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<html lang="en">
<head>
<title>Gzip User's Manual</title>
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<h1 class="settitle">Gzip User's Manual</h1>
<div class="node">
<p><hr>
<a name="Top"></a>Up: <a rel="up" accesskey="u" href="#dir">(dir)</a>
<br>
</div>
<ul class="menu">
<li><a accesskey="1" href="#Copying">Copying</a>: How you can copy and share <code>gzip</code>.
<li><a accesskey="2" href="#Overview">Overview</a>: Preliminary information.
<li><a accesskey="3" href="#Sample">Sample</a>: Sample output from <code>gzip</code>.
<li><a accesskey="4" href="#Invoking-gzip">Invoking gzip</a>: How to run <code>gzip</code>.
<li><a accesskey="5" href="#Advanced-usage">Advanced usage</a>: Concatenated files.
<li><a accesskey="6" href="#Environment">Environment</a>: The <code>GZIP</code> environment variable
<li><a accesskey="7" href="#Tapes">Tapes</a>: Using <code>gzip</code> on tapes.
<li><a accesskey="8" href="#Problems">Problems</a>: Reporting bugs.
<li><a accesskey="9" href="#Concept-Index">Concept Index</a>: Index of concepts.
</ul>
<div class="node">
<p><hr>
<a name="Copying"></a>Next: <a rel="next" accesskey="n" href="#Overview">Overview</a>,
Up: <a rel="up" accesskey="u" href="#Top">Top</a>
<br>
</div>
<h2 class="unnumbered">GNU GENERAL PUBLIC LICENSE</h2>
<div align="center">Version 2, June 1991</div>
<pre class="display"> Copyright © 1989, 1991 Free Software Foundation, Inc.
675 Mass Ave, Cambridge, MA 02139, USA
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
</pre>
<h3 class="unnumberedsec">Preamble</h3>
<p>The licenses for most software are designed to take away your
freedom to share and change it. By contrast, the GNU General Public
License is intended to guarantee your freedom to share and change free
software—to make sure the software is free for all its users. This
General Public License applies to most of the Free Software
Foundation's software and to any other program whose authors commit to
using it. (Some other Free Software Foundation software is covered by
the GNU Library General Public License instead.) You can apply it to
your programs, too.
<p>When we speak of free software, we are referring to freedom, not
price. Our General Public Licenses are designed to make sure that you
have the freedom to distribute copies of free software (and charge for
this service if you wish), that you receive source code or can get it
if you want it, that you can change the software or use pieces of it
in new free programs; and that you know you can do these things.
<p>To protect your rights, we need to make restrictions that forbid
anyone to deny you these rights or to ask you to surrender the rights.
These restrictions translate to certain responsibilities for you if you
distribute copies of the software, or if you modify it.
<p>For example, if you distribute copies of such a program, whether
gratis or for a fee, you must give the recipients all the rights that
you have. You must make sure that they, too, receive or can get the
source code. And you must show them these terms so they know their
rights.
<p>We protect your rights with two steps: (1) copyright the software, and
(2) offer you this license which gives you legal permission to copy,
distribute and/or modify the software.
<p>Also, for each author's protection and ours, we want to make certain
that everyone understands that there is no warranty for this free
software. If the software is modified by someone else and passed on, we
want its recipients to know that what they have is not the original, so
that any problems introduced by others will not reflect on the original
authors' reputations.
<p>Finally, any free program is threatened constantly by software
patents. We wish to avoid the danger that redistributors of a free
program will individually obtain patent licenses, in effect making the
program proprietary. To prevent this, we have made it clear that any
patent must be licensed for everyone's free use or not licensed at all.
<p>The precise terms and conditions for copying, distribution and
modification follow.
<ol type=1 start=1>
<li>This License applies to any program or other work which contains
a notice placed by the copyright holder saying it may be distributed
under the terms of this General Public License. The “Program”, below,
refers to any such program or work, and a “work based on the Program”
means either the Program or any derivative work under copyright law:
that is to say, a work containing the Program or a portion of it,
either verbatim or with modifications and/or translated into another
language. (Hereinafter, translation is included without limitation in
the term “modification”.) Each licensee is addressed as “you”.
