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
       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 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
       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 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 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
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .