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gzip(1)
NAME
gzip, gunzip, gzcat - Compresses or expands files.
SYNOPSIS
gzip [-acdfhlLnNrtvV19] [-S suffix] [name...]
gunzip [-acfhlLnNrtvV] [-S suffix] [name...]
gzcat [-afhlLnNrtvV] [-S suffix] [name...]
OPTIONS
-a (--ascii)
Specifies ascii text mode; converts end-of-line using local
conventions. This option is supported only on some nonUnix systems. For
MSDOS, CR LF is converted to LF when compressing, and LF is converted
to CR LF when decompressing.
-c (--stdout --to--stdout)
Writes output on standard output; keeps 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.
The gzcat command is equivalent to the gunzip -c or gzip -cd command.
-d (--decompress --uncompress)
Specifies an uncompress operation.
The gunzip command is equivalent to the gzip -d command.
-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 the gzip command, and if the -c option is also
specified, copy the input data without change to the standard output;
that is, let the gzcat command behave as the cat command. If the -f
option is not specified, and when not running in the background, the
gzip command prompts to verify whether an existing file should be
overwritten.
-h (--help)
Displays a help screen and quits.
-l (--list)
Lists the following fields for each compressed file:
compressed size
Specifies size of the compressed file.
uncompressed size
Specifies size of the uncompressed file.
ratio
Specifies compression ratio (or 0.0% if unknown).
uncompressed_name
Specifies the name of the uncompressed file.
The uncompressed size is given as -1 for files that are not in the gzip
format, such as compressed .Z files.
When used with the -v option, the following fields are also displayed:
method
Specifies compression method.
crc Specifies the 32-bit CRC of the uncompressed data.
date & time
Specifies the time stamp for the uncompressed file.
The compression methods currently supported are deflate, compress, lzh
(SCO compress -H) and pack. The crc value is given as ffffffff for a
file that is not in the gzip format. When used with the -N option,
the uncompressed name and the date and time are those stored within the
compressed file, if present.
When used with the -v option, the size totals and compression ratio for
all files is also displayed, unless some sizes are unknown. When the -q
option is specified, the title and totals lines are not displayed.
-L (--License)
Displays the gzip license and quits.
-n (--no-name)
Specifies that the original file name and time stamp are not saved when
compressing 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.
-N (--name)
Specifies, when compressing, to always save the original file name and
time stamp; this is the default. Specifies, when decompressing, to
restore the original file name and time stamp, if present. This option
is useful on systems which have a limit on the length of a file name or
when the time stamp has been lost after a file transfer.
-q (--quiet)
Suppresses all warnings.
-r (--recursive)
Travels the directory structure recursively. If any of the file names
specified on the command line are directories, the gzip command
descends into the directory and compresses all the files it finds there
(or decompresses them in the case of the gunzip command).
-S .suf (--suffix .suf)
Uses the suffix .suf instead of .gz. Any suffix can be given, but
suffixes other than .z and .gz should be avoided to remove confusion
when files are transferred to other systems. A null suffix forces the
gunzip command to attempt decompressing all given files regardless of
the suffix, as follows:
gunzip -S * (*.* for MSDOS)
Previous versions of the gzip command used the .z suffix. This was
changed to avoid a conflict with the pack command.
-t (--test)
Specifies that the compressed file's integrity be tested.
-v (--verbose)
Specifies verbose mode. Displays the name and percentage reduction for
each file compressed or decompressed.
-V (--Version)
Specifies the version number and compilation options and then quits.
-# (--fast --best)
Regulates the speed of compression by using the specified digit #, for
which -1 or --fast indicates the fastest compression method (less
compression) and -9 or --best indicates the slowest compression method
(best compression). The default compression level is -6 (that is,
biased towards high compression at the expense of speed).
DESCRIPTION
The gzip command reduces the size of specified files using the 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.
If files are not specified, or if a file name is "-", the standard input is
compressed to the standard output. The gzip command only attempts to
compress regular files. In particular, it ignores symbolic links.
By default, the gzip command 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.
Compressed files can be restored to their original form using the gzip
command with the -d option, or by using the gunzip command.
