


COMPRESS(1)	    UNIX Programmer's Manual	      COMPRESS(1)



NAME
     compress, uncompress, zcat - compress and expand data

SYNOPSIS
     compress [ -f ] [ -v ] [ -c ] [ -b _b_i_t_s ] [ _n_a_m_e ... ]
     uncompress [ -f ] [ -v ] [ -c ] [ _n_a_m_e ... ]
     zcat [ _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, even if it
     does not actually shrink or the corresponding _n_a_m_e.Z file
     already exists.  Except when run in the background under
     /_b_i_n/_s_h, if -f is not given the user is prompted as to
     whether an existing _n_a_m_e.Z file should be overwritten.

     The -c (``cat'') option makes _c_o_m_p_r_e_s_s/_u_n_c_o_m_p_r_e_s_s write to
     the standard output; no files are changed.  The nondestruc-
     tive 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.





Printed 11/26/99	  May 11, 1986				1






COMPRESS(1)	    UNIX Programmer's Manual	      COMPRESS(1)



     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.

     The -v option causes the printing of the percentage reduc-
     tion 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.
     _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 is 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.
     -- 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.



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COMPRESS(1)	    UNIX Programmer's Manual	      COMPRESS(1)



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.)

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















































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