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GZIP
!!!GZIP
NAME
SYNOPSIS
DESCRIPTION
OPTIONS
ADVANCED USAGE
ENVIRONMENT
SEE ALSO
DIAGNOSTICS
CAVEATS
BUGS
COPYRIGHT NOTICE
----
!!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
specified, or if a file name is
__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, _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__ 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, 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 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 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 displayed:


method: compression method crc: the 32-bit CRC of the
uncompressed data date


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'' ).


__-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 transferred to other systems. A
null suffix forces gunzip to try decompression on all given
files regardless of suffix, as in:


gunzip -S


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


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


compresses better than


gzip -c file1 file2


If you want to recompress concatenated files to get better
compression, do:


gzip -cd old.gz | gzip


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=
''


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)


The ''gzip'' file format is specified in P. Deutsch,
GZIP file format specification version 4.3,
____,
Internet RFC 1952 (May 1996). The ''zip'' deflation
format is specified in P. Deutsch, DEFLATE
Compressed Data Format Specification version 1.3,
____,
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 [[-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'' ''


''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


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=
__


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 compression level (-6). On some
highly redundant files, ''compress'' compresses better
than ''gzip.''
!!COPYRIGHT NOTICE


Copyright 1998, 1999, 2001 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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Last edited on Tuesday, June 4, 2002 12:22:09 am by "perry"
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