gzip, gunzip, gzcat - Compresses or expands files.
gzip [-acdfhlLnNrtvV19] [-S suffix] [name...]
gunzip [-acfhlLnNrtvV] [-S suffix] [name...]
gzcat [-afhlLnNrtvV] [-S suffix] [name...]
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.
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. Specifies an uncompress operation.
The gunzip command is equivalent to the gzip -d
command. 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. Displays a help screen and
quits. Lists the following fields for each compressed
file: Specifies size of the compressed
file. Specifies size of the uncompressed file.
Specifies compression ratio (or 0.0% if unknown).
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
files.
When used with the -v option, the following fields
are also displayed: Specifies compression method.
Specifies the 32-bit CRC of the uncompressed data.
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. Displays the gzip license and quits.
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. 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. Suppresses
all warnings. 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). Uses the suffix
instead of and 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 suffix.
This was changed to avoid a conflict with the
pack command. Specifies that the compressed file's
integrity be tested. Specifies verbose mode. Displays
the name and percentage reduction for each
file compressed or decompressed. Specifies the
version number and compilation options and then
quits. 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).
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 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, -z, _z or with an uncompressed
file without the original extension. The gunzip
command also recognizes the special extensions and as
shorthands for and respectively. When compressing, the
gzip command uses the extension if necessary instead of
truncating a file with a 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 file in cases where the uncompress command
does not. Therefore, if you get an error when uncompressing
a file, do not assume that the 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 [Toc] [Back]
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 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 [Toc] [Back]
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)
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.
Success. An error occurred. A warning is encountered.
Commands: compress(1), pack(1)
Files: acl(4)
gzip(1)
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