1.\" $NetBSD: compress.1,v 1.13 2003/01/23 08:53:46 wiz Exp $ 2.\" 3.\" Copyright (c) 1986, 1990, 1993 4.\" The Regents of the University of California. All rights reserved. 5.\" 6.\" This code is derived from software contributed to Berkeley by 7.\" James A. Woods, derived from original work by Spencer Thomas 8.\" and Joseph Orost. 9.\" 10.\" Redistribution and use in source and binary forms, with or without 11.\" modification, are permitted provided that the following conditions 12.\" are met: 13.\" 1. Redistributions of source code must retain the above copyright 14.\" notice, this list of conditions and the following disclaimer. 15.\" 2. Redistributions in binary form must reproduce the above copyright 16.\" notice, this list of conditions and the following disclaimer in the 17.\" documentation and/or other materials provided with the distribution. 18.\" 3. All advertising materials mentioning features or use of this software 19.\" must display the following acknowledgement: 20.\" This product includes software developed by the University of 21.\" California, Berkeley and its contributors. 22.\" 4. Neither the name of the University nor the names of its contributors 23.\" may be used to endorse or promote products derived from this software 24.\" without specific prior written permission. 25.\" 26.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 27.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 28.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 29.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 30.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 31.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 32.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 33.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 34.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 35.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 36.\" SUCH DAMAGE. 37.\" 38.\" @(#)compress.1 8.2 (Berkeley) 4/18/94 39.\" 40.Dd January 23, 2003 41.Dt COMPRESS 1 42.Os 43.Sh NAME 44.Nm compress , 45.\".Nm uncompress , 46.Nm uncompress 47.\".Nm zcat 48.Nd compress and expand data 49.Sh SYNOPSIS 50.Nm 51.Op Fl cdfv 52.Op Fl b Ar bits 53.Op Ar 54.Nm uncompress 55.Op Fl cdfv 56.Op Ar 57.\".Nm zcat 58.\".Op Ar 59.Sh DESCRIPTION 60.Nm 61reduces the size of the named files using adaptive Lempel-Ziv coding. 62Each 63.Ar file 64is renamed to the same name plus the extension 65.Dq .Z . 66As many of the modification time, access time, file flags, file mode, 67user ID, and group ID as allowed by permissions are retained in the 68new file. 69If compression would not reduce the size of a 70.Ar file , 71the file is ignored. 72.Pp 73.Nm uncompress 74restores the compressed files to their original form, renaming the 75files by deleting the 76.Dq .Z 77extension. 78.\".Pp 79.\".Nm Zcat 80.\"is an alias for 81.\".Dq "uncompress -c" . 82.Pp 83If renaming the files would cause files to be overwritten and the standard 84input device is a terminal, the user is prompted (on the standard error 85output) for confirmation. 86If prompting is not possible or confirmation is not received, the files 87are not overwritten. 88.Pp 89If no files are specified, the standard input is compressed or uncompressed 90to the standard output. 91If either the input and output files are not regular files, the checks for 92reduction in size and file overwriting are not performed, the input file is 93not removed, and the attributes of the input file are not retained. 94.Pp 95The options are as follows: 96.Bl -tag -width Ds 97.It Fl b 98Specify the 99.Ar bits 100code limit (see below). 101.It Fl c 102Compressed or uncompressed output is written to the standard output. 103No files are modified. 104.It Fl d 105Force decompression. 106.It Fl f 107Force compression of 108.Ar file , 109even if it is not actually reduced in size. 110Additionally, files are overwritten without prompting for confirmation. 111.It Fl v 112Print the percentage reduction of each file. 113.El 114.Pp 115.Nm 116uses a modified Lempel-Ziv algorithm. 117Common substrings in the file are first replaced by 9-bit codes 257 and up. 118When code 512 is reached, the algorithm switches to 10-bit codes and 119continues to use more bits until the 120limit specified by the 121.Fl b 122flag is reached (the default is 16). 123.Ar Bits 124must be between 9 and 16. 125.Pp 126After the 127.Ar bits 128limit is reached, 129.Nm 130periodically checks the compression ratio. 131If it is increasing, 132.Nm 133continues to use the existing code dictionary. 134However, if the compression ratio decreases, 135.Nm 136discards the table of substrings and rebuilds it from scratch. This allows 137the algorithm to adapt to the next "block" of the file. 138.Pp 139The 140.Fl b 141flag is omitted for 142.Ar uncompress 143since the 144.Ar bits 145parameter specified during compression 146is encoded within the output, along with 147a magic number to ensure that neither decompression of random data nor 148recompression of compressed data is attempted. 149.Pp 150The amount of compression obtained depends on the size of the 151input, the number of 152.Ar bits 153per code, and the distribution of common substrings. 154Typically, text such as source code or English is reduced by 50\-60%. 155Compression is generally much better than that achieved by Huffman 156coding (as used in the historical command pack), or adaptive Huffman 157coding (as used in the historical command compact), and takes less 158time to compute. 159.Pp 160The 161.Nm 162utility exits 0 on success, and \*[Gt]0 if an error occurs. 163.Sh SEE ALSO 164.Xr zcat 1 165.Rs 166.%A Welch, Terry A. 167.%D June, 1984 168.%T "A Technique for High Performance Data Compression" 169.%J "IEEE Computer" 170.%V 17:6 171.%P pp. 8-19 172.Re 173.Sh HISTORY 174The 175.Nm 176command appeared in 177.Bx 4.3 . 178