440 lines
12 KiB
Groff
440 lines
12 KiB
Groff
.PU
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.TH GZIP 1
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.SH NAME
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gzip, gunzip, zcat \- compress or expand files
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.SH SYNOPSIS
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.ll +8
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.B gzip
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.RB [ " \-acdfhlLnrtvV19 " ]
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.RB [ \-S\ suffix ]
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[
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.I "name \&..."
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]
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.ll -8
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.br
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.B gunzip
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.RB [ " \-acfhlLnrtvV " ]
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.RB [ \-S\ suffix ]
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[
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.I "name \&..."
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]
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.br
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.B zcat
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.RB [ " \-fhLV " ]
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[
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.I "name \&..."
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]
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.SH DESCRIPTION
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.I Gzip
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reduces the size of the named files using Lempel-Ziv coding (LZ77).
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Whenever possible,
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each file is replaced by one with the extension
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.B "\&.gz,"
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while keeping the same ownership modes, access and modification times.
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(The default extension is
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.B "\-gz"
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for VMS,
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.B "z"
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for MSDOS, OS/2 FAT and Atari.)
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If no files are specified, the standard input is compressed to the
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standard output.
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.I Gzip
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will only attempt to compress regular files.
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In particular, it will ignore symbolic links.
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.PP
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If the new file name is too long for its file system,
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.I gzip
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truncates it and keeps the original file name in the compressed file.
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.I Gzip
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attempts to truncate only the parts of the file name longer than 3 characters.
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(A part is delimited by dots.) If the name consists of small parts only,
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the longest parts are truncated. For example, if file names are limited
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to 14 characters, gzip.msdos.exe is compressed to gzi.msd.exe.gz.
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Names are not truncated on systems which do not have a limit on file name
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length.
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.PP
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Compressed files can be restored to their original form using
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.I gzip -d
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or
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.I gunzip
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or
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.I zcat.
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If the original name saved in the compressed file is not suitable for its
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file system, a new name is constructed from the original one to make it
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legal.
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.PP
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.I gunzip
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takes a list of files on its command line and replaces each
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file whose name ends with .gz, -gz, .z, -z, _z or .Z
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and which begins with the correct magic number with an uncompressed
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file without the original extension.
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.I gunzip
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also recognizes the special extensions
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.B "\&.tgz"
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and
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.B "\&.taz"
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as shorthands for
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.B "\&.tar.gz"
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and
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.B "\&.tar.Z"
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respectively.
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When compressing,
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.I gzip
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uses the
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.B "\&.tgz"
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extension if necessary instead of truncating a file with a
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.B "\&.tar"
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extension.
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.PP
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.I gunzip
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can currently decompress files created by
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.I gzip, zip, compress, compress -H
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or
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.I pack.
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The detection of the input format is automatic. When using
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the first two formats,
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.I gunzip
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checks a 32 bit CRC. For
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.I pack, gunzip
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checks the uncompressed length. The standard
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.I compress
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format was not designed to allow consistency checks. However
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.I gunzip
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is sometimes able to detect a bad .Z file. If you get an error
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when uncompressing a .Z file, do not assume that the .Z file is
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correct simply because the standard
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.I uncompress
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does not complain. This generally means that the standard
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.I uncompress
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does not check its input, and happily generates garbage output.
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The SCO compress -H format (lzh compression method) does not include a CRC
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but also allows some consistency checks.
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.PP
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Files created by
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.I zip
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can be uncompressed by gzip only if they have a single member compressed
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with the 'deflation' method. This feature is only intended to help
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conversion of tar.zip files to the tar.gz format. To extract zip files
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with several members, use
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.I unzip
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instead of
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.I gunzip.
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.PP
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.I zcat
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is identical to
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.I gunzip
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.B \-c.
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(On some systems,
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.I zcat
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may be installed as
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.I gzcat
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to preserve the original link to
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.I compress.)
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.I zcat
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uncompresses either a list of files on the command line or its
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standard input and writes the uncompressed data on standard output.
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.I zcat
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will uncompress files that have the correct magic number whether
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they have a
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.B "\&.gz"
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suffix or not.
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.PP
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.I Gzip
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uses the Lempel-Ziv algorithm used in
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.I zip
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and PKZIP.
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The amount of compression obtained depends on the size of the
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input and the distribution of common substrings.
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Typically, text such as source code or English
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is reduced by 60\-70%.
