4.4BSD/usr/share/man/cat1/zcat.0

COMPRESS(1)                  BSD Reference Manual                  COMPRESS(1)

NNAAMMEE
     ccoommpprreessss, uunnccoommpprreessss, zzccaatt - compress and expand data

SSYYNNOOPPSSIISS
     ccoommpprreessss [--ccffvv] [--bb _b_i_t_s] [_f_i_l_e _._._.]
     uunnccoommpprreessss [--ccffvv] [_f_i_l_e _._._.]
     zzccaatt [_f_i_l_e _._._.]

DDEESSCCRRIIPPTTIIOONN
     CCoommpprreessss reduces the size of the named files using adaptive Lempel-Ziv
     coding.  Each _f_i_l_e is renamed to the same name plus the extension ``.Z''.
     As many of the modification time, access time, file flags, file mode, us-
     er ID, and group ID as allowed by permissions are retained in the new
     file.  If compression would not reduce the size of a _f_i_l_e, the file is
     ignored.

     UUnnccoommpprreessss restores the compressed files to their original form, renaming
     the files by deleting the ``.Z'' extension.

     ZZccaatt is an alias for ``uncompress -c''.

     If renaming the files would cause files to be overwritten and the stan-
     dard input device is a terminal, the user is prompted (on the standard
     error output) for confirmation.  If prompting is not possible or confir-
     mation is not received, the files are not overwritten.

     If no files are specified, the standard input is compressed or uncom-
     pressed to the standard output.  If either the input and output files are
     not regular files, the checks for reduction in size and file overwriting
     are not performed, the input file is not removed, and the attributes of
     the input file are not retained.

     The options are as follows:

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

     --cc      Compressed or uncompressed output is written to the standard out-
             put.  No files are modified.

     --ff      Force compression of _f_i_l_e, even if it is not actually reduced in
             size.  Additionally, files are overwritten without prompting for
             confirmation.

     --vv      Print the percentage reduction of each file.

     CCoommpprreessss uses a modified Lempel-Ziv algorithm.  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 (the default is
     16).  _B_i_t_s must be between 9 and 16.

     After the _b_i_t_s limit is reached, ccoommpprreessss periodically checks the com-
     pression ratio.  If it is increasing, ccoommpprreessss continues to use the ex-
     isting 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.

     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 num-
     ber 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%.  Com-
     pression is generally much better than that achieved by Huffman coding
     (as used in the historical command pack), or adaptive Huffman coding (as
     used in the historical command compact), and takes less time to compute.

     The ccoommpprreessss utility exits 0 on success, and >0 if an error occurs.

SSEEEE AALLSSOO
     zopen(3)

     Welch, Terry A., "A Technique for High Performance Data Compression",
     _I_E_E_E _C_o_m_p_u_t_e_r, 17:6, pp. 8-19, June, 1984.

HHIISSTTOORRYY
     The ccoommpprreessss command appeared in 4.3BSD.

4.3 Berkeley Distribution        June 6, 1993                                2