NAME
gzip, gunzip, zcat - compress or expand files
SYNOPSIS
gzip [ -acdfhlLnrtvV19 ] [-S suffix] [ name ... ]
gunzip [ -acfhlLnrtvV ] [-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 and Atari.) If no
files are specified, 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 new file name is too long for its file system, gzip
truncates it and keeps the original file name in the
compressed file. Gzip attempts to truncate only the parts
of the file name longer than 3 characters. (A part is del-
imited 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.
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, gun-
zip 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 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. 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.
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
decompressing.
-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
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 --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 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.
-q --quiet
Suppress all warnings.
-r --recurse
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
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 transferred
to other. A null suffix forces gunzip 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 specified
digit #, where -1 or --fast indicates the fastest
compression method (less compression) and -9 or --best
indicates the slowest compression method (optimal
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:
zcat 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.
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="-8 -v"; export GZIP
for csh: setenv GZIP "-8 -v"
for MSDOS: set GZIP=-8 -v
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)
DIAGNOSTICS
Exit status is normally 0; if an error occurs, exit status
is 1. If a warning occurs, exit status is 2.
Usage: gzip [-cdfhlLnrtvV19] [-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. 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 direc-
tory, (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 boun-
dary. 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 warn-
ing 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 /dev/rmt/datn
for csh: (setenv GZIP "-q"; tar xfz /dev/rmt/datn)
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.
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.
