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head(P) head(P)
NAME
head - copy the first part of files
SYNOPSIS
head [-n number][file...]
DESCRIPTION
The head utility shall copy its input files to the stan-
dard output, ending the output for each file at a desig-
nated point.
Copying shall end at the point in each input file indi-
cated by the -n number option. The option-argument num-
ber shall be counted in units of lines.
OPTIONS
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:
-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.
OPERANDS
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.
STDIN
The standard input shall be used only if no file oper-
ands are specified. See the INPUT FILES section.
INPUT FILES
Input files shall be text files, but the line length is
not restricted to {LINE_MAX} bytes.
ENVIRONMENT VARIABLES
The following environment variables shall affect the
execution of head:
LANG Provide a default value for the internationaliza-
tion 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 cate-
gories.)
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 mes-
sages written to standard error.
NLSPATH
Determine the location of message catalogs for
the processing of LC_MESSAGES .
ASYNCHRONOUS EVENTS
Default.
STDOUT
The standard output shall contain designated portions of
the input files.
If multiple file operands are specified, head shall pre-
cede the output for each with the header:
"\n==> %s <==\n", <pathname>
except that the first header written shall not include
the initial <newline>.
STDERR
The standard error shall be used only for diagnostic
messages.
OUTPUT FILES
None.
EXTENDED DESCRIPTION
None.
EXIT STATUS
The following exit values shall be returned:
0 Successful completion.
>0 An error occurred.
CONSEQUENCES OF ERRORS
Default.
The following sections are informative.
APPLICATION USAGE
The obsolescent - number form is withdrawn in this ver-
sion. Applications should use the -n number option.
EXAMPLES
To write the first ten lines of all files (except those
with a leading period) in the directory:
head *
RATIONALE
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 war-
ranted its inclusion alongside tail.
This standard version of head follows the Utility Syntax
Guidelines. The -n option was added to this new inter-
face 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.
FUTURE DIRECTIONS
None.
SEE ALSO
sed , tail
COPYRIGHT
Portions of this text are reprinted and reproduced in
electronic form from IEEE Std 1003.1, 2003 Edition,
Standard for Information Technology -- Portable Operat-
ing System Interface (POSIX), The Open Group Base Speci-
fications Issue 6, Copyright (C) 2001-2003 by the Insti-
tute 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.open-
group.org/unix/online.html .
POSIX 2003 head(P)
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