Section: POSIX Programmer's Manual (1P)
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.
--- copy the first part of files
head [-n number] [file...]
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
option. The option-argument
shall be counted in units of lines.
utility shall conform to the Base Definitions volume of POSIX.1-2017,
, Utility Syntax Guidelines.
The following option shall be supported:
- -n number
lines of each input file shall be copied to standard output. The
application shall ensure that the
option-argument is a positive decimal integer.
When a file contains less than
lines, it shall be copied to standard output in its entirety. This
shall not be an error.
If no options are specified,
shall act as if
had been specified.
The following operand shall be supported:
A pathname of an input file. If no
operands are specified, the standard input shall be used.
The standard input shall be used if no
operands are specified, and shall be used if a
and the implementation treats the
as meaning standard input.
Otherwise, the standard input shall not be used.
See the INPUT FILES section.
Input files shall be text files, but the line length is not restricted
The following environment variables shall affect the execution of
Provide a default value for the internationalization variables that are
unset or null. (See the Base Definitions volume of POSIX.1-2017,
Section 8.2, Internationalization Variables
for the precedence of internationalization variables used to determine
the values of locale categories.)
If set to a non-empty string value, override the values of all the
other internationalization variables.
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).
Determine the locale that should be used to affect the format and
contents of diagnostic messages written to standard error.
Determine the location of message catalogs for the processing of
The standard output shall contain designated portions of the input
operands are specified,
shall precede the output for each with the header:
"\n==> %s <==\n", <pathname>
except that the first header written shall not include the initial
The standard error shall be used only for diagnostic messages.
The following exit values shall be returned:
An error occurred.
CONSEQUENCES OF ERRORS
The following sections are informative.
to process pathnames, it is recommended that LC_ALL, or at least
LC_CTYPE and LC_COLLATE, are set to POSIX or C in the environment,
since pathnames can contain byte sequences that do not form valid
characters in some locales, in which case the utility's behavior would
be undefined. In the POSIX locale each byte is a valid single-byte
character, and therefore this problem is avoided.
To write the first ten lines of all files (except those with a leading
period) in the directory:
head -- *
Although it is possible to simulate
10q for a single file, the standard developers decided that the
on historical BSD systems warranted its inclusion alongside
POSIX.1-2008 version of
follows the Utility Syntax Guidelines. The
option was added to this new interface so that
would be more logically related. Earlier versions of this standard
option. This form is no longer specified by POSIX.1-2008 but may
be present in some implementations.
There is no
option (as there is in
because it is not historical practice and because other utilities in
this volume of POSIX.1-2017 provide similar functionality.
The Base Definitions volume of POSIX.1-2017,
Chapter 8, Environment Variables,
Section 12.2, Utility Syntax Guidelines
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
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
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