COMPRESS(1)                OpenBSD Reference Manual                COMPRESS(1)

NAME
     compress, uncompress, gzip, gunzip - compress and expand data

SYNOPSIS
     compress [-LV]
     compress [-cdfghlOnNqrtv123456789] [-b bits] [-S suffix] [-o filename]
              [file ...]
     uncompress [-cfhlnNqrtv] [-o filename] [file ...]

     gzip [-LV]
     gzip [-cdfghlnNOqrtv123456789] [-b bits] [-S suffix] [-o filename]
              [file ...]
     gunzip [-cfhnNqrltv] [-o filename] [file ...]

     zcat [-fghqr] [file ...]
     gzcat [-fhqr] [file ...]

DESCRIPTION
     The compress and gzip utilities reduce the size of the named files using
     adaptive Lempel-Ziv coding.  They are functionally identical, but use
     different algorithms for compression.  If invoked as gzip or compress -g,
     the deflate mode of compression is chosen by default; otherwise the older
     method of compression (compress mode) is used.

     Each file is renamed to the same name plus the extension ``.Z'', or
     ``.gz'' (in deflate mode).  As many of the modification time, access
     time, file flags, file mode, user ID, and group ID as allowed by permis-
     sions are retained in the new file.  If compression would not reduce the
     size of a file, the file is ignored (unless -f is used).

     The uncompress and gunzip utilities restore compressed files to their
     original form, renaming the files by removing the extension (or by using
     the stored name if the -N flag is specified).  When decompressing, the
     following extensions are recognized: ``.Z'', ``-Z'', ``_Z'', ``.gz'',
     ``-gz'', ``_gz'', ``.tgz'', ``-tgz'', ``_tgz'', ``.taz'', ``-taz'', and
     ``_taz''.  Extensions ending in ``tgz'' and ``taz'' are not removed when
     decompressing, instead they are converted to ``tar''.

     The zcat command is equivalent in functionality to uncompress -c.  The
     gzcat command is equivalent in functionality to gunzip -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 or 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:

     -V      Display the program version (RCS IDs of the source files) and ex-
             it.

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

     -c      Compressed or uncompressed output is written to the standard out-
             put.  No files are modified (force zcat or gzcat mode).

     -d      Decompress the source files instead of compressing them (force
             uncompress mode).

     -f      Force compression of file, even if it is not actually reduced in
             size.  Additionally, files are overwritten without prompting for
             confirmation.  If the input data is not in a format recognized by
             compress and if the option -c is also given, copy the input data
             without change to the standard output: let zcat or gzcat behave
             as cat(1).

     -g      Use deflate scheme which reportedly provides better compression
             rates (force gzip mode).  This flag need not be specified when
             invoked as gzip.

     -h      Print a short help message.

     -l      List information for the specified compressed files.  The follow-
             ing information is listed:

             compressed size    Size of the compressed file.

             uncompressed size  Size of the file when uncompressed.

             compression ratio  Ratio of the difference between the compressed
                                and uncompressed sizes to the uncompressed
                                size.

             uncompressed name  Name the file will be saved as when uncom-
                                pressing.

             If the -v option is specified, the following additional informa-
             tion is printed:

             compression method  Name of the method used to compress the file.

             crc                 32-bit CRC (cyclic redundancy code) of the
                                 uncompressed file.

             time stamp          Date and time corresponding to the last data
                                 modification time (mtime) of the compressed
                                 file (if the -n option is specified, the time
                                 stamp stored in the compressed file is print-
                                 ed instead).

     -n      When compressing, do not save the original file name and time
             stamp.  This information is saved by default when the deflate
             scheme is used.  When uncompressing, do not restore the original
             file name and time stamp.  By default, the uncompressed file in-
             herits the time stamp of the compressed version and the uncom-
             pressed file name is generated from the name of the compressed
             file name as described above.

     -N      When compressing, save the original file name and time stamp in
             the compressed file.  This information is saved by default when
             the deflate scheme is used.  When uncompressing or listing, use
             the time stamp and file name stored in the compressed file, if
             any, for the uncompressed version.

     -1...9  Use deflate scheme with compression factor of -1 to -9.  Compres-
             sion factor -1 is the fastest, but provides a poorer level of
             compression.  Compression factor -9 provides the best level of
             compression, but is relatively slow.  The default is -6.  This
             option implies -g.

     -O      Use old compression method.

     -o filename
             Set the output file name.

     -S suffix
             Set suffix for compressed files.

     -t      Test the integrity of each file leaving any files intact.

     -r      Recursive mode, compress will descend into specified directories.

     -q      Be quiet, suppress all messages.

     -v      Print the percentage reduction of each file and other informa-
             tion.

     In normal mode, compress uses a modified Lempel-Ziv algorithm (LZW).
     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.  bits must be between 9 and 16 (the default is 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.

     gzip uses a slightly different version of the Lempel-Ziv algorithm
     (LZ77).  Common substrings are replaced by pointers to previous strings,
     and are found using a hash table.  Unique substrings are emitted as a
     string of literal bytes, and compressed as Huffman trees.

     The -b flag is omitted for uncompress or gunzip 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 re-
     compression 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% using
     compress and by 60 - 70% using gzip.  Compression is generally much bet-
     ter than that achieved by Huffman coding (as used in the historical com-
     mand pack), or adaptive Huffman coding (as used in the historical command
     compact), and takes less time to compute.

     The compress and gzip utilities exit with 0 on success, 1 if an error oc-
     curred, or 2 if one or more files were not compressed because they would
     have grown in size (and -f was not specified).

RETURN VALUES
     The compress utility exits with one of the following values:

     0     The file was compressed successfully.
     1     An error occurred.
     2     A warning occurred.

SEE ALSO
     compress(3)

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

     RFC 1950      ZLIB Compressed Data Format Specification.
     RFC 1951      DEFLATE Compressed Data Format Specification.
     RFC 1952      GZIP File Format Specification.

STANDARDS
     The compress utility is compliant with the IEEE Std 1003.2-1992
     (``POSIX.2'') specification.

     The gzip and gunzip utilities are extensions.

HISTORY
     The compress command appeared in 4.3BSD.  The deflate compression support
     was added in OpenBSD 2.1.  Full gzip compatibility was added in OpenBSD
     3.4.  The `g' in this version of gzip stands for ``gratis''.

OpenBSD 3.5                     April 18, 1994                               4