<p>Activities other than copying, distribution and modification are not
covered by this License; they are outside its scope. The act of
running the Program is not restricted, and the output from the Program
is covered only if its contents constitute a work based on the
Program (independent of having been made by running the Program).
Whether that is true depends on what the Program does.
<li>You may copy and distribute verbatim copies of the Program's
source code as you receive it, in any medium, provided that you
conspicuously and appropriately publish on each copy an appropriate
copyright notice and disclaimer of warranty; keep intact all the
notices that refer to this License and to the absence of any warranty;
and give any other recipients of the Program a copy of this License
along with the Program.
<p>You may charge a fee for the physical act of transferring a copy, and
you may at your option offer warranty protection in exchange for a fee.
<li>You may modify your copy or copies of the Program or any portion
of it, thus forming a work based on the Program, and copy and
distribute such modifications or work under the terms of Section 1
above, provided that you also meet all of these conditions:
<ol type=a start=1>
<li>You must cause the modified files to carry prominent notices
stating that you changed the files and the date of any change.
<li>You must cause any work that you distribute or publish, that in
whole or in part contains or is derived from the Program or any
part thereof, to be licensed as a whole at no charge to all third
parties under the terms of this License.
<li>If the modified program normally reads commands interactively
when run, you must cause it, when started running for such
interactive use in the most ordinary way, to print or display an
announcement including an appropriate copyright notice and a
notice that there is no warranty (or else, saying that you provide
a warranty) and that users may redistribute the program under
these conditions, and telling the user how to view a copy of this
License. (Exception: if the Program itself is interactive but
does not normally print such an announcement, your work based on
the Program is not required to print an announcement.)
</ol>
<p>These requirements apply to the modified work as a whole. If
identifiable sections of that work are not derived from the Program,
and can be reasonably considered independent and separate works in
themselves, then this License, and its terms, do not apply to those
sections when you distribute them as separate works. But when you
distribute the same sections as part of a whole which is a work based
on the Program, the distribution of the whole must be on the terms of
this License, whose permissions for other licensees extend to the
entire whole, and thus to each and every part regardless of who wrote it.
<p>Thus, it is not the intent of this section to claim rights or contest
your rights to work written entirely by you; rather, the intent is to
exercise the right to control the distribution of derivative or
collective works based on the Program.
<p>In addition, mere aggregation of another work not based on the Program
with the Program (or with a work based on the Program) on a volume of
a storage or distribution medium does not bring the other work under
the scope of this License.
<li>You may copy and distribute the Program (or a work based on it,
under Section 2) in object code or executable form under the terms of
Sections 1 and 2 above provided that you also do one of the following:
<ol type=a start=1>
<li>Accompany it with the complete corresponding machine-readable
source code, which must be distributed under the terms of Sections
1 and 2 above on a medium customarily used for software interchange; or,
<li>Accompany it with a written offer, valid for at least three
years, to give any third party, for a charge no more than your
cost of physically performing source distribution, a complete
machine-readable copy of the corresponding source code, to be
distributed under the terms of Sections 1 and 2 above on a medium
customarily used for software interchange; or,
<li>Accompany it with the information you received as to the offer
to distribute corresponding source code. (This alternative is
allowed only for noncommercial distribution and only if you
received the program in object code or executable form with such
an offer, in accord with Subsection b above.)
</ol>
<p>The source code for a work means the preferred form of the work for
making modifications to it. For an executable work, complete source
code means all the source code for all modules it contains, plus any
associated interface definition files, plus the scripts used to
control compilation and installation of the executable. However, as a
special exception, the source code distributed need not include
anything that is normally distributed (in either source or binary
form) with the major components (compiler, kernel, and so on) of the
operating system on which the executable runs, unless that component
itself accompanies the executable.
