gzip.1

gzip源码

.TH GZIP 1 local
.SH NAME
gzip, gunzip, zcat \- compress or expand files
.SH SYNOPSIS
.ll +8
.B gzip
.RB [ " \-acdfhlLnNrtvV19 " ]
.RB [ \-S\ suffix ]
[
.I "name \&..."
]
.ll -8
.br
.B gunzip
.RB [ " \-acfhlLnNrtvV " ]
.RB [ \-S\ suffix ]
[
.I "name \&..."
]
.br
.B zcat
.RB [ " \-fhLV " ]
[
.I "name \&..."
]
.SH DESCRIPTION
.I Gzip
reduces the size of the named files using Lempel-Ziv coding (LZ77).
Whenever possible,
each file is replaced by one with the extension
.B "\&.gz,"
while keeping the same ownership modes, access and modification times.
(The default extension is
.B "\-gz"
for VMS,
.B "z"
for MSDOS, OS/2 FAT, Windows NT FAT and Atari.)
If no files are specified, or if a file name is "-", the standard input is
compressed to the standard output.
.I Gzip
will only attempt to compress regular files.
In particular, it will ignore symbolic links.
.PP
If the compressed file name is too long for its file system,
.I gzip
truncates it.
.I 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.
.PP
By default,
.I gzip
keeps the original file name and timestamp in the compressed file. These
are used when decompressing the file with the
.B \-N
option. This is useful when the compressed file name was truncated or
when the time stamp was not preserved after a file transfer.
.PP
Compressed files can be restored to their original form using
.I gzip -d
or
.I gunzip
or
.I 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.
.PP
.I gunzip
takes a list of files on its command line and replaces each
file whose name ends with .gz, -gz, .z, -z, _z or .Z
and which begins with the correct magic number with an uncompressed
file without the original extension.
.I gunzip
also recognizes the special extensions
.B "\&.tgz"
and
.B "\&.taz"
as shorthands for
.B "\&.tar.gz"
and
.B "\&.tar.Z"
respectively.
When compressing,
.I gzip
uses the
.B "\&.tgz"
extension if necessary instead of truncating a file with a
.B "\&.tar"
extension.
.PP
.I gunzip
can currently decompress files created by
.I gzip, zip, compress, compress -H
or
.I pack.
The detection of the input format is automatic.  When using
the first two formats,
.I gunzip
checks a 32 bit CRC. For
.I pack, gunzip
checks the uncompressed length. The standard
.I compress
format was not designed to allow consistency checks. However
.I 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
.I uncompress
does not complain. This generally means that the standard
.I 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.
.PP
Files created by
.I 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
.I unzip
instead of
.I gunzip.
.PP
.I zcat
is identical to
.I gunzip
.B \-c.
(On some systems,
.I zcat
may be installed as
.I gzcat
to preserve the original link to
.I compress.)
.I zcat
uncompresses either a list of files on the command line or its
standard input and writes the uncompressed data on standard output.
.I zcat
will uncompress files that have the correct magic number whether
they have a
.B "\&.gz"
suffix or not.
.PP
.I Gzip
uses the Lempel-Ziv algorithm used in
.I 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
.IR compress ),
Huffman coding (as used in
.IR pack ),
or adaptive Huffman coding
.RI ( compact ).
.PP
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.
.I gzip
preserves the mode, ownership and timestamps of files when compressing
or decompressing.

.SH OPTIONS
.TP
.B \-a --ascii
Ascii text mode: convert end-of-lines using local conventions. This option
is supported only on some non-Unix systems. For MSDOS, CR LF is converted
to LF when compressing, and LF is converted to CR LF when decompressing.
.TP
.B \-c --stdout --to-stdout
Write output on standard output; keep original files unchanged.
If there are several input files, the output consists of a sequence of
independently compressed members. To obtain better compression,
concatenate all input files before compressing them.
.TP
.B \-d --decompress --uncompress
Decompress.
.TP
.B \-f --force
Force compression or decompression even if the file has multiple links
or the corresponding file already exists, or if the compressed data
is read from or written to a terminal. If the input data is not in
a format recognized by
.I gzip,
and if the option --stdout is also given, copy the input data without change
to the standard ouput: let
.I zcat
behave as
.I cat.
If
.B \-f
is not given,
and when not running in the background,
.I gzip
prompts to verify whether an existing file should be overwritten.
.TP
.B \-h --help
Display a help screen and quit.
.TP
.B \-l --list
For each compressed file, list the following fields:

    compressed size: size of the compressed file
    uncompressed size: size of the uncompressed file
    ratio: compression ratio (0.0% if unknown)
    uncompressed_name: name of the uncompressed file

The uncompressed size is given as -1 for files not in gzip format,
such as compressed .Z files. To get the uncompressed size for such a file,
you can use:

    zcat file.Z | wc -c

In combination with the --verbose option, the following fields are also
displayed:

    method: compression method
    crc: the 32-bit CRC of the uncompressed data
    date & time: time stamp for the uncompressed file

The compression methods currently supported are deflate, compress, lzh
(SCO compress -H) and pack.  The crc is given as ffffffff for a file
not in gzip format.

With --name, the uncompressed name,  date and time  are
those stored within the compress file if present.

With --verbose, the size totals and compression ratio for all files
is also displayed, unless some sizes are unknown. With --quiet,
the title and totals lines are not displayed.
.TP
.B \-L --license
Display the
.I gzip
license and quit.
.TP
.B \-n --no-name
When compressing, do not save the original file name and time stamp by
default. (The original name is always saved if the name had to be