This manual is for GNU Gzip (version 1.13, 5 February 2023), and documents commands for compressing and decompressing data.
Copyright © 1998–1999, 2001–2002, 2006–2007, 2009–2023 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 1.3 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
‘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 in the
compressed file. This can be useful when decompressing the file with
-N if the compressed file name was truncated after a file
transfer.
If the original is a regular file, gzip by default keeps its
timestamp in the compressed file. This can be useful when
decompressing the file with -N if the timestamp was not
preserved after a file transfer.
However, due to limitations in the current gzip file
format, fractional seconds are discarded. Also, timestamps must fall
within the range 1970-01-01 00:00:01 through 2106-02-07 06:28:15
UTC, and hosts whose operating systems use 32-bit timestamps
are further restricted to timestamps 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’
‘-gz’, ‘-z’, or ‘_z’ (ignoring case)
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 per 32 KiB block, or
an expansion ratio of 0.015% for large files. The actual number of
used disk blocks almost never increases.
gzip normally preserves the mode and modification timestamp
of a file when compressing or decompressing. If you have appropriate
privileges, it also preserves the file’s owner and group.
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’:
Usage: gzip [OPTION]... [FILE]...
Compress or uncompress FILEs (by default, compress FILES in-place).
Mandatory arguments to long options are mandatory for short options too.
-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
-k, --keep keep (don't delete) input files
-l, --list list compressed file contents
-L, --license display software license
-n, --no-name do not save or restore the original name and timestamp
-N, --name save or restore the original name and timestamp
-q, --quiet suppress all warnings
-r, --recursive operate recursively on directories
--rsyncable make rsync-friendly archive
-S, --suffix=SUF use suffix SUF on compressed files
--synchronous synchronous output (safer if system crashes, but slower)
-t, --test test compressed file integrity
-v, --verbose verbose mode
-V, --version display version number
-1, --fast compress faster
-9, --best compress better
With no FILE, or when FILE is -, read standard input.
Report bugs to <bug-gzip@gnu.org>.
This is the output of the command ‘gzip -v texinfo.tex’:
texinfo.tex: 69.3% -- replaced with texinfo.tex.gz
The following command will find all regular ‘.gz’ files in the current directory and subdirectories (skipping file names that contain newlines), and extract them in place without destroying the original, stopping on the first failure:
find . -name '*
*' -prune -o -name '*.gz' -type f -print |
sed "
s/'/'\\\\''/g
s/^\\(.*\\)\\.gz$/gunzip <'\\1.gz' >'\\1'/
" |
sh -e
gzip ¶The format for running the gzip program is:
gzip option ...
gzip supports the following options:
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.
Decompress.
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 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.
Print an informative help message describing the options then quit.
Keep (don’t delete) input files during compression or decompression.
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
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: timestamp 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.
Display the gzip license then quit.
When compressing, do not save the original file name and timestamp 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
timestamp if present (copy it from the compressed file). This option
is the default when decompressing.
When compressing, always save the original file name, and save the seconds part of the original modification timestamp if the original is a regular file and its timestamp is at least 1 (1970-01-01 00:00:01 UTC) and is less than 232 (2106-02-07 06:28:16 UTC, assuming leap seconds are not counted); this is the default. When decompressing, restore from the saved file name and timestamp if present. This option is useful on systems which have a limit on file name length or when the timestamp has been lost after a file transfer.
Suppress all warning messages.
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).
Cater better to the rsync program by periodically resetting
the internal structure of the compressed data stream. This lets the
rsync program take advantage of similarities in the uncompressed
input when synchronizing two files compressed with this flag. The cost:
the compressed output is usually about one percent larger.
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:
gunzip -S "" * (*.* for MSDOS)
Previous versions of gzip used the ‘.z’ suffix. This was changed to
avoid a conflict with pack.
Use synchronous output, by transferring output data to the output
file’s storage device when the file system supports this. Because
file system data can be cached, without this option if the system
crashes around the time a command like ‘gzip FOO’ is run the user
might lose both FOO and FOO.gz; this is the default with
gzip, just as it is the default with most applications that
move data. When this option is used, gzip is safer but can
be considerably slower.
Test. Check the compressed file integrity.
Verbose. Display the name and percentage reduction for each file compressed.
Version. Display the version number and compilation options, then quit.
Regulate the speed of compression using the specified digit n, where -1 or --fast indicates the fastest compression method (less compression) and --best or -9 indicates the slowest compression method (optimal compression). The default compression level is -6 (that is, biased towards high compression at expense of speed).
An exit status of 0 indicates success, 1 indicates failure, and 2 indicates a warning but not failure.
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 obsolescent 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. As
this can cause problems when using scripts, this feature is supported
only for options that are reasonably likely to not cause too much
harm, and gzip warns if it is used. This feature will be
removed in a future release of gzip.
You can use an alias or script instead. For example, if
gzip is in the directory ‘/usr/bin’ you can prepend
$HOME/bin to your PATH and create an executable script
$HOME/bin/gzip containing the following:
#! /bin/sh export PATH=/usr/bin exec gzip -9 "$@"
The following environment variables are applicable only when using
gzip on IBM Z mainframes supporting DEFLATE COMPRESSION CALL
instruction:
Whether DEFLATE COMPRESSION CALL should be used. Default value is ‘1’. Set this to ‘0’ to disable DEFLATE COMPRESSION CALL altogether.
Compression levels on which DEFLATE COMPRESSION CALL should be used. Represented as a bit mask in decimal or hexadecimal form, where each bit corresponds to a compression level. Default value is ‘2’, which means level 1 only. In order to make use of DEFLATE COMPRESSION CALL by default, that is, on levels 1-6, set this to ‘0x7e’.
Size of deflate blocks produced by DEFLATE COMPRESSION CALL in bytes in decimal or hexadecimal form. Default value is ‘1048576’ (1 megabyte). When using DEFLATE COMPRESSION CALL to compress a file containing heterogeneous data (e.g. a ‘.tar’ archive containing text and binary files), setting this to a smaller value may improve compression ratio.
Size of the first fixed deflate block produced by DEFLATE COMPRESSION CALL in bytes in decimal or hexadecimal form. Default value is ‘4096’ (4 kilobytes). When using DEFLATE COMPRESSION CALL to compress a small file, setting this to a larger value may improve compression ratio.
Value of "Reserved for IBM" field of DEFLATE COMPRESSION CALL parameter block. Default value is ‘0’.
If set to any value, disables compression with DEFLATE COMPRESSION CALL. This variable is normally set during reproducible builds, where DEFLATE COMPRESSION CALL must be disabled, because its output may not be reproducible.
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 can use the --quiet option to suppress
the warning.
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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If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with…Texts.” line with this:
with the Invariant Sections being list their titles, with
the Front-Cover Texts being list, and with the Back-Cover Texts
being list.
If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.
If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.