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gzip
Developer(s) GNU Project
Stable release 1.3.13 / 2009-09-30; 3 months ago
Written in C
Operating system Cross-platform
Type data compression
License GNU GPL
Website http://www.gnu.org/software/gzip/

gzip is a software application used for file compression. gzip is short for GNU zip; as the program was created 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' in this specific version stands for gratis.[1]

FreeBSD and NetBSD use BSD-licensed implementation instead of the GNU version.[2].

Contents

File format

gzip
Filename extension .gz
Internet media type application/x-gzip
Uniform Type Identifier org.gnu.gnu-zip-archive
Developed by Jean-Loup Gailly and Mark Adler
Type of format data compression
Free file format? Yes

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 either .tar.bz2 or simply .tbz.

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.

gunzip and zcat

The gzip utility on UNIX systems has some alternative names.

When gzip invoked as gunzip, it decompresses the data (a file or stdin) as if -d option was enabled.

When gzip invoked as zcat, it also decompresses the data, but behaves similarly to cat. It decompresses individual files and concatenates them to standard output. This may be enabled as gzip -c -d.[3]

Examples

  • To gzip with a maximum compression rate, use the -9 argument:
$> gzip -9 file.txt

The command will then replace file.txt by file.txt.gz, which would have a smaller size.

  • To gzip several files with one command, use find. For example, to compress all .txt files older than 24 hours files in the current directory:
$> find . -name "*.txt" -mtime 1 -type f -exec gzip {} \;

This will create a single .gz file for each .txt file. Multiple files can be gzipped into one with the use of tar.

  • To uncompress, use gunzip:
$> gunzip file.txt.gz

See also

References

  • RFC 1952 - GZIP file format specification version 4.3

External links








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