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Linux 2.6 - man page for compress (linux section 1posix)

COMPRESS(P)			    POSIX Programmer's Manual			      COMPRESS(P)

NAME
       compress - compress data

SYNOPSIS
       compress [-fv][-b bits][file ...]

       compress [-cfv][-b bits][file]

DESCRIPTION
       The compress utility shall attempt to reduce the size of the named files by using adaptive
       Lempel-Ziv coding algorithm.

       Note:  Lempel-Ziv is US Patent 4464650, issued to William Eastman, Abraham  Lempel,  Jacob
	      Ziv, Martin Cohn on August 7th, 1984, and assigned to Sperry Corporation.

       Lempel-Ziv-Welch  compression is covered by US Patent 4558302, issued to Terry A. Welch on
       December 10th, 1985, and assigned to Sperry Corporation.

       On systems not supporting adaptive Lempel-Ziv coding algorithm, the input files shall  not
       be  changed  and an error value greater than two shall be returned. Except when the output
       is to the standard output, each file shall be replaced by one with the  extension  .Z.  If
       the  invoking  process  has appropriate privileges, the ownership, modes, access time, and
       modification time of the original file are preserved. If appending the .Z to the  filename
       would make the name exceed {NAME_MAX} bytes, the command shall fail. If no files are spec-
       ified, the standard input shall be compressed to the standard output.

OPTIONS
       The compress utility shall conform to the Base Definitions volume of IEEE Std 1003.1-2001,
       Section 12.2, Utility Syntax Guidelines.

       The following options shall be supported:

       -b  bits
	      Specify  the maximum number of bits to use in a code. For a conforming application,
	      the bits argument shall be:

	      9 <= bits <= 14

       The implementation may allow bits values of greater than 14.  The default is  14,  15,  or
       16.

       -c     Cause  compress to write to the standard output; the input file is not changed, and
	      no .Z files are created.

       -f     Force compression of file, even if it does not actually  reduce  the  size  of  the
	      file,  or if the corresponding file .Z file already exists. If the -f option is not
	      given, and the process is not running in the background, the user is prompted as to
	      whether an existing file .Z file should be overwritten.

       -v     Write the percentage reduction of each file to standard error.

OPERANDS
       The following operand shall be supported:

       file   A pathname of a file to be compressed.

STDIN
       The  standard input shall be used only if no file operands are specified, or if a file op-
       erand is '-' .

INPUT FILES
       If file operands are specified, the input files contain the data to be compressed.

ENVIRONMENT VARIABLES
       The following environment variables shall affect the execution of compress:

       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, Inter-
	      nationalization 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 interna-
	      tionalization 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 argu-
	      ments).

       LC_MESSAGES
	      Determine the locale that should be used to affect the format and contents of diag-
	      nostic messages written to standard error.

       NLSPATH
	      Determine the location of message catalogs for the processing of LC_MESSAGES .

ASYNCHRONOUS EVENTS
       Default.

STDOUT
       If  no  file  operands are specified, or if a file operand is '-' , or if the -c option is
       specified, the standard output contains the compressed output.

STDERR
       The standard error shall be used only for diagnostic and prompt messages  and  the  output
       from -v.

OUTPUT FILES
       The  output  files  shall contain the compressed output. The format of compressed files is
       unspecified and interchange of such files between implementations  (including  access  via
       unspecified file sharing mechanisms) is not required by IEEE Std 1003.1-2001.

EXTENDED DESCRIPTION
       None.

EXIT STATUS
       The following exit values shall be returned:

	0     Successful completion.

	1     An error occurred.

	2     One  or  more  files  were not compressed because they would have increased in size
	      (and the -f option was not specified).

       >2     An error occurred.

CONSEQUENCES OF ERRORS
       The input file shall remain unmodified.

       The following sections are informative.

APPLICATION USAGE
       The amount of compression obtained depends on the size of the input, the  number  of  bits
       per  code,  and the distribution of common substrings. Typically, text such as source code
       or English is reduced by 50-60%. Compression is generally much better than  that  achieved
       by Huffman coding or adaptive Huffman coding ( compact), and takes less time to compute.

       Although compress strictly follows the default actions upon receipt of a signal or when an
       error occurs, some unexpected results may occur. In some implementations it is likely that
       a partially compressed file is left in place, alongside its uncompressed input file. Since
       the general operation of compress is to delete the uncompressed file  only  after  the  .Z
       file  has  been successfully filled, an application should always carefully check the exit
       status of compress before arbitrarily deleting files that have like-named  neighbors  with
       .Z suffixes.

       The  limit  of  14  on  the  bits option-argument is to achieve portability to all systems
       (within the restrictions imposed by the lack of an explicit published file  format).  Some
       implementations based on 16-bit architectures cannot support 15 or 16-bit uncompression.

EXAMPLES
       None.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       uncompress , zcat

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 orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003				      COMPRESS(P)


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