<p>If distribution of executable or object code is made by offering
access to copy from a designated place, then offering equivalent
access to copy the source code from the same place counts as
distribution of the source code, even though third parties are not
compelled to copy the source along with the object code.
<li>You may not copy, modify, sublicense, or distribute the Program
except as expressly provided under this License. Any attempt
otherwise to copy, modify, sublicense or distribute the Program is
void, and will automatically terminate your rights under this License.
However, parties who have received copies, or rights, from you under
this License will not have their licenses terminated so long as such
parties remain in full compliance.
<li>You are not required to accept this License, since you have not
signed it. However, nothing else grants you permission to modify or
distribute the Program or its derivative works. These actions are
prohibited by law if you do not accept this License. Therefore, by
modifying or distributing the Program (or any work based on the
Program), you indicate your acceptance of this License to do so, and
all its terms and conditions for copying, distributing or modifying
the Program or works based on it.
<li>Each time you redistribute the Program (or any work based on the
Program), the recipient automatically receives a license from the
original licensor to copy, distribute or modify the Program subject to
these terms and conditions. You may not impose any further
restrictions on the recipients' exercise of the rights granted herein.
You are not responsible for enforcing compliance by third parties to
this License.
<li>If, as a consequence of a court judgment or allegation of patent
infringement or for any other reason (not limited to patent issues),
conditions are imposed on you (whether by court order, agreement or
otherwise) that contradict the conditions of this License, they do not
excuse you from the conditions of this License. If you cannot
distribute so as to satisfy simultaneously your obligations under this
License and any other pertinent obligations, then as a consequence you
may not distribute the Program at all. For example, if a patent
license would not permit royalty-free redistribution of the Program by
all those who receive copies directly or indirectly through you, then
the only way you could satisfy both it and this License would be to
refrain entirely from distribution of the Program.
<p>If any portion of this section is held invalid or unenforceable under
any particular circumstance, the balance of the section is intended to
apply and the section as a whole is intended to apply in other
circumstances.
<p>It is not the purpose of this section to induce you to infringe any
patents or other property right claims or to contest validity of any
such claims; this section has the sole purpose of protecting the
integrity of the free software distribution system, which is
implemented by public license practices. Many people have made
generous contributions to the wide range of software distributed
through that system in reliance on consistent application of that
system; it is up to the author/donor to decide if he or she is willing
to distribute software through any other system and a licensee cannot
impose that choice.
<p>This section is intended to make thoroughly clear what is believed to
be a consequence of the rest of this License.
<li>If the distribution and/or use of the Program is restricted in
certain countries either by patents or by copyrighted interfaces, the
original copyright holder who places the Program under this License
may add an explicit geographical distribution limitation excluding
those countries, so that distribution is permitted only in or among
countries not thus excluded. In such case, this License incorporates
the limitation as if written in the body of this License.
<li>The Free Software Foundation may publish revised and/or new versions
of the General Public License from time to time. Such new versions will
be similar in spirit to the present version, but may differ in detail to
address new problems or concerns.
<p>Each version is given a distinguishing version number. If the Program
specifies a version number of this License which applies to it and “any
later version”, you have the option of following the terms and conditions
either of that version or of any later version published by the Free
Software Foundation. If the Program does not specify a version number of
this License, you may choose any version ever published by the Free Software
Foundation.
<li>If you wish to incorporate parts of the Program into other free
programs whose distribution conditions are different, write to the author
to ask for permission. For software which is copyrighted by the Free
Software Foundation, write to the Free Software Foundation; we sometimes
make exceptions for this. Our decision will be guided by the two goals
of preserving the free status of all derivatives of our free software and
of promoting the sharing and reuse of software generally.
<li>BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN
OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
PROVIDE THE PROGRAM “AS IS” WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED
OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS
TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE
PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING,
REPAIR OR CORRECTION.
<li>IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES,
INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING
OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED
TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY
YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER
PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.
</ol>
<h3 class="unnumberedsec">How to Apply These Terms to Your New Programs</h3>
<p>If you develop a new program, and you want it to be of the greatest
possible use to the public, the best way to achieve this is to make it
free software which everyone can redistribute and change under these terms.
