zcat
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 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
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 in-
stalled as gzcat to preserve the original link to compress.) zcat un-
compresses 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 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
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 for-
mat, 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, 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
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 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
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 transpar-
ently. gzip is designed as a complement to tar, not as a replacement.
ENVIRONMENT
The obsolescent environment variable GZIP can hold a set of default op-
tions for gzip. These options are interpreted first and can be over-
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
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 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 (C) 1998-1999, 2001-2002, 2012, 2015-2018 Free Software Foun-
dation, Inc.
Copyright (C) 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.
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