formats(5) Standards, Environments, and Macros formats(5)
formats - file format notation
Utility descriptions use a syntax to describe the data organization within files--stdin, stdout, stderr, input files, and output files--
when that organization is not otherwise obvious. The syntax is similar to that used by the printf(3C) function. When used for stdin or
input file descriptions, this syntax describes the format that could have been used to write the text to be read, not a format that could
be used by the scanf(3C) function to read the input file.
The description of an individual record is as follows:
"<format>", [<arg1>, <arg2>, ..., <argn>]
The format is a character string that contains three types of objects defined below:
characters Characters that are not escape sequences or conversion specifications, as described below, are copied to the out-
escape sequences Represent non-graphic characters.
conversion specificationSpecifies the output format of each argument. (See below.)
The following characters have the following special meaning in the format string:
`` '' (An empty character position.) One or more blank characters.
/ Exactly one space character.
The notation for spaces allows some flexibility for application output. Note that an empty character position in format represents one or
more blank characters on the output (not white space, which can include newline characters). Therefore, another utility that reads that
output as its input must be prepared to parse the data using scanf(3C), awk(1), and so forth. The character is used when exactly one space
character is output.
The following table lists escape sequences and associated actions on display devices capable of the action.
Sequence Character Terminal Action
\ backslash None.
a alert Attempts to alert the user
through audible or visible
backspace Moves the printing position to
one column before the current
position, unless the current
position is the start of a
f form-feed Moves the printing position to
the initial printing position
of the next logical page.
newline Moves the printing position to
the start of the next line.
carriage-return Moves the printing position to
the start of the current line.
tab Moves the printing position to
the next tab position on the
current line. If there are no
more tab positions left on the
line, the behavior is unde-
v vertical-tab Moves the printing position to
the start of the next vertical
tab position. If there are no
more vertical tab positions
left on the page, the behavior
Each conversion specification is introduced by the percent-sign character (%). After the character %, the following appear in sequence:
flags Zero or more flags, in any order, that modify the meaning of the conversion specification.
field width An optional string of decimal digits to specify a minimum field width. For an output field, if the converted value
has fewer bytes than the field width, it is padded on the left (or right, if the left-adjustment flag (-),
described below, has been given to the field width).
precision Gives the minimum number of digits to appear for the d, o, i, u, x or X conversions (the field is padded with lead-
ing zeros), the number of digits to appear after the radix character for the e and f conversions, the maximum num-
ber of significant digits for the g conversion; or the maximum number of bytes to be written from a string in s
conversion. The precision takes the form of a period (.) followed by a decimal digit string; a null digit string is
treated as zero.
conversion characters A conversion character (see below) that indicates the type of conversion to be applied.
The flags and their meanings are:
- The result of the conversion is left-justified within the field.
+ The result of a signed conversion always begins with a sign (+ or -).
<space> If the first character of a signed conversion is not a sign, a space character is prefixed to the result. This means that
if the space character and + flags both appear, the space character flag is ignored.
# The value is to be converted to an alternative form. For c, d, i, u, and s conversions, the behaviour is undefined. For o
conversion, it increases the precision to force the first digit of the result to be a zero. For x or X conversion, a non-
zero result has 0x or 0X prefixed to it, respectively. For e, E, f, g, and G conversions, the result always contains a
radix character, even if no digits follow the radix character. For g and G conversions, trailing zeros are not removed from
the result as they usually are.
0 For d, i, o, u, x, X, e, E, f, g, and G conversions, leading zeros (following any indication of sign or base) are used to
pad to the field width; no space padding is performed. If the 0 and - flags both appear, the 0 flag is ignored. For d, i,
o, u, x and X conversions, if a precision is specified, the 0 flag is ignored. For other conversions, the behaviour is
Each conversion character results in fetching zero or more arguments. The results are undefined if there are insufficient arguments for the
format. If the format is exhausted while arguments remain, the excess arguments are ignored.
The conversion characters and their meanings are:
d,i,o,u,x,X The integer argument is written as signed decimal (d or i), unsigned octal (o), unsigned decimal (u), or unsigned hexadeci-
mal notation (x and X). The d and i specifiers convert to signed decimal in the style [-]dddd. The x conversion uses the
numbers and letters 0123456789abcdef and the X conversion uses the numbers and letters 0123456789ABCDEF. The precision com-
ponent of the argument specifies the minimum number of digits to appear. If the value being converted can be represented in
fewer digits than the specified minimum, it is expanded with leading zeros. The default precision is 1. The result of con-
verting a zero value with a precision of 0 is no characters. If both the field width and precision are omitted, the imple-
mentation may precede, follow or precede and follow numeric arguments of types d, i and u with blank characters; arguments
of type o (octal) may be preceded with leading zeros.
The treatment of integers and spaces is different from the printf(3C) function in that they can be surrounded with blank
characters. This was done so that, given a format such as:
the implementation could use a printf() call such as:
and still conform. This notation is thus somewhat like scanf() in addition to printf().
f The floating point number argument is written in decimal notation in the style [-]ddd.ddd, where the number of digits after
the radix character (shown here as a decimal point) is equal to the precision specification. The LC_NUMERIC locale category
determines the radix character to use in this format. If the precision is omitted from the argument, six digits are written
after the radix character; if the precision is explicitly 0, no radix character appears.
e,E The floating point number argument is written in the style [-]d.ddde+-dd (the symbol +- indicates either a plus or minus
sign), where there is one digit before the radix character (shown here as a decimal point) and the number of digits after
it is equal to the precision. The LC_NUMERIC locale category determines the radix character to use in this format. When
the precision is missing, six digits are written after the radix character; if the precision is 0, no radix character
appears. The E conversion character produces a number with E instead of e introducing the exponent. The exponent always
contains at least two digits. However, if the value to be written requires an exponent greater than two digits, additional
exponent digits are written as necessary.
g,G The floating point number argument is written in style f or e (or in style E in the case of a G conversion character), with
the precision specifying the number of significant digits. The style used depends on the value converted: style g is used
only if the exponent resulting from the conversion is less than -4 or greater than or equal to the precision. Trailing
zeros are removed from the result. A radix character appears only if it is followed by a digit.
c The integer argument is converted to an unsigned char and the resulting byte is written.
s The argument is taken to be a string and bytes from the string are written until the end of the string or the number of
bytes indicated by the precision specification of the argument is reached. If the precision is omitted from the argument,
it is taken to be infinite, so all bytes up to the end of the string are written.
% Write a % character; no argument is converted.
In no case does a non-existent or insufficient field width cause truncation of a field; if the result of a conversion is wider than the
field width, the field is simply expanded to contain the conversion result. The term field width should not be confused with the term pre-
cision used in the description of %s.
One difference from the C function printf() is that the l and h conversion characters are not used. There is no differentiation between
decimal values for type int, type long, or type short. The specifications %d or %i should be interpreted as an arbitrary length sequence
of digits. Also, no distinction is made between single precision and double precision numbers (float or double in C). These are simply
referred to as floating point numbers.
Many of the output descriptions use the term line, such as:
"%s", <input line>
Since the definition of line includes the trailing newline character already, there is no need to include a
in the format; a double new-
line character would otherwise result.
Example 1: To represent the output of a program that prints a date and time in the form Sunday, July 3, 10:02, where <weekday> and <month>
Example 2: To show pi written to 5 decimal places:
",<value of pi>
Example 3: To show an input file format consisting of five colon-separated fields:
awk(1), printf(1), printf(3C), scanf(3C)
SunOS 5.10 28 Mar 1995 formats(5)