1.\" Copyright (c) 1986, 1990, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" This code is derived from software contributed to Berkeley by 5.\" James A. Woods, derived from original work by Spencer Thomas 6.\" and Joseph Orost. 7.\" 8.\" Redistribution and use in source and binary forms, with or without 9.\" modification, are permitted provided that the following conditions 10.\" are met: 11.\" 1. Redistributions of source code must retain the above copyright 12.\" notice, this list of conditions and the following disclaimer. 13.\" 2. Redistributions in binary form must reproduce the above copyright 14.\" notice, this list of conditions and the following disclaimer in the 15.\" documentation and/or other materials provided with the distribution. 16.\" 3. All advertising materials mentioning features or use of this software 17.\" must display the following acknowledgement: 18.\" This product includes software developed by the University of 19.\" California, Berkeley and its contributors. 20.\" 4. Neither the name of the University nor the names of its contributors 21.\" may be used to endorse or promote products derived from this software 22.\" without specific prior written permission. 23.\" 24.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 25.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 26.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 27.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 28.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 29.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 30.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 31.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 32.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 33.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 34.\" SUCH DAMAGE. 35.\" 36.\" @(#)compress.1 8.2 (Berkeley) 4/18/94 37.\" $FreeBSD$ 38.\" 39.Dd April 18, 1994 40.Dt COMPRESS 1 41.Os 42.Sh NAME 43.Nm compress , 44.Nm uncompress , 45.Nd compress and expand data 46.Sh SYNOPSIS 47.Nm 48.Op Fl cfv 49.Op Fl b Ar bits 50.Op Ar 51.Nm uncompress 52.Op Fl cfv 53.Op Ar 54.Sh DESCRIPTION 55The 56.Nm 57utility reduces the size of the named files using adaptive Lempel-Ziv coding. 58Each 59.Ar file 60is renamed to the same name plus the extension 61.Dq .Z . 62As many of the modification time, access time, file flags, file mode, 63user ID, and group ID as allowed by permissions are retained in the 64new file. 65If compression would not reduce the size of a 66.Ar file , 67the file is ignored. 68.Pp 69The 70.Nm uncompress 71utility restores the compressed files to their original form, renaming the 72files by deleting the 73.Dq .Z 74extension. 75.Pp 76If renaming the files would cause files to be overwritten and the standard 77input device is a terminal, the user is prompted (on the standard error 78output) for confirmation. 79If prompting is not possible or confirmation is not received, the files 80are not overwritten. 81.Pp 82If no files are specified, the standard input is compressed or uncompressed 83to the standard output. 84If either the input and output files are not regular files, the checks for 85reduction in size and file overwriting are not performed, the input file is 86not removed, and the attributes of the input file are not retained. 87.Pp 88The options are as follows: 89.Bl -tag -width indent 90.It Fl b 91Specify the 92.Ar bits 93code limit (see below). 94.It Fl c 95Compressed or uncompressed output is written to the standard output. 96No files are modified. 97.It Fl f 98Force compression of 99.Ar file , 100even if it is not actually reduced in size. 101Additionally, files are overwritten without prompting for confirmation. 102.It Fl v 103Print the percentage reduction of each file. 104.El 105.Pp 106The 107.Nm 108utility uses a modified Lempel-Ziv algorithm. 109Common substrings in the file are first replaced by 9-bit codes 257 and up. 110When code 512 is reached, the algorithm switches to 10-bit codes and 111continues to use more bits until the 112limit specified by the 113.Fl b 114flag is reached (the default is 16). 115.Ar Bits 116must be between 9 and 16. 117.Pp 118After the 119.Ar bits 120limit is reached, 121.Nm 122periodically checks the compression ratio. 123If it is increasing, 124.Nm 125continues to use the existing code dictionary. 126However, if the compression ratio decreases, 127.Nm 128discards the table of substrings and rebuilds it from scratch. This allows 129the algorithm to adapt to the next "block" of the file. 130.Pp 131The 132.Fl b 133flag is omitted for 134.Nm uncompress 135since the 136.Ar bits 137parameter specified during compression 138is encoded within the output, along with 139a magic number to ensure that neither decompression of random data nor 140recompression of compressed data is attempted. 141.Pp 142The amount of compression obtained depends on the size of the 143input, the number of 144.Ar bits 145per code, and the distribution of common substrings. 146Typically, text such as source code or English is reduced by 50\-60%. 147Compression is generally much better than that achieved by Huffman 148coding (as used in the historical command pack), or adaptive Huffman 149coding (as used in the historical command compact), and takes less 150time to compute. 151.Sh DIAGNOSTICS 152.Ex -std compress uncompress 153.Sh SEE ALSO 154.Xr gunzip 1 , 155.Xr gzexe 1 , 156.Xr gzip 1 , 157.Xr zcat 1 , 158.Xr zmore 1 , 159.Xr znew 1 160.Rs 161.%A Welch, Terry A. 162.%D June, 1984 163.%T "A Technique for High Performance Data Compression" 164.%J "IEEE Computer" 165.%V 17:6 166.%P pp. 8-19 167.Re 168.Sh HISTORY 169The 170.Nm 171command appeared in 172.Bx 4.3 . 173