<p>To do so, attach the following notices to the program. It is safest
to attach them to the start of each source file to most effectively
convey the exclusion of warranty; and each file should have at least
the “copyright” line and a pointer to where the full notice is found.
<pre class="smallexample"> <var>one line to give the program's name and an idea of what it does.</var>
Copyright (C) 19<var>yy</var> <var>name of author</var>
This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License
as published by the Free Software Foundation; either version 2
of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
</pre>
<p>Also add information on how to contact you by electronic and paper mail.
<p>If the program is interactive, make it output a short notice like this
when it starts in an interactive mode:
<pre class="smallexample"> Gnomovision version 69, Copyright (C) 19<var>yy</var> <var>name of author</var>
Gnomovision comes with ABSOLUTELY NO WARRANTY; for details
type `show w'. This is free software, and you are welcome
to redistribute it under certain conditions; type `show c'
for details.
</pre>
<p>The hypothetical commands <span class="samp">show w</span> and <span class="samp">show c</span> should show
the appropriate parts of the General Public License. Of course, the
commands you use may be called something other than <span class="samp">show w</span> and
<span class="samp">show c</span>; they could even be mouse-clicks or menu items—whatever
suits your program.
<p>You should also get your employer (if you work as a programmer) or your
school, if any, to sign a “copyright disclaimer” for the program, if
necessary. Here is a sample; alter the names:
<pre class="example"> Yoyodyne, Inc., hereby disclaims all copyright
interest in the program `Gnomovision'
(which makes passes at compilers) written
by James Hacker.
<var>signature of Ty Coon</var>, 1 April 1989
Ty Coon, President of Vice
</pre>
<p>This General Public License does not permit incorporating your program into
proprietary programs. If your program is a subroutine library, you may
consider it more useful to permit linking proprietary applications with the
library. If this is what you want to do, use the GNU Library General
Public License instead of this License.
<div class="node">
<p><hr>
<a name="Overview"></a>Next: <a rel="next" accesskey="n" href="#Sample">Sample</a>,
Previous: <a rel="previous" accesskey="p" href="#Copying">Copying</a>,
Up: <a rel="up" accesskey="u" href="#Top">Top</a>
<br>
</div>
<h2 class="chapter">1 Overview</h2>
<p><a name="index-overview-1"></a>
<code>gzip</code> reduces the size of the named files using Lempel-Ziv coding
(LZ77). Whenever possible, each file is replaced by one with the
extension <span class="samp">.gz</span>, while keeping the same ownership modes, access and
modification times. (The default extension is <span class="samp">-gz</span> for VMS,
<span class="samp">z</span> for MSDOS, OS/2 FAT and Atari.) If no files are specified or
if a file name is "-", the standard input is compressed to the standard
output. <code>gzip</code> will only attempt to compress regular files. In
particular, it will ignore symbolic links.
<p>If the new file name is too long for its file system, <code>gzip</code>
truncates it. <code>gzip</code> 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.
<p>By default, <code>gzip</code> keeps the original file name and timestamp in
the compressed file. These are used when decompressing the file with the
<span class="samp">-N</span> option. This is useful when the compressed file name was
truncated or when the time stamp was not preserved after a file
transfer.
<p>Compressed files can be restored to their original form using <span class="samp">gzip -d</span>
or <code>gunzip</code> or <code>zcat</code>. 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.
<p><code>gunzip</code> takes a list of files on its command line and replaces
each file whose name ends with <span class="samp">.gz</span>, <span class="samp">.z</span>, <span class="samp">.Z</span>,
<span class="samp">-gz</span>, <span class="samp">-z</span> or <span class="samp">_z</span> and which begins with the correct
magic number with an uncompressed file without the original extension.
<code>gunzip</code> also recognizes the special extensions <span class="samp">.tgz</span> and
<span class="samp">.taz</span> as shorthands for <span class="samp">.tar.gz</span> and <span class="samp">.tar.Z</span>
respectively. When compressing, <code>gzip</code> uses the <span class="samp">.tgz</span>
extension if necessary instead of truncating a file with a <span class="samp">.tar</span>
extension.
