compress man page on DigitalUNIX

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compress(1)							   compress(1)

       compress, uncompress, zcat - Compresses and expands data

       compress [-cCdfFnqvV] [-b bits] [file...]

       uncompress [-cCfFnqvV] [file...]

       zcat [-n] [file...]

       Interfaces  documented on this reference page conform to industry stan‐
       dards as follows:

       compress:  XCU5.0

       uncompress:  XCU5.0

       zcat:  XCU5.0

       Refer to the standards(5) reference page	 for  more  information	 about
       industry standards and associated tags.

       Specifies  the  maximum	number	of  bits to use to replace common sub‐
       strings in the file.  The default for bits is  16,  with	 values	 of  9
       through	16  acceptable.	  First,  the  algorithm  uses 9-bit codes 257
       through 512.  Then it uses 10-bit  codes,  continuing  until  the  bits
       limit is reached.  (This option applies to the compress command only.)

	      After  the bits limit is attained, the compress command periodi‐
	      cally checks the compression ratio.  If it is  increasing,  com‐
	      press  continues	to use the existing code dictionary.  However,
	      if the compression ratio decreases, compress discards the	 table
	      of substrings and rebuilds it from the beginning.	 This lets the
	      algorithm adapt to the next block of the file.

	      [Tru64 UNIX]  The -b option must be the last option on the  com‐
	      mand  line.  Makes compress and uncompress write to the standard
	      output; no files are changed.  The  nondestructive  behavior  of
	      zcat  is identical to that of uncompress -c.  [Tru64 UNIX]  Pro‐
	      duces output compatible with compress 2.0.  [Tru64 UNIX]	Uncom‐
	      presses a file.  When used with the compress command, forces the
	      compression of file even if no reduction in the size of the file
	      will occur.

	      With  both  the compress and uncompress commands, if you run the
	      command in the background and  -f	 is  not  specified,  you  are
	      prompted	as  to whether an existing file should be overwritten.
	      [Tru64 UNIX]  Specifies that no header has been  added.	[Tru64
	      UNIX]  Specifies	a quiet operation. This is the default.	 Diag‐
	      nostics messages, which display if you specify the -v option, do
	      not print. If the -q and -v options are both specified, the last
	      one on the command  line	is  honored.   Prints  the  percentage
	      reduction	 of  each file when compressing the file.  Prints mes‐
	      sages to standard error concerning the expansion	of  each  file
	      when  uncompressing  the file. If the -q and -v options are both
	      specified, the last one on the command line is honored.	[Tru64
	      UNIX]  Specifies a version.

       The  path name of a file to be processed.  If - (hyphen) is used, stan‐
       dard input will be read.

	      The compress command will create an output file name by  append‐
	      ing a suffix of to the input file name.

	      With  the	 uncompress  and zcat commands, if the file name has a
	      suffix, the command will look for a file of that name.   If  the
	      suffix  is  not  specified,  the	command will append the suffix
	      before searching for the file.

	      The uncompress command will store output in a  file  named  with
	      the  suffix removed if it was specified.	If no suffix is speci‐
	      fied, the uncompress command will accept file as the name of the
	      output file and append a suffix before searching for the file.

       The compress command reduces the size of the named files using adaptive
       Lempel-Ziv coding.

       Whenever possible, each file is replaced	 by  one  with	the  extension
       while keeping the same ownership modes, access, and modification times.
       If no files are specified, standard input is compressed to the standard

       Compressed  files  can  be restored to their original form by using the
       uncompress or zcat command.

       The uncompress command replaces the  compressed	file  with  an	uncom‐
       pressed	version of the file, identical to the file that was originally
       compressed with compress; the  suffix  is  removed.   When  issuing  an
       uncompress command, you can refer to the compressed target file with or
       without the suffix. If  you  do	not  specify  the  suffix,  uncompress
       assumes it.

       The  zcat  command writes the uncompressed version of a compressed file
       to standard output.  The compressed (.Z) files remain intact. The  zcat
       command	is  identical  to uncompress -c.  When issuing a zcat command,
       you can refer to the compressed target file with or without the suffix;
       if you do not specify the suffix, zcat assumes it.

       [Tru64  UNIX]  The  compress command uses the modified Lempel-Ziv algo‐
       rithm popularized in “A Technique for High  Performance	Data  Compres‐
       sion,”  Terry  A. Welch, IEEE Computer, vol. 17, no. 6 (June 1984), pp.

       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, files containing source code or plain text are reduced by 50
       to 60 percent.  Compression is generally much better than that achieved
       by Huffman coding (as used in the pack  command)	 or  adaptive  Huffman
       coding, and takes less time to compute.

       [Tru64  UNIX]  The  compress command may not preserve the extended file
       attributes (property list) of a file, including the access control list
       (ACL)  if  any.	Verify that any ACLs have not been removed or modified
       after using compress or uncompress.

       The following exit values are returned:

   compress command
       Successful completion.  An error occurred.  One or more files were  not
       compressed because they became larger.  An error occurred.

   uncompress and zcat commands
       Successful completion.  An error occurred.

       Invalid	arguments  were	 specified  on the command line.  maxbits must
       follow -b.  The file cannot be uncompressed because it was  never  com‐
       pressed.	  The  file  was  compressed by a program that could deal with
       more bits than the compress code on this machine. Recompress  the  file
       with  smaller  bits.  file is assumed to be already compressed.	Rename
       the file and try again.	Respond y, or the locale's equivalent of a  y,
       if  you want the output file to be replaced; n, or the locale's equiva‐
       lent of a n, if not.  (The LC_MESSAGES variable determines the locale's
       equivalent of y or n.)  A SIGSEGV violation was detected, which usually
       means that the input file is corrupted.	Percentage of the input	 saved
       by  compression	(relevant  only for -v).  When the input file is not a
       regular file, (for example, a directory), it is	left  unaltered.   The
       input  file  has	 links; it is left unchanged.  (See the ln command for
       more information.)  No saving is achieved by  compression.   The	 input
       remains unchanged.

       To compress folder and print the savings, enter: compress -v folder

	      The  system  responds  with a message like: folder: Compression:
	      43.94% -- replaced with folder.Z The following command  displays
	      the uncompressed version of the testlog.Z file: zcat testlog.Z

       The  following  environment variables affect the execution of compress,
       uncompress, and zcat: Provides a default value for  the	international‐
       ization variables that are unset or null. If LANG is unset or null, the
       corresponding value from the default locale is used.   If  any  of  the
       internationalization  variables contain an invalid setting, the utility
       behaves as if none of the variables had been defined.  If set to a non-
       empty  string value, overrides the values of all the other internation‐
       alization variables.  Determines the locale for the  interpretation  of
       sequences of bytes of text data as characters (for example, single-byte
       as opposed to  multibyte	 characters  in	 arguments).   Determines  the
       locale  for  the	 format and contents of diagnostic messages written to
       standard error.	Determines the location of message catalogues for  the
       processing of LC_MESSAGES.

       Commands:  pack(1), pcat(1), unpack(1)

       Files:  acl(4)

       Standards:  standards(5)


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