path: root/usr.bin/printf/printf.1

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.\"	from: @(#)printf.1	5.11 (Berkeley) 7/24/91
.\"
.Dd $Mdocdate: August 2 2019 $
.Dt PRINTF 1
.Os
.Sh NAME
.Nm printf
.Nd formatted output
.Sh SYNOPSIS
.Nm printf
.Ar format
.Op Ar argument ...
.Sh DESCRIPTION
.Nm printf
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 arguments after the first are treated as strings
if the corresponding format is
.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 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:
.Pp
.Bl -tag -width Ds -offset indent -compact
.It Cm \ea
Write a <bell> character.
.It Cm \eb
Write a <backspace> character.
.It Cm \ee
Write an <escape> 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\(aq
Write a <single 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 .
.El
.Pp
Each format specification is introduced by the percent
.Pq Sq \&%
character.
The remainder of the format specifiers include,
in the following order:
.Bl -tag -width Ds
.It "Zero or more of the following flags:"
.Bl -tag -width Ds
.It Cm #
Specifies that the value should be printed in an
.Dq alternate form .
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
.Li 0x
.Pq Li 0X
prepended to it.
For
.Cm a ,
.Cm A ,
.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.
For all other formats, behaviour is undefined.
.It Cm \&\-
Specifies the
.Em left adjustment
of the output in the indicated field.
.It Cm \&+
Specifies that there should always be
a sign placed before the number when using signed formats.
.It Sq \&\ \&
A space specifies that a blank should be left before a positive number
for a signed format.
A
.Ql +
overrides a space if both are used.
.It Cm \&0
A zero character specifies that zero-padding should be used
rather than blank-padding.
This flag is ignored if used with a precision
specifier and any of the
.Cm d , i , o , u ,
or
.Cm x
.Pq Cm X
formats.
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 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
.Pq Sq \&. ,
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 diouxXfFeEgGaAbcs ) .
.El
.Pp
A field width or precision may be
.Ql \&*
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
.Pq Cm d No or Cm i ,
unsigned octal, unsigned decimal,
or unsigned hexadecimal
.Pq Cm x No or Cm X ,
respectively.
.It Cm fF
The
.Ar argument
is printed in the style
.Sm off
.Pf [\-]ddd Cm \&. No ddd
.Sm on
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.
.Pp
If the argument is infinity, it will be converted to [-]inf
.Pq Cm f
or [-]INF
.Pq Cm F ,
respectively.
If the argument is not-a-number (NaN), it will be converted to
[-]nan
.Pq Cm f
or [-]NAN
.Pq Cm F ,
respectively.
.It Cm eE
The
.Ar argument
is printed in the style
.Sm off
.Pf [\-]d Cm \&. No ddd Cm e No \(+-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.
An upper-case
.Sq E
is used for an
.Cm E
format.
.Pp
If the argument is infinity, it will be converted to [-]inf
.Pq Cm e
or [-]INF
.Pq Cm E ,
respectively.
If the argument is not-a-number (NaN), it will be converted to
[-]nan
.Pq Cm e
or [-]NAN
.Pq Cm E ,
respectively.
.It Cm gG
The
.Ar argument
is printed in style
.Cm f
or in style
.Cm e
.Pq Cm E
whichever gives full precision in minimum space.
.Pp
If the argument is infinity, it will be converted to [-]inf
.Pq Cm g
or [-]INF
.Pq Cm G ,
respectively.
If the argument is not-a-number (NaN), it will be converted to
[-]nan
.Pq Cm g
or [-]NAN
.Pq Cm G ,
respectively.
.It Cm aA
The
.Ar argument
is printed in style
.Sm off
.Pf [\-]0xh Cm \&. No hhh Cm p No [\(+-]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.
.Pp
If the argument is infinity, it will be converted to [-]inf
.Pq Cm a
or [-]INF
.Pq Cm A ,
respectively.
If the argument is not-a-number (NaN), it will be converted to
[-]nan
.Pq Cm a
or [-]NAN
.Pq Cm A ,
respectively.
.It Cm b
Characters from the string
.Ar argument
are printed with backslash-escape sequences expanded.
In the
.Ar argument ,
ASCII characters can be octally encoded either as
.Cm \e0 Ns Ar num
or as
.Cm \e Ns Ar num
like in the
.Ar format
string.
If the
.Ar argument
contains the special escape sequence
.Cm \ec ,
this escape sequence is discarded together with
all remaining characters in this argument, all further arguments,
and all remaining characters in the
.Ar format
string.
.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 bytes
indicated by the precision specification is reached; however if the
precision is 0 or missing, all characters in the string are printed.
.It Cm \&%
Print a
.Ql \&% ;
no argument is used.
.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.
.Sh EXIT STATUS
.Ex -std printf
.Sh EXAMPLES
Convert a hexadecimal value to decimal and print it out:
.Pp
.Dl $ printf \&"%d\en\&" 0x20
.Pp
Print the decimal representation of the character 'a' (see
.Xr ascii 7 ) :
.Pp
.Dl $ printf \&"%d\en\&" \e'a
.Sh SEE ALSO
.Xr echo 1 ,
.Xr printf 3
.Sh STANDARDS
The
.Nm
utility is compliant with the
.St -p1003.1-2008
specification, but in order to produce predictable output
it deliberately ignores the
.Xr locale 1
and always operates as if
.Ev LC_ALL Ns =C
were set.
.Pp
The escape sequences
.Cm \ee
and
.Cm \e' ,
as well as omitting the leading digit
.Cm 0
from
.Cm \e0 Ns Ar num
octal escape sequences in
.Cm %b
arguments, are extensions to that specification.
.Sh HISTORY
The
.Nm
command appeared in
.Bx 4.3 Reno .
.Sh CAVEATS
It is important never to pass a string with user-supplied data as a
format without using
.Ql %s .
An attacker can put format specifiers in the string to mangle your stack,
leading to a possible security hole.
.Pp
Always be sure to use the proper secure idiom:
.Bd -literal -offset indent
printf "%s" "$STRING"
.Ed
.Sh BUGS
Since arguments are translated from ASCII to floating-point,
and then back again, floating-point precision may be lost.