<p><code>gunzip</code> can currently decompress files created by <code>gzip</code>,
<code>zip</code>, <code>compress</code> or <code>pack</code>. The detection of the input
format is automatic. When using the first two formats, <code>gunzip</code>
checks a 32 bit CRC (cyclic redundancy check). For <code>pack</code>,
<code>gunzip</code> checks the uncompressed length. The <code>compress</code> format
was not designed to allow consistency checks. However <code>gunzip</code> is
sometimes able to detect a bad <span class="samp">.Z</span> file. If you get an error when
uncompressing a <span class="samp">.Z</span> file, do not assume that the <span class="samp">.Z</span> file is
correct simply because the standard <code>uncompress</code> does not complain.
This generally means that the standard <code>uncompress</code> does not check
its input, and happily generates garbage output. The SCO <span class="samp">compress
-H</span> format (<code>lzh</code> compression method) does not include a CRC but
also allows some consistency checks.
<p>Files created by <code>zip</code> can be uncompressed by <code>gzip</code> only if
they have a single member compressed with the 'deflation' method. This
feature is only intended to help conversion of <code>tar.zip</code> files to
the <code>tar.gz</code> format. To extract <code>zip</code> files with several
members, use <code>unzip</code> instead of <code>gunzip</code>.
<p><code>zcat</code> is identical to <span class="samp">gunzip -c</span>. <code>zcat</code>
uncompresses either a list of files on the command line or its standard
input and writes the uncompressed data on standard output. <code>zcat</code>
will uncompress files that have the correct magic number whether they
have a <span class="samp">.gz</span> suffix or not.
<p><code>gzip</code> uses the Lempel-Ziv algorithm used in <code>zip</code> 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 <code>compress</code>), Huffman
coding (as used in <code>pack</code>), or adaptive Huffman coding
(<code>compact</code>).
<p>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 <code>gzip</code> 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. <code>gzip</code> preserves the mode,
ownership and timestamps of files when compressing or decompressing.
<div class="node">
<p><hr>
<a name="Sample"></a>Next: <a rel="next" accesskey="n" href="#Invoking-gzip">Invoking gzip</a>,
Previous: <a rel="previous" accesskey="p" href="#Overview">Overview</a>,
Up: <a rel="up" accesskey="u" href="#Top">Top</a>
<br>
</div>
<h2 class="chapter">2 Sample Output</h2>
<p><a name="index-sample-2"></a>
Here are some realistic examples of running <code>gzip</code>.
<p>This is the output of the command <span class="samp">gzip -h</span>:
<pre class="example"> gzip 1.2.4 (18 Aug 93)
usage: gzip [-cdfhlLnNrtvV19] [-S suffix] [file ...]
-c --stdout write on standard output, keep original files unchanged
-d --decompress decompress
-f --force force overwrite of output file and compress links
-h --help give this help
-l --list list compressed file contents
-L --license display software license
-n --no-name do not save or restore the original name and time stamp
-N --name save or restore the original name and time stamp
-q --quiet suppress all warnings
-r --recursive operate recursively on directories
-S .suf --suffix .suf use suffix .suf on compressed files
-t --test test compressed file integrity
-v --verbose verbose mode
-V --version display version number
-1 --fast compress faster
-9 --best compress better
file... files to (de)compress. If none given, use standard input.
