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GZIP(1) General Commands Manual GZIP(1)
1mNAME0m
gzip, gunzip, zcat - compress or expand files
1mSYNOPSIS0m
1mgzip 22m[ 1m-acdfhklLnNrtvV19 22m] [1m-S suffix22m] [ 4mname24m 4m...24m ]
1mgunzip 22m[ 1m-acfhklLnNrtvV 22m] [1m-S suffix22m] [ 4mname24m 4m...24m ]
1mzcat 22m[ 1m-fhLV 22m] [ 4mname24m 4m...24m ]
1mDESCRIPTION0m
4mGzip24m reduces the size of the named files using Lempel-Ziv coding
(LZ77). Whenever possible, each file is replaced by one with the
extension 1m.gz22m, while keeping the same ownership modes, access and modi-
fication times. (The default extension is 1m-gz 22mfor VMS, 1mz 22mfor 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. 4mGzip24m will only attempt to compress regular files. In particu-
lar, it will ignore symbolic links.
If the compressed file name is too long for its file system, 4mgzip24m trun-
cates it. 4mGzip24m 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, 4mgzip24m keeps the original file name and timestamp in the com-
pressed file. These are used when decompressing the file with the 1m-N0m
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 4mgzip24m 4m-d0m
or 4mgunzip24m or 4mzcat24m. 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.
4mgunzip24m 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. 4mgunzip24m also recognizes the special extensions
1m.tgz 22mand 1m.taz 22mas shorthands for 1m.tar.gz 22mand 1m.tar.Z 22mrespectively. When
compressing, 4mgzip24m uses the 1m.tgz 22mextension if necessary instead of trun-
cating a file with a 1m.tar 22mextension.
4mgunzip24m can currently decompress files created by 4mgzip24m, 4mzip24m, 4mcompress24m,
4mcompress24m 4m-H24m or 4mpack24m. The detection of the input format is automatic.
When using the first two formats, 4mgunzip24m checks a 32 bit CRC. For 4mpack0m
and 4mgunzip24m checks the uncompressed length. The standard 4mcompress24m format
was not designed to allow consistency checks. However 4mgunzip24m 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 4muncompress24m does not complain. This generally means
that the standard 4muncompress24m 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 4mzip24m 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 4mzip24m file with a single member, use a command like 4mgunzip0m
4m<foo.zip24m or 4mgunzip24m 4m-S24m 4m.zip24m 4mfoo.zip24m. To extract zip files with several
members, use 4munzip24m instead of 4mgunzip24m.
4mzcat24m is identical to 4mgunzip24m 1m-c22m. (On some systems, 4mzcat24m may be
installed as 4mgzcat24m to preserve the original link to 4mcompress24m.) 4mzcat0m
uncompresses either a list of files on the command line or its standard
input and writes the uncompressed data on standard output. 4mzcat24m will
uncompress files that have the correct magic number whether they have a
1m.gz 22msuffix or not.
4mGzip24m uses the Lempel-Ziv algorithm used in 4mzip24m 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 4mcompress24m), Huffman coding (as
used in 4mpack24m), or adaptive Huffman coding (4mcompact24m).
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. 4mgzip24m preserves the mode,
ownership and timestamps of files when compressing or decompressing.
1mOPTIONS0m
1m-a --ascii0m
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 con-
verted to CR LF when decompressing.
1m-c --stdout --to-stdout0m
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.
1m-d --decompress --uncompress0m
Decompress.
1m-f --force0m
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 4mgzip24m, and if the option
--stdout is also given, copy the input data without change to
the standard output: let 4mzcat24m behave as 4mcat24m. If 1m-f 22mis not
given, and when not running in the background, 4mgzip24m prompts to
verify whether an existing file should be overwritten.
1m-h --help0m
Display a help screen and quit.
1m-k --keep0m
Keep (don't delete) input files during compression or decompres-
sion.
1m-l --list0m
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: time stamp 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.
1m-L --license0m
Display the 4mgzip24m license and quit.
1m-n --no-name0m
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 4mgzip24m 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.
1m-N --name0m
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.
1m-q --quiet0m
Suppress all warnings.
1m-r --recursive0m
Travel the directory structure recursively. If any of the file
names specified on the command line are directories, 4mgzip24m will
descend into the directory and compress all the files it finds
there (or decompress them in the case of 4mgunzip24m ).
1m-S .suf --suffix .suf0m
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.
1m-t --test0m
Test. Check the compressed file integrity.
1m-v --verbose0m
Verbose. Display the name and percentage reduction for each file
compressed or decompressed.
1m-V --version0m
Version. Display the version number and compilation options then
quit.
1m-# --fast --best0m
Regulate the speed of compression using the specified digit 4m#24m,
where 1m-1 22mor 1m--fast 22mindicates the fastest compression method
(less compression) and 1m-9 22mor 1m--best 22mindicates the slowest com-
pression method (best compression). The default compression
level is 1m-6 22m(that is, biased towards high compression at expense
of speed).
1mADVANCED USAGE0m
Multiple compressed files can be concatenated. In this case, 4mgunzip0m
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.
1mENVIRONMENT0m
The environment variable 1mGZIP 22mcan hold a set of default options for
4mgzip24m. 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.
1mSEE ALSO0m
znew(1), zcmp(1), zmore(1), zforce(1), gzexe(1), zip(1), unzip(1), com-
press(1)
The 4mgzip24m file format is specified in P. Deutsch, GZIP file format spec-
ification version 4.3, 1m<http://www.ietf.org/rfc/rfc1952.txt>22m, Internet
RFC 1952 (May 1996). The 4mzip24m deflation format is specified in P.
Deutsch, DEFLATE Compressed Data Format Specification version 1.3,
1m<http://www.ietf.org/rfc/rfc1951.txt>22m, Internet RFC 1951 (May 1996).
1mDIAGNOSTICS0m
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.
4mfile24m: not in gzip format
The file specified to 4mgunzip24m has not been compressed.
4mfile24m: 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 4mfile24m > recover
4mfile24m: compressed with 4mxx24m bits, can only handle 4myy24m bits
4mFile24m was compressed (using LZW) by a program that could deal
with more 4mbits24m than the decompress code on this machine. Recom-
press the file with gzip, which compresses better and uses less
memory.
4mfile24m: already has .gz suffix -- no change
The file is assumed to be already compressed. Rename the file
and try again.
4mfile24m 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.
4mxx.x%24m Percentage of the input saved by compression.
(Relevant only for 1m-v 22mand 1m-l22m.)
-- 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 4mxx24m other links: unchanged
The input file has links; it is left unchanged. See 4mln24m(1) for
more information. Use the 1m-f 22mflag to force compression of multi-
ply-linked files.
1mCAVEATS0m
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 4mgunzip24m for decompression, 4mgunzip0m
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 1mGZIP 22menvironment
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.)
1mBUGS0m
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, 4mcom-0m
4mpress24m compresses better than 4mgzip24m.
1mCOPYRIGHT NOTICE0m
Copyright (C) 1998, 1999, 2001, 2002, 2012 Free Software Foundation,
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
entire resulting derived work is distributed under the terms of a per-
mission 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.
local GZIP(1)
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