PRINTF(1)

NAME
printf - formatted output

SYNOPSIS
printf format [argument ...]

DESCRIPTION
printf formats and prints its arguments, after the first, under control of the format. The 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 argument.

The arguments after the first are treated as strings if the corresponding format is b, c or s; otherwise it is evaluated as a C constant, with the following extensions:

•   A leading plus or minus sign is allowed.

•   If the leading character is a single or double quote, the value is the ASCII code of the next character.

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.

Character escape sequences are in backslash notation as defined in ansiC. The characters and their meanings are as follows:

\a  Write a <bell> character.
\b  Write a <backspace> character.
\e  Write an <escape> character.
\f  Write a <form-feed> character.
\n  Write a <new-line> character.
\r  Write a <carriage return> character.
\t  Write a <tab> character.
\v  Write a <vertical tab> character.
\'  Write a <single quote> character.
\\  Write a backslash character.
\num
    Write an 8-bit character whose ASCII value is the 1-, 2-, or 3-digit octal number num.

Each format specification is introduced by the percent (‘%’) character. The remainder of the format specifiers include, in the following order:

Zero or more of the following flags:

    #   Specifies that the value should be printed in an “alternate form”. For the o format the precision of the number is increased to force the first character of the output string to a zero. For the x (X) format, a non-zero result has the string 0x (0X) prepended to it. For a, A, e, E, f, F, g, and 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 g and G formats, trailing zeros are not removed from the result as they would otherwise be. For all other formats, behaviour is undefined.

    -   Specifies the left adjustment of the output in the indicated field.

    +   Specifies that there should always be a sign placed before the number when using signed formats.

    ‘ ’ A space specifies that a blank should be left before a positive number for a signed format. A ‘+’ overrides a space if both are used.

    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 d, i, o, u, or x (X) formats. A ‘-’ overrides a ‘0’ if both are used.

Field Width:
    An optional digit string specifying a 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).

Precision:
    An optional period (‘’)., followed by an optional digit string giving a precision which specifies the number of digits to appear after the decimal point, for e and 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.

Format:
    A character which indicates the type of format to use (one of diouxXfFeEgGaAbcs).

A field width or precision may be ‘*’ instead of a digit string. In this case an argument supplies the field width or precision.

The format characters and their meanings are:

diouXx
    The argument is printed as a signed decimal (d or i), unsigned octal, unsigned decimal, or unsigned hexadecimal (x or X), respectively.

fF  The 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.

    If the argument is infinity, it will be converted to [-]inf (f) or [-]INF (F), respectively. If the argument is not-a-number (NaN), it will be converted to [-]nan (f) or [-]NAN (F), respectively.

eE  The argument is printed in the style [-]d.ddde±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 ‘E’ is used for an E format.

    If the argument is infinity, it will be converted to [-]inf (e) or [-]INF (E), respectively. If the argument is not-a-number (NaN), it will be converted to [-]nan (e) or [-]NAN (E), respectively.

gG  The argument is printed in style f or in style e (E) whichever gives full precision in minimum space.

    If the argument is infinity, it will be converted to [-]inf (g) or [-]INF (G), respectively. If the argument is not-a-number (NaN), it will be converted to [-]nan (g) or [-]NAN (G), respectively.

aA  The argument is printed in style [-]0xh.hhhp[±]d 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.

    If the argument is infinity, it will be converted to [-]inf (a) or [-]INF (A), respectively. If the argument is not-a-number (NaN), it will be converted to [-]nan (a) or [-]NAN (A), respectively.

b   Characters from the string argument are printed with backslash-escape sequences expanded. In the argument, ASCII characters can be octally encoded either as \0num or as \num like in the format string. If the argument contains the special escape sequence \c, this escape sequence is discarded together with all remaining characters in this argument, all further arguments, and all remaining characters in the format string.

c   The first character of argument is printed.

s   Characters from the string 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.

%   Print a ‘%’; no argument is used.

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.

EXIT STATUS

EXAMPLES
Convert a hexadecimal value to decimal and print it out:

    $ printf "%d\n" 0x20

Print the decimal representation of the character 'a' (see ascii(7)):

    $ printf "%d\n" \'a

SEE ALSO
echo(1), printf(3)

STANDARDS
The printf utility is compliant with the p1003.1-2008 specification, but in order to produce predictable output it deliberately ignores the locale(1) and always operates as if LC_ALL=C were set.

The escape sequences \e and \', as well as omitting the leading digit 0 from \0num octal escape sequences in %b arguments, are extensions to that specification.

HISTORY
The printf command appeared in 4.3BSD Reno.

CAVEATS
It is important never to pass a string with user-supplied data as a format without using ‘%s’. An attacker can put format specifiers in the string to mangle your stack, leading to a possible security hole.

Always be sure to use the proper secure idiom:

    printf "%s" "$STRING"

BUGS
Since arguments are translated from ASCII to floating-point, and then back again, floating-point precision may be lost.