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, Windows NT 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 compressed 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, -gz, .z, -z, _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,
compress -H or pack. The detection of the input format is automatic.
When using the first two formats, gunzip checks a 32 bit CRC. For
pack, gunzip checks the uncompressed length. The standard 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 zip files with several members, use unzip instead
of gunzip.
zcat is identical to gunzip -c. (On some systems, zcat may be
installed as gzcat to preserve the original link to compress.) 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.
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