</pre>
<p>This is the output of the command <span class="samp">gzip -v texinfo.tex</span>:
<pre class="example"> texinfo.tex: 71.6% -- replaced with texinfo.tex.gz
</pre>
<p>The following command will find all <code>gzip</code> files in the current
directory and subdirectories, and extract them in place without
destroying the original:
<pre class="example"> find . -name '*.gz' -print | sed 's/^\(.*\)[.]gz$/gunzip < "&" > "\1"/' | sh
</pre>
<div class="node">
<p><hr>
<a name="Invoking-gzip"></a>Next: <a rel="next" accesskey="n" href="#Advanced-usage">Advanced usage</a>,
Previous: <a rel="previous" accesskey="p" href="#Sample">Sample</a>,
Up: <a rel="up" accesskey="u" href="#Top">Top</a>
<br>
</div>
<h2 class="chapter">3 Invoking <code>gzip</code></h2>
<p><a name="index-invoking-3"></a><a name="index-options-4"></a>
The format for running the <code>gzip</code> program is:
<pre class="example"> gzip <var>option</var> ...
</pre>
<p><code>gzip</code> supports the following options:
<dl>
<dt><span class="samp">--stdout</span><dt><span class="samp">--to-stdout</span><dt><span class="samp">-c</span><dd>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.
<br><dt><span class="samp">--decompress</span><dt><span class="samp">--uncompress</span><dt><span class="samp">-d</span><dd>Decompress.
<br><dt><span class="samp">--force</span><dt><span class="samp">-f</span><dd>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 <code>gzip</code>, and if the option <span class="samp">--stdout</span> is also
given, copy the input data without change to the standard output: let
<code>zcat</code> behave as <code>cat</code>. If <span class="samp">-f</span> is not given, and
when not running in the background, <code>gzip</code> prompts to verify
whether an existing file should be overwritten.
<br><dt><span class="samp">--help</span><dt><span class="samp">-h</span><dd>Print an informative help message describing the options then quit.
<br><dt><span class="samp">--list</span><dt><span class="samp">-l</span><dd>For each compressed file, list the following fields:
<pre class="example"> 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
</pre>
<p>The uncompressed size is given as <span class="samp">-1</span> for files not in <code>gzip</code>
format, such as compressed <span class="samp">.Z</span> files. To get the uncompressed size for
such a file, you can use:
<pre class="example"> zcat file.Z | wc -c
</pre>
<p>In combination with the <span class="samp">--verbose</span> option, the following fields are also
displayed:
<pre class="example"> method: compression method (deflate,compress,lzh,pack)
crc: the 32-bit CRC of the uncompressed data
date & time: time stamp for the uncompressed file
</pre>
<p>The crc is given as ffffffff for a file not in gzip format.
<p>With <span class="samp">--verbose</span>, the size totals and compression ratio for all files
is also displayed, unless some sizes are unknown. With <span class="samp">--quiet</span>,
the title and totals lines are not displayed.
<br><dt><span class="samp">--license</span><dt><span class="samp">-L</span><dd>Display the <code>gzip</code> license then quit.
<br><dt><span class="samp">--no-name</span><dt><span class="samp">-n</span><dd>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 <code>gzip</code>
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.
<br><dt><span class="samp">--name</span><dt><span class="samp">-N</span><dd>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.
<br><dt><span class="samp">--quiet</span><dt><span class="samp">-q</span><dd>Suppress all warning messages.
<br><dt><span class="samp">--recursive</span><dt><span class="samp">-r</span><dd>Travel the directory structure recursively. If any of the file names
specified on the command line are directories, <code>gzip</code> will descend
into the directory and compress all the files it finds there (or
decompress them in the case of <code>gunzip</code>).
<br><dt><span class="samp">--suffix </span><var>suf</var><dt><span class="samp">-S </span><var>suf</var><dd>Use suffix <var>suf</var> instead of <span class="samp">.gz</span>. Any suffix can be
given, but suffixes other than <span class="samp">.z</span> and <span class="samp">.gz</span> 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:
<pre class="example"> gunzip -S "" * (*.* for MSDOS)
</pre>
<p>Previous versions of gzip used the <span class="samp">.z</span> suffix. This was changed to
avoid a conflict with <code>pack</code>.
<br><dt><span class="samp">--test</span><dt><span class="samp">-t</span><dd>Test. Check the compressed file integrity.
<br><dt><span class="samp">--verbose</span><dt><span class="samp">-v</span><dd>Verbose. Display the name and percentage reduction for each file compressed.
