Tribblix: manual page: gzip.1

GZIP(1) User Commands 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 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.
The gzip command 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. 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 valid.

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
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 and 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.

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 uncompressed data on standard output.
zcat will uncompress files that have the correct magic number whether
they have a .gz suffix 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 appropriate
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 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 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 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 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 timestamp 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, restore 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. Although
any non-empty suffix can be given so long as it does not
contain "/", 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 considerably
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 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 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 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 overridden by
explicit command line parameters. As this can cause problems when
using scripts, this feature is supported only for --rsyncable,
--synchronous, and options like -9 that set the compression level;
any other options or operands in GZIP are silently ignored.

You can use an alias or script instead. For example, instead of
setting GZIP="-9" in the environment, 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 "$@"

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 redundant 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/>

COPYRIGHT NOTICE

Copyright (C) 1998-1999, 2001-2002, 2012, 2015-2025 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
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.

local GZIP(1)