Computer Science
GZIP(1) GZIP(1)
NAME
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 own-
ership 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 regu-
lar files. In particular, it will ignore symbolic links.
If the compressed file name is too long for its file sys-
tem, 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 times-
tamp 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 neces-
sary 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 for-
mats, 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 compres-
sion 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 uncom-
press 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. Typ-
ically, 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.
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
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 pre-
sent.
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).
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 uncom-
pressed 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: 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
invocation of the program.
SEE ALSO
znew(1), zcmp(1), zmore(1), zforce(1), gzexe(1), zip(1),
unzip(1), compress(1), pack(1), compact(1)
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 com-
pressed.
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 --list option reports incorrect sizes if they exceed 2
gigabytes. 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 compres-
sion than the default compression level (-6). On some
highly redundant files, compress compresses better than
gzip.
local 1
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