COMPRESS(1)              USER COMMANDS                COMPRESS(1)



NAME
     compress, uncompress, zcat - compress and expand data

SYNOPSIS
     compress [ -f ] [ -v ] [ -c ] [ -V ] [ -b _b_i_t_s ] [ _n_a_m_e ... ]
     uncompress [ -f ] [ -v ] [ -c ] [ -V ] [ _n_a_m_e ... ]
     zcat [ -V ] [ _n_a_m_e ... ]

DESCRIPTION
     _C_o_m_p_r_e_s_s 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  owner-
     ship  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 _u_n_c_o_m_p_r_e_s_s or _z_c_a_t.

     The -f option will force compression of _n_a_m_e. This is useful
     for  compressing  an  entire  directory, even if some of the
     files do not actually  shrink.   If  -f  is  not  given  and
     _c_o_m_p_r_e_s_s  is  run in the foreground, the user is prompted as
     to whether an existing file should be overwritten.

     The -c option makes _c_o_m_p_r_e_s_s/_u_n_c_o_m_p_r_e_s_s write to  the  stan-
     dard  output;  no  files  are  changed.   The nondestructive
     behavior of _z_c_a_t is identical to that of _u_n_c_o_m_p_r_e_s_s -c.

     _C_o_m_p_r_e_s_s uses the modified Lempel-Ziv algorithm  popularized
     in  "A  Technique  for  High  Performance Data Compression",
     Terry A. Welch, _I_E_E_E _C_o_m_p_u_t_e_r, 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).   _B_i_t_s must be between 9 and 16.  The default
     can be changed in the source to allow _c_o_m_p_r_e_s_s to be run  on
     a smaller machine.

     After the _b_i_t_s  limit  is  attained,  _c_o_m_p_r_e_s_s  periodically
     checks the compression ratio.  If it is increasing, _c_o_m_p_r_e_s_s
     continues to use the existing code dictionary.  However,  if
     the compression ratio decreases, _c_o_m_p_r_e_s_s 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 _u_n_c_o_m_p_r_e_s_s,  since  the
     _b_i_t_s  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.





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     The amount of compression obtained depends on  the  size  of
     the input, the number of _b_i_t_s 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
     _p_a_c_k),  or adaptive Huffman coding (_c_o_m_p_a_c_t), and takes less
     time to compute.

     Under the -v option, a message is printed yielding the  per-
     centage of reduction for each file compressed.

     If the -V option is specified, the current version and  com-
     pile options are printed on stderr.

     Exit status is normally 0; if the last file is larger  after
     (attempted)  compression,  the  status  is  2;  if  an error
     occurs, exit status is 1.

SEE ALSO
     pack(1), compact(1)

DIAGNOSTICS
     Usage: compress [-dfvcV] [-b maxbits] [file ...]
             Invalid options were specified on the command line.
     Missing maxbits
             Maxbits must follow -b.
     _f_i_l_e: not in compressed format
             The  file  specified  to  _u_n_c_o_m_p_r_e_s_s  has  not  been
             compressed.
     _f_i_l_e: compressed with _x_x bits, can only handle _y_y bits
             _F_i_l_e was compressed by a  program  that  could  deal
             with  more  _b_i_t_s  than  the  compress  code  on this
             machine.  Recompress the file with smaller _b_i_t_s.
     _f_i_l_e: already has .Z suffix -- no change
             The  file  is  assumed  to  be  already  compressed.
             Rename the file and try again.
     _f_i_l_e: 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.
     _f_i_l_e 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 has been corrupted.
     Compression: _x_x._x_x%
             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.



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     -- has _x_x other links: unchanged
             The input file has links; it is left unchanged.  See
             _l_n(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 (64KB or less,
     as exhibited by the DEC PDP series, the Intel 80286, etc.)











































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