gzip: new option --synchronous

[debian/gzip] / doc / gzip.texi

\input texinfo @c -*-texinfo-*-
@c %**start of header
@setfilename gzip.info
@documentencoding UTF-8
@include version.texi
@settitle GNU Gzip
@finalout
@setchapternewpage odd
@c %**end of header
@copying
This manual is for GNU Gzip
(version @value{VERSION}, @value{UPDATED}),
and documents commands for compressing and decompressing data.

Copyright @copyright{} 1998-1999, 2001-2002, 2006-2007, 2009-2016 Free Software
Foundation, Inc.

Copyright @copyright{} 1992, 1993 Jean-loup Gailly

@quotation
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''.
@end quotation
@end copying

@dircategory Compression
@direntry
* Gzip: (gzip).                 General (de)compression of files (lzw).
@end direntry

@dircategory Individual utilities
@direntry
* gunzip: (gzip)Overview.                       Decompression.
* gzexe: (gzip)Overview.                        Compress executables.
* zcat: (gzip)Overview.                         Decompression to stdout.
* zdiff: (gzip)Overview.                        Compare compressed files.
* zforce: (gzip)Overview.                       Force .gz extension on files.
* zgrep: (gzip)Overview.                        Search compressed files.
* zmore: (gzip)Overview.                        Decompression output by pages.
@end direntry

@titlepage
@title GNU gzip
@subtitle The data compression program
@subtitle for Gzip version @value{VERSION}
@subtitle @value{UPDATED}
@author by Jean-loup Gailly

@page
@vskip 0pt plus 1filll
@insertcopying
@end titlepage

@contents

@ifnottex
@node Top
@top GNU Gzip: General file (de)compression

@insertcopying
@end ifnottex

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

@node Overview
@chapter Overview
@cindex overview

@command{gzip} reduces the size of the named files using Lempel--Ziv coding
(LZ77).  Whenever possible, each file is replaced by one with the
extension @samp{.gz}, while keeping the same ownership modes, access and
modification times.  (The default extension is @samp{-gz} for @abbr{VMS},
@samp{z} for @abbr{MSDOS}, @abbr{OS/2} @abbr{FAT} and Atari.)
If no files are specified or
if a file name is @file{-}, the standard input is compressed to the standard
output.  @command{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, @command{gzip}
truncates it.  @command{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, @command{gzip} keeps the original file name and time stamp in
the compressed file.  These are used when decompressing the file with the
@option{-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 @command{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
@abbr{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 @abbr{UTC}.  The upper bounds assume the typical case
where leap seconds are ignored.

Compressed files can be restored to their original form using @samp{gzip -d}
or @command{gunzip} or @command{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.

@command{gunzip} takes a list of files on its command line and replaces
each file whose name ends with @samp{.gz}, @samp{.z}
@samp{-gz}, @samp{-z}, or @samp{_z} (ignoring case)
and which begins with the correct
magic number with an uncompressed file without the original extension.
@command{gunzip} also recognizes the special extensions @samp{.tgz} and
@samp{.taz} as shorthands for @samp{.tar.gz} and @samp{.tar.Z}
respectively.  When compressing, @command{gzip} uses the @samp{.tgz}
extension if necessary instead of truncating a file with a @samp{.tar}
extension.

@command{gunzip} can currently decompress files created by @command{gzip},
@command{zip}, @command{compress} or @command{pack}.  The detection of the input
format is automatic.  When using the first two formats, @command{gunzip}
checks a 32 bit @abbr{CRC} (cyclic redundancy check).  For @command{pack},
@command{gunzip} checks the uncompressed length.  The @command{compress} format
was not designed to allow consistency checks.  However @command{gunzip} is
sometimes able to detect a bad @samp{.Z} file.  If you get an error when
uncompressing a @samp{.Z} file, do not assume that the @samp{.Z} file is
correct simply because the standard @command{uncompress} does not complain.
This generally means that the standard @command{uncompress} does not check
its input, and happily generates garbage output.  The @abbr{SCO} @samp{compress
-H} format (@abbr{LZH} compression method) does not include a @abbr{CRC} but
also allows some consistency checks.

