From Linux Man Pages
gzip, gunzip, zcat - compress or expand files
SYNOPSIS
gzip [ -acdfhlLnNrtvV19 ] [-S suffix] [ name ... ]
gunzip [ -acfhlLnNrtvV ] [-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 -gz for VMS, z for MSDOS, OS/2 FAT, Windows NT FAT and Atari.) If no files are speci-
fied, 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 decom-
pressing the file with the -N option. This is useful when the compressed file name was truncated or when the time
stamp 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, _z
or .Z 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, gunzip checks
the uncompressed length. The standard compress format was not designed to allow consistency checks. However gun-
zip 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 for-
mat (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 'defla-
tion' 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.
The gzip file format is specified in P. Deutsch, GZIP file format specification version 4.3,
<ftp://ftp.isi.edu/in-notes/rfc1952.txt>, Internet RFC 1952 (May 1996). The zip deflation format is specified in
P. Deutsch, DEFLATE Compressed Data Format Specification version 1.3, <ftp://ftp.isi.edu/in-notes/rfc1951.txt>,
Internet RFC 1951 (May 1996).
OPTIONS
-a --ascii
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.
-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, concate-
nate 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 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 ouput: 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.
-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
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: time stamp 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 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 decompress-
ing, restore the original file name and time stamp if present. This option is useful on systems which have
a limit on file name length or when the time stamp 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
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(1).
-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 com-
pression). The default compression level is -6 (that is, biased towards high compression at expense of
speed).
ADVANCED USAGE
Multiple compressed files can be concatenated. In this case, gunzip will extract all members at once. For exam-
ple:
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 indepen-
dently, 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 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. For example:
for sh: GZIP="-8v --name"; export GZIP
for csh: setenv GZIP "-8v --name"
for MSDOS: set GZIP=-8v --name
On Vax/VMS, the name of the environment variable is GZIP_OPT, to avoid a conflict with the symbol set for invoca-
tion of the program.
RELATED
znew(1), zcmp(1), zmore(1), zforce(1), gzexe(1), zip(1), unzip(1), compress(1), pack(1), compact(1)
The gzip file format is specified in P. Deutsch, GZIP file format specification version 4.3,
<ftp://ftp.isi.edu/in-notes/rfc1952.txt>, Internet RFC 1952 (May 1996). The zip deflation format is specified in
P. Deutsch, DEFLATE Compressed Data Format Specification version 1.3, <ftp://ftp.isi.edu/in-notes/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 [-cdfhlLnNrtvV19] [-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 -- no change
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 decompression, gunzip detects that
there is extra trailing garbage after the compressed data and emits a warning by default. You have to use the
--quiet option to suppress the warning. This option can be set in the GZIP environment variable as in:
for sh: GZIP="-q" tar -xfz --block-compress /dev/rst0
for csh: (setenv GZIP -q; tar -xfz --block-compr /dev/rst0
In the above example, gzip is invoked implicitly by the -z option of GNU tar. Make sure that the same block size
(-b option of tar) is used for reading and writing compressed data on tapes. (This example assumes you are using
the GNU version of tar.)
BUGS
The gzip format represents the 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 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 modified versions, except that this permission notice may be stated in a translation approved by
the Foundation.
CATEGORY