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printf - formatted output


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printf format [arguments ...]


The printf utility 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 either c, b 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 the -ansiC, with extensions. The characters and their meanings are as follows:

\a Write a <bell> character.
\b Write a <backspace> character.
\c Ignore remaining characters in this string.
\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
\0 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 specification includes, in the following order:

"Zero or more of the following flags:"
# A ‘#’ character specifying that the value should be printed in an ‘‘alternate form’’. For c, d, and s, formats, this option has no effect. For the o formats 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 e, E, 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 ‘+’ character specifying that there should always be a sign placed before the number when using signed formats.
' '
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;
0 A zero ‘0’ character indicating that zero-padding should be used rather than blank-padding. 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);
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 characters to be printed from a string; if the digit string is missing, the precision is treated as zero;
A character which indicates the type of format to use (one of 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 (fFeEgGaA) may be prefixed by an L to request that additional precision be used, if available.

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:

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. The values If] and Na] are printed as ‘inf’ and ‘nan’, respectively.
eE The argument is printed in the style e
.Sm off '[- d.ddd ± 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 ‘inf’ and ‘nan’, respectively.
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 printed in style
.Sm off '[- h.hhh ± p 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 ‘inf’ and ‘nan’, respectively.
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 characters indicated by the precision specification is reached; however if the precision is 0 or missing, all characters in the string are printed.
b As for s, but interpret character escapes in backslash notation in the string argument.
% Print a ‘%’; no argument is used.

The decimal point character is defined in the program’s locale (category LC_NUMERIC).

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.


.Ex -std


The traditional BSD behavior of converting arguments of numeric formats not beginning with a digit to the ASCII code of the first character is not supported.


echo(1), printf(3)





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