Files created by @command{zip} can be uncompressed by @command{gzip} only if
they have a single member compressed with the ``deflation'' method.  This
feature is only intended to help conversion of @file{tar.zip} files to
the @file{tar.gz} format.  To extract a @command{zip} file with a single
member, use a command like @samp{gunzip <foo.zip} or @samp{gunzip -S
.zip foo.zip}.  To extract @command{zip} files with several
members, use @command{unzip} instead of @command{gunzip}.

@command{zcat} is identical to @samp{gunzip -c}.  @command{zcat}
uncompresses either a list of files on the command line or its standard
input and writes the uncompressed data on standard output.  @command{zcat}
will uncompress files that have the correct magic number whether they
have a @samp{.gz} suffix or not.

@command{gzip} uses the Lempel--Ziv algorithm used in @command{zip} and
@abbr{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 @abbr{LZW} (as used in @command{compress}), Huffman
coding (as used in @command{pack}), or adaptive Huffman coding
(@command{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 @command{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.  @command{gzip} normally preserves the mode,
ownership and time stamps of files when compressing or decompressing.

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

@node Sample
@chapter Sample output
@cindex sample

Here are some realistic examples of running @command{gzip}.

This is the output of the command @samp{gzip -h}:

@example
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
      --synchronous synchronous output (safer if system crashes, but slower)
  -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>.
@end example

This is the output of the command @samp{gzip -v texinfo.tex}:

@example
texinfo.tex:     69.3% -- replaced with texinfo.tex.gz
@end example

The following command will find all regular @samp{.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:

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

@node Invoking gzip
@chapter Invoking @command{gzip}
@cindex invoking
@cindex options

The format for running the @command{gzip} program is:

@example
gzip @var{option} @dots{}
@end example

@command{gzip} supports the following options:

@table @option
@item --stdout
@itemx --to-stdout
@itemx -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.

@item --decompress
@itemx --uncompress
@itemx -d
Decompress.

@item --force
@itemx -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 @command{gzip}, and if the option @option{--stdout} is also
given, copy the input data without change to the standard output: let
@command{zcat} behave as @command{cat}.  If @option{-f} is not given, and
when not running in the background, @command{gzip} prompts to verify
whether an existing file should be overwritten.

@item --help
@itemx -h
Print an informative help message describing the options then quit.

@item --keep
@itemx -k
Keep (don't delete) input files during compression or decompression.

@item --list
@itemx -l
For each compressed file, list the following fields:

@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
@end example

The uncompressed size is given as @minus{}1 for files not in @command{gzip}
format, such as compressed @samp{.Z} files.  To get the uncompressed size for
such a file, you can use:

@example
zcat file.Z | wc -c
@end example

In combination with the @option{--verbose} option, the following fields are also
displayed:

@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
@end example

The @abbr{CRC} is given as ffffffff for a file not in gzip format.

With @option{--verbose}, the size totals and compression ratio for all files
is also displayed, unless some sizes are unknown.  With @option{--quiet},
the title and totals lines are not displayed.

The @command{gzip} format represents the input size modulo
@math{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:

@example
zcat file.gz | wc -c
@end example

@item --license
@itemx -L
Display the @command{gzip} license then quit.

@item --no-name
@itemx -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 @command{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.

@item --name
@itemx -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.

@item --quiet
@itemx -q
Suppress all warning messages.

@item --recursive
@itemx -r
Travel the directory structure recursively.  If any of the file names
specified on the command line are directories, @command{gzip} will descend
into the directory and compress all the files it finds there (or
decompress them in the case of @command{gunzip}).

@item --suffix @var{suf}
@itemx -S @var{suf}
Use suffix @var{suf} instead of @samp{.gz}.  Any suffix can be
given, but suffixes other than @samp{.z} and @samp{.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:

@example
gunzip -S "" *        (*.* for MSDOS)
@end example

Previous versions of gzip used the @samp{.z} suffix.  This was changed to
avoid a conflict with @command{pack}.

