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

NAME

       gzip, gunzip, zcat - compress or expand files

SYNOPSIS

       gzip [ -acdfhklLnNrtvV19 ] [--rsyncable] [-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 -gz for VMS, 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 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 time stamp 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 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.

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. 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 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 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: 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 --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 time stamp 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 time stamp 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 time stamp; this is the default. When
decompressing, restore the original file name and time stamp if present. This option is useful on
systems which have a limit on file name length or when the time stamp 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 ).

--rsyncable
While compressing, synchronize the output occasionally based on the input. This increases size by
less than 1 percent most cases, but means that the rsync(1) program can take advantage of
similarities in the uncompressed input when synchronizing two files compressed with this flag.
gunzip cannot tell the difference between a compressed file created with this option, and one
created without it.

-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.

-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 (best 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:

             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 overwritten by explicit command line parameters. For example:
             for sh:    GZIP="-8v --name"; export GZIP
             for csh:   setenv GZIP "-8v --name"
             for MSDOS: set GZIP=-8v --name

       On Vax/VMS, the name of the environment variable is GZIP_OPT, to avoid a conflict with the symbol set for
       invocation of the program.

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 -- 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 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  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 --block-compress /dev/rst0
         for csh: (setenv GZIP -q; tar -xfz --block-compr /dev/rst0

       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.  (This example
       assumes you are using the GNU version of tar.)

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.

COPYRIGHT NOTICE

       Copyright © 1998, 1999, 2001, 2002, 2012 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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