gzip



GZIP(1)                     General Commands Manual                    GZIP(1)


NAME

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


SYNOPSIS

     gzip [-123456789cdfhLlNnOqrtVv] [-b bits] [-o filename] [-S suffix]
          [file ...]
     gunzip [-cfhLlNnqrtVv] [-o filename] [file ...]
     gzcat [-fhqr] [file ...]


DESCRIPTION

     The gzip utility reduces the size of the named files using adaptive
     Lempel-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,
     renaming 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
     compressed by both gzip and compress(1), recognising the following
     extensions: ``.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
     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 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.

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

     -c      Compressed or uncompressed output is written to the standard
             output.  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).

     -h      Print a short help message.

     -L      A no-op which exists for compatibility only.  On GNU gzip, it
             displays the program's license.

     -l      List information for the specified compressed files.  The
             following 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
                                uncompressing.

             If the -v option is specified, the following additional
             information 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
                                 printed instead).

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

     -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      A no-op which exists for compatibility only.  On GNU gzip, it
             displays version information.

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

     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
     dictionary.  However, if the compression ratio decreases, gzip 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 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
     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 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 coding (as used in the historical command compact), and takes
     less time to compute.


ENVIRONMENT

     GZIP    Options which are passed to gzip, gunzip, and gzcat
             automatically.


EXIT STATUS

     The gzip utility exits with one of the following values:

           0       Success.
           1       An error occurred.
           2       At least one of the specified files was not compressed
                   since -f was not specified and compression would have
                   resulted in a size increase.
           >2      An error occurred.

     The gunzip and gzcat utilities exit 0 on success, and >0 if an error
     occurs.


SEE ALSO

     compress(1), gzexe(1), zdiff(1), zforce(1), zmore(1), znew(1),
     compress(3)


STANDARDS

     P. Deutsch and J-L. Gailly, ZLIB Compressed Data Format Specification
     version 3.3, RFC 1950, May 1996.

     P. Deutsch, DEFLATE Compressed Data Format Specification version 1.3, RFC
     1951, May 1996.

     P. Deutsch, GZIP file format specification version 4.3, RFC 1952, May
     1996.


HISTORY

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

OpenBSD 5.9                     October 7, 2014                    OpenBSD 5.9

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