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GNU Gzip: General file (de)compression

This manual is for GNU Gzip (version 1.6), and documents commands for
compressing and decompressing data.

   Copyright (C) 1998-1999, 2001-2002, 2006-2007, 2009-2013 Free
Software Foundation, Inc.

   Copyright (C) 1992, 1993 Jean-loup Gailly

     Permission is granted to copy, distribute and/or modify this
     document under the terms of the GNU Free Documentation License,
     Version 1.3 or any later version published by the Free Software
     Foundation; with no Invariant Sections, with no Front-Cover Texts,
     and with no Back-Cover Texts.  A copy of the license is included in
     the section entitled "GNU Free Documentation License".

* Menu:

* Overview::		Preliminary information.
* Sample::		Sample output from 'gzip'.
* Invoking gzip::	How to run 'gzip'.
* Advanced usage::	Concatenated files.
* Environment::		The 'GZIP' environment variable
* Tapes::               Using 'gzip' on tapes.
* Problems::		Reporting bugs.
* GNU Free Documentation License:: Copying and sharing this manual.
* Concept index::       Index of concepts.

File: gzip.info,  Node: Overview,  Next: Sample,  Prev: Top,  Up: Top

1 Overview

'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 and Atari.)  If no files are specified 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 new 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 time stamp 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.
However, due to limitations in the current 'gzip' file format,
fractional seconds are discarded.  Also, time stamps must fall within
the range 1970-01-01 00:00:00 through 2106-02-07 06:28:15 UTC, and hosts
whose operating systems use 32-bit time stamps are further restricted to
time stamps no later than 2038-01-19 03:14:07 UTC.  The upper bounds
assume the typical case where leap seconds are ignored.

   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', '.z' '-gz', '-z', or '_z' (ignoring
case) 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'

   'gunzip' can currently decompress files created by 'gzip', 'zip',
'compress' or 'pack'.  The detection of the input format is automatic.
When using the first two formats, 'gunzip' checks a 32 bit CRC (cyclic
redundancy check).  For 'pack', 'gunzip' checks the uncompressed length.
The '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 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'.  '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' normally preserves
the mode, ownership and time stamps of files when compressing or

   The 'gzip' file format is specified in P. Deutsch, GZIP file format
specification version 4.3, Internet RFC 1952
(http://www.ietf.org/rfc/rfc1952.txt) (May 1996).  The 'zip' deflation
format is specified in P. Deutsch, DEFLATE Compressed Data Format
Specification version 1.3, Internet RFC 1951
(http://www.ietf.org/rfc/rfc1951.txt) (May 1996).

File: gzip.info,  Node: Sample,  Next: Invoking gzip,  Prev: Overview,  Up: Top

2 Sample output

Here are some realistic examples of running 'gzip'.

   This is the output of the command 'gzip -h':

     Usage: gzip [OPTION]... [FILE]...
     Compress or uncompress FILEs (by default, compress FILES in-place).

     Mandatory arguments to long options are mandatory for short options too.

       -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
       -k, --keep        keep (don't delete) input files
       -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, --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
         --rsyncable   Make rsync-friendly archive

     With no FILE, or when FILE is -, read standard input.

     Report bugs to <bug-gzip AT gnu.org>.

   This is the output of the command 'gzip -v texinfo.tex':

     texinfo.tex:     69.3% -- replaced with texinfo.tex.gz

   The following command will find all regular '.gz' files in the
current directory and subdirectories (skipping file names that contain
newlines), and extract them in place without destroying the original,
stopping on the first failure:

     find . -name '*
     *' -prune -o -name '*.gz' -type f -print |
       sed "
         s/^\\(.*\\)\\.gz$/gunzip <'\\1.gz' >'\\1'/
       " |
       sh -e

File: gzip.info,  Node: Invoking gzip,  Next: Advanced usage,  Prev: Sample,  Up: Top

3 Invoking 'gzip'

The format for running the 'gzip' program is:

     gzip OPTION ...

