Gzip

gzip is short for GNU zip, a GNU free software file compression program. It 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.

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 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 rolling collections of files into a tar archive, and then compressing that archive with gzip. The final .tar.gz or .tgz file is usually called a "compressed 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.

Note that since the header of a gzip'd file contains a timestamp, it is necessary to use the utilities zcmp or zdiff to determine if two gzip'd files uncompress to the same data stream without getting false negatives.

Other uses

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.

The corresponding program for unzipping files zipped with gzip is gunzip.

See also

• List of archive formats
• List of file archivers
• Comparison of file archivers
• List of Unix programs
• .tar.gz.gpg
• Free file format

External links

• gzip file format specification
• gzip Home Page and http://www.gzip.org/
• Zlib Home Page
• List of GZIP related resources, tutorials, sources
• The gzip Recovery Toolkit

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