doc: correct a diagnostic in man page to match actual

[debian/gzip] / gzip.1

.TH GZIP 1 local
.SH NAME
gzip, gunzip, zcat \- compress or expand files
.SH SYNOPSIS
.ll +8
.B gzip
.RB [ " \-acdfhklLnNrtvV19 " ]
.RB [ \-S\ suffix ]
[
.I "name \&..."
]
.ll -8
.br
.B gunzip
.RB [ " \-acfhklLnNrtvV " ]
.RB [ \-S\ suffix ]
[
.I "name \&..."
]
.br
.B zcat
.RB [ " \-fhLV " ]
[
.I "name \&..."
]
.SH DESCRIPTION
.I Gzip
reduces the size of the named files using Lempel-Ziv coding (LZ77).
Whenever possible,
each file is replaced by one with the extension
.BR "\&.gz" ,
while keeping the same ownership modes, access and modification times.
(The default extension is
.B "\-gz"
for VMS,
.B "z"
for MSDOS, OS/2 FAT, Windows NT FAT and Atari.)
If no files are specified, or if a file name is "-", the standard input is
compressed to the standard output.
.I Gzip
will only attempt to compress regular files.
In particular, it will ignore symbolic links.
.PP
If the compressed file name is too long for its file system,
.I gzip
truncates it.
.I 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.
.PP
By default,
.I gzip
keeps the original file name and timestamp in the compressed file. These
are used when decompressing the file with the
.B \-N
option. This is useful when the compressed file name was truncated or
when the time stamp was not preserved after a file transfer.
.PP
Compressed files can be restored to their original form using
.I gzip -d
or
.I gunzip
or
.IR 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.
.PP
.I gunzip
takes a list of files on its command line and replaces each
file whose name ends with .gz, -gz, .z, -z, or _z (ignoring case)
and which begins with the correct magic number with an uncompressed
file without the original extension.
.I gunzip
also recognizes the special extensions
.B "\&.tgz"
and
.B "\&.taz"
as shorthands for
.B "\&.tar.gz"
and
.B "\&.tar.Z"
respectively.
When compressing,
.I gzip
uses the
.B "\&.tgz"
extension if necessary instead of truncating a file with a
.B "\&.tar"
extension.
.PP
.I gunzip
can currently decompress files created by
.IR gzip ,
.IR zip ,
.IR compress ,
.I "compress -H"
or
.IR pack .
The detection of the input format is automatic.  When using
the first two formats,
.I gunzip
checks a 32 bit CRC. For
.I pack
and
.I gunzip
checks the uncompressed length. The standard
.I compress
format was not designed to allow consistency checks. However
.I 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
.I uncompress
does not complain. This generally means that the standard
.I 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.
.PP
Files created by
.I 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
.I zip
file with a single member, use a command like
.I "gunzip <foo.zip"
or
.IR "gunzip -S .zip foo.zip" .
To extract zip files
with several members, use
.I unzip
instead of
.IR gunzip .
.PP
.I zcat
is identical to
.I gunzip
.BR \-c .
(On some systems,
.I zcat
may be installed as
.I gzcat
to preserve the original link to
.IR compress .)
.I zcat
uncompresses either a list of files on the command line or its
standard input and writes the uncompressed data on standard output.
.I zcat
will uncompress files that have the correct magic number whether
they have a
.B "\&.gz"
suffix or not.
.PP
.I Gzip
uses the Lempel-Ziv algorithm used in
.I 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
.IR compress ),
Huffman coding (as used in
.IR pack ),
or adaptive Huffman coding
.RI ( compact ).
.PP
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.
.I gzip
preserves the mode, ownership and timestamps of files when compressing
or decompressing.
.SH OPTIONS
.TP
.B \-a --ascii
Ascii text mode: convert end-of-lines using local conventions. This option
is supported only on some non-Unix systems. For MSDOS, CR LF is converted
to LF when compressing, and LF is converted to CR LF when decompressing.
.TP
.B \-c --stdout --to-stdout
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.
.TP
.B \-d --decompress --uncompress
Decompress.
.TP
.B \-f --force
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
.IR gzip ,
and if the option --stdout is also given, copy the input data without change
to the standard output: let
.I zcat
behave as
.IR cat .
If
.B \-f
is not given,
and when not running in the background,
.I gzip
prompts to verify whether an existing file should be overwritten.
.TP
.B \-h --help
Display a help screen and quit.
.TP
.B \-k --keep
Keep (don't delete) input files during compression or decompression.
.TP
.B \-l --list
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
    crc: the 32-bit CRC of the uncompressed data
    date & time: time stamp for the uncompressed file

The compression methods currently supported are deflate, compress, lzh
(SCO compress -H) and pack.  The crc is given as ffffffff for a file
not in gzip format.

With --name, the uncompressed name,  date and time  are
those stored within the compress file if present.

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.
.TP
.B \-L --license
Display the
.I gzip
license and quit.
.TP
.B \-n --no-name
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
.I 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.
.TP
.B \-N --name
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.
.TP
.B \-q --quiet
Suppress all warnings.
.TP
.B \-r --recursive
Travel the directory structure recursively. If any of the file names
specified on the command line are directories,
.I gzip
will descend into the directory and compress all the files it finds there
(or decompress them in the case of
.I gunzip
).
.TP
.B \-S .suf   --suffix .suf
When compressing, use suffix .suf instead of .gz.
Any non-empty suffix can be given, but suffixes
other than .z and .gz should be avoided to avoid confusion when files
are transferred to other systems.

