compress(1) - NetBSD Manual Pages

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COMPRESS(1)                 NetBSD Reference Manual                COMPRESS(1)


NAME

     compress, uncompress - compress and expand data


SYNOPSIS

     compress [-cfv] [-b bits] [file ...]
     uncompress [-cfv] [file ...]


DESCRIPTION

     compress reduces the size of the named files using adaptive Lempel-Ziv
     coding.  Each file 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 file, the file is
     ignored.

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

     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:

     -b      Specify the bits code limit (see below).

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

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

     -v      Print the percentage reduction of each file.

     compress 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 -b flag is reached (the default is
     16).  Bits must be between 9 and 16.

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

     The -b flag is omitted for uncompress since the bits 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 bits 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 compress utility exits 0 on success, and >0 if an error occurs.


SEE ALSO

     zcat(1)

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


HISTORY

     The compress command appeared in 4.3BSD.

4.3 Berkeley Distribution       April 18, 1994                               2