HEAD(P) POSIX Programmer's Manual HEAD(P)NAMEhead - copy the first part of files
SYNOPSIShead [-n number][file...]
DESCRIPTION
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.
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 speci‐
fied.
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 operands are speci‐
fied. 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 internationalization variables
that are unset or null. (See the Base Definitions volume of
IEEE Std 1003.1-2001, Section 8.2, Internationalization Vari‐
ables 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 .
ASYNCHRONOUS EVENTS
Default.
STDOUT
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>.
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 version. Applica‐
tions 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 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 histori‐
cal 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 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 .
IEEE/The Open Group 2003 HEAD(P)
