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gzip
Compress or decompress files
SYNTAX
gzip [ -acdfhlLnNrtvV19 ] [-S suffix] [ name ... ]
gunzip [ -acfhlLnNrtvV ] [-S suffix] [ name ... ]
zcat [ -fhLV ] [ name ... ]
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,
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 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
This option reports incorrect sizes if they exceed 2
gigabytes.
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 fol-
lowing 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
decompressing, 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 trans-
ferred to other systems. A null suffix forces gun-
zip 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 speci-
fied 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 compres-
sion at expense of speed).
In some rare cases, the --best option gives worse compres-
sion than the default compression level (-6). On some
highly redundant files, compress compresses better than
gzip.
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 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 time stamp 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 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 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 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.
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 mem-
bers can still be recovered (if the damaged member is
removed). However, you can get better compression by com-
pressing 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 sup-
ports 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/bash: GZIP="-8v --name"; export GZIP
for csh: setenv GZIP "-8v --name"
"Small is beautiful" - Schumacher's Dictum
Related commands:
znew(1), zcmp(1), zmore(1), zforce(1), gzexe(1), zip(1), unzip(1), compress(1),
pack(1), compact(1)
Equivalent BASH command:
gzip - Compress files
