gzcat(1L)
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 modi-
fication 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 particu-
lar, it will ignore symbolic links.
If the compressed file name is too long for its file system, gzip trun-
cates 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 com-
pressed 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 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 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 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 spec-
ification version 4.3, <ftp://ftp.isi.edu/in-notes/rfc1952.txt>, Inter-
net 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 con-
verted 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 com-
pressed 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 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.
-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 ).
--rsyncable
While compressing, synchronize the output occasionally based on
the input. This increases size by less than 1 percent most
cases, but means that the rsync(1) program can much more effi-
ciently syncronize files compressed with this flag. gunzip can-
not tell the difference between a compressed file created with
this option, and one created without it.
-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 confu-
sion when files are transferred to other systems. A null suffix
forces gunzip to try decompression on all given files regard-
less 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 com-
pression method (best compression). 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:
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.
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), com-
press(1), pack(1), compact(1)
The gzip file format is specified in P. Deutsch, GZIP file format spec-
ification version 4.3, <ftp://ftp.isi.edu/in-notes/rfc1952.txt>, Inter-
net 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. Recom-
press 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 multi-
ply-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 uncom-
pressed 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, com-
press compresses better than gzip.
COPYRIGHT NOTICE
Copyright (C) 1998, 1999, 2001, 2002 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.
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