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gzip

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gzip (software)
Original author(s)
Developer(s)GNU Project
Initial release31 October 1992; 31 years ago (1992-10-31)
Stable release
1.13[1] Edit this on Wikidata / 19 August 2023
Repositorygit.savannah.gnu.org/cgit/gzip.git
Written inC
Operating systemUnix-like, Plan 9, Inferno
TypeData compression
LicenseGPL-3.0-or-later
Websitewww.gnu.org/software/gzip/

gzip is a file format and a software application used for file compression and decompression. The program was created by Jean-loup Gailly and Mark Adler as a free software replacement for the compress program used in early Unix systems, and intended for use by GNU (from where the "g" of gzip is derived). Version 0.1 was first publicly released on 31 October 1992, and version 1.0 followed in February 1993.

The decompression of the gzip format can be implemented as a streaming algorithm, an important feature for Web protocols, data interchange and ETL (in standard pipes) applications.

File format

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gzip (file format)
Filename extension
.gz
Internet media type
application/gzip[2]
Uniform Type Identifier (UTI)org.gnu.gnu-zip-archive
Magic number1f 8b
Developed byJean-loup Gailly and Mark Adler
Type of formatData compression
Open format?Yes
Websitegzip.org (obsolete)

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 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 (1f 8b), the compression method (08 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.[3]
  • a body, containing a DEFLATE-compressed payload
  • an 8-byte trailer, containing a CRC-32 checksum and the length of the original uncompressed data, modulo 232.[4]
gzip can be combined with the tar program to compress multiple files.

Although its file format also allows for multiple such streams to be concatenated (gzipped files are simply decompressed concatenated as if they were originally one file),[5] gzip is normally used to compress just single files.[6] Compressed archives are typically created by assembling collections of files into a single tar archive (also called tarball),[7] and then compressing that archive with gzip. The final compressed file usually has the extension .tar.gz or .tgz.

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). The gzip file format is also not to be confused with the that of the compress utility, based on LZW, with extension .Z; however, the gunzip utility is able to decompress .Z files.[8]

Implementations

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NetBSD Gzip / FreeBSD Gzip
Developer(s)The NetBSD Foundation
Repositorycvsweb.netbsd.org/bsdweb.cgi/src/usr.bin/gzip/
Written inC
TypeData compression
LicenseSimplified BSD License

Various implementations of the program have been written. The most commonly known is the GNU Project's implementation using Lempel-Ziv coding (LZ77). 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.[9] FreeBSD, DragonFly BSD and NetBSD use a BSD-licensed implementation instead of the GNU version; it is actually a command-line interface for zlib intended to be compatible with the GNU implementations' options.[10] These implementations originally come from NetBSD, and support decompression of bzip2 and the Unix pack format.

An alternative compression program achieving 3-8% better compression is Zopfli. It achieves gzip-compatible compression using more exhaustive algorithms, at the expense of compression time required. It does not affect decompression time.

pigz, written by Mark Adler, is compatible with gzip and speeds up compression by using all available CPU cores and threads.[11]

Damage recovery

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Data in blocks prior to the first damaged part of the archive is usually fully readable. Data from blocks not demolished by damage that are located afterward may be recoverable through difficult workarounds.[12]

Derivatives and other uses

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The tar utility included in most Linux distributions can extract .tar.gz files by passing the z option, e.g., tar -zxf file.tar.gz, where -z instructs decompression, -x means extraction, and -f specifies the name of the compressed archive file to extract from. Optionally, -v (verbose) lists files as they are being extracted.[13]

zlib is an abstraction of the DEFLATE algorithm in library form which includes support both for the gzip file format and a lightweight data 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 gzip format is used in HTTP compression, a technique used to speed up the sending of HTML and other content on the World Wide Web. It is one of the three standard formats for HTTP compression as specified in RFC 2616. This RFC also specifies a zlib format (called "DEFLATE"), which is equal to the gzip format except that gzip adds eleven bytes of overhead in the form of headers and trailers. Still, the gzip format is sometimes recommended over zlib because Internet Explorer does not implement the standard correctly and cannot handle the zlib format as specified in RFC 1950.[14]

zlib DEFLATE is used internally by the Portable Network Graphics (PNG) format.

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).[15]

AdvanceCOMP, Zopfli, libdeflate 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.[citation needed]

Research published in 2023 showed that simple lossless compression techniques such as gzip could be combined with a k-nearest-neighbor classifier to create an attractive alternative to deep neural networks for text classification in natural language processing. This approach has been shown to equal and in some cases outperform conventional approaches such as BERT due to low resource requirements, e.g. no requirement for GPU hardware.[16]

See also

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Notes

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  1. ^ Jim Meyering (19 August 2023). "gzip-1.13 released [stable]". Retrieved 20 August 2023.
  2. ^ The 'application/zlib' and 'application/gzip' Media Types. Internet Engineering Task Force. doi:10.17487/RFC6713. RFC 6713. Retrieved 1 March 2014.
  3. ^ Deutsch, L. Peter (May 1996). "GZIP file format specification version 4.3". Internet Engineering Task Force. doi:10.17487/RFC1952. Retrieved 23 July 2019.
  4. ^ Jean-loup Gailly. "GNU Gzip". Gnu.org. Archived from the original on 15 October 2015. Retrieved 11 October 2015.
  5. ^ "GNU Gzip: Advanced usage". Gnu.org. Archived from the original on 24 December 2012. Retrieved 28 November 2012.
  6. ^ "Can gzip compress several files into a single archive?". Gnu.org. Archived from the original on 22 July 2010. Retrieved 27 January 2010.
  7. ^ "tarball, The Jargon File, version 4.4.7". Catb.org. Archived from the original on 20 March 2017. Retrieved 27 January 2010.
  8. ^ "GNU Gzip". The GNU Operating System and the Free Software Movement. 5 February 2023. Retrieved 3 April 2024. gunzip can currently decompress files created by gzip, zip, compress or pack. The detection of the input format is automatic.
  9. ^ "OpenBSD gzip(1) manual page". Openbsd.org. OpenBSD. Retrieved 4 February 2018.
  10. ^ "gzip". Man.freebsd.org. 9 October 2011. Archived from the original on 17 December 2019. Retrieved 1 March 2014.
  11. ^ Mark Adler (2017). "pigz: A parallel implementation of gzip for modern multi-processor, multi-core machines". zlib.net. Archived from the original on 18 December 2018. Retrieved 23 December 2018.
  12. ^ Recovering a damaged .gz file – Jean-loup Gailly, GZip.org
  13. ^ "How To Extract / Unzip tar.gz Files From Linux Command Line". Knowledge Base by phoenixNAP. 14 November 2019. Retrieved 12 January 2022.
  14. ^ Lawrence, Eric (21 November 2014). "Compressing the Web". MSDN Blogs > IEInternals. Microsoft. Archived from the original on 28 October 2015. Retrieved 2 November 2015.
  15. ^ "Comparison Tool: 7-zip vs bzip2 vs gzip". compressionratings.com. Archived from the original on 1 November 2014. Retrieved 1 November 2014.
  16. ^ Jiang, Zhiying; Yang, Matthew; Tsirlin, Mikhail; Tang, Raphael; Dai, Yiqin; Lin, Jimmy (July 2023). ""Low-Resource" Text Classification: A Parameter-Free Classification Method with Compressors". Findings of the Association for Computational Linguistics: ACL 2023. Toronto, Canada: Association for Computational Linguistics: 6810–6828. doi:10.18653/v1/2023.findings-acl.426. S2CID 260668487.

References

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