gzip
<h2>NAME</h2>
gzip, gunzip, zcat - compress or expand files
<a name“lbAC”> </a>
<h2>SYNOPSIS</h2>
gzip
[ -acdfhlLnNrtvV19 ]
[-S suffix]
[
name …
]
gunzip
[ -acfhlLnNrtvV ]
[-S suffix]
[
name …
]
zcat
[ -fhLV ]
[
name …
]
<a name“lbAD”> </a>
<h2>DESCRIPTION</h2>
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, 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. Gzip
will only attempt to compress regular files. In particular, it will ignore symbolic links.
<p>
</p><div id“wide”><script>zSB(3,3)</script></div>
If the compressed 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.
<p>
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.
</p><p>
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.
</p><p>
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.
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.
</p><p>
gunzip
can currently decompress files created by
gzip, zip, compress, compress -H
or
pack.
The detection of the input format is automatic. When using
the first two formats,
gunzip
checks a 32 bit CRC. For
pack, gunzip
checks the uncompressed length. The standard
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.
</p><p>
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.
</p><p>
zcat
is identical to
gunzip
-c.
(On some systems,
zcat
may be installed as
gzcat
to preserve the original link to
compress.)
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.
</p><p>
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).
</p><p>
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.
</p><p>
<a name“lbAE”> </a>
</p><h2>OPTIONS</h2>
<dl compact“compact”>
<dt>-a –ascii
</dt><dd>
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.
</dd><dt>-c –stdout –to-stdout
</dt><dd>
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.
</dd><dt>-d –decompress –uncompress
</dt><dd>
Decompress.
</dd><dt>-f –force
</dt><dd>
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
gzip,
and if the option –stdout is also given, copy the input data without change
to the standard ouput: let
zcat
behave as
cat.
If
-f
is not given,
and when not running in the background,
gzip
prompts to verify whether an existing file should be overwritten.
</dd><dt>-h –help
</dt><dd>
Display a help screen and quit.
</dd><dt>-l –list
</dt><dd>
For each compressed file, list the following fields:
<p>
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
</p><p>
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:
</p><p>
zcat file.Z | wc -c
</p><p>
In combination with the –verbose option, the following fields are also
displayed:
</p><p>
method: compression method
crc: the 32-bit CRC of the uncompressed data
date & time: time stamp for the uncompressed file
</p><p>
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.
</p><p>
With –name, the uncompressed name, date and time are
those stored within the compress file if present.
</p><p>
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.
</p></dd><dt>-L –license
</dt><dd>
Display the
gzip
license and quit.
</dd><dt>-n –no-name
</dt><dd>
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
truncated.) When decompressing, do not restore the original file name
if present (remove only the
gzip
suffix from the compressed file name) and do not restore the original
time stamp if present (copy it from the compressed file). This option
is the default when decompressing.
</dd><dt>-N –name
</dt><dd>
When compressing, always save the original file name and time stamp; this
is the default. When decompressing, restore the original file name and
time stamp if present. This option is useful on systems which have
a limit on file name length or when the time stamp has been lost after
a file transfer.
</dd><dt>-q –quiet
</dt><dd>
Suppress all warnings.
</dd><dt>-r –recursive
</dt><dd>
Travel the directory structure recursively. If any of the file names
specified on the command line are directories,
gzip
will descend into the directory and compress all the files it finds there
(or decompress them in the case of
gunzip
).
</dd><dt>-S .suf –suffix .suf
</dt><dd>
Use suffix .suf instead of .gz. Any suffix can be given, but suffixes
other than .z and .gz should be avoided to avoid confusion when files
are transferred to other systems. A null suffix forces gunzip to try
decompression on all given files regardless of suffix, as in:
<p>
gunzip -S ““ (. for MSDOS)
</p><p>
Previous versions of gzip used
the .z suffix. This was changed to avoid a conflict with
pack(1).
</p></dd><dt>-t –test
</dt><dd>
Test. Check the compressed file integrity.
</dd><dt>-v –verbose
</dt><dd>
Verbose. Display the name and percentage reduction for each file compressed
or decompressed.
</dd><dt>-V –version
</dt><dd>
Version. Display the version number and compilation options then quit.
</dd><dt>-# –fast –best
</dt><dd>
Regulate the speed of compression using the specified digit
#,
where
-1
or
–fast
indicates the fastest compression method (less compression)
and
-9
or
–best
indicates the slowest compression method (best compression).
The default compression level is
-6**
(that is, biased towards high compression at expense of speed).
</dd></dl>
<a name“lbAF”> </a>
<h2>ADVANCED USAGE</h2>
Multiple compressed files can be concatenated. In this case, gunzip
will extract all members at once. For example: <p>
gzip -c file1 > foo.gz
gzip -c file2 >> foo.gz
</p><p> Then </p><p>
gunzip -c foo
</p><p>
is equivalent to </p><p>
cat file1 file2
</p><p> In case of damage to one member of a .gz file, other members can still be recovered (if the damaged member is removed). However, you can get better compression by compressing all members at once: </p><p>
cat file1 file2 | gzip > foo.gz
</p><p> compresses better than </p><p>
gzip -c file1 file2 > foo.gz
</p><p> If you want to recompress concatenated files to get better compression, do: </p><p>
gzip -cd old.gz | gzip > new.gz
</p><p> If a compressed file consists of several members, the uncompressed size and CRC reported by the –list option applies to the last member only. If you need the uncompressed size for all members, you can use: </p><p>
gzip -cd file.gz | wc -c
</p><p> If you wish to create a single archive file with multiple members so that members can later be extracted independently, use an archiver such as tar or zip. GNU tar supports the -z option to invoke gzip transparently. gzip is designed as a complement to tar, not as a replacement. <a name''“lbAG”> </a> </p>
– Main.FredPettis - 2010-06-24