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

       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 Corpora-

       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.

       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.

       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  speci-
       fied, or if a file operand is '-' .

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

       The following environment variables shall affect the execution of  com-

       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.

              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).

              Determine the locale that should be used to  affect  the  format
              and contents of diagnostic messages written to standard error.

              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

       The standard error shall be used only for diagnostic  and  prompt  mes-
       sages 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 mecha-
       nisms) is not required by IEEE Std 1003.1-2001.


       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 cod-
       ing  or adaptive Huffman coding ( compact), and takes less time to com-

       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

       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 .

IEEE/The Open Group                  2003                          COMPRESS(P)

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