Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
GZIP(1)			FreeBSD	General	Commands Manual		       GZIP(1)

     gzip, gunzip, gzcat -- compress and expand	data (deflate mode)

     gzip [-123456789cdfhLlNnOqrtVv] [-b bits] [-o filename] [-S suffix]
	  [file	...]
     gunzip [-cfhLlNnqrtVv] [-o	filename] [file	...]
     gzcat [-fhqr] [file ...]

     The gzip utility reduces the size of the named files using	adaptive Lem-
     pel-Ziv coding, in	deflate	mode.  If invoked as gzip -O, the compress
     mode of compression is chosen; see	compress(1) for	more information.
     Each file is renamed to the same name plus	the extension ".gz".  As many
     of	the modification time, access time, file flags,	file mode, user	ID,
     and group ID as allowed by	permissions are	retained in the	new file.  If
     compression would not reduce the size of a	file, the file is ignored (un-
     less -f is	used).

     The gunzip	utility	restores compressed files to their original form, re-
     naming the	files by removing the extension	(or by using the stored	name
     if	the -N flag is specified).  It has the ability to restore files	com-
     pressed by	both gzip and compress(1), recognising the following exten-
     sions: ".Z", "-Z",	"_Z", ".gz", "-gz", "_gz", ".tgz", "-tgz", "_tgz",
     ".taz", "-taz", and "_taz".  Extensions ending in "tgz" and "taz" are not
     removed when decompressing, instead they are converted to "tar".

     The gzcat command is equivalent in	functionality to gunzip	-c.

     If	renaming the files would cause files to	be overwritten and the stan-
     dard input	device is a terminal, the user is prompted (on the standard
     error output) for confirmation.  If prompting is not possible or confir-
     mation is not received, the files are not overwritten.

     If	no files are specified,	the standard input is compressed or uncom-
     pressed to	the standard output.  If either	the input or output files are
     not regular files,	the checks for reduction in size and file overwriting
     are not performed,	the input file is not removed, and the attributes of
     the input file are	not retained.

     By	default, when compressing, the original	file name and time stamp are
     stored in the compressed file.  When uncompressing, this information is
     not used.	Instead, the uncompressed file inherits	the time stamp of the
     compressed	version	and the	uncompressed file name is generated from the
     name of the compressed file as described above.  These defaults may be
     overridden	by the -N and -n flags,	described below.

     The options are as	follows:

     -1...9  Use the deflate scheme, with compression factor of	-1 to -9.
	     Compression factor	-1 is the fastest, but provides	a poorer level
	     of	compression.  Compression factor -9 provides the best level of
	     compression, but is relatively slow.  The default is -6.

     -b	bits
	     Specify the bits code limit (see below).

     -c	     Compressed	or uncompressed	output is written to the standard out-
	     put.  No files are	modified (force	gzcat mode).

     -d	     Decompress	the source files instead of compressing	them (force
	     gunzip mode).

     -f	     Force compression of file,	even if	it is not actually reduced in
	     size.  Additionally, files	are overwritten	without	prompting for
	     confirmation.  If the input data is not in	a format recognized by
	     gzip and if the option -c is also given, copy the input data
	     without change to the standard output: let	gzcat behave as

     -h	     Print a short help	message.

     -L	     A no-op which exists for compatibility only.  On GNU gzip,	it
	     displays the program's license.

     -l	     List information for the specified	compressed files.  The follow-
	     ing information is	listed:

	     compressed	size	Size of	the compressed file.

	     uncompressed size	Size of	the file when uncompressed.

	     compression ratio	Ratio of the difference	between	the compressed
				and uncompressed sizes to the uncompressed

	     uncompressed name	Name the file will be saved as when uncom-

	     If	the -v option is specified, the	following additional informa-
	     tion is printed:

	     compression method	 Name of the method used to compress the file.

	     crc		 32-bit	CRC (cyclic redundancy code) of	the
				 uncompressed file.

	     time stamp		 Date and time corresponding to	the last data
				 modification time (mtime) of the compressed
				 file (if the -n option	is specified, the time
				 stamp stored in the compressed	file is
				 printed instead).

     -N	     When uncompressing	or listing, use	the time stamp and file	name
	     stored in the compressed file, if any, for	the uncompressed ver-

     -n	     When compressing, do not store the	original file name and time
	     stamp in the gzip header.

     -O	     Use old compression method	(force compress(1) mode).

     -o	filename
	     Set the output file name.

     -q	     Be	quiet: suppress	all messages.

     -r	     Recursive mode: gzip will descend into specified directories.

     -S	suffix
	     Set the suffix for	compressed files.

     -t	     Test the integrity	of each	file leaving any files intact.

     -V	     A no-op which exists for compatibility only.  On GNU gzip,	it
	     displays version information.

     -v	     Print the percentage reduction of each file and other informa-

     gzip uses a modified Lempel-Ziv algorithm (LZW).  Common substrings are
     replaced by pointers to previous strings, and are found using a hash ta-
     ble.  Unique substrings are emitted as a string of	literal	bytes, and
     compressed	as Huffman trees.  When	code 512 is reached, the algorithm
     switches to 10-bit	codes and continues to use more	bits until the limit
     specified by the -b flag is reached.  bits	must be	between	9 and 16 (the
     default is	16).

     After the bits limit is reached, gzip periodically	checks the compression
     ratio.  If	it is increasing, gzip continues to use	the existing code dic-
     tionary.  However,	if the compression ratio decreases, gzip discards the
     table of substrings and rebuilds it from scratch.	This allows the	algo-
     rithm to adapt to the next	"block"	of the file.

     The -b flag is omitted for	gunzip since the bits parameter	specified dur-
     ing compression is	encoded	within the output, along with a	magic number
     to	ensure that neither decompression of random data nor recompression of
     compressed	data is	attempted.

     The amount	of compression obtained	depends	on the size of the input, the
     number of bits per	code, and the distribution of common substrings.  Typ-
     ically, text such as source code or English is reduced by 60 - 70%	using
     gzip.  Compression	is generally much better than that achieved by Huffman
     coding (as	used in	the historical command pack), or adaptive Huffman cod-
     ing (as used in the historical command compact), and takes	less time to

     GZIP    Options which are passed to gzip, gunzip, and gzcat automati-

     The gzip utility exits with one of	the following values:

	   0	   Success.
	   1	   An error occurred.
	   2	   At least one	of the specified files was not compressed
		   since -f was	not specified and compression would have re-
		   sulted in a size increase.
	   >2	   An error occurred.

     The gunzip	and gzcat utilities exit 0 on success, and >0 if an error oc-

     compress(1), gzexe(1), zdiff(1), zforce(1), zmore(1), znew(1),

     P.	Deutsch	and J-L. Gailly, ZLIB Compressed Data Format Specification
     version 3.3, RFC 1950, May	1996.

     P.	Deutsch, DEFLATE Compressed Data Format	Specification version 1.3, RFC
     1951, May 1996.

     P.	Deutsch, GZIP file format specification	version	4.3, RFC 1952, May

     gzip compatibility	was added to compress(1) in OpenBSD 3.4.  The `g' in
     this version of gzip stands for "gratis".

FreeBSD	13.0			October	7, 2014			  FreeBSD 13.0


Want to link to this manual page? Use this URL:

home | help