<br><dt><span class="samp">--version</span><dt><span class="samp">-V</span><dd>Version. Display the version number and compilation options, then quit.
<br><dt><span class="samp">--fast</span><dt><span class="samp">--best</span><dt><span class="samp">-</span><var>n</var><dd>Regulate the speed of compression using the specified digit <var>n</var>,
where <span class="samp">-1</span> or <span class="samp">--fast</span> indicates the fastest compression
method (less compression) and <span class="samp">--best</span> or <span class="samp">-9</span> indicates the
slowest compression method (optimal compression). The default
compression level is <span class="samp">-6</span> (that is, biased towards high compression at
expense of speed).
</dl>
<div class="node">
<p><hr>
<a name="Advanced-usage"></a>Next: <a rel="next" accesskey="n" href="#Environment">Environment</a>,
Previous: <a rel="previous" accesskey="p" href="#Invoking-gzip">Invoking gzip</a>,
Up: <a rel="up" accesskey="u" href="#Top">Top</a>
<br>
</div>
<h2 class="chapter">4 Advanced usage</h2>
<p><a name="index-concatenated-files-5"></a>
Multiple compressed files can be concatenated. In this case,
<code>gunzip</code> will extract all members at once. If one member is
damaged, other members might still be recovered after removal of the
damaged member. Better compression can be usually obtained if all
members are decompressed and then recompressed in a single step.
<p>This is an example of concatenating <code>gzip</code> files:
<pre class="example"> gzip -c file1 > foo.gz
gzip -c file2 >> foo.gz
</pre>
<p>Then
<pre class="example"> gunzip -c foo
</pre>
<p>is equivalent to
<pre class="example"> cat file1 file2
</pre>
<p>In case of damage to one member of a <span class="samp">.gz</span> 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:
<pre class="example"> cat file1 file2 | gzip > foo.gz
</pre>
<p>compresses better than
<pre class="example"> gzip -c file1 file2 > foo.gz
</pre>
<p>If you want to recompress concatenated files to get better compression, do:
<pre class="example"> zcat old.gz | gzip > new.gz
</pre>
<p>If a compressed file consists of several members, the uncompressed
size and CRC reported by the <span class="samp">--list</span> option applies to the last member
only. If you need the uncompressed size for all members, you can use:
<pre class="example"> zcat file.gz | wc -c
</pre>
<p>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 <code>tar</code> or <code>zip</code>. GNU <code>tar</code> supports the <span class="samp">-z</span>
option to invoke <code>gzip</code> transparently. <code>gzip</code> is designed as a
complement to <code>tar</code>, not as a replacement.
<div class="node">
<p><hr>
<a name="Environment"></a>Next: <a rel="next" accesskey="n" href="#Tapes">Tapes</a>,
Previous: <a rel="previous" accesskey="p" href="#Advanced-usage">Advanced usage</a>,
Up: <a rel="up" accesskey="u" href="#Top">Top</a>
<br>
</div>
<h2 class="chapter">5 Environment</h2>
<p><a name="index-Environment-6"></a>
The environment variable <code>GZIP</code> can hold a set of default options for
<code>gzip</code>. These options are interpreted first and can be overwritten by
explicit command line parameters. For example:
<pre class="example"> for sh: GZIP="-8v --name"; export GZIP
for csh: setenv GZIP "-8v --name"
for MSDOS: set GZIP=-8v --name
</pre>
<p>On Vax/VMS, the name of the environment variable is <code>GZIP_OPT</code>, to
avoid a conflict with the symbol set for invocation of the program.
