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PROLOG | NAME | SYNOPSIS | DESCRIPTION | OPTIONS | OPERANDS | STDIN | INPUT FILES | ENVIRONMENT VARIABLES | ASYNCHRONOUS EVENTS | STDOUT | STDERR | OUTPUT FILES | EXTENDED DESCRIPTION | EXIT STATUS | CONSEQUENCES OF ERRORS | APPLICATION USAGE | EXAMPLES | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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HEAD(1P) POSIX Programmer's Manual HEAD(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.
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
POSIX.1‐2017, 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
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 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 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:
LANG 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.)
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.
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:
0 Successful completion.
>0 An error occurred.
Default.
The following sections are informative.
When using head 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 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
historical practice and because other utilities in this volume of
POSIX.1‐2017 provide similar functionality.
None.
sed(1p), tail(1p)
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 http://www.opengroup.org/unix/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.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 HEAD(1P)
Pages that refer to this page: tail(1p)
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HTML rendering created 2023-12-22 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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