gzip - phpMan

GZIP(1) GZIP(1)

NAME
gzip, gunzip, zcat - compress or expand files

SYNOPSIS
gzip [ -acdfhlLnNrtvV19 ] [-S suffix] [ name ... ]
gunzip [ -acfhlLnNrtvV ] [-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 speci-
fied, 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 charac-
ters, 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, _z or .Z 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 respec-
tively. 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, 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 con-
version 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 compres-
sion obtained depends on the size of the input and the distribution of common sub-
strings. 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 com-
press), 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.

The gzip file format is specified in P. Deutsch, GZIP file format specification
version 4.3, <ftp://ftp.isi.edu/in-notes/rfc1952.txt>, Internet RFC 1952 (May
1996). The zip deflation format is specified in P. Deutsch, DEFLATE Compressed
Data Format Specification version 1.3, <ftp://ftp.isi.edu/in-notes/rfc1951.txt>,
Internet RFC 1951 (May 1996).

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 recog-
nized by gzip, and if the option --stdout is also given, copy the input data
without change to the standard ouput: 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.

-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 dis-
played:

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 trun-
cated.) 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 speci-
fied on the command line are directories, gzip will descend into the direc-
tory and compress all the files it finds there (or decompress them in the
case of gunzip ).

-S .suf --suffix .suf
Use suffix .suf instead of .gz. Any suffix can be given, but suffixes other
than .z and .gz should be avoided to avoid confusion when files are trans-
ferred to other systems. A null suffix forces gunzip to try decompression
on all given files regardless of suffix, as in:

gunzip -S "" * (*.* for MSDOS)

Previous versions of gzip used the .z suffix. This was changed to avoid a
conflict with pack(1).

-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 com-
pressing 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 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.

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

DIAGNOSTICS
Exit status is normally 0; if an error occurs, exit status is 1. If a warning
occurs, exit status is 2.

Usage: gzip [-cdfhlLnNrtvV19] [-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 informa-
tion. 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 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 com-
pression level (-6). On some highly redundant files, compress compresses better
than gzip.

Permission is granted to make and distribute verbatim copies of this manual pro-
vided 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)

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