<div class="node">
<p><hr>
<a name="Tapes"></a>Next: <a rel="next" accesskey="n" href="#Problems">Problems</a>,
Previous: <a rel="previous" accesskey="p" href="#Environment">Environment</a>,
Up: <a rel="up" accesskey="u" href="#Top">Top</a>
<br>
</div>
<h2 class="chapter">6 Using <code>gzip</code> on tapes</h2>
<p><a name="index-tapes-7"></a>
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 <code>gunzip</code> for decompression,
<code>gunzip</code> detects that there is extra trailing garbage after the
compressed data and emits a warning by default. You have to use the
<span class="samp">--quiet</span> option to suppress the warning. This option can be set in the
<code>GZIP</code> environment variable, as in:
<pre class="example"> for sh: GZIP="-q" tar -xfz --block-compress /dev/rst0
for csh: (setenv GZIP "-q"; tar -xfz --block-compress /dev/rst0)
</pre>
<p>In the above example, <code>gzip</code> is invoked implicitly by the <span class="samp">-z</span>
option of GNU <code>tar</code>. Make sure that the same block size (<span class="samp">-b</span>
option of <code>tar</code>) is used for reading and writing compressed data on
tapes. (This example assumes you are using the GNU version of
<code>tar</code>.)
<div class="node">
<p><hr>
<a name="Problems"></a>Next: <a rel="next" accesskey="n" href="#Concept-Index">Concept Index</a>,
Previous: <a rel="previous" accesskey="p" href="#Tapes">Tapes</a>,
Up: <a rel="up" accesskey="u" href="#Top">Top</a>
<br>
</div>
<h2 class="chapter">7 Reporting Bugs</h2>
<p><a name="index-bugs-8"></a>
If you find a bug in <code>gzip</code>, please send electronic mail to
<span class="samp">bug-gzip@gnu.org</span><!-- /@w -->. Include the version number,
which you can find by running <span class="samp">gzip -V</span><!-- /@w -->. Also include in your
message the hardware and operating system, the compiler used to compile
<code>gzip</code>,
a description of the bug behavior, and the input to <code>gzip</code> that triggered
the bug.
<div class="node">
<p><hr>
<a name="Concept-Index"></a>Previous: <a rel="previous" accesskey="p" href="#Problems">Problems</a>,
Up: <a rel="up" accesskey="u" href="#Top">Top</a>
<br>
</div>
<h2 class="unnumbered">Concept Index</h2>
<ul class="index-cp" compact>
<li><a href="#index-bugs-8">bugs</a>: <a href="#Problems">Problems</a></li>
<li><a href="#index-concatenated-files-5">concatenated files</a>: <a href="#Advanced-usage">Advanced usage</a></li>
<li><a href="#index-Environment-6">Environment</a>: <a href="#Environment">Environment</a></li>
<li><a href="#index-invoking-3">invoking</a>: <a href="#Invoking-gzip">Invoking gzip</a></li>
<li><a href="#index-options-4">options</a>: <a href="#Invoking-gzip">Invoking gzip</a></li>
<li><a href="#index-overview-1">overview</a>: <a href="#Overview">Overview</a></li>
<li><a href="#index-sample-2">sample</a>: <a href="#Sample">Sample</a></li>
<li><a href="#index-tapes-7">tapes</a>: <a href="#Tapes">Tapes</a></li>
</ul>
<div class="contents">
<h2>Table of Contents</h2>
<ul>
<li><a name="toc_Copying" href="#Copying">GNU GENERAL PUBLIC LICENSE</a>
<ul>
<li><a href="#Copying">Preamble</a>
<li><a href="#Copying">How to Apply These Terms to Your New Programs</a>
</li></ul>
<li><a name="toc_Overview" href="#Overview">1 Overview</a>
<li><a name="toc_Sample" href="#Sample">2 Sample Output</a>
<li><a name="toc_Invoking-gzip" href="#Invoking-gzip">3 Invoking <code>gzip</code></a>
<li><a name="toc_Advanced-usage" href="#Advanced-usage">4 Advanced usage</a>
<li><a name="toc_Environment" href="#Environment">5 Environment</a>
<li><a name="toc_Tapes" href="#Tapes">6 Using <code>gzip</code> on tapes</a>
<li><a name="toc_Problems" href="#Problems">7 Reporting Bugs</a>
<li><a name="toc_Concept-Index" href="#Concept-Index">Concept Index</a>
</li></ul>
</div>
</body></html>
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