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

FreeBSD Manual Pages


home | help
COMPRESS(1)		    General Commands Manual		   COMPRESS(1)

       compress, uncompress, zcat - compress and expand	data (version 4.1)

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

       Compress	 reduces the size of the named files using adaptive Lempel-Ziv
       coding.	Whenever possible, each	file is	replaced by one	with  the  ex-
       tension	.Z, while keeping the same ownership modes, access and modifi-
       cation times.  If no files are specified, the standard  input  is  com-
       pressed to the standard output.	Compress will only attempt to compress
       regular files.  In particular, it will ignore symbolic links. If	a file
       has multiple hard links,	compress will refuse to	compress it unless the
       -f flag is given.

       If -f is	not given and compress is run in the foreground, the  user  is
       prompted	as to whether an existing file should be overwritten.

       Compressed  files  can  be restored to their original form using	uncom-
       press or	zcat.

       uncompress takes	a list of files	on its command line and	replaces  each
       file  whose  name  ends with .Z and which begins	with the correct magic
       number with an uncompressed file	without	the .Z.	 The uncompressed file
       will have the mode, ownership and timestamps of the compressed file.

       The  -c	option makes compress/uncompress write to the standard output;
       no files	are changed.

       zcat is identical to uncompress -c.  zcat uncompresses either a list of
       files  on  the command line or its standard input and writes the	uncom-
       pressed data on standard	output.	 zcat will uncompress files that  have
       the correct magic number	whether	they have a .Z suffix or not.

       If  the -r flag is specified, compress will operate recursively.	If any
       of the file names specified on the command line are  directories,  com-
       press  will  descend  into  the directory and compress all the files it
       finds there.

       The -V flag tells each of these	programs  to  print  its  version  and
       patchlevel, along with any preprocessor flags specified during compila-
       tion, on	stderr before doing any	compression or uncompression.

       Compress	uses the modified Lempel-Ziv algorithm popularized in "A Tech-
       nique for High Performance Data Compression", Terry A. Welch, IEEE Com-
       puter, 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 com-
       pression	ratio.	If it is increasing, compress continues	to use the ex-
       isting  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 parame-
       ter 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.

       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 cod-
       ing (as used in pack), or adaptive Huffman coding (compact), and	 takes
       less time to compute.

       Under  the  -v  option, a message is printed yielding the percentage of
       reduction for each file compressed.

       Exit status is normally 0; if the last file is larger after (attempted)
       compression, the	status is 2; if	an error occurs, exit status is	1.

       pack(1),	compact(1)

       Usage: compress [-dfvcVr] [-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 has been corrupted.
       Compression: xx.xx%
	       Percentage of the input saved by	compression.   (Relevant  only
	       for -v.)
       -- not a	regular	file or	directory: ignored
	       When the	input file is not a regular file or directory, (e.g. a
	       symbolic	link, socket, FIFO, device file),  it  is  left	 unal-
       -- has xx other links: unchanged
	       The  input file has links; it is	left unchanged.	 See ln(1) for
	       more information. Use the -f flag to force compression of  mul-
	       tiply-linked files.
       -- file unchanged
	       No  savings is achieved by compression.	The input remains vir-

       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 se-
       ries, the Intel 80286, etc.)

       Invoking	 compress with a -r flag will occasionally cause it to produce
       spurious	error warnings of the form

	"<filename>.Z already has .Z suffix - ignored"

       These warnings can be ignored. See the comments in compress.c:compdir()
       for an explanation.

				     local			   COMPRESS(1)


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

home | help