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

       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  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(1P)

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