User Commands 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 ownership
modes, access and modification times. (The default exten-
sion 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 out-
put. 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 com-
pressed to gzi.msd.exe.gz. Names are not truncated on sys-
tems 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 decompressing
the file with the -N option. This is useful when the com-
pressed 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 com-
pressing, 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,
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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 out-
put. 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 installed as gzcat to preserve the original link to com-
press.) zcat uncompresses either a list of files on the
command line or its standard input and writes the uncom-
pressed 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. Typi-
cally, 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 expan-
sion 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 Com-
pressed Data Format Specification version 1.3,
<ftp://ftp.isi.edu/in-notes/rfc1951.txt>, Internet RFC 1951
(May 1996).
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OPTIONS
-a --ascii
Ascii text mode: convert end-of-lines using local con-
ventions. 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 decom-
pressing.
-c --stdout --to-stdout
Write output on standard output; keep original files
unchanged. If there are several input files, the out-
put 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 writ-
ten 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 stan-
dard 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
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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 decom-
pressing, do not restore the original file name if pre-
sent (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 direc-
tories, gzip will descend into the directory and com-
press 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:
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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 com-
pression method (less compression) and -9 or --best
indicates the slowest compression method (best compres-
sion). 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 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:
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gzip -cd old.gz | gzip > new.gz
If a compressed file consists of several members, the uncom-
pressed 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 invo-
cation 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)
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 Com-
pressed 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
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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 com-
presses 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 nec-
essary to pad the output with zeroes up to a block boundary.
When the data is read and the whole block is passed to gun-
zip for decompression, gunzip detects that there is extra
trailing garbage after the compressed data and emits a warn-
ing by default. You have to use the --quiet option to sup-
press the warning. This option can be set in the GZIP envi-
ronment 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.)
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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 fol-
lowing 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 compres-
sion than the default compression level (-6). On some highly
redundant files, compress compresses better than gzip.
COPYRIGHT NOTICE
Copyright (C) 1998, 1999, 2001, 2002 Free Software Founda-
tion, 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 per-
mission notice are preserved on all copies.
Permission is granted to copy and distribute modified ver-
sions of this manual under the conditions for verbatim copy-
ing, provided that the entire resulting derived work is dis-
tributed 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 condi-
tions for modified versions, except that this permission
notice may be stated in a translation approved by the Foun-
dation.
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