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COMPRESS(1)             FreeBSD General Commands Manual            COMPRESS(1)

     compress, uncompress, - compress and expand data

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

     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,
     user 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

     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
     standard input device is a terminal, the user is prompted (on the
     standard error output) for confirmation.  If prompting is not possible or
     confirmation is not received, the files are not overwritten.

     If no files are specified, the standard input is compressed or
     uncompressed 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
             output.  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

     -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
     compression ratio.  If it is increasing, compress continues to use the
     existing 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
     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 bits 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 the historical command pack), or adaptive Huffman coding (as
     used in the historical command compact), and takes less time to compute.

     The compress and uncompress utilities exit 0 on success, and >0 if an
     error occurs.

     gunzip(1), gzexe(1), gzip(1), zcat(1), zmore(1), znew(1)

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

     The compress command appeared in 4.3BSD.

FreeBSD 11.0-PRERELEASE         April 18, 1994         FreeBSD 11.0-PRERELEASE


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