HEAD(1) General Commands Manual HEAD(1)

head — copy the first part of files

head [-n number][file ...]

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.

The head utility shall conform to the Base Definitions volume of IEEE Std 1003.1-2001 (“POSIX.1”), xbd_chap12(7).

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 specified.

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.

The standard input shall be used only if no file operands are specified. See the INPUT FILES section.

Input files shall be text files, but the line length is not restricted to {LINE_MAX} bytes.

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 (“POSIX.1”), 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.
[ XSI] Determine the location of message catalogs for the processing of LC_MESSAGES .

Default.

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>.

The standard error shall be used only for diagnostic messages.

None.

None.

The following exit values shall be returned:

Successful completion.
An error occurred.

Default.

The obsolescent - number form is withdrawn in this version. Applications should use the -n number option.

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 head with sed(1) 10q for a single file, the standard developers decided that the popularity of head on historical BSD systems warranted its inclusion alongside tail(1).

This standard version of head follows the Utility Syntax Guidelines. The -n option was added to this new interface so that head and tail(1) would be more logically related.

There is no -c option (as there is in tail(1)) because it is not historical practice and because other utilities in this volume of IEEE Std 1003.1-2001 (“POSIX.1”) provide similar functionality.

None.

sed(1), tail(1)

First released in Issue 4.

The obsolescent -number form is withdrawn.

The normative text is reworded to avoid use of the term "must" for application requirements.

The DESCRIPTION is updated to clarify that when a file contains less than the number of lines requested, the entire file is copied to standard output.

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December 6, 2001 posix.fail