The gunzip command takes a list of specified files and replaces each file
that begins with the correct magic number and whose name ends with .gz,
-gz, .z, -z, _z or .Z with an uncompressed file without the original
extension. The gunzip command also recognizes the special extensions .tgz
and .taz as shorthands for .tar.gz and .tar.Z respectively. When
compressing, the gzip command uses the .tgz extension if necessary instead
of truncating a file with a .tar extension.
The gunzip command can currently decompress files that are created by the
gzip, zip, compress, compress -H or pack commands. The detection of the
input format is automatic. When using the first two formats, the gunzip
command checks a 32-bit CRC. For the pack command, the gunzip command
checks the uncompressed length. Although the standard compress format was
not designed to allow consistency checks, the gunzip command is sometimes
able to detect a bad .Z file in cases where the uncompress command does
not. Therefore, if you get an error when uncompressing a .Z file, do not
assume that the .Z file is correct if the same file can be decompressed
without error by the uncompress command. In this case, the uncompress
command probably did not process the input file correctly, and the
generated output file is not useful.
The gzip command uses the Lempel-Ziv algorithm used in the zip and PKZIP
commands. 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 the compress command), Huffman
coding (as used in the pack command), or adaptive Huffman coding (in the
compact command).
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. The gzip command preserves the mode, ownership and
time stamps of files when compressing or decompressing.
Advanced Usage
Multiple compressed files can be concatenated. In this case, the gunzip
command extracts all members at once. For example:
gzip -c file1 > foo.gz
gzip -c file2 >> foo.gz
Using the previous example, gunzip -c foo is equivalent to cat file1 file2.
In case a member of a .gz file is damaged, other members can still be
recovered (if the damaged member is removed). However, you gain better
compression by compressing all members at once as follows:
cat file1 file2 | gzip > foo.gz
The preceding command line compresses better than the following one:
gzip -c file1 file2 > foo.gz
If you want to recompress concatenated files to gain better compression, do
the following:
gzip -cd old.gz | gzip > new.gz
If a compressed file consists of several members, the uncompressed size and
CRC reported by the -l option applies to the last member only. If you need
the uncompressed size for all members, use the following command:
gzip -cd file.gz | wc -c
To create a single archive file with multiple members so that members can
later be extracted independently, use an archiver such as the tar or zip
commands. GNU tar supports the -z option to invoke the gzip command
transparently. The gzip command is designed as a complement to the tar
command, not as a replacement.
Environment
The environment variable GZIP can hold a set of default options for the
gzip command. These options are interpreted first and can be overwritten
by explicit command line options as follows.
GZIP="-8v --name"; export GZIP (for sh)
setenv GZIP "-8v --name" (for csh)
RESTRICTIONS
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 the gunzip command for decompression, the
gunzip command detects that there is extra trailing garbage after the
compressed data and emits a warning by default. You have to use the -q
option to suppress warnings. This option can be set in the GZIP environment
variable as follows:
GZIP="-q" tar -xfz --block-compress /dev/tape/tape0_d1 (for sh)
(setenv GZIP -q; tar -xfz --block-compr /dev/tape/tape0_d1 (for csh)
In the previous example, the gzip command is invoked implicitly by the -z
option of the GNU tar command. Make sure that the same block size
(specified by the -b option of the tar command) is used for reading and
writing compressed data on tapes. (This example assumes that you are using
the GNU version of the tar command.)
The --list flag reports incorrect sizes if they exceed 2 gigabytes. The --
list flag reports sizes as -1 and crc as ffffffff if the compressed file is
on a nonseekable media.
In rare cases, the --best flag gives worse compression than the default
compression level (-6). On some highly redundant files, the compress
command compresses better than gzip command.
[Tru64 UNIX] The gzip command may not preserve the extended file
attributes (property list) of a file, including any access control lists
(ACL). Verify that any ACLs are not removed or modified by using gzip.
EXIT STATUS
0 (Zero)
Success.
1 An error occurred.
2 A warning is encountered.
SEE ALSO
Commands: compress(1), pack(1)
Files: acl(4)
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