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
‘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 in the
compressed file. This can be useful when decompressing the file with
-N if the compressed file name was truncated after a file
transfer.
If the original is a regular file, gzip by default keeps its
timestamp in the compressed file. This can be useful when
decompressing the file with -N if the timestamp was not
preserved after a file transfer.
However, due to limitations in the current gzip file
format, fractional seconds are discarded. Also, timestamps must fall
within the range 1970-01-01 00:00:01 through 2106-02-07 06:28:15
UTC, and hosts whose operating systems use 32-bit timestamps
are further restricted to timestamps no later than 2038-01-19
03:14:07 UTC. The upper bounds assume the typical case
where leap seconds are ignored.
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’
‘-gz’, ‘-z’, or ‘_z’ (ignoring case)
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 per 32 KiB block, or
an expansion ratio of 0.015% for large files. The actual number of
used disk blocks almost never increases.
gzip normally preserves the mode and modification timestamp
of a file when compressing or decompressing. If you have appropriate
privileges, it also preserves the file’s owner and group.
The gzip file format is specified in P. Deutsch, GZIP file
format specification version 4.3,
Internet RFC 1952 (May
1996). The zip deflation format is specified in P. Deutsch,
DEFLATE Compressed Data Format Specification version 1.3,
Internet RFC 1951 (May
1996).