gzip.info [plain text]

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

This manual is for Gzip (version 1.3.10, 29 December 2006), and
documents commands for compressing and decompressing data.

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".

INFO-DIR-SECTION Utilities
START-INFO-DIR-ENTRY
* Gzip: (gzip). The gzip command for compressing files.
END-INFO-DIR-ENTRY

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

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

Compressing Files
*****************

This manual is for Gzip (version 1.3.10, 29 December 2006), and
documents commands for compressing and decompressing data.

* 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, 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 timestamp 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
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
decompressing.

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':

gzip VERSION-NUMBER
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.
Report bugs to <bug-gzip@gnu.org>.

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

texinfo.tex: 69.7% -- replaced with texinfo.tex.gz

The following command will find all `gzip' files in the current
directory and subdirectories, and extract them in place without
destroying the original:

find . -name '*.gz' -print | sed 's/^$.*$[.]gz$/gunzip < "&" > "\1"/' | sh

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:

`--stdout'
`--to-stdout'
`-c'
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.

`--decompress'
`--uncompress'
`-d'
Decompress.

`--force'
`-f'
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.

`--help'
`-h'
Print an informative help message describing the options then quit.

`--list'
`-l'
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 GB 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

`--license'
`-L'
Display the `gzip' license then quit.

`--no-name'
`-n'
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.

`--name'
`-N'
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.

`--quiet'
`-q'
Suppress all warning messages.

`--recursive'
`-r'
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'
`-t'
Test. Check the compressed file integrity.

`--verbose'
`-v'
Verbose. Display the name and percentage reduction for each file
compressed.

`--version'
`-V'
Version. Display the version number and compilation options, then
quit.

`--fast'
`--best'
`-N'
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

Then

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
replacement.

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 Vax/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

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

0. PREAMBLE

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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1. APPLICABILITY AND DEFINITIONS

This License applies to any manual or other work, in any medium,
that contains a notice placed by the copyright holder saying it
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Preserve the Title of the section, and preserve in the
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5. COMBINING DOCUMENTS

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unmodified, and list them all as Invariant Sections of your
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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."

6. COLLECTIONS OF DOCUMENTS

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.

7. AGGREGATION WITH INDEPENDENT WORKS

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.

8. TRANSLATION

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
prevail.

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.

9. TERMINATION

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.

10. FUTURE REVISIONS OF THIS LICENSE

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
`http://www.gnu.org/copyleft/'.

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.

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
situation.

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
************************

[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: Top1013
Node: Overview2137
Node: Sample7313
Node: Invoking gzip8968
Node: Advanced usage13946
Node: Environment15536
Node: Tapes16105
Node: Problems17120
Node: Copying This Manual17579
Node: GNU Free Documentation License17817
Node: Concept Index40213

End Tag Table