Originální popis anglicky:
head - copy the first part of files
Návod, kniha: POSIX Programmer's Manual
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, 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 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:
- 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
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.
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:
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
historical practice and because other utilities in this volume of
IEEE Std 1003.1-2001 provide similar functionality.
None.
sed ,
tail
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
.
