(gzip.info) Overview

Info Catalog

(gzip.info) Top

(gzip.info) Sample

 
 Overview
 ********
 
    `gzip' reduces the size of the named files using Lempel-Ziv coding
 (LZ77).  Whenever possible, each file is replaced by one with the
 extension `.gz', while keeping the same ownership modes, access and
 modification times.  (The default extension is `-gz' for VMS, `z' for
 MSDOS, OS/2 FAT and Atari.)  If no files are specified or if a file
 name is "-", the standard input is compressed to the standard output.
 `gzip' will only attempt to compress regular files.  In particular, it
 will ignore symbolic links.
 
    If the new file name is too long for its file system, `gzip'
 truncates it.  `gzip' attempts to truncate only the parts of the file
 name longer than 3 characters.  (A part is delimited by dots.) If the
 name consists of small parts only, the longest parts are truncated.
 For example, if file names are limited to 14 characters, gzip.msdos.exe
 is compressed to gzi.msd.exe.gz.  Names are not truncated on systems
 which do not have a limit on file name length.
 
    By default, `gzip' keeps the original file name and timestamp in the
 compressed file. These are used when decompressing the file with the
 `-N' option. This is useful when the compressed file name was truncated
 or when the time stamp was not preserved after a file transfer.
 
    Compressed files can be restored to their original form using `gzip
 -d' or `gunzip' or `zcat'.  If the original name saved in the
 compressed file is not suitable for its file system, a new name is
 constructed from the original one to make it legal.
 
    `gunzip' takes a list of files on its command line and replaces each
 file whose name ends with `.gz', `.z', `.Z', `-gz', `-z' or `_z' and
 which begins with the correct magic number with an uncompressed file
 without the original extension.  `gunzip' also recognizes the special
 extensions `.tgz' and `.taz' as shorthands for `.tar.gz' and `.tar.Z'
 respectively. When compressing, `gzip' uses the `.tgz' extension if
 necessary instead of truncating a file with a `.tar' extension.
 
    `gunzip' can currently decompress files created by `gzip', `zip',
 `compress' or `pack'. The detection of the input format is automatic.
 When using the first two formats, `gunzip' checks a 32 bit CRC (cyclic
 redundancy check). For `pack', `gunzip' checks the uncompressed length.
 The `compress' format was not designed to allow consistency checks.
 However `gunzip' is sometimes able to detect a bad `.Z' file. If you
 get an error when uncompressing a `.Z' file, do not assume that the
 `.Z' file is correct simply because the standard `uncompress' does not
 complain.  This generally means that the standard `uncompress' does not
 check its input, and happily generates garbage output.  The SCO
 `compress -H' format (`lzh' compression method) does not include a CRC
 but also allows some consistency checks.
 
    Files created by `zip' can be uncompressed by `gzip' only if they
 have a single member compressed with the 'deflation' method. This
 feature is only intended to help conversion of `tar.zip' files to the
 `tar.gz' format.  To extract a `zip' file with a single member, use a
 command like `gunzip <foo.zip' or `gunzip -S .zip foo.zip'.  To extract
 `zip' files with several members, use `unzip' instead of `gunzip'.
 
    `zcat' is identical to `gunzip -c'.  `zcat' uncompresses either a
 list of files on the command line or its standard input and writes the
 uncompressed data on standard output.  `zcat' will uncompress files
 that have the correct magic number whether they have a `.gz' suffix or
 not.
 
    `gzip' uses the Lempel-Ziv algorithm used in `zip' and PKZIP.  The
 amount of compression obtained depends on the size of the input and the
 distribution of common substrings.  Typically, text such as source code
 or English is reduced by 60-70%.  Compression is generally much better
 than that achieved by LZW (as used in `compress'), Huffman coding (as
 used in `pack'), or adaptive Huffman coding (`compact').
 
    Compression is always performed, even if the compressed file is
 slightly larger than the original. The worst case expansion is a few
 bytes for the `gzip' file header, plus 5 bytes every 32K block, or an
 expansion ratio of 0.015% for large files. Note that the actual number
 of used disk blocks almost never increases. `gzip' preserves the mode,
 ownership and timestamps of files when compressing or decompressing.
 
    The `gzip' file format is specified in P. Deutsch, GZIP file format
 specification version 4.3, Internet RFC 1952
 (ftp://ftp.isi.edu/in-notes/rfc1952.txt) (May 1996).  The `zip'
 deflation format is specified in P. Deutsch, DEFLATE Compressed Data
 Format Specification version 1.3, Internet RFC 1951
 (ftp://ftp.isi.edu/in-notes/rfc1951.txt) (May 1996).
 

Info Catalog

(gzip.info) Top

(gzip.info) Sample