gzip, gunzip, zcat - compress or expand files
gzip [ -acdfhklLnNrtvV19 ]
[-S suffix] [ name ...
]
gunzip [ -acfhklLnNrtvV ] [-S
suffix] [ name ...
]
zcat [ -fhLV ] [ name ...
]
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, 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
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 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
truncating 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 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.
(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.
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
preserves the
mode, ownership and timestamps of files when compressing or
decompressing.
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.
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.
Display a help screen and 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
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: timestamp 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.
Display the gzip
license and 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 timestamp; 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.
Suppress all warnings.
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
).
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.
Use synchronous output. With this option, gzip
is less
likely to lose data during a system crash, but it can be considerably
slower.
Test. Check the compressed file integrity.
Verbose. Display the name and percentage reduction for each file compressed or decompressed.
Version. Display the version number and compilation options then quit.
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).
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 compression 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.
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
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:
gzip -cd 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 $HOME/bin/gzip containing the
following:
#! /bin/sh export PATH=/usr/bin exec gzip -9 "$@"
znew(1), zcmp(1), zmore(1), zforce(1), gzexe(1), zip(1), unzip(1), compress(1)
The gzip
file format is specified in P. Deutsch, GZIP file
format specification 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).
Exit status is normally 0; if an error occurs, exit status is 1. If a warning occurs, exit status is 2.
Invalid options were specified on the command line.
file
: not in gzip formatThe 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
bitsFile
was compressed (using LZW) by a program that could deal
with more bits
than the decompress code on this machine.
Recompress the file with gzip, which compresses better and uses less
memory.
file
: already has .gz suffix -- unchangedThe 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 not.
A SIGSEGV violation was detected which usually means that the input file has been corrupted.
xx.x%
Percentage of the input saved by compression.(Relevant only for -v and -l.)
When the input file is not a regular file or directory, (e.g. a symbolic link, socket, FIFO, device file), it is left unaltered.
xx
other links: unchangedThe input file has links; it is left unchanged. See ln
(1)
for more information. Use the -f flag to force
compression of multiply-linked files.
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 suppress the warning.
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 compressed 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,
compress
compresses better than gzip
.
Copyright © 1998-1999, 2001-2002, 2012, 2015-2018 Free Software
Foundation, Inc.
Copyright © 1992, 1993 Jean-loup Gailly
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