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COMPRESS(1P)               POSIX Programmer's Manual              COMPRESS(1P)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may dif-
       fer (consult the corresponding Linux manual page for details of Linux behavior), or the interface  may  not  be
       implemented on Linux.

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  owner-
       ship,  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  specified,  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, Util-
       ity 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 cre-
              ated.

       -f     Force compression of file, even if it does not actually reduce the size of the file, or  if  the  corre-
              sponding  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 operand 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, Internationalization Variables for the prece-
              dence 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 exam-
              ple, single-byte as opposed to multi-byte characters in arguments).

       LC_MESSAGES
              Determine the locale that should be used to affect the format and contents of diagnostic messages  writ-
              ten 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  inter-
       change  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 dis-
       tribution 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 ( com-
       pact), 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  uncom-
       pressed  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, Stan-
       dard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base  Specifica-
       tions  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 Stan-
       dard,  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                         COMPRESS(1P)