gzip, gunzip, zcat - compress or expand files

Version

1.3.5

Description

gzip (GNU zip) is a compression utility designed to be a replacement for 'compress'. Its main advantages over compress are much better compression and freedom from patented algorithms. The GNU Project uses it as the standard compression program for its system.

gzip currently uses by default the LZ77 algorithm used in zip 1.9 (the portable pkzip compatible archiver). The gzip format was however designed to accommodate several compression algorithms. See below for a comparison of zip and gzip.

gunzip can currently decompress files created by gzip, compress or pack. The detection of the input format is automatic. For the gzip format, gunzip checks a 32 bit CRC. 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 because there is some redundancy in the .Z compression format. 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.

gzip produces files with a .gz extension. Previous versions of gzip used the .z extension, which was already used by the 'pack' Huffman encoder. gunzip is able to decompress .z files (packed or gzip'ed).

How to use

In order to simply uncompress a compressed file issue the command, e.g. 
   gunzip foo.gz
In order to look for the contents of a tar-archived and gzip-compressed file named foo.tar.gz (sometimes abbreviated foo.tgz) issue the command 
gzip -dc foo.tar.gz | tar -tvf - | more
See the gzip manual pages or the gzip info node for more details. Correlated commands are

Note about zip vs. gzip:

The name 'gzip' was a very unfortunate choice, because zip and gzip are two really different programs, although the actual compression and decompression sources were written by the same persons. A different name should have been used for gzip, but it is too late to change now.

zip is an archiver: it compresses several files into a single archive file. gzip is a simple compressor: each file is compressed separately. Both share the same compression and decompression code for the 'deflate' method. unzip can also decompress old zip archives (implode, shrink and reduce methods). gunzip can also decompress files created by compress and pack. zip 1.9 and gzip do not support compression methods other than deflation. (zip 1.0 supports shrink and implode). Better compression methods may be added in future versions of gzip. zip will always stick to absolute compatibility with pkzip, it is thus constrained by PKWare, which is a commercial company. The gzip header format is deliberately different from that of pkzip to avoid such a constraint.

On Unix, gzip is mostly useful in combination with tar. GNU tar 1.11.2 has a -z option to invoke gzip automatically. "tar -z" compresses better than zip, since gzip can then take advantage of redundancy between distinct files. The drawback is that you must scan the whole tar.gz file in order to extract a single file near the end; unzip can directly seek to the end of the zip file. There is no overhead when you extract the whole archive anyway. If a member of a .zip archive is damaged, other files can still be recovered. If a .tar.gz file is damaged, files beyond the failure point cannot be recovered. (Future versions of gzip will have error recovery features.)

gzip and gunzip are distributed as a single program. zip and unzip are, for historical reasons, two separate programs, although the authors of these two programs work closely together in the info-zip team. zip and unzip are not associated with the GNU project.