4.3BSD - /usr/man/cat1/compress.0

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 modi‐
       fication times.  If no files are specified, the standard input is  com‐
       pressed  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 actu‐
       ally  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 overwrit‐
       ten.

       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 Tech‐
       nique for High Performance Data Compression", Terry A. Welch, IEEE Com���
       puter, 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 com‐
       pression 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 parame‐
       ter 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 cod‐
       ing (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  vir‐
               gin.

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  (64KB or less, as exhibited by the DEC PDP
       series, the Intel 80286, etc.)

       compress should be more flexible about the existence of the  ‘.Z’  suf‐
       fix.


4.3 Berkeley Distribution        May 11, 1986                      COMPRESS(1)