gzip.info   [plain text]

This is gzip.info, produced by makeinfo version 4.8 from gzip.texi.

   This manual is for Gzip (version 1.3.12, 5 February 2007), and
documents commands for compressing and decompressing data.

   Copyright (C) 1998, 1999, 2001, 2002, 2006, 2007 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 1.2 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".

* Gzip: (gzip).                 The gzip command for compressing files.

INFO-DIR-SECTION Individual utilities
* gzip: (gzip)Invoking gzip.                    Compress files.

File: gzip.info,  Node: Top,  Next: Overview,  Up: (dir)

Compressing Files

This manual is for Gzip (version 1.3.12, 5 February 2007), and
documents commands for compressing and decompressing data.

   Copyright (C) 1998, 1999, 2001, 2002, 2006, 2007 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 1.2 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.
* Copying This Manual:: How to make copies of 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', `.Z', `-gz', `-z' or `_z' and
which begins with the correct magic number with an uncompressed file
without the original extension.  `gunzip' also recognizes the special
extensions `.tgz' and `.taz' as shorthands for `.tar.gz' and `.tar.Z'
respectively.  When compressing, `gzip' uses the `.tgz' extension if
necessary instead of truncating a file with a `.tar' extension.

   `gunzip' can currently decompress files created by `gzip', `zip',
`compress' 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

   `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
(ftp://ftp.isi.edu/in-notes/rfc1952.txt) (May 1996).  The `zip'
deflation format is specified in P. Deutsch, DEFLATE Compressed Data
Format Specification version 1.3, Internet RFC 1951
(ftp://ftp.isi.edu/in-notes/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
       -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

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

     Report bugs to <bug-gzip@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.

     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').

`--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: Copying This Manual,  Prev: Tapes,  Up: Top

7 Reporting Bugs

If you find a bug in `gzip', please send electronic mail to
<bug-gzip@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: Copying This Manual,  Next: Concept Index,  Prev: Problems,  Up: Top

Appendix A Copying This Manual

* Menu:

* GNU Free Documentation License::  License for copying this manual.

File: gzip.info,  Node: GNU Free Documentation License,  Up: Copying This Manual

A.1 GNU Free Documentation License

                      Version 1.2, November 2002

     Copyright (C) 2000,2001,2002 Free Software Foundation, Inc.
     51 Franklin St, Fifth Floor, Boston, MA  02110-1301, USA

     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
     instruction or reference.


     This License applies to any manual or other work, in any medium,
     that contains a notice placed by the copyright holder saying it
     can be distributed under the terms of this License.  Such a notice
     grants a world-wide, royalty-free license, unlimited in duration,
     to use that work under the conditions stated herein.  The
     "Document", below, refers to any such manual or work.  Any member
     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 requiring permission under copyright law.

     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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     The "Invariant Sections" are certain Secondary Sections whose
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       I. Preserve the section Entitled "History", Preserve its Title,
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       M. Delete any section Entitled "Endorsements".  Such a section
          may not be included in the Modified Version.

       N. Do not retitle any existing section to be Entitled
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       O. Preserve any Warranty Disclaimers.

     If the Modified Version includes new front-matter sections or
     appendices that qualify as Secondary Sections and contain no
     material copied from the Document, you may at your option
     designate some or all of these sections as invariant.  To do this,
     add their titles to the list of Invariant Sections in the Modified
     Version's license notice.  These titles must be distinct from any
     other section titles.

     You may add a section Entitled "Endorsements", provided it contains
     nothing but endorsements of your Modified Version by various
     parties--for example, statements of peer review or that the text
     has been approved by an organization as the authoritative
     definition of a standard.

     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 document.


     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
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     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
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     Translation is considered a kind of modification, so you may
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     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 for under this License.  Any other
     attempt to copy, modify, sublicense or distribute the Document 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.


     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
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     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.

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 1.2
       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: Copying This Manual,  Up: Top

Appendix B Concept Index

* Menu:

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

Tag Table:
Node: Top1018
Node: Overview2164
Node: Sample7362
Node: Invoking gzip9298
Node: Advanced usage14290
Node: Environment15877
Node: Tapes16442
Node: Problems17459
Node: Copying This Manual17918
Node: GNU Free Documentation License18156
Node: Concept Index40552

End Tag Table