compress(1)              USER COMMANDS                compress(1)



NAME
     compress, uncompress, zcat -  compress,  expand  or  display
     expanded files

SYNOPSIS
     compress [ -cfv ] [ -b bits ] [ filename...  ]
     uncompress [ -cv ] [ filename...  ]
     zcat [ filename...  ]

DESCRIPTION
     compress reduces the size of the named files using  adaptive
     Lempel-Ziv coding.  Whenever possible, each file is replaced
     by one with a .Z, extension.  The  ownership  modes,  access
     time  and modification time will stay the same.  If no files
     are specified, the standard input is compressed to the stan-
     dard  output.  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(1)], and takes less time to  compute.   The
     bits  parameter  specified  during  compression  is  encoded
     within the compressed file, along with  a  magic  number  to
     ensure   that  neither  decompression  of  random  data  nor
     recompression of compressed data  is  subsequently  allowed.
     Compressed  files  can  be  restored  to their original form
     using uncompress.  zcat produces uncompressed output on  the
     standard output, but leaves the compressed .Z file intact.

OPTIONS
     -c   Write to the standard output;  no  files  are  changed.
          The  nondestructive  behavior  of  zcat is identical to
          that of `uncompress -c'.

     -f   Force compression, even if the file does  not  actually
          shrink,  or  the  corresponding .Z file already exists.
          Except  when   running   in   the   background   (under
          /usr/bin/sh),  if  -f  is  not  given, prompt to verify
          whether an existing .Z file should be overwritten.

     -v   Verbose.  Display the  percentage  reduction  for  each
          file compressed.

     -b bits
          Set the upper limit  (in  bits)  for  common  substring
          codes.   bits  must  be  between  9  and  16 (16 is the
          default).  Lowering the number of bits will  result  in
          larger, less compressed files.

FILES
     /usr/bin/sh




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compress(1)              USER COMMANDS                compress(1)



SEE ALSO
     pack(1) A Technique for High Performance  Data  Compression,
     Terry  A.  Welch, IEEE Computer, vol. 17, no. 6 (June 1984),
     pp. 8-19.

DIAGNOSTICS
     Exit status is  normally  0.   If  the  last  file  was  not
     compressed because it became larger, the status is 2.  If an
     error occurs, exit status is 1.

     Usage: compress [-fvc] [-b maxbits] [filename ...]
               Invalid options  were  specified  on  the  command
               line.

     Missing maxbits
               Maxbits must follow -b.

     filename: not in compressed format
               The file specified  to  uncompress  has  not  been
               compressed.

     filename: compressed with xxbits, can only handle yybits
               filename was compressed by a  program  that  could
               deal with more bits than the compress code on this
               machine.  Recompress the file with smaller bits.

     filename: already has .Z suffix -- no change
               The file is  assumed  to  be  already  compressed.
               Rename the file and try again.

     filename: 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,  (such
               as 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 are achieved by compression.  The input



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compress(1)              USER COMMANDS                compress(1)



               remains uncompressed.

NOTES
     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  (64KB  or
     less).  compress should be more flexible about the existence
     of the .Z suffix.















































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