@item --synchronous
Use synchronous output, by transferring output data to the output
file's storage device when the file system supports this.  Because
file system data can be cached, without this option if the system
crashes around the time a command like @samp{gzip FOO} is run the user
might lose both @file{FOO} and @file{FOO.gz}; this is the default with
@command{gzip}, just as it is the default with most applications that
move data.  When this option is used, @command{gzip} is safer but can
be considerably slower.

@item --test
@itemx -t
Test.  Check the compressed file integrity.

@item --verbose
@itemx -v
Verbose.  Display the name and percentage reduction for each file compressed.

@item --version
@itemx -V
Version.  Display the version number and compilation options, then quit.

@item --fast
@itemx --best
@itemx -@var{n}
Regulate the speed of compression using the specified digit @var{n},
where @option{-1} or @option{--fast} indicates the fastest compression
method (less compression) and @option{--best} or @option{-9} indicates the
slowest compression method (optimal compression).  The default
compression level is @option{-6} (that is, biased towards high compression at
expense of speed).
@end table

@node Advanced usage
@chapter Advanced usage
@cindex concatenated files

Multiple compressed files can be concatenated.  In this case,
@command{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 @command{gzip} files:

@example
gzip -c file1  > foo.gz
gzip -c file2 >> foo.gz
@end example

@noindent
Then

@example
gunzip -c foo
@end example

@noindent
is equivalent to

@example
cat file1 file2
@end example

In case of damage to one member of a @samp{.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:

@example
cat file1 file2 | gzip > foo.gz
@end example

@noindent
compresses better than

@example
gzip -c file1 file2 > foo.gz
@end example

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

@example
zcat old.gz | gzip > new.gz
@end example

If a compressed file consists of several members, the uncompressed
size and @abbr{CRC} reported by the @option{--list} option applies to
the last member
only.  If you need the uncompressed size for all members, you can use:

@example
zcat file.gz | wc -c
@end example

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 @command{tar} or @command{zip}.  @acronym{GNU} @command{tar}
supports the @option{-z}
option to invoke @command{gzip} transparently.  @command{gzip} is designed as a
complement to @command{tar}, not as a replacement.

@node Environment
@chapter Environment
@cindex Environment

The obsolescent environment variable @env{GZIP} can hold a set of
default options for @command{gzip}.  These options are interpreted
first and can be overwritten by explicit command line parameters.  As
this can cause problems when using scripts, this feature is supported
only for options that are reasonably likely to not cause too much
harm, and @command{gzip} warns if it is used.  This feature will be
removed in a future release of @command{gzip}.

You can use an alias or script instead.  For example, if
@command{gzip} is in the directory @samp{/usr/bin} you can prepend
@file{$HOME/bin} to your @env{PATH} and create an executable script
@file{$HOME/bin/gzip} containing the following:

@example
#! /bin/sh
export PATH=/usr/bin
exec gzip -9 "$@@"
@end example

On @abbr{VMS}, the name of the obsolescent environment variable is
@env{GZIP_OPT}, to avoid a conflict with the symbol set for invocation
of the program.

@node Tapes
@chapter Using @command{gzip} on tapes
@cindex 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 @command{gunzip} for decompression,
@command{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 can use the @option{--quiet} option to suppress
the warning.

@node Problems
@chapter Reporting Bugs
@cindex bugs

If you find a bug in @command{gzip}, please send electronic mail to
@email{bug-gzip@@gnu.org}.  Include the version number,
which you can find by running @w{@samp{gzip -V}}.  Also include in your
message the hardware and operating system, the compiler used to compile
@command{gzip},
a description of the bug behavior, and the input to @command{gzip}
that triggered
the bug.@refill

@node GNU Free Documentation License
@appendix GNU Free Documentation License

@include fdl.texi

@node Concept index
@appendix Concept index

@printindex cp

@bye