   'gzip' supports the following options:

     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.


     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 output: 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.

     Print an informative help message describing the options then quit.

     Keep (don't delete) input files during compression or

     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 (deflate,compress,lzh,pack)
          crc: the 32-bit CRC of the uncompressed data
          date & time: time stamp for the uncompressed file

     The CRC is given as ffffffff for a file not in gzip format.

     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.

     The 'gzip' format represents the input size modulo 2^32, so the
     uncompressed size and compression ratio are listed incorrectly for
     uncompressed files 4 GiB 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

     Display the 'gzip' license then quit.

     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.

     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.

     Suppress all warning messages.

     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').

     While compressing, synchronize the output occasionally based on the
     input.  This increases size by less than 1 percent most cases, but
     means that the 'rsync' program can much more efficiently
     synchronize files compressed with this flag.  'gunzip' cannot tell
     the difference between a compressed file created with this option,
     and one created without it.

'--suffix SUF'
'-S 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 "" *        (*.* for MSDOS)

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

     Test.  Check the compressed file integrity.

     Verbose.  Display the name and percentage reduction for each file

     Version.  Display the version number and compilation options, then

     Regulate the speed of compression using the specified digit N,
     where '-1' or '--fast' indicates the fastest compression method
     (less compression) and '--best' or '-9' indicates the slowest
     compression method (optimal compression).  The default compression
     level is '-6' (that is, biased towards high compression at expense
     of speed).

File: gzip.info,  Node: Advanced usage,  Next: Environment,  Prev: Invoking gzip,  Up: Top

4 Advanced usage

Multiple compressed files can be concatenated.  In this case, 'gunzip'
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.

   This is an example of concatenating 'gzip' files:

     gzip -c file1  > foo.gz
     gzip -c file2 >> foo.gz


     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 > foo.gz

compresses better than

     gzip -c file1 file2 > foo.gz

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

     zcat 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:

     zcat 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

File: gzip.info,  Node: Environment,  Next: Tapes,  Prev: Advanced usage,  Up: Top

5 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 VMS, the name of the environment variable is 'GZIP_OPT', to avoid
a conflict with the symbol set for invocation of the program.

File: gzip.info,  Node: Tapes,  Next: Problems,  Prev: Environment,  Up: Top

6 Using 'gzip' on tapes

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 if the garbage contains nonzero bytes.
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-compress /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'.)

File: gzip.info,  Node: Problems,  Next: GNU Free Documentation License,  Prev: Tapes,  Up: Top

7 Reporting Bugs

If you find a bug in 'gzip', please send electronic mail to
<bug-gzip AT gnu.org>.  Include the version number, which you can find by
running 'gzip -V'.  Also include in your message the hardware and
operating system, the compiler used to compile 'gzip', a description of
the bug behavior, and the input to 'gzip' that triggered the bug.

File: gzip.info,  Node: GNU Free Documentation License,  Next: Concept index,  Prev: Problems,  Up: Top

Appendix A GNU Free Documentation License

                     Version 1.3, 3 November 2008

     Copyright (C) 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.

     Everyone is permitted to copy and distribute verbatim copies
     of this license document, but changing it is not allowed.


     The purpose of this License is to make a manual, textbook, or other
     functional and useful document "free" in the sense of freedom: to
     assure everyone the effective freedom to copy and redistribute it,
     with or without modifying it, either commercially or
     noncommercially.  Secondarily, this License preserves for the
     author and publisher a way to get credit for their work, while not
     being considered responsible for modifications made by others.

     This License is a kind of "copyleft", which means that derivative
     works of the document must themselves be free in the same sense.
     It complements the GNU General Public License, which is a copyleft
     license designed for free software.

