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 May 17, 2002 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 or a 83.Ar file 84argument is a single dash 85.Pq Sq Fl , 86the standard input is compressed or uncompressed to the standard output. 87If either the input and output files are not regular files, the checks for 88reduction in size and file overwriting are not performed, the input file is 89not removed, and the attributes of the input file are not retained. 90.Pp 91The options are as follows: 92.Bl -tag -width indent 93.It Fl b 94Specify the 95.Ar bits 96code limit (see below). 97.It Fl c 98Compressed or uncompressed output is written to the standard output. 99No files are modified. 100.It Fl f 101Force compression of 102.Ar file , 103even if it is not actually reduced in size. 104Additionally, files are overwritten without prompting for confirmation. 105.It Fl v 106Print the percentage reduction of each file. 107.El 108.Pp 109The 110.Nm 111utility uses a modified Lempel-Ziv algorithm. 112Common substrings in the file are first replaced by 9-bit codes 257 and up. 113When code 512 is reached, the algorithm switches to 10-bit codes and 114continues to use more bits until the 115limit specified by the 116.Fl b 117flag is reached (the default is 16). 118.Ar Bits 119must be between 9 and 16. 120.Pp 121After the 122.Ar bits 123limit is reached, 124.Nm 125periodically checks the compression ratio. 126If it is increasing, 127.Nm 128continues to use the existing code dictionary. 129However, if the compression ratio decreases, 130.Nm 131discards the table of substrings and rebuilds it from scratch. 132This allows 133the algorithm to adapt to the next "block" of the file. 134.Pp 135The 136.Fl b 137flag is omitted for 138.Nm uncompress 139since the 140.Ar bits 141parameter specified during compression 142is encoded within the output, along with 143a magic number to ensure that neither decompression of random data nor 144recompression of compressed data is attempted. 145.Pp 146The amount of compression obtained depends on the size of the 147input, the number of 148.Ar bits 149per code, and the distribution of common substrings. 150Typically, text such as source code or English is reduced by 50\-60%. 151Compression is generally much better than that achieved by Huffman 152coding (as used in the historical command pack), or adaptive Huffman 153coding (as used in the historical command compact), and takes less 154time to compute. 155.Sh DIAGNOSTICS 156.Ex -std compress uncompress 157.Pp 158The 159.Nm compress 160utility exits 2 if attempting to compress the file would not reduce its size 161and the 162.Fl f 163option was not specified. 164.Sh SEE ALSO 165.Xr gunzip 1 , 166.Xr gzexe 1 , 167.Xr gzip 1 , 168.Xr zcat 1 , 169.Xr zmore 1 , 170.Xr znew 1 171.Rs 172.%A Welch, Terry A. 173.%D June, 1984 174.%T "A Technique for High Performance Data Compression" 175.%J "IEEE Computer" 176.%V 17:6 177.%P pp. 8-19 178.Re 179.Sh STANDARDS 180The 181.Nm compress 182and 183.Nm uncompress 184utilities conform to 185.St -p1003.1-2001 . 186.Sh HISTORY 187The 188.Nm 189command appeared in 190.Bx 4.3 . 191