340 lines
11 KiB
Plaintext
340 lines
11 KiB
Plaintext
\input texinfo @c -*-texinfo-*-
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@c %**start of header
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@setfilename gzip.info
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@settitle Gzip User's Manual
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@finalout
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@setchapternewpage odd
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@c %**end of header
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@ifinfo
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This file documents the the GNU @code{gzip} command for compressing files.
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Copyright (C) 1992-1993 Jean-loup Gailly
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Permission is granted to make and distribute verbatim copies of
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this manual provided the copyright notice and this permission notice
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are preserved on all copies.
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@ignore
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Permission is granted to process this file through TeX and print the
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results, provided the printed document carries copying permission
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notice identical to this one except for the removal of this paragraph
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(this paragraph not being relevant to the printed manual).
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@end ignore
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Permission is granted to copy and distribute modified versions of this
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manual under the conditions for verbatim copying, provided that the entire
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resulting derived work is distributed under the terms of a permission
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notice identical to this one.
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Permission is granted to copy and distribute translations of this manual
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into another language, under the above conditions for modified versions,
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except that this permission notice may be stated in a translation approved
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by the Foundation.
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@end ifinfo
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@titlepage
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@title gzip
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@subtitle The data compression program
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@subtitle Edition 1.0.7, for Gzip Version 1.0.7
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@subtitle March 1993
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@author by Jean-loup Gailly
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@page
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@vskip 0pt plus 1filll
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Copyright @copyright{} 1992-1993 Jean-loup Gailly
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Permission is granted to make and distribute verbatim copies of
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this manual provided the copyright notice and this permission notice
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are preserved on all copies.
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Permission is granted to copy and distribute modified versions of this
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manual under the conditions for verbatim copying, provided that the entire
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resulting derived work is distributed under the terms of a permission
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notice identical to this one.
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Permission is granted to copy and distribute translations of this manual
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into another language, under the above conditions for modified versions,
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except that this permission notice may be stated in a translation approved
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by the Foundation.
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@end titlepage
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@node Top, , , (dir)
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@ifinfo
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This file documents the @code{gzip} command to compress files.
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@end ifinfo
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@menu
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* Copying:: How you can copy and share @code{gzip}.
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* Overview:: Preliminary information.
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* Sample:: Sample output from @code{gzip}.
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* Invoking gzip:: How to run @code{gzip}.
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* Advanced usage:: Concatenated files.
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* Environment:: The @code{GZIP} environment variable
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* Problems:: Reporting bugs.
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* Concept Index:: Index of concepts.
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@end menu
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@node Copying, Overview, , Top
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@include gpl.texinfo
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@node Overview, Sample, Copying, Top
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@chapter Overview
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@cindex overview
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@code{Gzip} reduces the size of the named files using Lempel-Ziv coding
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(LZ77). Whenever possible, each file is replaced by one with the
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extension ".z", while keeping the same ownership modes, access and
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modification times. (The extension is "-z" for VMS, "z" for MSDOS, OS/2
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and Atari.) If no files are specified, the standard input is compressed
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to the standard output. If the new file name is too long, @code{gzip}
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truncates it and keeps the original file name in the compressed file.
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@code{gzip} will only attempt to compress regular files. In particular,
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it will ignore symbolic links.
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Compressed files can be restored to their original form using
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"@code{gzip} -d" or @code{gunzip} or @code{zcat}.
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@code{gunzip} takes a list of files on its command line and replaces
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each file whose name ends with ".z" or ".Z" and which begins with the
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correct magic number with an uncompressed file without the original
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extension. @code{gunzip} also recognizes the special extensions ".tgz"
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and ".taz" as shorthands for ".tar.z" or ".tar.Z".
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@code{gunzip} can currently decompress files created by @code{gzip},
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@code{zip}, @code{compress} or @code{pack}. The detection of the input
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format is automatic. When using the first two formats, @code{gunzip}
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checks a 32 bit CRC (cyclic redundancy check). For @code{pack},
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@code{gunzip} checks the uncompressed length. The @code{compress}
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format was not designed to allow consistency checks. However @code{gunzip}
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is sometimes able to detect a bad .Z file. If you get an error
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when uncompressing a .Z file, do not assume that the .Z file is
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correct simply because the standard @code{uncompress} does not complain.
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This generally means that the standard @code{uncompress} does not check its
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input, and happily generates garbage output.
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Files created by @code{zip} can be uncompressed by @code{gzip} only if
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they have a single member compressed with the 'deflation' method. This
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feature is only intended to help conversion of @code{tar.zip} files to
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the @code{tar.z} format. To extract @code{zip} files with several
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members, use @code{unzip} instead of @code{gunzip}.
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@code{zcat} is identical to "@code{gunzip} -c". @code{zcat}
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uncompresses either a list of files on the command line or its standard
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input and writes the uncompressed data on standard output. @code{zcat}
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will uncompress files that have the correct magic number whether they
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have a ".z" suffix or not.
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@code{gzip} uses the Lempel-Ziv algorithm used in @code{zip} and PKZIP.
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The amount of compression obtained depends on the size of the input and
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the distribution of common substrings. Typically, text such as source
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code or English is reduced by 60-70%. Compression is generally much
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better than that achieved by LZW (as used in @code{compress}), Huffman
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coding (as used in @code{pack}), or adaptive Huffman coding
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(@code{compact}).
