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COMPRESS(1)							   COMPRESS(1)

NAME
       compress, uncompress, zcat - compress and expand data

SYNOPSIS
       compress [ -f ] [ -v ] [ -c ] [ -b bits ] [ name ...  ]
       uncompress [ -f ] [ -v ] [ -c ] [ name ...  ]
       zcat [ name ...	]

DESCRIPTION
       Compress	 reduces the size of the named files using adaptive Lempel-Ziv
       coding.	Whenever possible, each file  is  replaced  by	one  with  the
       extension  .Z,  while  keeping  the  same  ownership  modes, access and
       modification times.  If no files are specified, the standard  input  is
       compressed to the standard output.  Compressed files can be restored to
       their original form using uncompress or zcat.

       The -f option will force compression of	name,  even  if	 it  does  not
       actually	 shrink	 or  the  corresponding	 name.Z	 file  already exists.
       Except when run in the background under /bin/sh, if -f is not given the
       user  is	 prompted  as  to  whether  an	existing name.Z file should be
       overwritten.

       The -c (``cat'') option makes compress/uncompress write to the standard
       output;	no  files are changed.	The nondestructive behavior of zcat is
       identical to that of uncompress -c.

       Compress uses the  modified  Lempel-Ziv	algorithm  popularized	in  "A
       Technique  for High Performance Data Compression", Terry A. Welch, IEEE
       Computer, vol. 17, no. 6 (June 1984), pp. 8-19.	Common	substrings  in
       the  file  are first replaced by 9-bit codes 257 and up.	 When code 512
       is reached, the algorithm switches to 10-bit codes and continues to use
       more  bits until the limit specified by the -b flag is reached (default
       16).  Bits must be between 9 and 16.  The default can be changed in the
       source to allow compress to be run on a smaller machine.

       After  the  bits	 limit	is  attained, compress periodically checks the
       compression ratio.  If it is increasing, compress continues to use  the
       existing code dictionary.  However, if the compression ratio decreases,
       compress discards the table of substrings and rebuilds it from scratch.
       This allows the algorithm to adapt to the next "block" of the file.

       Note  that  the	-b  flag  is  omitted  for  uncompress, since the bits
       parameter specified during compression is encoded  within  the  output,
       along  with  a  magic  number  to  ensure that neither decompression of
       random data nor recompression of compressed data is attempted.

       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 (as used in pack), or adaptive  Huffman  coding  (compact),  and
       takes less time to compute.

       The  -v	option causes the printing of the percentage reduction of each
       file.

       If an error occurs, exit status is 1, else if the  last	file  was  not
       compressed  because  it became larger, the status is 2; else the status
       is 0.

DIAGNOSTICS
       Usage: compress [-fvc] [-b maxbits] [file ...]
	       Invalid options were specified on the command line.
       Missing maxbits
	       Maxbits must follow -b.
       file: not in compressed format
	       The file specified to uncompress has not been compressed.
       file: compressed with xx bits, can only handle yy bits
	       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: already has .Z suffix -- no change
	       The file is assumed to be already compressed.  Rename the  file
	       and try again.
       file: filename too long to tack on .Z
	       The  file  cannot be compressed because its name is longer than
	       12 characters.  Rename and try again.  This  message  does  not
	       occur on BSD systems.
       file already exists; do you wish to overwrite (y or n)?
	       Respond	"y" if you want the output file to be replaced; "n" if
	       not.
       uncompress: corrupt input
	       A SIGSEGV violation was detected which usually means  that  the
	       input file is corrupted.
       Compression: xx.xx%
	       Percentage  of  the input saved by compression.	(Relevant only
	       for -v.)
       -- not a regular file: unchanged
	       When the input file is not a regular file, (e.g. a  directory),
	       it is left unaltered.
       -- has xx other links: unchanged
	       The  input file has links; it is left unchanged.	 See ln(1) for
	       more information.
       -- file unchanged
	       No savings is  achieved	by  compression.   The	input  remains
	       virgin.

BUGS
       Although	 compressed  files  are compatible between machines with large
       memory, -b12 should be used for file transfer to architectures  with  a
       small process data space of 64KB or less.

       compress	 should	 be  more  flexible  about  the	 existence of the `.Z'
       suffix.

4.3 Berkeley Distribution	 May 11, 1986			   COMPRESS(1)
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