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.\" 4. Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" @(#)compress.1 8.2 (Berkeley) 4/18/94 33.\" $FreeBSD$ 34.\" 35.Dd May 17, 2002 36.Dt COMPRESS 1 37.Os 38.Sh NAME 39.Nm compress , 40.Nm uncompress 41.Nd compress and expand data 42.Sh SYNOPSIS 43.Nm 44.Op Fl fv 45.Op Fl b Ar bits 46.Op Ar 47.Nm 48.Fl c 49.Op Fl b Ar bits 50.Op Ar file 51.Nm uncompress 52.Op Fl f 53.Op Ar 54.Nm uncompress 55.Fl c 56.Op Ar file 57.Sh DESCRIPTION 58The 59.Nm 60utility reduces the size of files using adaptive Lempel-Ziv coding. 61Each 62.Ar file 63is renamed to the same name plus the extension 64.Pa .Z . 65A 66.Ar file 67argument with a 68.Pa .Z 69extension will be ignored except it will cause an 70error exit after other arguments are processed. 71If compression would not reduce the size of a 72.Ar file , 73the file is ignored. 74.Pp 75The 76.Nm uncompress 77utility restores compressed files to their original form, renaming the 78files by deleting the 79.Pa .Z 80extensions. 81A file specification need not include the file's 82.Pa .Z 83extension. 84If a file's name in its file system does not have a 85.Pa .Z 86extension, it will not be uncompressed and it will cause 87an error exit after other arguments are processed. 88.Pp 89If renaming the files would cause files to be overwritten and the standard 90input device is a terminal, the user is prompted (on the standard error 91output) for confirmation. 92If prompting is not possible or confirmation is not received, the files 93are not overwritten. 94.Pp 95As many of the modification time, access time, file flags, file mode, 96user ID, and group ID as allowed by permissions are retained in the 97new file. 98.Pp 99If no files are specified or a 100.Ar file 101argument is a single dash 102.Pq Sq Fl , 103the standard input is compressed or uncompressed to the standard output. 104If either the input and output files are not regular files, the checks for 105reduction in size and file overwriting are not performed, the input file is 106not removed, and the attributes of the input file are not retained 107in the output file. 108.Pp 109The options are as follows: 110.Bl -tag -width ".Fl b Ar bits" 111.It Fl b Ar bits 112The code size (see below) is limited to 113.Ar bits , 114which must be in the range 9..16. 115The default is 16. 116.It Fl c 117Compressed or uncompressed output is written to the standard output. 118No files are modified. 119The 120.Fl v 121option is ignored. 122Compression is attempted even if the results will be larger than the 123original. 124.It Fl f 125Files are overwritten without prompting for confirmation. 126Also, for 127.Nm compress , 128files are compressed even if they are not actually reduced in size. 129.It Fl v 130Print the percentage reduction of each file. 131Ignored by 132.Nm uncompress 133or if the 134.Fl c 135option is also used. 136.El 137.Pp 138The 139.Nm 140utility uses a modified Lempel-Ziv algorithm. 141Common substrings in the file are first replaced by 9-bit codes 257 and up. 142When code 512 is reached, the algorithm switches to 10-bit codes and 143continues to use more bits until the 144limit specified by the 145.Fl b 146option or its default is reached. 147.Pp 148After the limit is reached, 149.Nm 150periodically checks the compression ratio. 151If it is increasing, 152.Nm 153continues to use the existing code dictionary. 154However, if the compression ratio decreases, 155.Nm 156discards the table of substrings and rebuilds it from scratch. 157This allows 158the algorithm to adapt to the next "block" of the file. 159.Pp 160The 161.Fl b 162option is unavailable for 163.Nm uncompress 164since the 165.Ar bits 166parameter specified during compression 167is encoded within the output, along with 168a magic number to ensure that neither decompression of random data nor 169recompression of compressed data is attempted. 170.Pp 171The amount of compression obtained depends on the size of the 172input, the number of 173.Ar bits 174per code, and the distribution of common substrings. 175Typically, text such as source code or English is reduced by 50\-60%. 176Compression is generally much better than that achieved by Huffman 177coding (as used in the historical command pack), or adaptive Huffman 178coding (as used in the historical command compact), and takes less 179time to compute. 180.Sh EXIT STATUS 181.Ex -std compress uncompress 182.Pp 183The 184.Nm compress 185utility exits 2 if attempting to compress a file would not reduce its size 186and the 187.Fl f 188option was not specified and if no other error occurs. 189.Sh SEE ALSO 190.Xr gunzip 1 , 191.Xr gzexe 1 , 192.Xr gzip 1 , 193.Xr zcat 1 , 194.Xr zmore 1 , 195.Xr znew 1 196.Rs 197.%A Welch, Terry A. 198.%D June, 1984 199.%T "A Technique for High Performance Data Compression" 200.%J "IEEE Computer" 201.%V 17:6 202.%P pp. 8-19 203.Re 204.Sh STANDARDS 205The 206.Nm compress 207and 208.Nm uncompress 209utilities conform to 210.St -p1003.1-2001 . 211.Sh HISTORY 212The 213.Nm 214command appeared in 215.Bx 4.3 . 216.Sh BUGS 217Some of these might be considered otherwise-undocumented features. 218.Pp 219.Nm compress : 220If the utility does not compress a file because doing so would not 221reduce its size, and a file of the same name except with an 222.Pa .Z 223extension exists, the named file is not really ignored as stated above; 224it causes a prompt to confirm the overwriting of the file with the extension. 225If the operation is confirmed, that file is deleted. 226.Pp 227.Nm uncompress : 228If an empty file is compressed (using 229.Fl f ) , 230the resulting 231.Pa .Z 232file is also empty. 233That seems right, but if 234.Nm uncompress 235is then used on that file, an error will occur. 236.Pp 237Both utilities: If a 238.Sq Fl 239argument is used and the utility prompts the user, the standard input 240is taken as the user's reply to the prompt. 241.Pp 242Both utilities: 243If the specified file does not exist, but a similarly-named one with (for 244.Nm compress ) 245or without (for 246.Nm uncompress ) 247a 248.Pa .Z 249extension does exist, the utility will waste the user's time by not 250immediately emitting an error message about the missing file and 251continuing. 252Instead, it first asks for confirmation to overwrite 253the existing file and then does not overwrite it. 254