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Compression is always performed, even if the compressed file is slightly
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larger than the original. The worst case expansion is a few bytes for
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the gzip file header, plus 5 bytes every 32K block, or an expansion
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ratio of 0.015% for large files. @code{gzip} preserves the mode,
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ownership and timestamps of files when compressing or decompressing.
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@node Sample, Invoking gzip, Overview, Top
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@chapter Sample Output
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@cindex sample
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Here are some realistic examples of running @code{gzip}.
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This is the output of the command @samp{gzip}:
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@example
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usage: gzip [-cdfhLrv19] [file ...]
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For more help, type: gzip -h
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@end example
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This is the output of the command @samp{gzip -h}:
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@example
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gzip 1.0.7 (18 Mar 93)
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usage: gzip [-cdfhLrtvV19] [file ...]
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-c --stdout write on standard output, keep original files unchanged
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-d --decompress decompress
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-f --force force overwrite of output file and compress links
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-h --help give this help
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-L --license display software license
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-q --quiet suppress all warnings
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-r --recurse recurse through directories
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-t --test test compressed file integrity (implies -d)
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-v --verbose verbose mode
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-V --version display version number
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-1 --fast compress faster
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-9 --best compress better
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file... files to (de)compress. If none given, use standard input
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@end example
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This is the output of the command @samp{gzip -v gzip.c}:
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@example
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gzip.c: 69.8% -- replaced with gzip.c.z
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@end example
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@node Invoking gzip, Advanced usage, Sample, Top
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@chapter Invoking @code{gzip}
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@cindex invoking
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@cindex options
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The format for running the @code{gzip} program is:
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@example
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gzip @var{option} @dots{}
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@end example
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@code{gzip} supports the following options:
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@table @samp
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@item --help
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@itemx -h
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Print an informative help message describing the options.
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@item --stdout
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@itemx -c
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Write output on standard output; keep original files unchanged.
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If there are several input files, the output consists of a sequence of
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independently compressed members. To obtain better compression,
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concatenate all input files before compressing them.
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@item --decompress
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@itemx -d
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Decompress.
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@item --force
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@itemx -f
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Force compression or decompression even if the file has multiple links
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or the corresponding file already exists. If -f is not given, and
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when not running in the background, @code{gzip} prompts to verify
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whether an existing file should be overwritten.
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@item --help
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@itemx -h
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Display a help screen.
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@item --license
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@itemx -L
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Display the @code{gzip} license.
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@item --recurse
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@itemx -r
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Travel the directory structure recursively. If any of the file names
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specified on the command line are directories, @code{gzip} will descend
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into the directory and compress all the files it finds there (or
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decompress them in the case of @code{gunzip}).
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@item --test
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@itemx -t
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Test. Check the compressed file integrity.
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@item --verbose
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@itemx -v
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Verbose. Display the name and percentage reduction for each file compressed.
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@item --version
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@itemx -V
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Version. Display the version number and compilation options.
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@item --fast
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@itemx --best
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@itemx -#
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Regulate the speed of compression using the specified digit #, where -1
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or --fast indicates the fastest compression method (less compression)
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and --best or -9 indicates the slowest compression method (optimal
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compression). The default compression level is -5.
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@end table
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@node Advanced usage, Environment, Invoking gzip, Top
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@chapter Advanced usage
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@cindex concatenated files
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Multiple compressed files can be concatenated. In this case,
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@code{gunzip} will extract all members at once. If one member is
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damaged, other members might still be recovered after removal of the
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damaged member. Better compression can be usually obtained if all
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members are decompressed then recompressed in a single step.
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This is an example of concatenating gzip files:
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@example
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gzip -c file1 > foo.z
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gzip -c file2 >> foo.z
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@end example
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Then
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@example
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gunzip -c foo
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@end example
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is equivalent to
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@example
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cat file1 file2
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@end example
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In case of damage to one member of a .z file, other members can
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still be recovered (if the damaged member is removed). However,
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you can get better compression by compressing all members at once:
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@example
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cat file1 file2 | gzip > foo.z
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@end example
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compresses better than
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@example
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gzip -c file1 file2 > foo.z
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@end example
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If you want to recompress concatenated files to get better compression, do:
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@example
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zcat old.z | gzip > new.z
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@end example
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@node Environment, Problems, Advanced usage, Top
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@chapter Environment
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@cindex Environment
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The environment variable @code{GZIP} can hold a set of default options for
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gzip. These options are interpreted first and can be overwritten by
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explicit command line parameters. For example:
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@example
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for sh: GZIP="-8 -v"; export GZIP
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for csh: setenv GZIP "-8 -v"
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for MSDOS: set GZIP=-8 -v
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@end example
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On Vax/VMS, the name of the environment variable is @code{GZIP_OPT}, to
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avoid a conflict with the symbol set for invocation of the program.
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@node Problems, Concept Index, Environment, Top
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@chapter Reporting Bugs
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@cindex bugs
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If you find a bug in @code{gzip}, please send electronic mail to
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@w{@samp{jloup@@chorus.fr}} or, if this fails, to
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@w{@samp{bug-gnu-utils@@prep.ai.mit.edu}}. Include the version number,
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which you can find by running @w{@samp{gzip -V}}. Also include in your
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message the hardware and operating system, the compiler used to compile,
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a description of the bug behavior, and the input to gzip that triggered
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the bug.@refill
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@node Concept Index, , Problems, Top
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@unnumbered Concept Index
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@printindex cp
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@shortcontents
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@contents
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@bye
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