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GZIP(1) General Commands Manual GZIP(1)
NAME
gzip, gunzip, zcat - compress or expand files
SYNOPSIS
gzip [ -acdfhklLnNrtvV19 ] [-S suffix] [ name ... ]
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
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 stan‐
dard 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 char‐
acters, 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 com‐
pressed 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 uncom‐
pressed file without the original extension. gunzip also recog‐
nizes 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, com‐
press, 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 uncom‐
press does not check its input, and happily generates garbage out‐
put. 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 fea‐
ture 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 gun‐
zip.
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 com‐
press), Huffman coding (as used in pack), or adaptive Huffman cod‐
ing (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 com‐
pressing or decompressing.
OPTIONS
-a --ascii
Ascii text mode: convert end-of-lines using local conven‐
tions. This option is supported only on some non-Unix sys‐
tems. For MSDOS, CR LF is converted to LF when compressing,
and LF is converted 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 sequence 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 compressed data is read from or written to a termi‐
nal. 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
decompression.
-l --list
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 uncom‐
pressed 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.
-L --license
Display the gzip license and quit.
-n --no-name
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 decom‐
pressing.
-N --name
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.
-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
Use synchronous output. With this option, gzip is less
likely to lose data during a system crash, but it can be
considerably slower.
-t --test
Test. Check the compressed file integrity.
-v --verbose
Verbose. Display the name and percentage reduction for each
file compressed or decompressed.
-V --version
Version. Display the version number and compilation options
then quit.
-# --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 compression method (best compression). The default
compression level is -6 (that is, biased towards high com‐
pression at expense of speed).
--rsyncable
When you synchronize a compressed file between two comput‐
ers, 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 ar‐
chive, 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.
ADVANCED USAGE
Multiple compressed files can be concatenated. In this case, gun‐
zip 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 com‐
pression, 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.
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 $HOME/bin/gzip containing the fol‐
lowing:
#! /bin/sh
export PATH=/usr/bin
exec gzip -9 "$@"
SEE ALSO
znew(1), zcmp(1), zmore(1), zforce(1), gzexe(1), zip(1), unzip(1),
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 speci‐
fied 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. Recompress 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 not.
gunzip: corrupt input
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.)
-- not a regular file or directory: ignored
When the input file is not a regular file or directory,
(e.g. a symbolic link, socket, FIFO, device file), it is
left unaltered.
-- has xx other links: unchanged
The 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.
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 decom‐
pression, 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.
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
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 NOTICE
Copyright © 1998-1999, 2001-2002, 2012, 2015-2018 Free Software
Foundation, 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 entire resulting derived work is distributed under the
terms of a permission notice identical to this one.
Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modi‐
fied versions, except that this permission notice may be stated in
a translation approved by the Foundation.
local GZIP(1)