gzip.1

GNU 系统开发的基于Iz77语法的压缩 学习lz77压缩算法

.PU
.TH GZIP 1 local
.SH NAME
gzip, gunzip, zcat \- compress or expand files
.SH SYNOPSIS
.ll +8
.B gzip
.RB [ " \-cdfhLrtvV19 " ]
[
.I "name \&..."
]
.ll -8
.br
.B gunzip
.RB [ " \-cfhLrtvV " ]
[
.I "name \&..."
]
.br
.B zcat
.RB [ " \-hLV " ]
[
.I "name \&..."
]
.SH DESCRIPTION
.I Gzip
reduces the size of the named files using Lempel-Ziv coding (LZ77).
Whenever possible,
each file is replaced by one with the extension
.B "\&.z,"
while keeping the same ownership modes, access and modification times.
(The extension is
.B "\-z"
for VMS,
.B "z"
for MSDOS, OS/2 and Atari.)
If no files are specified, the standard input is compressed to the
standard output. If the new file name is too long,
.I gzip
truncates it and keeps the original file name in the compressed file.
.I Gzip
will only attempt to compress regular files.
In particular, it will ignore symbolic links.
.PP
Compressed files can be restored to their original form using
.I gzip -d
or
.I gunzip
or
.I zcat.
.PP
.I gunzip
takes a list of files on its command line and replaces each
file whose name ends with
.B "\&.z"
or
.B "\&.Z"
or
.B "\&-z"
and which begins with the correct magic number with an uncompressed
file without the original extension.
.I gunzip
also recognizes the special extensions
.B "\&.tgz"
and
.B "\&.taz"
as shorthands for
.B "\&.tar.z"
or
.B "\&.tar.Z"
.PP
.I gunzip
can currently decompress files created by
.I gzip, zip, compress
or
.I pack.
The detection of the input format is automatic.  When using
the first two formats,
.I gunzip
checks a 32 bit CRC. For
.I pack, gunzip
checks the uncompressed length. The
.I compress
format was not designed to allow consistency checks. However
.I 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
.I uncompress
does not complain. This generally means that the standard
.I uncompress
does not check its input, and happily generates garbage output.
.PP
Files created by
.I 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.z format. To extract zip files
with several members, use
.I unzip
instead of
.I gunzip.
.PP
.I zcat
is identical to
.I gunzip
.B \-c.
(On some systems,
.I zcat
may be installed as
.I gzcat
to preserve the original link to
.I compress.)
.I zcat
uncompresses either a list of files on the command line or its
standard input and writes the uncompressed data on standard output.
.I zcat
will uncompress files that have the correct magic number whether
they have a
.B "\&.z"
suffix or not.
.PP
.I Gzip
uses the Lempel-Ziv algorithm used in
.I 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 
.IR compress ),
Huffman coding (as used in
.IR pack ),
or adaptive Huffman coding
.RI ( compact ).
.PP
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.
.I gzip
preserves the mode, ownership and timestamps of files when compressing
or decompressing.

.SH OPTIONS
.TP
.B \-c --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.
.TP
.B \-d --decompress
Decompress.
.TP
.B \-f --force
Force compression or decompression even if the file has multiple links
or the corresponding file already exists.
If
.B \-f
is not given,
and when not running in the background,
.I gzip
prompts to verify whether an existing file should be overwritten.
.TP
.B \-h --help
Display a help screen.
.TP
.B \-L --license
Display the
.I gzip
license.
.TP
.B \-q --quiet
Suppress all warnings.
.TP
.B \-r --recurse
Travel the directory structure recursively. If any of the file names
specified on the command line are directories, 
.I gzip
will descend into the directory and compress all the files it finds there
(or decompress them in the case of
.I gunzip
).
.TP
.B \-t --test
Test. Check the compressed file integrity.
.TP
.B \-v --verbose
Verbose. Display the name and percentage reduction for each file compressed.
.TP
.B \-V --version
Version. Display the version number and compilation options.
.TP
.B \-# --fast --best
Regulate the speed of compression using the specified digit
.IR # ,
where
.B \-1
or
.B \-\-fast
indicates the fastest compression method (less compression)
and
.B \-9
or
.B \-\-best
indicates the slowest compression method (optimal compression).
The default compression level is
.BR \-5.
.SH "ADVANCED USAGE"
Multiple compressed files can be concatenated. In this case,
.I gunzip
will extract all members at once. For example:

      gzip -c file1  > foo.z
      gzip -c file2 >> foo.z
Then
      gunzip -c foo

is equivalent to

      cat file1 file2

In case of damage to one member of a .z 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.z

compresses better than

      gzip -c file1 file2 > foo.z

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

      zcat old.z | gzip > new.z
.SH "ENVIRONMENT"
The environment variable
.B GZIP
can hold a set of default options for
.I gzip.
These options are interpreted first and can be overwritten by
explicit command line parameters. For example:
      for sh:    GZIP="-8 -v"; export GZIP
      for csh:   setenv GZIP "-8 -v"
      for MSDOS: set GZIP=-8 -v

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.
.SH "SEE ALSO"
znew(1), zcmp(1), zmore(1), zforce(1), gzexe(1), zip(1), unzip(1), compress(1),
pack(1), compact(1)
.SH "DIAGNOSTICS"
Exit status is normally 0;
if an error occurs, exit status is 1. If a warning occurs, exit status is 2.
.PP
Usage: gzip [-cdfhLrtvV19] [file ...]
.in +8
Invalid options were specified on the command line.
.in -8
.IR file :
not in gzip format
.in +8
The file specified to
.I gunzip
has not been compressed.
.in -8
.IR file:
Corrupt input. Use zcat to recover some data.
.in +8
The compressed file has been damaged. The data up to the point of failure
can be recovered using
.in +8
zcat file > recover
.in -16
.IR file :
compressed with 
.I xx
bits, can only handle 
.I yy
bits
.in +8
.I File
was compressed (using LZW) by a program that could deal with
more 
.I bits
than the decompress code on this machine.
Recompress the file with gzip, which compresses better and uses
less memory.
.in -8
.IR file :
already has z suffix -- no change
.in +8
The file is assumed to be already compressed.
Rename the file and try again or use zcat.
.in -8
.I file
already exists; do you wish to overwrite (y or n)?
.in +8
Respond "y" if you want the output file to be replaced; "n" if not.
.in -8
gunzip: corrupt input
.in +8
A SIGSEGV violation was detected which usually means that the input file has
been corrupted.
.in -8
.I "xx.x%"
.in +8
Percentage of the input saved by compression.
(Relevant only for
.BR \-v \.)
.in -8
-- not a regular file or directory: ignored
.in +8
When the input file is not a regular file or directory,
(e.g. a symbolic link, socket, FIFO, device file), it is
left unaltered.
.in -8
-- has 
.I xx 
other links: unchanged
.in +8
The input file has links; it is left unchanged.  See
.IR ln "(1)"
for more information. Use the
.B \-f
flag to force compression of multiply-linked files.
.in -8
.SH CAVEATS
The .z extension is already used by
.IR pack "(1)".
You can link
.I gzip
to
.I pcat
to get transparent decompression for programs expecting .z files to be in
.IR pack
format.