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COMPRESS(1)		FreeBSD	General	Commands Manual		   COMPRESS(1)

     compress, uncompress -- compress and expand data

     compress [-fv] [-b	bits] [file ...]
     compress -c [-b bits] [file]
     uncompress	[-f] [file ...]
     uncompress	-c [file]

     The compress utility reduces the size of files using adaptive Lempel-Ziv
     coding.  Each file	is renamed to the same name plus the extension .Z.  A
     file argument with	a .Z extension will be ignored except it will cause an
     error exit	after other arguments are processed.  If compression would not
     reduce the	size of	a file,	the file is ignored.

     The uncompress utility restores compressed	files to their original	form,
     renaming the files	by deleting the	.Z extensions.	A file specification
     need not include the file's .Z extension.	If a file's name in its	file
     system does not have a .Z extension, it will not be uncompressed and it
     will cause	an error exit after other arguments are	processed.

     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.

     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

     If	no files are specified or a file argument is a single dash (`-'), the
     standard input is compressed or uncompressed to the standard output.  If
     either the	input and 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 in
     the output	file.

     The options are as	follows:

     -b	bits  The code size (see below)	is limited to bits, which must be in
	      the range	9..16.	The default is 16.

     -c	      Compressed or uncompressed output	is written to the standard
	      output.  No files	are modified.  The -v option is	ignored.  Com-
	      pression is attempted even if the	results	will be	larger than
	      the original.

     -f	      Files are	overwritten without prompting for confirmation.	 Also,
	      for compress, files are compressed even if they are not actually
	      reduced in size.

     -v	      Print the	percentage reduction of	each file.  Ignored by
	      uncompress or if the -c option is	also used.

     The compress utility uses a modified Lempel-Ziv algorithm.	 Common	sub-
     strings in	the file are first replaced by 9-bit codes 257 and up.	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 option or its	de-
     fault is reached.

     After the limit is	reached, compress periodically checks the compression
     ratio.  If	it is increasing, compress continues to	use the	existing code
     dictionary.  However, if the compression ratio decreases, compress	dis-
     cards the table of	substrings and rebuilds	it from	scratch.  This allows
     the algorithm to adapt to the next	"block"	of the file.

     The -b option is unavailable for uncompress since the bits	parameter
     specified during compression is encoded within the	output,	along with a
     magic number to ensure that neither decompression of random data nor re-
     compression 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 50-60%.	Com-
     pression is generally much	better than that achieved by Huffman coding
     (as used in the historical	command	pack), or adaptive Huffman coding (as
     used in the historical command compact), and takes	less time to compute.

     The compress and uncompress utilities exit	0 on success, and >0 if	an er-
     ror occurs.

     The compress utility exits	2 if attempting	to compress a file would not
     reduce its	size and the -f	option was not specified and if	no other error

     Create a file test_file with a single line	of text:

	   echo	"This is a test" > test_file

     Try to reduce the size of the file	using a	10-bit code and	show the exit

	   $ compress -b 10 test_file
	   $ echo $?

     Try to compress the file and show compression percentage:

	   $ compress -v test_file
	   test_file: file would grow; left unmodified

     Same as above but forcing compression:

	   $ compress -f -v test_file
	   test_file.Z:	79% expansion

     Compress and uncompress the string	`hello'	on the fly:

	   $ echo "hello" | compress | uncompress

     gunzip(1),	gzexe(1), gzip(1), zcat(1), zmore(1), znew(1)

     Welch, Terry A., "A Technique for High Performance	Data Compression",
     IEEE Computer, 17:6, pp. 8-19, June, 1984.

     The compress and uncompress utilities conform to IEEE Std 1003.1-2001

     The compress command appeared in 4.3BSD.

     Some of these might be considered otherwise-undocumented features.

     compress: If the utility does not compress	a file because doing so	would
     not reduce	its size, and a	file of	the same name except with an .Z	exten-
     sion exists, the named file is not	really ignored as stated above;	it
     causes a prompt to	confirm	the overwriting	of the file with the exten-
     sion.  If the operation is	confirmed, that	file is	deleted.

     uncompress: If an empty file is compressed	(using -f), the	resulting .Z
     file is also empty.  That seems right, but	if uncompress is then used on
     that file,	an error will occur.

     Both utilities: If	a `-' argument is used and the utility prompts the
     user, the standard	input is taken as the user's reply to the prompt.

     Both utilities: If	the specified file does	not exist, but a similarly-
     named one with (for compress) or without (for uncompress) a .Z extension
     does exist, the utility will waste	the user's time	by not immediately
     emitting an error message about the missing file and continuing.  In-
     stead, it first asks for confirmation to overwrite	the existing file and
     then does not overwrite it.

FreeBSD	13.0		       October 20, 2020			  FreeBSD 13.0


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