Editing Gzip (section)

== File format ==
{{Infobox file format
| name = gzip (file format)
| extension = .gz
| mime = application/gzip<ref>{{cite IETF|title=The 'application/zlib' and 'application/gzip' Media Types|rfc=6713|publisher=Internet Engineering Task Force|access-date=2014-03-01}}</ref>
| uniform type = org.gnu.gnu-zip-archive
| magic = <code>1f 8b</code>
| owner = Jean-loup Gailly and Mark Adler
| genre = [[Data compression]]
| url = {{URL|http://gzip.org}} (obsolete)
| open = Yes
}}

gzip is based on the [[DEFLATE]] algorithm, which is a combination of [[LZ77 and LZ78|LZ77]] and [[Huffman coding]]. DEFLATE was intended as a replacement for [[LZW]] and other [[patent]]-encumbered [[data compression]] [[algorithm]]s which, at the time, limited the usability of the [[compress]] utility and other popular archivers.

"gzip" is often also used to refer to the gzip file format, which is:
* a 10-byte header, containing a [[magic number (programming)|magic number]] (<code>1f 8b</code>), the compression method (<code>08</code> for DEFLATE), 1-byte of header flags, a 4-byte timestamp, compression flags and the operating system ID.
* optional extra headers as allowed by the header flags, including the original filename, a comment field, an "extra" field, and the lower half of a [[CRC-32]] checksum for the header section.<ref>{{Cite web |last=Deutsch |first=L. Peter |date=May 1996 |title=GZIP file format specification version 4.3 |url=https://tools.ietf.org/html/rfc1952.html |journal= |language=en |publisher=Internet Engineering Task Force |doi=10.17487/RFC1952 |access-date=2019-07-23}}</ref>
* a body, containing a DEFLATE-compressed [[Payload (computing)|payload]]
* an 8-byte trailer, containing a [[Cyclic redundancy check|CRC-32]] checksum and the length of the original uncompressed data, modulo 2<sup>32</sup>.<ref>{{cite web|author1=Jean-loup Gailly|title=GNU Gzip|url=https://www.gnu.org/software/gzip/manual/gzip.html#index-options-4|website=Gnu.org|access-date=11 October 2015|archive-date=15 October 2015|archive-url=https://web.archive.org/web/20151015064748/http://www.gnu.org/software/gzip/manual/gzip.html#index-options-4|url-status=live}}</ref>

[[File:Targzip.svg|thumb|gzip can be combined with the [[tar (file format)|tar]] program to compress multiple files.]]
Although its file format also allows for multiple such streams to be [[Concatenation|concatenated]] (gzipped files are simply decompressed concatenated as if they were originally one file),<ref>{{cite web| title=GNU Gzip: Advanced usage| url=https://www.gnu.org/software/gzip/manual/html_node/Advanced-usage.html| website=Gnu.org| access-date=2012-11-28| archive-date=24 December 2012| archive-url=https://web.archive.org/web/20121224211417/https://www.gnu.org/software/gzip/manual/html_node/Advanced-usage.html| url-status=live}}</ref> gzip is normally used to compress just single files.<ref>{{cite web| title=Can gzip compress several files into a single archive?| url=http://www.gzip.org/#faq16|website=Gnu.org| access-date=2010-01-27 |archive-url=https://web.archive.org/web/20100722004012/http://www.gzip.org/#faq16 |archive-date=2010-07-22}}</ref> Compressed archives are typically created by assembling collections of files into a single [[tar (file format)|tar]] archive (also called [[tar (file format)|tarball]]),<ref>{{cite web|title=tarball, The Jargon File, version 4.4.7|url=http://www.catb.org/jargon/html/T/tarball.html|website=Catb.org|access-date=2010-01-27|archive-date=20 March 2017|archive-url=https://web.archive.org/web/20170320183848/http://www.catb.org/jargon/html/T/tarball.html|url-status=live}}</ref> and then compressing that archive with gzip. The final compressed file usually has the extension {{mono|.tar.gz}} or {{mono|.tgz}}.

gzip is not to be confused with the [[ZIP (file format)|ZIP]] archive format, which also uses [[Deflate|DEFLATE]]. The ZIP format can hold collections of files without an external archiver, but is less compact than compressed [[tar (computing)|tarballs]] holding the same data, because it compresses files individually and cannot take advantage of redundancy between files ([[solid compression]]).
The gzip file format is also not to be confused with that of the compress utility, based on LZW, with extension {{mono|.Z}}; however, the gunzip utility is able to decompress .Z files.<ref>{{cite web | title=GNU Gzip | website=The GNU Operating System and the Free Software Movement | date=2023-02-05 | url=https://www.gnu.org/software/gzip/manual/gzip.html | access-date=2024-04-03 |quote=gunzip can currently decompress files created by gzip, zip, compress or pack. The detection of the input format is automatic. }}</ref>