Gzip

infobox file format
name = gzip
extension = .gz
mime = application/x-gzip
owner = Jean-Loup Gailly and Mark Adler
genre = data compression
website = http://www.gzip.org/Infobox_Software
name = gzip
developer = GNU Project
latest_release_version = 1.3.12
latest_release_date = release date|2007|04|13
operating_system = Cross-platform
genre = data compression
programming_language = C
license = GPL
website = http://www.gnu.org/software/gzip/

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.

OpenBSD's version of gzip is actually the compress program, to which support for the gzip format was added in OpenBSD 3.4 - the 'g' here stands for . [cite web|url=http://www.openbsd.org/cgi-bin/man.cgi?query=gzip#HISTORY|title=OpenBSD gzip(1) manual page|accessdate=2007-07-23|publisher=OpenBSD]

File format

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 (zipped files are simply decompressed concatenated as if they were originally one file), 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.

Other uses

The “Content-Encoding” header in HTTP/1.1 allows clients to optionally receive compressed HTTP responses and (less commonly) to send compressed requests. The standard itself specifies two compression methods: “gzip” (RFC 1952; the content wrapped in a gzip stream) and “deflate” (RFC 1950; the content wrapped in a zlib-formatted stream). Compressed responses are supported by many HTTP client libraries, almost all modern browsers and both of the major HTTP server platforms, Apache and Microsoft IIS. Many server implementations, however, incorrectly implement the protocol by using the raw DEFLATE stream format (RFC 1951) instead. The bug is sufficiently pervasive that most modern browsers will accept both RFC 1951 and RFC 1950-formatted data for the “deflate” compressed method.

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 and 7-Zip can produce gzip-compatible files, using an internal DEFLATE implementation with better compression ratios than gzip itself—at the cost of more processor time compared to the reference implementation.

References

* RFC 1952 - GZIP file format specification version 4.3

See also

* List of archive formats
* List of file archivers
* Comparison of file archivers
* List of Unix programs
* Free file format

External links

* [http://www.gzip.org/ original gzip Home Page]