     We have designed this License in order to use it for manuals for
     free software, because free software needs free documentation: a
     free program should come with manuals providing the same freedoms
     that the software does.  But this License is not limited to
     software manuals; it can be used for any textual work, regardless
     of subject matter or whether it is published as a printed book.  We
     recommend this License principally for works whose purpose is
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     This License applies to any manual or other work, in any medium,
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     of the public is a licensee, and is addressed as "you".  You accept
     the license if you copy, modify or distribute the work in a way
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     A "Modified Version" of the Document means any work containing the
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     A "Secondary Section" is a named appendix or a front-matter section
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     You may add a passage of up to five words as a Front-Cover Text,
     and a passage of up to 25 words as a Back-Cover Text, to the end of
     the list of Cover Texts in the Modified Version.  Only one passage
     of Front-Cover Text and one of Back-Cover Text may be added by (or
     through arrangements made by) any one entity.  If the Document
     already includes a cover text for the same cover, previously added
     by you or by arrangement made by the same entity you are acting on
     behalf of, you may not add another; but you may replace the old
     one, on explicit permission from the previous publisher that added
     the old one.

     The author(s) and publisher(s) of the Document do not by this
     License give permission to use their names for publicity for or to
     assert or imply endorsement of any Modified Version.


     You may combine the Document with other documents released under
     this License, under the terms defined in section 4 above for
     modified versions, provided that you include in the combination all
     of the Invariant Sections of all of the original documents,
     unmodified, and list them all as Invariant Sections of your
     combined work in its license notice, and that you preserve all
     their Warranty Disclaimers.

     The combined work need only contain one copy of this License, and
     multiple identical Invariant Sections may be replaced with a single
     copy.  If there are multiple Invariant Sections with the same name
     but different contents, make the title of each such section unique
     by adding at the end of it, in parentheses, the name of the
     original author or publisher of that section if known, or else a
     unique number.  Make the same adjustment to the section titles in
     the list of Invariant Sections in the license notice of the
     combined work.

     In the combination, you must combine any sections Entitled
     "History" in the various original documents, forming one section
     Entitled "History"; likewise combine any sections Entitled
     "Acknowledgements", and any sections Entitled "Dedications".  You
     must delete all sections Entitled "Endorsements."


     You may make a collection consisting of the Document and other
     documents released under this License, and replace the individual
     copies of this License in the various documents with a single copy
     that is included in the collection, provided that you follow the
     rules of this License for verbatim copying of each of the documents
     in all other respects.

     You may extract a single document from such a collection, and
     distribute it individually under this License, provided you insert
     a copy of this License into the extracted document, and follow this
     License in all other respects regarding verbatim copying of that


     A compilation of the Document or its derivatives with other
     separate and independent documents or works, in or on a volume of a
     storage or distribution medium, is called an "aggregate" if the
     copyright resulting from the compilation is not used to limit the
     legal rights of the compilation's users beyond what the individual
     works permit.  When the Document is included in an aggregate, this
     License does not apply to the other works in the aggregate which
     are not themselves derivative works of the Document.

     If the Cover Text requirement of section 3 is applicable to these
     copies of the Document, then if the Document is less than one half
     of the entire aggregate, the Document's Cover Texts may be placed
     on covers that bracket the Document within the aggregate, or the
     electronic equivalent of covers if the Document is in electronic
     form.  Otherwise they must appear on printed covers that bracket
     the whole aggregate.


     Translation is considered a kind of modification, so you may
     distribute translations of the Document under the terms of section
     4.  Replacing Invariant Sections with translations requires special
     permission from their copyright holders, but you may include
     translations of some or all Invariant Sections in addition to the
     original versions of these Invariant Sections.  You may include a
     translation of this License, and all the license notices in the
     Document, and any Warranty Disclaimers, provided that you also
     include the original English version of this License and the
     original versions of those notices and disclaimers.  In case of a
     disagreement between the translation and the original version of
     this License or a notice or disclaimer, the original version will

     If a section in the Document is Entitled "Acknowledgements",
     "Dedications", or "History", the requirement (section 4) to
     Preserve its Title (section 1) will typically require changing the
     actual title.


