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 -nnumber option. The option-argument number shall be counted in
units of lines.
The head utility shall conform to the Base Definitions volume of
IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines.
The following option shall be supported:
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
The following environment variables shall affect the execution of head:
Provide a default value for the internationalization
variables that are unset or null. (See the Base Definitions volume of
IEEE Std 1003.1-2001, 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 LC_MESSAGES.
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.
This standard 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.
There is no -c option (as there is in tail) because it is not
historical practice and because other utilities in this volume of
IEEE Std 1003.1-2001 provide similar functionality.
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 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 http://www.opengroup.org/unix/online.html