Ubuntu Manpage: gzip, gunzip, zcat - compress or expand files

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. 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 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 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 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 © 1998–1999, 2001–2002, 2012, 2015–2023 Free Software Foundation, Inc.
       Copyright © 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.

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