HEAD(1P) POSIX Programmer's Manual HEAD(1P)PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the corresponding
Linux manual page for details of Linux behavior), or the interface may
not be implemented on Linux.
NAMEhead — copy the first part of files
SYNOPSIShead [−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 speci‐
fied.
OPERANDS
The following operand shall be supported:
file A pathname of an input file. If no file operands are speci‐
fied, 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 vari‐
ables that are unset or null. (See the Base Definitions vol‐
ume of POSIX.1‐2008, Section 8.2, Internationalization Vari‐
ables 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 histori‐
cal practice and because other utilities in this volume of POSIX.1‐2008
provide similar functionality.
FUTURE DIRECTIONS
None.
SEE ALSO
sed, tail
The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment
Variables, Section 12.2, Utility Syntax Guidelines
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the source
files to man page format. To report such errors, see https://www.ker‐
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 HEAD(1P)