     You may not copy, modify, sublicense, or distribute the Document
     except as expressly provided under this License.  Any attempt
     otherwise to copy, modify, sublicense, or distribute it is void,
     and will automatically terminate your rights under this License.

     However, if you cease all violation of this License, then your
     license from a particular copyright holder is reinstated (a)
     provisionally, unless and until the copyright holder explicitly and
     finally terminates your license, and (b) permanently, if the
     copyright holder fails to notify you of the violation by some
     reasonable means prior to 60 days after the cessation.

     Moreover, your license from a particular copyright holder is
     reinstated permanently if the copyright holder notifies you of the
     violation by some reasonable means, this is the first time you have
     received notice of violation of this License (for any work) from
     that copyright holder, and you cure the violation prior to 30 days
     after your receipt of the notice.

     Termination of your rights under this section does not terminate
     the licenses of parties who have received copies or rights from you
     under this License.  If your rights have been terminated and not
     permanently reinstated, receipt of a copy of some or all of the
     same material does not give you any rights to use it.


     The Free Software Foundation may publish new, revised versions of
     the GNU Free Documentation 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.  See

     Each version of the License is given a distinguishing version
     number.  If the Document specifies that a particular numbered
     version of this License "or any later version" applies to it, you
     have the option of following the terms and conditions either of
     that specified version or of any later version that has been
     published (not as a draft) by the Free Software Foundation.  If the
     Document does not specify a version number of this License, you may
     choose any version ever published (not as a draft) by the Free
     Software Foundation.  If the Document specifies that a proxy can
     decide which future versions of this License can be used, that
     proxy's public statement of acceptance of a version permanently
     authorizes you to choose that version for the Document.


     "Massive Multiauthor Collaboration Site" (or "MMC Site") means any
     World Wide Web server that publishes copyrightable works and also
     provides prominent facilities for anybody to edit those works.  A
     public wiki that anybody can edit is an example of such a server.
     A "Massive Multiauthor Collaboration" (or "MMC") contained in the
     site means any set of copyrightable works thus published on the MMC

     "CC-BY-SA" means the Creative Commons Attribution-Share Alike 3.0
     license published by Creative Commons Corporation, a not-for-profit
     corporation with a principal place of business in San Francisco,
     California, as well as future copyleft versions of that license
     published by that same organization.

     "Incorporate" means to publish or republish a Document, in whole or
     in part, as part of another Document.

     An MMC is "eligible for relicensing" if it is licensed under this
     License, and if all works that were first published under this
     License somewhere other than this MMC, and subsequently
     incorporated in whole or in part into the MMC, (1) had no cover
     texts or invariant sections, and (2) were thus incorporated prior
     to November 1, 2008.

     The operator of an MMC Site may republish an MMC contained in the
     site under CC-BY-SA on the same site at any time before August 1,
     2009, provided the MMC is eligible for relicensing.

ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and license
notices just after the title page:

       Copyright (C)  YEAR  YOUR NAME.
       Permission is granted to copy, distribute and/or modify this document
       under the terms of the GNU Free Documentation License, Version 1.3
       or any later version published by the Free Software Foundation;
       with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
       Texts.  A copy of the license is included in the section entitled ``GNU
       Free Documentation License''.

   If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the "with...Texts." line with this:

         with the Invariant Sections being LIST THEIR TITLES, with
         the Front-Cover Texts being LIST, and with the Back-Cover Texts
         being LIST.

   If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the

   If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of free
software license, such as the GNU General Public License, to permit
their use in free software.

File: gzip.info,  Node: Concept index,  Prev: GNU Free Documentation License,  Up: Top

Appendix B Concept index

* Menu:

* bugs:                                  Problems.              (line 6)
* concatenated files:                    Advanced usage.        (line 6)
* Environment:                           Environment.           (line 6)
* invoking:                              Invoking gzip.         (line 6)
* options:                               Invoking gzip.         (line 6)
* overview:                              Overview.              (line 6)
* sample:                                Sample.                (line 6)
* tapes:                                 Tapes.                 (line 6)

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