more detailed info-page available further down.
gunzip [ -acfhklLnNrtvV ] [-S suffix] [ name ... ]
zcat [ -fhLV ] [ name ... ]
DESCRIPTION
Gzip reduces the size of the named files using Lempel-Ziv coding
(LZ77). Whenever possible, each file is replaced by one with the ex-
tension .gz, while keeping the same ownership modes, access and modifi-
cation times. (The default extension is 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.
If the compressed file name is too long for its file system, gzip trun-
cates 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 ex-
ample, 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 com-
pressed 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 timestamp was not preserved after a file transfer.
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, -gz, .z, -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 trun-
cating a file with a .tar extension.
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
and gunzip checks the uncompressed length. The standard compress format
was not designed to allow consistency checks. However gunzip is some-
times able to detect a bad .Z file. If you get an error when uncom-
pressing 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 gen-
erates 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
of compression obtained depends on the size of the input and the dis-
tribution 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 ex-
pansion ratio of 0.015% for large files. Note that the actual number of
used disk blocks almost never increases. gzip preserves the mode, own-
ership and timestamps of files when compressing or decompressing.
OPTIONS
-a --ascii
Ascii text mode: convert end-of-lines using local conventions.
This option is supported only on some non-Unix systems. For MS-
DOS, CR LF is converted to LF when compressing, and LF is con-
verted to CR LF when decompressing.
-c --stdout --to-stdout
Write output on standard output; keep original files unchanged.
If there are several input files, the output consists of a se-
quence of independently compressed members. To obtain better
compression, concatenate all input files before compressing
them.
-d --decompress --uncompress
Decompress.
-f --force
Force compression or decompression even if the file has multiple
links or the corresponding file already exists, or if the com-
pressed 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.
-h --help
Display a help screen and quit.
-k --keep
Keep (don't delete) input files during compression or decompres-
sion.
-l --list
For each compressed file, list the following fields:
compressed size: size of the compressed file
uncompressed size: size of the uncompressed file
date & time: timestamp for the uncompressed file
The compression methods currently supported are deflate, com-
press, 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.
-L --license
Display the gzip license and quit.
-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 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.
-N --name
When compressing, always save the original file name and time-
stamp; this is the default. When decompressing, restore the
original file name and timestamp if present. This option is use-
ful on systems which have a limit on file name length or when
the timestamp has been lost after a file transfer.
-q --quiet
Suppress all warnings.
-r --recursive
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 ).
-S .suf --suffix .suf
When compressing, use suffix .suf instead of .gz. Any non-empty
suffix can be given, but suffixes other than .z and .gz should
be avoided to avoid confusion when files are transferred to
other systems.
When decompressing, add .suf to the beginning of the list of
suffixes to try, when deriving an output file name from an input
file name.
--synchronous
-# --fast --best
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 com-
pression method (best compression). The default compression
level is -6 (that is, biased towards high compression at expense
of speed).
--rsyncable
When you synchronize a compressed file between two computers,
this option allows rsync to transfer only files that were
changed in the archive instead of the entire archive. Normally,
after a change is made to any file in the archive, the compres-
sion algorithm can generate a new version of the archive that
does not match the previous version of the archive. In this
case, rsync transfers the entire new version of the archive to
the remote computer. With this option, rsync can transfer only
the changed files as well as a small amount of metadata that is
required to update the archive structure in the area that was
changed.
ADVANCED USAGE
Multiple compressed files can be concatenated. In this case, gunzip
will extract all members at once. For example:
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:
gzip -cd old.gz | gzip > new.gz
written by explicit command line parameters. As this can cause prob-
lems 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 $HOME/bin/gzip containing the following:
#! /bin/sh
export PATH=/usr/bin
exec gzip -9 "$@"
SEE ALSO
znew(1), zcmp(1), zmore(1), zforce(1), gzexe(1), zip(1), unzip(1), com-
press(1)
The gzip file format is specified in P. Deutsch, GZIP file format spec-
ification version 4.3, <https://www.ietf.org/rfc/rfc1952.txt>, Internet
RFC 1952 (May 1996). The zip deflation format is specified in P.
Deutsch, DEFLATE Compressed Data Format Specification version 1.3,
<https://www.ietf.org/rfc/rfc1951.txt>, Internet RFC 1951 (May 1996).
DIAGNOSTICS
Exit status is normally 0; if an error occurs, exit status is 1. If a
warning occurs, exit status is 2.
