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.\" from: @(#)compress.1 8.2 (Berkeley) 4/18/94 37.\" $Id: compress.1,v 1.4 1994/10/17 21:43:16 cgd Exp $ 38.\" 39.Dd April 18, 1994 40.Dt COMPRESS 1 41.Os BSD 4.3 42.Sh NAME 43.Nm compress , 44.\".Nm uncompress , 45.Nm uncompress 46.\".Nm zcat 47.Nd compress and expand data 48.Sh SYNOPSIS 49.Nm compress 50.Op Fl cfv 51.Op Fl b Ar bits 52.Op Ar 53.Nm uncompress 54.Op Fl cfv 55.Op Ar 56.\".Nm zcat 57.\".Op Ar 58.Sh DESCRIPTION 59.Nm Compress 60reduces the size of the named files using adaptive Lempel-Ziv coding. 61Each 62.Ar file 63is renamed to the same name plus the extension 64.Dq .Z . 65As many of the modification time, access time, file flags, file mode, 66user ID, and group ID as allowed by permissions are retained in the 67new file. 68If compression would not reduce the size of a 69.Ar file , 70the file is ignored. 71.Pp 72.Nm Uncompress 73restores the compressed files to their original form, renaming the 74files by deleting the 75.Dq .Z 76extension. 77.\".Pp 78.\".Nm Zcat 79.\"is an alias for 80.\".Dq "uncompress -c" . 81.Pp 82If renaming the files would cause files to be overwritten and the standard 83input device is a terminal, the user is prompted (on the standard error 84output) for confirmation. 85If prompting is not possible or confirmation is not received, the files 86are not overwritten. 87.Pp 88If no files are specified, the standard input is compressed or uncompressed 89to the standard output. 90If either the input and output files are not regular files, the checks for 91reduction in size and file overwriting are not performed, the input file is 92not removed, and the attributes of the input file are not retained. 93.Pp 94The options are as follows: 95.Bl -tag -width Ds 96.It Fl b 97Specify the 98.Ar bits 99code limit (see below). 100.It Fl c 101Compressed or uncompressed output is written to the standard output. 102No files are modified. 103.It Fl f 104Force compression of 105.Ar file , 106even if it is not actually reduced in size. 107Additionally, files are overwritten without prompting for confirmation. 108.It Fl v 109Print the percentage reduction of each file. 110.El 111.Pp 112.Nm Compress 113uses a modified Lempel-Ziv algorithm. 114Common substrings in the file are first replaced by 9-bit codes 257 and up. 115When code 512 is reached, the algorithm switches to 10-bit codes and 116continues to use more bits until the 117limit specified by the 118.Fl b 119flag is reached (the default is 16). 120.Ar Bits 121must be between 9 and 16. 122.Pp 123After the 124.Ar bits 125limit is reached, 126.Nm compress 127periodically checks the compression ratio. 128If it is increasing, 129.Nm compress 130continues to use the existing code dictionary. 131However, if the compression ratio decreases, 132.Nm compress 133discards the table of substrings and rebuilds it from scratch. This allows 134the algorithm to adapt to the next "block" of the file. 135.Pp 136The 137.Fl b 138flag is omitted for 139.Ar uncompress 140since the 141.Ar bits 142parameter specified during compression 143is encoded within the output, along with 144a magic number to ensure that neither decompression of random data nor 145recompression of compressed data is attempted. 146.Pp 147.ne 8 148The amount of compression obtained depends on the size of the 149input, the number of 150.Ar bits 151per code, and the distribution of common substrings. 152Typically, text such as source code or English is reduced by 50\-60%. 153Compression is generally much better than that achieved by Huffman 154coding (as used in the historical command pack), or adaptive Huffman 155coding (as used in the historical command compact), and takes less 156time to compute. 157.Pp 158The 159.Nm compress 160utility exits 0 on success, and >0 if an error occurs. 161.Sh SEE ALSO 162.Xr zcat 1 163.Rs 164.%A Welch, Terry A. 165.%D June, 1984 166.%T "A Technique for High Performance Data Compression" 167.%J "IEEE Computer" 168.%V 17:6 169.%P pp. 8-19 170.Re 171.Sh HISTORY 172The 173.Nm 174command appeared in 175.Bx 4.3 . 176