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       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.


       compress — compress data


       compress [−fv] [−b bits] [file...]

       compress [−cfv] [−b bits] [file]


       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.


       The compress utility shall conform to the Base Definitions volume of POSIX.1‐2008, 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. If the
                 response is affirmative, the existing file will be overwritten.

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


       The following operand shall be supported:

       file      A pathname of a file to be compressed.


       The standard input shall be used only if no file operands are  specified,  or  if  a  file
       operand is '−'.


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


       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   POSIX.1‐2008,   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.

                 Determine the locale for the behavior of ranges, equivalence classes, and multi-
                 character collating elements used in the extended regular expression defined for
                 the yesexpr locale keyword in the LC_MESSAGES category.

       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),  the  behavior  of  character  classes  used in the extended regular
                 expression defined for the yesexpr locale keyword in the LC_MESSAGES category.

                 Determine the locale used to process affirmative responses, and the locale  used
                 to  affect  the  format  and  contents  of diagnostic messages, prompts, and the
                 output from the −v option written to standard error.

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




       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.


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


       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 POSIX.1‐2008.




       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.


       The input file shall remain unmodified.

       The following sections are informative.


       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.








       uncompress, zcat

       The Base Definitions volume of POSIX.1‐2008, Chapter  8,  Environment  Variables,  Section
       12.2, Utility Syntax Guidelines


       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1, 2013 Edition, Standard for Information Technology  --  Portable  Operating  System
       Interface  (POSIX),  The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc  and  The  Open  Group.   (This  is
       POSIX.1-2008  with  the  2013  Technical  Corrigendum  1  applied.)  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 .

       Any typographical or formatting errors that appear in this page are most  likely  to  have
       been  introduced  during  the conversion of the source files to man page format. To report
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