4.3BSD-Reno/share/man/cat1/compress.0

COMPRESS(1)		    UNIX Reference Manual		   COMPRESS(1)

NNAAMMEE
     ccoommpprreessss, uunnccoommpprreessss, zzccaatt - compress and expand data

SSYYNNOOPPSSIISS
     ccoommpprreessss [--ff] [--vv] [--cc] [--bb _b_i_t_s] [_f_i_l_e ...]
     uunnccoommpprreessss [--ff] [--vv] [--cc] [_f_i_l_e ...]
     zzccaatt [_f_i_l_e ...]

DDEESSCCRRIIPPTTIIOONN
     CCoommpprreessss reduces the size of the named files using adaptive Lempel-Ziv
     coding.  Whenever possible, each _f_i_l_e is replaced by one with the exten-
     sion ._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 uunnccoommpprreessss or zzccaatt

     --ff    Force compression of _f_i_l_e, even if it does not actually shrink or
	   the corresponding _f_i_l_e._Z file already exists.  Except when run in
	   the background under /_b_i_n/_s_h, if --ff is not given the user is
	   prompted as to whether an existing _f_i_l_e._Z file should be overwrit-
	   ten.

     --cc    (``cat'') makes ccoommpprreessss//uunnccoommpprreessss write to the standard output;
	   no files are changed.  The nondestructive behavior of zzccaatt is
	   identical to that of uunnccoommpprreessss --cc..

     --bb    Specify _b_i_t_s code limit (see below).

     --vv    Print the percentage reduction of each file.

     CCoommpprreessss uses the modified Lempel-Ziv algorithm popularized in "A Tech-
     nique 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 --bb 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 ccoommpprreessss to be run on a smaller machine.

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

     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.  Typ-
     ically, 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 (as used in compact), and
     takes less time to compute.

     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.

DDIIAAGGNNOOSSTTIICCSS
     Usage: compress [--ffvvcc] [--bb _m_a_x_b_i_t_s] [_f_i_l_e ...]
	   Invalid options were specified on the command line.
     Missing maxbits
	   Maxbits must follow --bb.

     _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
	   --vv.)

     -- 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 ln(1) for more
	   information.

     -- file unchanged
	   No savings is achieved by compression.  The input remains virgin.

FFIILLEESS
     _f_i_l_e._Z   compressed file is file.Z

HHIISSTTOORRYY
     Appeared in 4.3 BSD.

BBUUGGSS
     Although compressed files are compatible between machines with large
     memory, --bb_1_2 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.)

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