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.\" 38.Dd April 18, 1994 39.Dt COMPRESS 1 40.Os BSD 4.3 41.Sh NAME 42.Nm compress , 43.Nm uncompress , 44.Nm zcat 45.Nd compress and expand data 46.Sh SYNOPSIS 47.Nm compress 48.Op Fl cfv 49.Op Fl b Ar bits 50.Op Ar 51.Nm uncompress 52.Op Fl cfv 53.Op Ar 54.Nm zcat 55.Op Ar 56.Sh DESCRIPTION 57.Nm Compress 58reduces the size of the named files using adaptive Lempel-Ziv coding. 59Each 60.Ar file 61is renamed to the same name plus the extension 62.Dq .Z . 63As many of the modification time, access time, file flags, file mode, 64user ID, and group ID as allowed by permissions are retained in the 65new file. 66If compression would not reduce the size of a 67.Ar file , 68the file is ignored. 69.Pp 70.Nm Uncompress 71restores the compressed files to their original form, renaming the 72files by deleting the 73.Dq .Z 74extension. 75.Pp 76.Nm Zcat 77is an alias for 78.Dq "uncompress -c" . 79.Pp 80If renaming the files would cause files to be overwritten and the standard 81input device is a terminal, the user is prompted (on the standard error 82output) for confirmation. 83If prompting is not possible or confirmation is not received, the files 84are not overwritten. 85.Pp 86If no files are specified, the standard input is compressed or uncompressed 87to the standard output. 88If either the input and output files are not regular files, the checks for 89reduction in size and file overwriting are not performed, the input file is 90not removed, and the attributes of the input file are not retained. 91.Pp 92The options are as follows: 93.Bl -tag -width Ds 94.It Fl b 95Specify the 96.Ar bits 97code limit (see below). 98.It Fl c 99Compressed or uncompressed output is written to the standard output. 100No files are modified. 101.It Fl f 102Force compression of 103.Ar file , 104even if it is not actually reduced in size. 105Additionally, files are overwritten without prompting for confirmation. 106.It Fl v 107Print the percentage reduction of each file. 108.El 109.Pp 110.Nm Compress 111uses 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 compress 125periodically checks the compression ratio. 126If it is increasing, 127.Nm compress 128continues to use the existing code dictionary. 129However, if the compression ratio decreases, 130.Nm compress 131discards the table of substrings and rebuilds it from scratch. This allows 132the algorithm to adapt to the next "block" of the file. 133.Pp 134The 135.Fl b 136flag is omitted for 137.Ar uncompress 138since the 139.Ar bits 140parameter specified during compression 141is encoded within the output, along with 142a magic number to ensure that neither decompression of random data nor 143recompression of compressed data is attempted. 144.Pp 145.ne 8 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.Pp 156The 157.Nm compress 158utility exits 0 on success, and >0 if an error occurs. 159.Sh SEE ALSO 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