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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 differ (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  Corpora-
       tion.
       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 invok-
       ing  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 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, Utility Syntax Guidelines.
       The following options shall be supported:
       -b  bits
              Specify the maximum number of bits to use in a code. For a  con-
              forming 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 speci-
       fied, 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 com-
       press:
       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).
       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
       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 mes-
       sages 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  mecha-
       nisms) 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 cod-
       ing or adaptive Huffman coding ( compact), and takes less time to  com-
       pute.
       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 original Standard can be  obtained  online
       at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group                  2003                         COMPRESS(1P)