This manual is for Gzip (version 1.3.8, 8 December 2006), and documents commands for compressing and decompressing data.
Copyright © 1998, 1999, 2001, 2002, 2006 Free Software Foundation, Inc.
Copyright © 1992, 1993 Jean-loup Gailly
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”.
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. However, due to limitations in the current gzip file
format, fractional seconds are discarded. Also, time stamps must fall
within the range 1970-01-01 00:00:00 through 2106-02-07 06:28:15
UTC, and hosts whose operating systems use 32-bit time
stamps are further restricted to time stamps no later than 2038-01-19
03:14:07 UTC. The upper bounds assume the typical case
where leap seconds are ignored.
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 a zip file with a single
member, use a command like `gunzip <foo.zip' or `gunzip -S
.zip foo.zip'. 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 normally preserves the mode,
ownership and time stamps of files when compressing or decompressing.
The gzip file format is specified in P. Deutsch, gzip file
format specification version 4.3,
Internet RFC 1952 (May
1996). The zip deflation format is specified in P. Deutsch,
deflate Compressed Data Format Specification version 1.3,
Internet RFC 1951 (May
1996).
Here are some realistic examples of running gzip.
This is the output of the command `gzip -h':
gzip version-number
usage: gzip [-cdfhlLnNrtvV19] [-S suffix] [file ...]
-c --stdout write on standard output, keep original files unchanged
-d --decompress decompress
-f --force force overwrite of output file and compress links
-h --help give this help
-l --list list compressed file contents
-L --license display software license
-n --no-name do not save or restore the original name and time stamp
-N --name save or restore the original name and time stamp
-q --quiet suppress all warnings
-r --recursive operate recursively on directories
-S .suf --suffix .suf use suffix .suf on compressed files
-t --test test compressed file integrity
-v --verbose verbose mode
-V --version display version number
-1 --fast compress faster
-9 --best compress better
file... files to (de)compress. If none given, use standard input.
Report bugs to <bug-gzip@gnu.org>.
This is the output of the command `gzip -v texinfo.tex':
texinfo.tex: 69.7% -- replaced with texinfo.tex.gz
The following command will find all gzip files in the current
directory and subdirectories, and extract them in place without
destroying the original:
find . -name '*.gz' -print | sed 's/^\(.*\)[.]gz$/gunzip < "&" > "\1"/' | sh
gzip
The format for running the gzip program is:
gzip option ...
gzip supports the following options:
gzip, and if the option `--stdout' is also
given, copy the input data without change to the standard output: 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.
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 (deflate,compress,lzh,pack)
crc: the 32-bit CRC of the uncompressed data
date & time: time stamp for the uncompressed file
The crc is given as ffffffff for a file not in gzip format.
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.
The gzip format represents the input size modulo
2^32, so the uncompressed size and compression ratio are listed
incorrectly for uncompressed files 4 GB and larger. To work around
this problem, you can use the following command to discover a large
uncompressed file's true size:
zcat file.gz | wc -c
gzip license then quit.
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.
gzip will descend
into the directory and compress all the files it finds there (or
decompress them in the case of gunzip).
gunzip -S "" * (*.* for MSDOS)
Previous versions of gzip used the `.z' suffix. This was changed to
avoid a conflict with pack.
Multiple compressed files can be concatenated. In this case,
gunzip will extract all members at once. If one member is
damaged, other members might still be recovered after removal of the
damaged member. Better compression can be usually obtained if all
members are decompressed and then recompressed in a single step.
This is an example of concatenating gzip files:
gzip -c file1 > foo.gz
gzip -c file2 >> foo.gz
Then
gunzip -c foo
is equivalent to
cat file1 file2
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:
cat file1 file2 | gzip > foo.gz
compresses better than
gzip -c file1 file2 > foo.gz
If you want to recompress concatenated files to get better compression, do:
zcat old.gz | gzip > new.gz
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:
zcat file.gz | wc -c
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.
The environment variable GZIP can hold a set of default options for
gzip. These options are interpreted first and can be overwritten by
explicit command line parameters. For example:
for sh: GZIP="-8v --name"; export GZIP
for csh: setenv GZIP "-8v --name"
for MSDOS: set GZIP=-8v --name
On Vax/VMS, the name of the environment variable is GZIP_OPT, to
avoid a conflict with the symbol set for invocation of the program.
gzip on tapes
When writing compressed data to a tape, it is generally necessary to pad
the output with zeroes up to a block boundary. When the data is read and
the whole block is passed to gunzip for decompression,
gunzip detects that there is extra trailing garbage after the
compressed data and emits a warning by default if the garbage contains
nonzero bytes. You have to use the
`--quiet' option to suppress the warning. This option can be set in the
GZIP environment variable, as in:
for sh: GZIP="-q" tar -xfz --block-compress /dev/rst0
for csh: (setenv GZIP "-q"; tar -xfz --block-compress /dev/rst0)
In the above example, gzip is invoked implicitly by the `-z'
option of GNU tar. Make sure that the same block size (`-b'
option of tar) is used for reading and writing compressed data on
tapes. (This example assumes you are using the GNU version of
tar.)
If you find a bug in gzip, please send electronic mail to
bug-gzip@gnu.org. Include the version number,
which you can find by running `gzip -V'. Also include in your
message the hardware and operating system, the compiler used to compile
gzip,
a description of the bug behavior, and the input to gzip that triggered
the bug.
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