1.\" $NetBSD: compress.1,v 1.16 2017/07/04 06:58:55 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. Neither the name of the University nor the names of its contributors 19.\" may be used to endorse or promote products derived from this software 20.\" without specific prior written permission. 21.\" 22.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 23.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 24.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 25.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 26.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 27.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 28.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 29.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 30.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 31.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 32.\" SUCH DAMAGE. 33.\" 34.\" @(#)compress.1 8.2 (Berkeley) 4/18/94 35.\" 36.Dd January 23, 2003 37.Dt COMPRESS 1 38.Os 39.Sh NAME 40.Nm compress , 41.\".Nm uncompress , 42.Nm uncompress 43.\".Nm zcat 44.Nd compress and expand data 45.Sh SYNOPSIS 46.Nm 47.Op Fl cdfv 48.Op Fl b Ar bits 49.Op Ar 50.Nm uncompress 51.Op Fl cdfv 52.Op Ar 53.\".Nm zcat 54.\".Op Ar 55.Sh DESCRIPTION 56.Nm 57reduces 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 69.Nm uncompress 70restores the compressed files to their original form, renaming the 71files by deleting the 72.Dq .Z 73extension. 74.\".Pp 75.\".Nm Zcat 76.\"is an alias for 77.\".Dq "uncompress -c" . 78.Pp 79If renaming the files would cause files to be overwritten and the standard 80input device is a terminal, the user is prompted (on the standard error 81output) for confirmation. 82If prompting is not possible or confirmation is not received, the files 83are not overwritten. 84.Pp 85If no files are specified, the standard input is compressed or uncompressed 86to 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 Ds 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 d 101Force decompression. 102.It Fl f 103Force compression of 104.Ar file , 105even if it is not actually reduced in size. 106Additionally, files are overwritten without prompting for confirmation. 107.It Fl v 108Print the percentage reduction of each file. 109.El 110.Pp 111.Nm 112uses a modified Lempel-Ziv algorithm. 113Common substrings in the file are first replaced by 9-bit codes 257 and up. 114When code 512 is reached, the algorithm switches to 10-bit codes and 115continues to use more bits until the 116limit specified by the 117.Fl b 118flag is reached (the default is 16). 119.Ar Bits 120must be between 9 and 16. 121.Pp 122After the 123.Ar bits 124limit is reached, 125.Nm 126periodically checks the compression ratio. 127If it is increasing, 128.Nm 129continues to use the existing code dictionary. 130However, if the compression ratio decreases, 131.Nm 132discards the table of substrings and rebuilds it from scratch. 133This allows the algorithm to adapt to the next "block" of the file. 134.Pp 135The 136.Fl b 137flag is omitted for 138.Ar 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 EXIT STATUS 156.Ex -std compress 157.Sh SEE ALSO 158.Xr zcat 1 159.Rs 160.%A Welch, Terry A. 161.%D June, 1984 162.%T "A Technique for High Performance Data Compression" 163.%J "IEEE Computer" 164.%V 17:6 165.%P pp. 8-19 166.Re 167.Sh HISTORY 168The 169.Nm 170command appeared in 171.Bx 4.3 . 172