uncompress(1) - man - phpMan

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GZIP(1) General Commands Manual GZIP(1)

NAME
gzip, gunzip, zcat - compress or expand files

SYNOPSIS
gzip [ -acdfhklLnNrtvV19 ] [-S suffix] [ name ... ]
gunzip [ -acfhklLnNrtvV ] [-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 extension is 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
particular, it will ignore symbolic links.

If the compressed file name is too long for its file system, gzip truncates it. Gzip at‐
tempts to truncate only the parts of the file name longer than 3 characters. (A part is de‐
limited 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 compressed 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 timestamp 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, or _z (ignoring case) and which begins with the correct magic number with
an uncompressed file without the original extension. gunzip also recognizes the special ex‐
tensions .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 and gunzip checks the uncompressed length. The standard com‐
press 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 gener‐
ally 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 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. 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 adap‐
tive 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 ev‐
ery 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 time‐
stamps of files when compressing or decompressing.

OPTIONS
-a --ascii
Ascii text mode: convert end-of-lines using local conventions. This option is sup‐
ported only on some non-Unix systems. For MSDOS, CR LF is converted to LF when com‐
pressing, 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 corre‐
sponding 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 output: 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.

-k --keep
Keep (don't delete) input files during compression or decompression.

-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: timestamp 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 com‐
press 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 dis‐
played.

-L --license
Display the gzip license and quit.

-n --no-name
When compressing, do not save the original file name and timestamp 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 timestamp 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 timestamp; this is the de‐
fault. When decompressing, restore the original file name and timestamp if present.
This option is useful on systems which have a limit on file name length or when the
timestamp 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
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 de‐
riving an output file name from an input file name.

--synchronous
Use synchronous output. With this option, gzip is less likely to lose data during a
system crash, but it can be considerably slower.

-t --test
Test. Check the compressed file integrity.

-v --verbose
Verbose. Display the name and percentage reduction for each file compressed or decom‐
pressed.

-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 in‐
dicates 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 compression at expense of speed).

--rsyncable
When you synchronize a compressed file between two computers, this option allows rsync
to transfer only files that were changed in the archive instead of the entire archive.
Normally, after a change is made to any file in the archive, the compression algorithm
can generate a new version of the archive that does not match the previous version of
the archive. In this case, rsync transfers the entire new version of the archive to
the remote computer. With this option, rsync can transfer only the changed files as
well as a small amount of metadata that is required to update the archive structure in
the area that was changed.

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 mem‐
bers 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 op‐
tion to invoke gzip transparently. gzip is designed as a complement to tar, not as a replace‐
ment.

ENVIRONMENT
The obsolescent 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. As
this can cause problems when using scripts, this feature is supported only for options that
are reasonably likely to not cause too much harm, and gzip warns if it is used. This feature
will be removed in a future release of gzip.

You can use an alias or script instead. For example, if gzip is in the directory /usr/bin
you can prepend $HOME/bin to your PATH and create an executable script $HOME/bin/gzip con‐
taining the following:

#! /bin/sh
export PATH=/usr/bin
exec gzip -9 "$@"

SEE ALSO
znew(1), zcmp(1), zmore(1), zforce(1), gzexe(1), zip(1), unzip(1), compress(1)

The gzip file format is specified in P. Deutsch, GZIP file format specification version 4.3,
<https://www.ietf.org/rfc/rfc1952.txt>, Internet RFC 1952 (May 1996). The zip deflation for‐
mat is specified in P. Deutsch, DEFLATE Compressed Data Format Specification version 1.3,
<https://www.ietf.org/rfc/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 [-cdfhklLnNrtvV19] [-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 re‐
covered 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 bet‐
ter and uses less memory.

file: already has .gz suffix -- unchanged
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 cor‐
rupted.

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 ze‐
roes 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 can use the --quiet option to suppress the warning.

BUGS
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 compressed 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, compress compresses better than gzip.

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 con‐
ditions for verbatim copying, provided that the entire resulting derived work is distributed
under the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual into another lan‐
guage, under the above conditions for modified versions, except that this permission notice
may be stated in a translation approved by the Foundation.

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