When decompressing, add .suf to the beginning of the list of
suffixes to try, when deriving an output file name from an input file name.
.TP
.B \-t --test
Test. Check the compressed file integrity.
.TP
.B \-v --verbose
Verbose. Display the name and percentage reduction for each file compressed
or decompressed.
.TP
.B \-V --version
Version. Display the version number and compilation options then quit.
.TP
.B \-# --fast --best
Regulate the speed of compression using the specified digit
.IR # ,
where
.B \-1
or
.B \-\-fast
indicates the fastest compression method (less compression)
and
.B \-9
or
.B \-\-best
indicates the slowest compression method (best compression).
The default compression level is
.BR \-6
(that is, biased towards high compression at expense of speed).
.SH "ADVANCED USAGE"
Multiple compressed files can be concatenated. In this case,
.I gunzip
will extract all members at once. For example:

      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:

      gzip -cd 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:

      gzip -cd 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.
.SH "ENVIRONMENT"
The obsolescent environment variable
.B GZIP
can hold a set of default options for
.IR 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
.I gzip
warns if it is used.
This feature will be removed in a future release of
.IR gzip .
.PP
You can use an alias or script instead.  For example, if
.I gzip
is in the directory
.B /usr/bin
you can prepend
.B $HOME/bin
to your
.B PATH
and create an executable script
.B $HOME/bin/gzip
containing the following:

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

On VMS, the name of the obsolescent environment variable is GZIP_OPT, to
avoid a conflict with the symbol set for invocation of the program.
.SH "SEE ALSO"
znew(1), zcmp(1), zmore(1), zforce(1), gzexe(1), zip(1), unzip(1), compress(1)
.PP
The
.I gzip
file format is specified in P. Deutsch, \s-1GZIP\s0 file format
specification version 4.3,
.BR <http://www.ietf.org/rfc/rfc1952.txt> ,
Internet RFC 1952 (May 1996).
The
.I zip
deflation format is specified in P. Deutsch, \s-1DEFLATE\s0 Compressed
Data Format Specification version 1.3,
.BR <http://www.ietf.org/rfc/rfc1951.txt> ,
Internet RFC 1951 (May 1996).
.SH "DIAGNOSTICS"
Exit status is normally 0;
if an error occurs, exit status is 1. If a warning occurs, exit status is 2.
.TP
Usage: gzip [-cdfhklLnNrtvV19] [-S suffix] [file ...]
Invalid options were specified on the command line.
.TP
\fIfile\fP\^: not in gzip format
The file specified to
.I gunzip
has not been compressed.
.TP
\fIfile\fP\^: Corrupt input. Use zcat to recover some data.
The compressed file has been damaged. The data up to the point of failure
can be recovered using

      zcat \fIfile\fP > recover
.TP
\fIfile\fP\^: compressed with \fIxx\fP bits, can only handle \fIyy\fP bits
.I File
was compressed (using LZW) by a program that could deal with
more
.I bits
than the decompress code on this machine.
Recompress the file with gzip, which compresses better and uses
less memory.
.TP
\fIfile\fP\^: already has .gz suffix -- unchanged
The file is assumed to be already compressed.
Rename the file and try again.
.TP
\fIfile\fP already exists; do you wish to overwrite (y or n)?
Respond "y" if you want the output file to be replaced; "n" if not.
.TP
gunzip: corrupt input
A SIGSEGV violation was detected which usually means that the input file has
been corrupted.
.TP
\fIxx.x%\fP Percentage of the input saved by compression.
(Relevant only for
.BR \-v
and
.BR \-l \.)
.TP
-- not a regular file or directory: ignored
When the input file is not a regular file or directory,
(e.g. a symbolic link, socket, FIFO, device file), it is
left unaltered.
.TP
-- has \fIxx\fP other links: unchanged
The input file has links; it is left unchanged.  See
.IR ln "(1)"
for more information. Use the
.B \-f
flag to force compression of multiply-linked files.
.SH CAVEATS
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
.I gunzip
for decompression,
.I gunzip
detects that there is extra trailing garbage after the compressed data
and emits a warning by default.  You can use the --quiet option to
suppress the warning.
.SH BUGS
The gzip format represents the input size modulo 2^32, so the
--list option reports incorrect uncompressed sizes and compression
ratios 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

The --list option reports sizes as -1 and crc as ffffffff if the
compressed file is on a non seekable media.

In some rare cases, the --best option gives worse compression than
the default compression level (-6). On some highly redundant files,
.I compress
compresses better than
.IR gzip .
.SH "COPYRIGHT NOTICE"
Copyright \(co 1998-1999, 2001-2002, 2012, 2015-2016 Free Software Foundation,
Inc.
.br
Copyright \(co 1992, 1993 Jean-loup Gailly
.PP
Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.
.ig
Permission is granted to process this file through troff and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).
..
.PP
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the entire
resulting derived work is distributed under the terms of a permission
notice identical to this one.
.PP
Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that this permission notice may be stated in a translation approved
by the Foundation.