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
The gzip command reduces the size of the named files using Lempel-Ziv coding (LZ77). Whenever possible, each file is re‐
placed 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. The gzip command will only attempt to compress regular files. In par‐
ticular, it will ignore symbolic links.

If the compressed file name is too long for its file system, gzip truncates it. The gzip command 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 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 (ignor‐
ing case) 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 for‐
mat is automatic. When using the first two formats, gunzip checks a 32 bit CRC. For pack and gunzip checks the uncompressed
length. The standard compress format was not designed to allow consistency checks. However gunzip is sometimes able to de‐
tect a bad .Z file. If you get an error when uncompressing a .Z file, do not assume that the .Z file is correct simply be‐
cause 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.

The zcat command 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 uncom‐
pressed data on standard output. zcat will uncompress files that have the correct magic number whether they have a .gz suf‐
fix or not.

The gzip command 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 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 per 32 KiB block, or an expansion ratio of 0.015% for large files. The
actual number of used disk blocks almost never increases.

gzip normally preserves the mode and modification timestamp of a file when compressing or decompressing. If you have appro‐
priate privileges, it also preserves the file's owner and group.

OPTIONS
-a --ascii
Ascii text mode: convert end-of-lines using local conventions. This option is supported only on some non-Unix sys‐
tems. 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 output con‐
sists 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 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 exist‐
ing 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 uncom‐
pressed 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 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 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 save the seconds part of the original modification time‐
stamp if the original is a regular file and its timestamp is at least 1 (1970-01-01 00:00:01 UTC) and is less than
2**32 (2106-02-07 06:28:16 UTC, assuming leap seconds are not counted); this is the default. When decompressing, re‐
store from the saved 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 deriving 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 con‐
siderably slower.

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

-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 (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 ar‐
chive, 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 up‐
date 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). How‐
ever, 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 transparently. gzip is designed as a comple‐
ment to tar, not as a replacement.

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 fea‐
ture 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 containing 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 format 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 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 -- 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 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 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 can use the --quiet option to suppress the warning.

BUGS
In some rare cases, the --best option gives worse compression than the default compression level (-6). On some highly redun‐
dant files, compress compresses better than gzip.

REPORTING BUGS
Report bugs to: bug-gzip@gnu.org
GNU gzip home page: <https://www.gnu.org/software/gzip/>
General help using GNU software: <https://www.gnu.org/gethelp/>

Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permis‐
sion 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 permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual into another language, 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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