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Compression is generally much better than that achieved by
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LZW (as used in
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.IR compress ),
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Huffman coding (as used in
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.IR pack ),
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or adaptive Huffman coding
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.RI ( compact ).
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.PP
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Compression is always performed, even if the compressed file is
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slightly larger than the original. The worst case expansion is
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a few bytes for the gzip file header, plus 5 bytes every 32K block,
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or an expansion ratio of 0.015% for large files. Note that the actual
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number of used disk blocks almost never increases.
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.I gzip
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preserves the mode, ownership and timestamps of files when compressing
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or decompressing.
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.SH OPTIONS
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.TP
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.B \-a --ascii
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Ascii text mode: convert end-of-lines using local conventions. This option
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is supported only on some non-Unix systems. For MSDOS, CR LF is converted
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to LF when compressing, and LF is converted to CR LF when decompressing.
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.TP
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.B \-c --stdout --to-stdout
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Write output on standard output; keep original files unchanged.
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If there are several input files, the output consists of a sequence of
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independently compressed members. To obtain better compression,
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concatenate all input files before compressing them.
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.TP
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.B \-d --decompress --uncompress
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Decompress.
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.TP
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.B \-f --force
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Force compression or decompression even if the file has multiple links
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or the corresponding file already exists, or if the compressed data
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is read from or written to a terminal. If the input data is not in
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a format recognized by
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.I gzip,
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and if the option --stdout is also given, copy the input data without change
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to the standard ouput: let
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.I zcat
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behave as
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.I cat.
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If
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.B \-f
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is not given,
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and when not running in the background,
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.I gzip
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prompts to verify whether an existing file should be overwritten.
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.TP
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.B \-h --help
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Display a help screen and quit.
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.TP
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.B \-l --list
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For each compressed file, list the following fields:
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compressed size: size of the compressed file
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uncompressed size: size of the uncompressed file
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ratio: compression ratio (0.0% if unknown)
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uncompressed_name: name of the uncompressed file
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The uncompressed size is given as -1 for files not in gzip format,
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such as compressed .Z files. To get the uncompressed size for such a file,
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you can use:
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zcat file.Z | wc -c
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In combination with the --verbose option, the following fields are also
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displayed:
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method: compression method
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crc: the 32-bit CRC of the uncompressed data
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date & time: time stamp for the uncompressed file
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The compression methods currently supported are deflate, compress, lzh
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(SCO compress -H) and pack. The crc is given as ffffffff for a file
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not in gzip format.
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With --verbose, the size totals and compression ratio for all files
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is also displayed, unless some sizes are unknown. With --quiet,
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the title and totals lines are not displayed.
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.TP
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.B \-L --license
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Display the
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.I gzip
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license and quit.
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.TP
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.B \-n --no-name
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When compressing, do not save the original file name by default. (The
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original name is always saved if the name had to be truncated.) When
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decompressing, do not restore the original file name if present: remove
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only the
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.I gzip
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suffix from the compressed file name.
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.TP
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.B \-q --quiet
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Suppress all warnings.
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.TP
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.B \-r --recurse
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Travel the directory structure recursively. If any of the file names
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specified on the command line are directories,
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.I gzip
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will descend into the directory and compress all the files it finds there
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(or decompress them in the case of
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.I gunzip
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).
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.TP
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.B \-S .suf --suffix .suf
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Use suffix .suf instead of .gz. Any suffix can be given, but suffixes
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other than .z and .gz should be avoided to avoid confusion when files
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are transferred to other. A null suffix forces gunzip to try
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decompression on all given files regardless of suffix, as in:
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gunzip -S "" * (*.* for MSDOS)
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Previous versions of gzip used
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the .z suffix. This was changed to avoid a conflict with
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.IR pack "(1)".
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.TP
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.B \-t --test
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Test. Check the compressed file integrity.
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.TP
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.B \-v --verbose
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Verbose. Display the name and percentage reduction for each file compressed
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or decompressed.
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.TP
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.B \-V --version
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Version. Display the version number and compilation options then quit.
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.TP
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.B \-# --fast --best
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Regulate the speed of compression using the specified digit
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.IR # ,
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where
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.B \-1
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or
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.B \-\-fast
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indicates the fastest compression method (less compression)
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and
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.B \-9
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or
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.B \-\-best
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indicates the slowest compression method (optimal compression).
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The default compression level is
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.BR \-6
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(that is, biased towards high compression at expense of speed).
