gzip

Gzip is any of several software applications used for file compression and decompression. The term usually refers to the GNU Project's implementation, "gzip" standing for GNU zip. It is based on the DEFLATE algorithm, which is a combination of Lempel-Ziv (LZ77) and Huffman coding. 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 the Project. Version 0.1 was first publicly released on October 30, 1992, and 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.^[2]

FreeBSD, DragonFlyBSD, 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 implementation's options.^[3] These implementations originally come from NetBSD, and supports decompression of bzip2 and Unix pack(1) format.

Other uses

The “Content-Encoding”/"Accept-Encoding" and "Transfer-Encoding"/"TE" headers in HTTP/1.1 allow clients to optionally receive compressed HTTP responses and (less commonly) to send compressed requests. The specification for HTTP/1.1 (RFC 2616) specifies three compression methods: “gzip” (RFC 1952; the content wrapped in a gzip stream), “deflate” (RFC 1950; the content wrapped in a zlib-formatted stream), and "compress" (explained in RFC 2616 section 3.5 as 'The encoding format produced by the common UNIX file compression program "compress". This format is an adaptive Lempel-Ziv-Welch coding (LZW).'). Many client libraries, browsers, and server platforms (including Apache and Microsoft IIS) support gzip. Many agents also support deflate, although several important players incorrectly implement deflate support using the format specified by RFC 1951 instead of the correct format specified by RFC 1950 (which encapsulates RFC 1951). Notably, Internet Explorer versions 6, 7, and 8 report deflate support but do not actually accept RFC 1950 format, making actual use of deflate highly unusual. Many clients accept both RFC 1951 and RFC 1950-formatted data for the "deflate" compressed method, but a server has no way to detect whether a client will correctly handle RFC 1950 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). bzip2-compressed archive files 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.

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 time stamp
• 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.^[4] 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".^[5]

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.

gunzip and zcat

The gzip utility on UNIX systems has some alternative names.

When gzip is invoked as gunzip, it decompresses the data (a file or stdin). gunzip is equivalent to gzip -d.

When gzip is invoked as zcat, it also decompresses the data, but behaves similarly to cat. It decompresses individual files and concatenates them to standard output. zcat is equivalent to gzip -d -c. ^[6]

Examples

• To compress a file with gzip, pass the filename as an argument:

gzip file.txt

The command will then replace the original file with a new, usually smaller file called file.txt.gz. To keep the original file file.txt, it is necessary to use the -c option and redirect the output to a new file.

• To uncompress, use gunzip:

gunzip file.txt.gz

• Generally, multiple files can be compressed by combining tar with gzip:

tar czf files.tar.gz *.txt

See also

• List of archive formats
• List of file archivers
• Comparison of file archivers
• List of Unix programs
• Free file format
• Info-ZIP's funzip can gunzip gzip-ped data

References

External links

• GNU Gzip home page
• original gzip Home Page
• pigz - Parallel gzip (and Windows version)