head (1p) - Linux Man Pages
head: copy the first part of files
PROLOGThis 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.
head - copy the first part of files
The head utility shall copy its input files to the standard output, ending the output for each file at a designated point.
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.
The following operand shall be supported:
A pathname of an input file. If no file operands are specified,
the standard input shall be used.
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
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>
The following exit values shall be returned:
- Successful completion.
- An error occurred.
CONSEQUENCES OF ERRORS
To write the first ten lines of all files (except those with a leading period) in the directory:
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.
COPYRIGHTPortions 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 .