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PROGRAM:
NAME
head — copy the first part of files
SYNOPSIS
head [−n number] [file...]
DESCRIPTION
The head utility shall copy its input files to the standard output, ending the output for
each file at a designated point.
Copying shall end at the point in each input file indicated by the −n number option. The
option-argument number shall be counted in units of lines.
OPTIONS
The head utility shall conform to the Base Definitions volume of POSIX.1‐2008, Section
12.2, Utility Syntax Guidelines.
The following option shall be supported:
−n number The first number lines of each input file shall be copied to standard output.
The application shall ensure that the number option-argument is a positive
decimal integer.
When a file contains less than number lines, it shall be copied to standard output in its
entirety. This shall not be an error.
If no options are specified, head shall act as if −n 10 had been specified.
OPERANDS
The following operand shall be supported:
file A pathname of an input file. If no file operands are specified, the standard
input shall be used.
STDIN
The standard input shall be used if no file operands are specified, and shall be used if a
file operand is '−' and the implementation treats the '−' as meaning standard input.
Otherwise, the standard input shall not be used. See the INPUT FILES section.
INPUT FILES
Input files shall be text files, but the line length is not restricted to {LINE_MAX}
bytes.
ENVIRONMENT VARIABLES
The following environment variables shall affect the execution of head:
LANG Provide a default value for the internationalization variables that are unset or
null. (See the Base Definitions volume of POSIX.1‐2008, Section 8.2,
Internationalization Variables for the precedence of internationalization
variables used to determine the values of locale categories.)
LC_ALL If set to a non-empty string value, override the values of all the other
internationalization variables.
LC_CTYPE Determine the locale for the interpretation of sequences of bytes of text data
as characters (for example, single-byte as opposed to multi-byte characters in
arguments and input files).
LC_MESSAGES
Determine the locale that should be used to affect the format and contents of
diagnostic messages written to standard error.
NLSPATH Determine the location of message catalogs for the processing of LC_MESSAGES.
ASYNCHRONOUS EVENTS
Default.
STDOUT
The standard output shall contain designated portions of the input files.
If multiple file operands are specified, head shall precede the output for each with the
header:
"\n==> %s <==\n", <pathname>
except that the first header written shall not include the initial <newline>.
STDERR
The standard error shall be used only for diagnostic messages.
OUTPUT FILES
None.
EXTENDED DESCRIPTION
None.
EXIT STATUS
The following exit values shall be returned:
0 Successful completion.
>0 An error occurred.
CONSEQUENCES OF ERRORS
Default.
The following sections are informative.
APPLICATION USAGE
None.
EXAMPLES
To write the first ten lines of all files (except those with a leading period) in the
directory:
head −− *
RATIONALE
Although it is possible to simulate head with sed 10q for a single file, the standard
developers decided that the popularity of head on historical BSD systems warranted its
inclusion alongside tail.
POSIX.1‐2008 version of head follows the Utility Syntax Guidelines. The −n option was
added to this new interface so that head and tail would be more logically related. Earlier
versions of this standard allowed a −number option. This form is no longer specified by
POSIX.1‐2008 but may be present in some implementations.
There is no −c option (as there is in tail) because it is not historical practice and
because other utilities in this volume of POSIX.1‐2008 provide similar functionality.
FUTURE DIRECTIONS
None.
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