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 the GNU Project (the "g" is from "GNU"). Version 0.1 was first publicly released on 31 October 1992, and 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 (1f 8b), 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).
Implementations
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. 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. These implementations originally come from NetBSD, and supports decompression of bzip2 and the Unix format.
