Gzip User's Manual - 2. Overview

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# 2. Overview

`gzip` reduces the size of the named files using Lempel-Ziv coding (LZ77). Whenever possible, each file is replaced by one with the extension ``.gz'`, while keeping the same ownership modes, access and modification times. (The default extension is ``-gz'` for VMS, ``z'` for MSDOS, OS/2 FAT and Atari.) If no files are specified or if a file name is "-", the standard input is compressed to the standard output. `gzip` will only attempt to compress regular files. In particular, it will ignore symbolic links.

If the new file name is too long for its file system, `gzip` truncates it. `gzip` attempts to truncate only the parts of the file name longer than 3 characters. (A part is delimited by dots.) If the name consists of small parts only, the longest parts are truncated. For example, if file names are limited to 14 characters, gzip.msdos.exe is compressed to gzi.msd.exe.gz. Names are not truncated on systems which do not have a limit on file name length.

By default, `gzip` keeps the original file name and timestamp in the compressed file. These are used when decompressing the file with the ``-N'` option. This is useful when the compressed file name was truncated or when the time stamp was not preserved after a file transfer.

Compressed files can be restored to their original form using ``gzip -d'` or `gunzip` or `zcat`. If the original name saved in the compressed file is not suitable for its file system, a new name is constructed from the original one to make it legal.

`gunzip` takes a list of files on its command line and replaces each file whose name ends with ``.gz'`, ``.z'`, ``.Z'`, ``-gz'`, ``-z'` or ``_z'` and which begins with the correct magic number with an uncompressed file without the original extension. `gunzip` also recognizes the special extensions ``.tgz'` and ``.taz'` as shorthands for ``.tar.gz'` and ``.tar.Z'` respectively. When compressing, `gzip` uses the ``.tgz'` extension if necessary instead of truncating a file with a ``.tar'` extension.

`gunzip` can currently decompress files created by `gzip`, `zip`, `compress` or `pack`. The detection of the input format is automatic. When using the first two formats, `gunzip` checks a 32 bit CRC (cyclic redundancy check). For `pack`, `gunzip` checks the uncompressed length. The `compress` format was not designed to allow consistency checks. However `gunzip` is sometimes able to detect a bad ``.Z'` file. If you get an error when uncompressing a ``.Z'` file, do not assume that the ``.Z'` file is correct simply because the standard `uncompress` does not complain. This generally means that the standard `uncompress` does not check its input, and happily generates garbage output. The SCO ``compress -H'` format (`lzh` compression method) does not include a CRC but also allows some consistency checks.

Files created by `zip` can be uncompressed by `gzip` only if they have a single member compressed with the 'deflation' method. This feature is only intended to help conversion of `tar.zip` files to the `tar.gz` format. To extract `zip` files with several members, use `unzip` instead of `gunzip`.

`zcat` is identical to ``gunzip -c'`. `zcat` uncompresses either a list of files on the command line or its standard input and writes the uncompressed data on standard output. `zcat` will uncompress files that have the correct magic number whether they have a ``.gz'` suffix or not.

`gzip` uses the Lempel-Ziv algorithm used in `zip` and PKZIP. The amount of compression obtained depends on the size of the input and the distribution of common substrings. Typically, text such as source code or English is reduced by 60-70%. Compression is generally much better than that achieved by LZW (as used in `compress`), Huffman coding (as used in `pack`), or adaptive Huffman coding (`compact`).

Compression is always performed, even if the compressed file is slightly larger than the original. The worst case expansion is a few bytes for the `gzip` file header, plus 5 bytes every 32K block, or an expansion ratio of 0.015% for large files. Note that the actual number of used disk blocks almost never increases. `gzip` preserves the mode, ownership and timestamps of files when compressing or decompressing.

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