Usage: gzip [-cdfhklLnNrtvV19] [-S suffix] [file ...]
Invalid options were specified on the command line.
file: not in gzip format
The file specified to gunzip has not been compressed.
file: Corrupt input. Use zcat to recover some data.
The compressed file has been damaged. The data up to the point
of failure can be recovered using
zcat file > recover
file: compressed with xx bits, can only handle yy bits
File was compressed (using LZW) by a program that could deal
with more bits than the decompress code on this machine. Recom-
press the file with gzip, which compresses better and uses less
memory.
file: already has .gz suffix -- unchanged
The file is assumed to be already compressed. Rename the file
and try again.
file already exists; do you wish to overwrite (y or n)?
Respond "y" if you want the output file to be replaced; "n" if
The input file has links; it is left unchanged. See ln(1) for
more information. Use the -f flag to force compression of multi-
ply-linked files.
CAVEATS
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. You can use the --quiet option to sup-
press the warning.
BUGS
The gzip format represents the input size modulo 2^32, so the --list
option reports incorrect uncompressed sizes and compression ratios 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
The --list option reports sizes as -1 and crc as ffffffff if the com-
pressed file is on a non seekable media.
In some rare cases, the --best option gives worse compression than the
default compression level (-6). On some highly redundant files, com-
press compresses better than gzip.
COPYRIGHT NOTICE
Copyright © 1998-1999, 2001-2002, 2012, 2015-2018 Free Software Founda-
tion, Inc.
Copyright © 1992, 1993 Jean-loup Gailly
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the en-
tire resulting derived work is distributed under the terms of a permis-
sion notice identical to this one.
Permission is granted to copy and distribute translations of this man-
ual into another language, under the above conditions for modified ver-
sions, except that this permission notice may be stated in a transla-
tion approved by the Foundation.
local GZIP(1)
Copyright © 1998-1999, 2001-2002, 2006-2007, 2009-2018 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".
* Menu:
* Overview:: Preliminary information.
* Sample:: Sample output from 'gzip'.
* Invoking gzip:: How to run 'gzip'.
* Advanced usage:: Concatenated files.
* Environment:: Environment variables.
* Tapes:: Using 'gzip' on tapes.
* Problems:: Reporting bugs.
* GNU Free Documentation License:: Copying and sharing this manual.
* Concept index:: Index of concepts.
File: gzip.info, Node: Overview, Next: Sample, Prev: Top, Up: Top
1 Overview
**********
'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
'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 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 timestamps 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
(https://www.ietf.org/rfc/rfc1952.txt) (May 1996). The 'zip' deflation
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
File: gzip.info, Node: Invoking gzip, Next: Advanced usage, Prev: Sample, Up: Top
3 Invoking 'gzip'
*****************
The format for running the 'gzip' program is:
'-d'
Decompress.
'--force'
'-f'
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.
'--help'
'-h'
Print an informative help message describing the options then quit.
'--keep'
'-k'
Keep (don't delete) input files during compression or
decompression.
'--list'
'-l'
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 (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.
The 'gzip' format represents the input size modulo 2^32, so the
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.
'--name'
'-N'
When compressing, always save the original file name, and save the
original timestamp if the original is a regular file; this is the
default. When decompressing, restore the original 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.
'--quiet'
'-q'
Suppress all warning messages.
'--recursive'
'-r'
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').
'--rsyncable'
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.
'--suffix SUF'
'-S SUF'
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'.
'--synchronous'
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
'--version'
'-V'
Version. Display the version number and compilation options, then
quit.
'--fast'
'--best'
'-N'
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).
File: gzip.info, Node: Advanced usage, Next: Environment, Prev: Invoking gzip, Up: Top
4 Advanced usage
****************
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:
File: gzip.info, Node: Environment, Next: Tapes, Prev: Advanced usage, Up: Top
5 Environment
*************
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:
'DFLTCC'
Whether DEFLATE COMPRESSION CALL should be used. Default value is
'1'. Set this to '0' to disable DEFLATE COMPRESSION CALL
altogether.
'DFLTCC_LEVEL_MASK'
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'.
'DFLTCC_BLOCK_SIZE'
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.
'DFLTCC_FIRST_FHT_BLOCK_SIZE'
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.
***********************
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.
File: gzip.info, Node: Problems, Next: GNU Free Documentation License, Prev: Tapes, Up: Top
7 Reporting Bugs
****************
If you find a bug in 'gzip', please send electronic mail to
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