PRINTF(1)

NAME
printf - formatted output

SYNOPSIS
printf [-L] format [argument ...]

DESCRIPTION
printf formats and prints its arguments, 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. Each argument is used only once.

If the first character of format is a dash (‘-’), format must (and always may) be preceded by a word consisting of two dashes (‘- -’) to prevent it from being interpreted as an option string. See getopt(3).

The -L option causes all floating point values resulting from format conversions to be printed using long double formats, rather than the default double.

The arguments after the first are treated as strings if the corresponding format is either b, B, c, or s; otherwise each 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, as part of the argument rather than just quoting it for the shell, the value is the code of the character following that quote character. No further characters are permitted.

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 <double 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.

\xXX
    Write an 8-bit character whose ASCII value is the 1- or 2-digit hexadecimal number XX.

Each format specification is introduced by the percent character (‘%’). To produce a literal percent (‘%’) in the output, write the percent character twice: (‘%%’). This is not a format conversion, but a special escape sequence, and consumes no arguments. The remainder of the format specification includes, in the following order:

Zero or more of the following unordered flags:

    #   A ‘#’ character specifying that the value should be printed in an “alternative form”. For b, c, C, d, and s formats, this option has no effect. For the o format the precision of the number is increased to cause the first character of the numeric output string to be a zero. For the x (X) format, a non-zero result has the string ‘0x’ (‘0X’) prepended to it. For 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.

    -   A minus sign which specifies left adjustment of the output in the indicated field;

    +   A plus sign which specifies that there should always be a sign (‘+’ or ‘-’) placed before the number when using signed formats;

    ‘ ’ A <space> character which specifies that a space should be left in the position a minus sign would otherwise appear, before a non-negative number for a signed format. A ‘+’ overrides a <space> if both are used;

    0   A digit zero character which specifies that zero-padding should be used rather than space-padding to the left of right adjusted fields. In this case, a sign (‘+’, ‘-’ or ‘ ’), if included, precedes the padding. Padding to the right of left adjusted fields always uses spaces. A ‘-’ overrides a ‘0’ if both are used;

    Specifying a flag more than once is not an error, but has no additional effect.

Field Width:
    An optional digit string specifying a field width; if the output string has fewer bytes than the field width it will be 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 a zero appearing after another digit 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 (b, B, and s formats); 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 diouxXfFeEgGaAbBcCs).

A field width or precision may be ‘*’ instead of a digit string. In this case the next argument, preceding the value to be converted, supplies the field width or precision. It must be an unsigned integer constant. If both the field width and precision are ‘*’ then two arguments are required, with the field width preceding the precision, and the value to be converted following those.

The format characters and their meanings are:

diouXx
    The argument, which must represent an integer constant, with an optional leading plus or minus sign, is printed as a signed decimal (d or i), unsigned octal (o), unsigned decimal (u), or unsigned hexadecimal (X or x).

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 number is Infinity, or Not-a-Number, then ‘inf’ or ‘nan’ is printed for f format, and ‘INF’ or ‘NAN’ for F format.

eE  The argument is printed in the style [-]d.dddedd 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, and upper-case for Infinity and NaN as for F format.

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

aA  The argument is treated as a floating point number, for which the underlying hexadecimal representation is printed. See printf(3) for the details.

b   Characters from the string argument are printed with backslash-escape sequences expanded.

    The following additional backslash-escape sequences are supported:

    \c  Causes printf to ignore any remaining characters in the string operand containing it, any remaining operands, and any additional characters in the format operand.

    \0num
        Write an 8-bit character whose ASCII value is the 1-, 2-, or 3-digit octal number num.

    \^c Write the control character c. Generates characters ‘\000’ through ‘\037’, and ‘\177’ (from ‘\^?’).

    \M^c
        Write the control character c with the 8th bit set. Generates characters ‘\200’ through ‘\237’, and ‘\377’ (from ‘\M^?’).

    \M-c
        Write the character c with the 8th bit set. Generates characters ‘\240’ through ‘\376’.

B   Characters from the string argument are printed with unprintable characters backslash-escaped using the ‘\c’, ‘\^c’, ‘\M^c’, or ‘\M-c’ formats described above.

c   The first character of the next argument is printed.

C   The argument, which must represent an integer constant, with an optional leading plus or minus sign, is treated as a wide character code point, and 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; if the precision is omitted, all characters in the string are printed.

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.

If an argument required by a format string is present, but is not in a format expected by the format string conversion, a warning will be printed to standard error, identifying the argument by number (1 is the first after the format string) and usually also by content. In such a case a value will still be produced, and printed, and processing will continue, but printf will eventually exit with a failure status.

EXIT STATUS

SEE ALSO
echo(1), printf(3), vis(3), printf(9)

STANDARDS
The printf utility conforms to p1003.1-2001.

Support for the floating point formats and ‘*’ as a field width and precision are optional in POSIX.

The behaviour of the %B and %C formats and the \', \, \e, \xXX, and \[M][-|^]c escape sequences are undefined in POSIX.

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

Hexadecimal character constants given as escapes in strings are restricted to, and should be specified as, two hexadecimal characters. This is contrary to the ISO C standard but does guarantee detection of the end of the constant.

NOTES
All formats which treat the argument as a number first convert the 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 %c format to do likewise, but in fact it does not.

To convert a string representation of a decimal, octal, or hexadecimal number into the corresponding character, using a portable invocation, two nested printf 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:

    printf $(printf '\\%o' 0x0a)

In this implementation of printf it is possible to achieve the same result using one invocation:

    printf %C 0x0a