Gzip

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The correct title of this article is gzip. The initial letter is shown capitalized due to technical restrictions.

gzip
File extension:	`.gz`
MIME type:	`application/x-gzip`
Developed by:	Jean-loup Gailly and Mark Adler
Type of format:	data compression

gzip is a software application used for file compression. gzip is short for GNU zip; the program is a free software replacement for the compress program used in early Unix systems, intended for use by the GNU Project.

gzip was created by Jean-loup Gailly and Mark Adler. Version 0.1 was first publicly released on October 31, 1992. Version 1.0 followed in February 1993.

1 File format
2 Other uses
3 References
4 See also
5 External links

gzip is based on the DEFLATE algorithm, which is a combination of LZ77 and Huffman coding. DEFLATE was intended as a replacement for LZW and other patent-encumbered data compression algorithms which, at the time, limited the usability of compress 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, a version number and a timestamp
optional extra headers, such as the original file name,
a body, containing a DEFLATE-compressed payload
an 8-byte footer, containing a CRC-32 checksum and the length of the original uncompressed data

Although its file format also allows for multiple such streams to be concatenated together (these are simply decompressed concatenated as if they were one), gzip is normally used to compress just single files. Compressed archives are typically created by assembling collections of files into a single tar archive, and then compressing that archive with gzip. The final .tar.gz or .tgz file is usually called a tarball.

gzip is not to be confused with the ZIP archive format, which also uses DEFLATE. The ZIP format can hold collections of files without an external archiver, but is less compact than compressed tarballs holding the same data because it compresses files individually and cannot take advantage of redundancy between files (solid compression).

zlib is an abstraction of the DEFLATE algorithm in library form which includes support both for the gzip file format and a lightweight stream format in its API. The zlib stream format, DEFLATE and the gzip file format were standardized respectively as RFC 1950, RFC 1951 and RFC 1952.

The corresponding program for uncompressing gzipped files is gunzip. Both commands call the same binary; gunzip has the same effect as gzip -d.

The HTTP/1.1 protocol allows for clients to optionally request the compression of content from the server. The standard itself specifies two compression methods: “gzip” (the content wrapped in a gzip stream) and “deflate” (the content in a raw, headerless DEFLATE stream). Both are supported by many HTTP client libraries and almost all modern browsers.

Since the late 1990s, bzip2, a file compression utility based on a block-sorting algorithm, has gained some popularity as a gzip replacement. It produces considerably smaller files (especially for source code and other structured text), but at the cost of memory and processing time (up to a factor of 4). bzip2-compressed tarballs are conventionally named .tar.bz2.

AdvanceCOMP has a DEFLATE implementation which produces gzip-compatible files with better compression ratios than gzip itself.