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.SH "ADVANCED USAGE"
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Multiple compressed files can be concatenated. In this case,
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.I gunzip
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will extract all members at once. For example:
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gzip -c file1 > foo.gz
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gzip -c file2 >> foo.gz
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Then
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gunzip -c foo
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is equivalent to
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cat file1 file2
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In case of damage to one member of a .gz file, other members can
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still be recovered (if the damaged member is removed). However,
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you can get better compression by compressing all members at once:
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cat file1 file2 | gzip > foo.gz
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compresses better than
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gzip -c file1 file2 > foo.gz
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If you want to recompress concatenated files to get better compression, do:
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zcat old.gz | gzip > new.gz
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.SH "ENVIRONMENT"
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The environment variable
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.B GZIP
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can hold a set of default options for
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.I gzip.
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These options are interpreted first and can be overwritten by
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explicit command line parameters. For example:
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for sh: GZIP="-8 -v"; export GZIP
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for csh: setenv GZIP "-8 -v"
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for MSDOS: set GZIP=-8 -v
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On Vax/VMS, the name of the environment variable is GZIP_OPT, to
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avoid a conflict with the symbol set for invocation of the program.
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.SH "SEE ALSO"
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znew(1), zcmp(1), zmore(1), zforce(1), gzexe(1), zip(1), unzip(1), compress(1),
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pack(1), compact(1)
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.SH "DIAGNOSTICS"
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Exit status is normally 0;
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if an error occurs, exit status is 1. If a warning occurs, exit status is 2.
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.PP
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Usage: gzip [-cdfhlLnrtvV19] [-S suffix] [file ...]
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.in +8
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Invalid options were specified on the command line.
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.in -8
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.IR file :
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not in gzip format
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.in +8
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The file specified to
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.I gunzip
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has not been compressed.
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.in -8
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.IR file:
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Corrupt input. Use zcat to recover some data.
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.in +8
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The compressed file has been damaged. The data up to the point of failure
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can be recovered using
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.in +8
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zcat file > recover
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.in -16
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.IR file :
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compressed with
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.I xx
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bits, can only handle
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.I yy
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bits
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.in +8
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.I File
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was compressed (using LZW) by a program that could deal with
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more
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.I bits
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than the decompress code on this machine.
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Recompress the file with gzip, which compresses better and uses
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less memory.
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.in -8
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.IR file :
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already has .gz suffix -- no change
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.in +8
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The file is assumed to be already compressed.
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Rename the file and try again.
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.in -8
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.I file
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already exists; do you wish to overwrite (y or n)?
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.in +8
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Respond "y" if you want the output file to be replaced; "n" if not.
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.in -8
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gunzip: corrupt input
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.in +8
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A SIGSEGV violation was detected which usually means that the input file has
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been corrupted.
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.in -8
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.I "xx.x%"
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.in +8
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Percentage of the input saved by compression.
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(Relevant only for
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.BR \-v
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and
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.BR \-l \.)
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.in -8
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-- not a regular file or directory: ignored
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.in +8
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When the input file is not a regular file or directory,
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(e.g. a symbolic link, socket, FIFO, device file), it is
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left unaltered.
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.in -8
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-- has
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.I xx
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other links: unchanged
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.in +8
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The input file has links; it is left unchanged. See
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.IR ln "(1)"
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for more information. Use the
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.B \-f
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flag to force compression of multiply-linked files.
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.in -8
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.SH CAVEATS
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When writing compressed data to a tape, it is generally necessary to
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pad the output with zeroes up to a block boundary. When the data is
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read and the whole block is passed to
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.I gunzip
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for decompression,
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.I gunzip
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detects that there is extra trailing garbage after the compressed data
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and emits a warning by default. You have to use the --quiet option to
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suppress the warning. This option can be set in the
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.B GZIP
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environment variable as in:
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for sh: GZIP="-q" tar xfz /dev/rmt/datn
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for csh: (setenv GZIP "-q"; tar xfz /dev/rmt/datn)
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In the above example, gzip is invoked implicitly by the -z option
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of GNU tar. Make sure that the same block size (-b option of
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tar) is used for reading and writing compressed data on tapes.
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.SH BUGS
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The --list option reports incorrect sizes if they exceed 2 gigabytes.
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The --list option reports sizes as -1 and crc as ffffffff if the
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compressed file is on a non seekable media.
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