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 April 18, 1994 40.Dt COMPRESS 1 41.Os BSD 4.3 42.Sh NAME 43.Nm compress , 44.Nm uncompress , 45.Nm zcat 46.Nd compress and expand data 47.Sh SYNOPSIS 48.Nm compress 49.Op Fl cfv 50.Op Fl b Ar bits 51.Op Ar 52.Nm uncompress 53.Op Fl cfv 54.Op Ar 55.Nm zcat 56.Op Ar 57.Sh DESCRIPTION 58.Nm Compress 59reduces the size of the named files using adaptive Lempel-Ziv coding. 60Each 61.Ar file 62is renamed to the same name plus the extension 63.Dq .Z . 64As many of the modification time, access time, file flags, file mode, 65user ID, and group ID as allowed by permissions are retained in the 66new file. 67If compression would not reduce the size of a 68.Ar file , 69the file is ignored. 70.Pp 71.Nm Uncompress 72restores the compressed files to their original form, renaming the 73files by deleting the 74.Dq .Z 75extension. 76.Pp 77.Nm Zcat 78is an alias for 79.Dq "uncompress -c" . 80.Pp 81If renaming the files would cause files to be overwritten and the standard 82input device is a terminal, the user is prompted (on the standard error 83output) for confirmation. 84If prompting is not possible or confirmation is not received, the files 85are not overwritten. 86.Pp 87If no files are specified, the standard input is compressed or uncompressed 88to the standard output. 89If either the input and output files are not regular files, the checks for 90reduction in size and file overwriting are not performed, the input file is 91not removed, and the attributes of the input file are not retained. 92.Pp 93The options are as follows: 94.Bl -tag -width indent 95.It Fl b 96Specify the 97.Ar bits 98code limit (see below). 99.It Fl c 100Compressed or uncompressed output is written to the standard output. 101No files are modified. 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 Compress 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. This 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 146.ne 8 147The amount of compression obtained depends on the size of the 148input, the number of 149.Ar bits 150per code, and the distribution of common substrings. 151Typically, text such as source code or English is reduced by 50\-60%. 152Compression is generally much better than that achieved by Huffman 153coding (as used in the historical command pack), or adaptive Huffman 154coding (as used in the historical command compact), and takes less 155time to compute. 156.Sh DIAGNOSTICS 157The 158.Nm 159utility exits 0 on success, and >0 if an error occurs. 160.Sh SEE ALSO 161.Rs 162.%A Welch, Terry A. 163.%D June, 1984 164.%T "A Technique for High Performance Data Compression" 165.%J "IEEE Computer" 166.%V 17:6 167.%P pp. 8-19 168.Re 169.Sh HISTORY 170The 171.Nm 172command appeared in 173.Bx 4.3 . 174