Overview (GNU Gzip)
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.
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
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 ‘
-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 ‘
gunzip can currently decompress files created by
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
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
zcat is identical to ‘
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 (
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 normally preserves the mode, ownership and timestamps of files when compressing or decompressing.
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).