GZIP(1)                    OpenBSD Reference Manual                    GZIP(1)

NAME
     gzip, gunzip, gzcat - compress and expand data (deflate mode)

SYNOPSIS
     gzip [-123456789cdfghLlNnOqrtVv] [-b bits] [-o filename] [-S suffix]
          [file ...]
     gunzip [-cfhlNnqrtVv] [-o filename] [file ...]
     gzcat [-fghqr] [file ...]

DESCRIPTION
     The gzip utility reduces the size of the named files using adaptive Lem-
     pel-Ziv coding, in deflate mode.  If invoked as gzip -O, the compress
     mode of compression is chosen; see compress(1) for more information.
     Each file is renamed to the same name plus the extension ``.gz''.  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 ignored
     (unless -f is used).

     The gunzip utility restores compressed files to their original form, re-
     naming the files by removing the extension (or by using the stored name
     if the -N flag is specified).  It has the ability to restore files com-
     pressed by both gzip and compress(1), recognising the following exten-
     sions: ``.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 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.

     By default, when compressing, the original file name and time stamp are
     stored in the compressed file.  When uncompressing, this information is
     not used.  Instead, the uncompressed file inherits the time stamp of the
     compressed version and the uncompressed file name is generated from the
     name of the compressed file as described above.  These defaults may be
     overridden by the -N and -n flags, described below.

     The options are as follows:

     -1...9  Use the deflate scheme, with compression factor of -1 to -9.
             Compression 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.

     -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 gzcat mode).

     -d      Decompress the source files instead of compressing them (force
             gunzip 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
             gzip and if the option -c is also given, copy the input data
             without change to the standard output: let gzcat behave as
             cat(1).

     -g      Use the deflate scheme, which reportedly provides better compres-
             sion rates (the default).

     -h      Print a short help message.

     -L      Print the license.

     -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 uncompressing or listing, use the time stamp and file name
             stored in the compressed file, if any, for the uncompressed ver-
             sion.

     -n      When compressing, do not store the original file name and time
             stamp in the gzip header.

     -O      Use old compression method (force compress(1) mode).

     -o filename
             Set the output file name.

     -q      Be quiet: suppress all messages.

     -r      Recursive mode: gzip will descend into specified directories.

     -S suffix
             Set the suffix for compressed files.

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

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

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

     gzip uses a modified Lempel-Ziv algorithm (LZW).  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.  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, gzip periodically checks the compression
     ratio.  If it is increasing, gzip continues to use the existing code dic-
     tionary.  However, if the compression ratio decreases, gzip discards the
     table of substrings and rebuilds it from scratch.  This allows the algo-
     rithm to adapt to the next ``block'' of the file.

     The -b flag is omitted for gunzip since the bits parameter specified dur-
     ing 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.  Typ-
     ically, text such as source code or English is reduced by 60 - 70% using
     gzip.  Compression is generally much better than that achieved by Huffman
     coding (as used in the historical command pack), or adaptive Huffman cod-
     ing (as used in the historical command compact), and takes less time to
     compute.

     The gzip, gunzip, and gzcat utilities exit with 0 on success; 1 if an er-
     ror occurred; or 2 if a warning occurred.

ENVIRONMENT
     GZIP    Options which are passed to gzip, gunzip, and gzcat automatical-
             ly.

SEE ALSO
     compress(1), gzexe(1), gzsig(1), zdiff(1), zforce(1), zmore(1), znew(1),
     compress(3)

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

HISTORY
     gzip compatibility was added to compress(1) in OpenBSD 3.4.  The `g' in
     this version of gzip stands for ``gratis''.

OpenBSD 4.7                     April 18, 2009                               3
