh EXIT STATUS
.Ex -std
.Sh EXAMPLES
Print the string
.Qq hello :
.Bd -literal -offset indent
$ printf "%s\en" hello
hello
.Ed
.Pp
Same as above, but notice that the format string is not quoted and hence we
do not get the expected behavior:
.Bd -literal -offset indent
$ printf %s\en hello
hellon$
.Ed
.Pp
Print arguments forcing sign only for the first argument:
.Bd -literal -offset indent
$ printf "%+d\en%d\en%d\en" 1 -2 13
+1
-2
13
.Ed
.Pp
Same as above, but the single format string will be applied to the three
arguments:
.Bd -literal -offset indent
$ printf "%+d\en" 1 -2 13
+1
-2
+13
.Ed
.Pp
Print number using only two digits after the decimal point:
.Bd -literal -offset indent
$ printf "%.2f\en" 31.7456
31.75
.Ed
.Sh COMPATIBILITY
The traditional
.Bx
behavior of converting arguments of numeric formats not beginning
with a digit to the ASCII
code of the first character is not supported.
.Sh SEE ALSO
.Xr builtin 1 ,
.Xr echo 1 ,
.Xr sh 1 ,
.Xr printf 3
.Sh STANDARDS
The
.Nm
command is expected to be compatible with the
.St -p1003.2
specification.
The
.Cm \ex Ns Ar hex-num
backslash-escape is an extension.
.Sh HISTORY
The
.Nm
command appeared in
.Bx 4.3 Reno .
It is modeled
after the standard library function,
.Xr printf 3 .
.Sh CAVEATS
ANSI hexadecimal character constants were deliberately not provided.
.Pp
Trying to print a dash ("-") as the first character causes
.Nm
to interpret the dash as a program argument.
.Nm --
must be used before
.Ar format .
.Pp
If the locale contains multibyte characters
(such as UTF-8),
the
.Cm c
format and
.Cm b
and
.Cm s
formats with a precision
may not operate as expected.
.Sh BUGS
Since the floating point numbers are translated from ASCII
to floating-point and then back again, floating-point precision may be lost.
(By default, the number is translated to an IEEE-754 double-precision
value before being printed.
The
.Cm L
modifier may produce additional precision, depending on the hardware platform.)
.Pp
The escape sequence \e000 is the string terminator.
When present in the argument for the
.Cm b
format, the argument will be truncated at the \e000 character.
.Pp
Multibyte characters are not recognized in format strings (this is only
a problem if
.Ql %
can appear inside a multibyte character).