.\" Copyright (c) 1986, 1990, 1993 .\" The Regents of the University of California. All rights reserved. .\" .\" This code is derived from software contributed to Berkeley by .\" James A. Woods, derived from original work by Spencer Thomas .\" and Joseph Orost. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. All advertising materials mentioning features or use of this software .\" must display the following acknowledgement: .\" This product includes software developed by the University of .\" California, Berkeley and its contributors. .\" 4. Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" @(#)compress.1 8.2 (Berkeley) 4/18/94 .\" $FreeBSD: src/usr.bin/compress/compress.1,v 1.19 2004/07/05 17:12:53 ru Exp $ .\" .Dd May 17, 2002 .Dt COMPRESS 1 .Os .Sh NAME .Nm compress , .Nm uncompress .Nd compress and expand data .Sh SYNOPSIS .Nm .Op Fl cfv .Op Fl b Ar bits .Op Ar .Nm uncompress .Op Fl cfv .Op Ar .Sh DESCRIPTION The .Nm utility reduces the size of the named files using adaptive Lempel-Ziv coding. Each .Ar file is renamed to the same name plus the extension .Dq .Z . 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 .Ar file , the file is ignored. .Pp The .Nm uncompress utility restores the compressed files to their original form, renaming the files by deleting the .Dq .Z extension. .Pp If renaming the files would cause files to be overwritten and the standard input device is a terminal, the user is prompted (on the standard error output) for confirmation. If prompting is not possible or confirmation is not received, the files are not overwritten. .Pp If no files are specified or a .Ar file argument is a single dash .Pq Sq Fl , 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. .Pp The options are as follows: .Bl -tag -width indent .It Fl b Specify the .Ar bits code limit (see below). .It Fl c Compressed or uncompressed output is written to the standard output. No files are modified. .It Fl f Force compression of .Ar file , even if it is not actually reduced in size. Additionally, files are overwritten without prompting for confirmation. .It Fl v Print the percentage reduction of each file. .El .Pp The .Nm utility uses a modified Lempel-Ziv algorithm. 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 .Fl b flag is reached (the default is 16). .Ar Bits must be between 9 and 16. .Pp After the .Ar bits limit is reached, .Nm periodically checks the compression ratio. If it is increasing, .Nm continues to use the existing code dictionary. However, if the compression ratio decreases, .Nm discards the table of substrings and rebuilds it from scratch. This allows the algorithm to adapt to the next "block" of the file. .Pp The .Fl b flag is omitted for .Nm uncompress since the .Ar 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. .Pp The amount of compression obtained depends on the size of the input, the number of .Ar 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 the historical command pack), or adaptive Huffman coding (as used in the historical command compact), and takes less time to compute. .Sh DIAGNOSTICS .Ex -std compress uncompress .Pp The .Nm compress utility exits 2 if attempting to compress the file would not reduce its size and the .Fl f option was not specified. .Sh SEE ALSO .Xr gunzip 1 , .Xr gzexe 1 , .Xr gzip 1 , .Xr zcat 1 , .Xr zmore 1 , .Xr znew 1 .Rs .%A Welch, Terry A. .%D June, 1984 .%T "A Technique for High Performance Data Compression" .%J "IEEE Computer" .%V 17:6 .%P pp. 8-19 .Re .Sh STANDARDS The .Nm compress and .Nm uncompress utilities conform to .St -p1003.1-2001 . .Sh HISTORY The .Nm command appeared in .Bx 4.3 .