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COMPRESS(1)							   COMPRESS(1)

NAME    [Toc]    [Back]

     compress, uncompress, zcat	- compress and expand data

SYNOPSIS    [Toc]    [Back]

     compress [	-f ] [ -v ] [ -c ] [ -V	] [ -d ] [ -b bits ] [ name ...	]
     uncompress	[ -f ] [ -v ] [	-c ] [ -V ] [ name ... ]
     zcat [ name ... ]

DESCRIPTION    [Toc]    [Back]

     Compress reduces the size of the named files using	adaptive Lempel-Ziv
     coding.  Whenever possible, each file is replaced by one with the
     extension .Z, while keeping the same permission modes, access and
     modification times.  When invoked by the superuser	the ownership modes
     are also retained.	 If no files are specified, the	standard input is
     compressed	to the standard	output.	 Compressed files can be restored to
     their original form using uncompress or zcat. The -d option causes
     compress to uncompress (they are the same program,	normally looking at
     the name with which they are invoked to determine whether to compress or

     The -f option will	force compression of name, even	if it does not
     actually shrink or	the corresponding name.Z file already exists.  Except
     when run in the background	under /bin/sh, if -f is	not given the user is
     prompted as to whether an existing	name.Z file should be overwritten.

     The -c (``cat'') option makes compress/uncompress write to	the standard
     output; no	files are changed.  The	nondestructive behavior	of zcat	is
     identical to that of uncompress -c.

     Compress uses the modified	Lempel-Ziv algorithm popularized in "A
     Technique for High	Performance Data Compression", Terry A.	Welch, IEEE
     Computer, vol. 17,	no. 6 (June 1984), pp. 8-19.  Common substrings	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 flag is reached (default 16).
     Bits must be between 9 and	16.  The default can be	changed	in the source
     to	allow compress to be run on a smaller machine.

     After the bits limit is attained, 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 discards the table of substrings and rebuilds it from scratch.
     This allows the algorithm to adapt	to the next "block" of the file.

     Note that the -b flag is omitted 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
     recompression of compressed data is attempted.

									Page 1

COMPRESS(1)							   COMPRESS(1)

     The amount	of compression obtained	depends	on the size of the input, the
     number of bits per	code, and the distribution of common substrings.
     Typically,	text such as source code or English is reduced by 50-60%.
     Compression is generally much better than that achieved by	Huffman	coding
     (as used in pack),	or adaptive Huffman coding (compact), and takes	less
     time to compute.

     The -v option causes the printing of the percentage reduction of each
     file and additional error message(s) information.	The -V option prints
     the version of compress.

     If	an error occurs, exit status is	1, else	if the last file was not
     compressed	because	it became larger, the status is	2; else	the status is
     0.	 No error message is printed if	the compression	failed.

DIAGNOSTICS    [Toc]    [Back]

     Usage: compress [-fvc] [-b	maxbits] [file ...]
	     Invalid options were specified on the command line.
     Missing maxbits
	     Maxbits must follow -b.
     file:  not	in compressed format
	     The file specified	to uncompress has not been compressed.
     file:  compressed with xx bits, can only handle yy	bits
	     File was compressed by a program that could deal with more	bits
	     than the compress code on this machine.  Recompress the file with
	     smaller bits.
     file:  already has	.Z suffix -- no	change
	     The file is assumed to be already compressed.  Rename the file
	     and try again.
     file:  filename too long to tack on .Z
	     The file cannot be	compressed because its name is longer than 12
	     characters.  Rename and try again.	 This message does not occur
	     on	BSD systems.
     file already exists; do you wish to overwrite (y or n)?
	     Respond "y" if you	want the output	file to	be replaced; "n" if
     uncompress: corrupt input
	     A SIGSEGV violation was detected which usually means that the
	     input file	is corrupted.
     Compression: xx.xx%
	     Percentage	of the input saved by compression.  (Relevant only for
     --	not a regular file: unchanged
	     When the input file is not	a regular file,	(e.g. a	directory), it
	     is	left unaltered.
     --	has xx other links: unchanged
	     The input file has	links; it is left unchanged.  See ln(1)	for
	     more information.
     --	file unchanged
	     No	savings	is achieved by compression.  The input remains virgin.
	     (Relevant only for	-v.)

									Page 2

COMPRESS(1)							   COMPRESS(1)

NOTES    [Toc]    [Back]

     The suffix	.Z is used by the commands compress/uncompress.	The suffix .z
     is	used by	the commands pack/unpack. The suffix .gz is used by the
     commands gzip/gunzip.

SEE ALSO    [Toc]    [Back]

     pack(1), unpack(1), gzip(1), gunzip(1), zcat(1).

BUGS    [Toc]    [Back]

     Although compressed files are compatible between machines with large
     memory, -b12 should be used for file transfer to architectures with a
     small process data	space (64KB or less, as	exhibited by the DEC PDP
     series, the Intel 80286, etc.)

     compress should be	more flexible about the	existence of the `.Z' suffix.

									PPPPaaaaggggeeee 3333
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