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. 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.\" 34.Dd March 4, 2021 35.Dt COMPRESS 1 36.Os 37.Sh NAME 38.Nm compress , 39.Nm uncompress 40.Nd compress and expand data 41.Sh SYNOPSIS 42.Nm 43.Op Fl fv 44.Op Fl b Ar bits 45.Op Ar 46.Nm 47.Fl c 48.Op Fl b Ar bits 49.Op Ar file 50.Nm uncompress 51.Op Fl f 52.Op Ar 53.Nm uncompress 54.Fl c 55.Op Ar file 56.Sh DESCRIPTION 57The 58.Nm 59utility reduces the size of files using adaptive Lempel-Ziv coding. 60Each 61.Ar file 62is renamed to the same name plus the extension 63.Pa .Z . 64A 65.Ar file 66argument with a 67.Pa .Z 68extension will be ignored except it will cause an 69error exit after other arguments are processed. 70If compression would not reduce the size of a 71.Ar file , 72the file is ignored. 73.Pp 74The 75.Nm uncompress 76utility restores compressed files to their original form, renaming the 77files by deleting the 78.Pa .Z 79extensions. 80A file specification need not include the file's 81.Pa .Z 82extension. 83If a file's name in its file system does not have a 84.Pa .Z 85extension, it will not be uncompressed and it will cause 86an error exit after other arguments are processed. 87.Pp 88If renaming the files would cause files to be overwritten and the standard 89input device is a terminal, the user is prompted (on the standard error 90output) for confirmation. 91If prompting is not possible or confirmation is not received, the files 92are not overwritten. 93.Pp 94As many of the modification time, access time, file flags, file mode, 95user ID, and group ID as allowed by permissions are retained in the 96new file. 97.Pp 98If no files are specified or a 99.Ar file 100argument is a single dash 101.Pq Sq Fl , 102the standard input is compressed or uncompressed to the standard output. 103If either the input and output files are not regular files, the checks for 104reduction in size and file overwriting are not performed, the input file is 105not removed, and the attributes of the input file are not retained 106in the output file. 107.Pp 108The options are as follows: 109.Bl -tag -width ".Fl b Ar bits" 110.It Fl b Ar bits 111The code size (see below) is limited to 112.Ar bits , 113which must be in the range 9..16. 114The default is 16. 115.It Fl c 116Compressed or uncompressed output is written to the standard output. 117No files are modified. 118The 119.Fl v 120option is ignored. 121Compression is attempted even if the results will be larger than the 122original. 123.It Fl f 124Files are overwritten without prompting for confirmation. 125Also, for 126.Nm compress , 127files are compressed even if they are not actually reduced in size. 128.It Fl v 129Print the percentage reduction of each file. 130Ignored by 131.Nm uncompress 132or if the 133.Fl c 134option is also used. 135.El 136.Pp 137The 138.Nm 139utility uses a modified Lempel-Ziv algorithm. 140Common substrings in the file are first replaced by 9-bit codes 257 and up. 141When code 512 is reached, the algorithm switches to 10-bit codes and 142continues to use more bits until the 143limit specified by the 144.Fl b 145option or its default is reached. 146.Pp 147After the limit is reached, 148.Nm 149periodically checks the compression ratio. 150If it is increasing, 151.Nm 152continues to use the existing code dictionary. 153However, if the compression ratio decreases, 154.Nm 155discards the table of substrings and rebuilds it from scratch. 156This allows 157the algorithm to adapt to the next "block" of the file. 158.Pp 159The 160.Fl b 161option is unavailable for 162.Nm uncompress 163since the 164.Ar bits 165parameter specified during compression 166is encoded within the output, along with 167a magic number to ensure that neither decompression of random data nor 168recompression of compressed data is attempted. 169.Pp 170The amount of compression obtained depends on the size of the 171input, the number of 172.Ar bits 173per code, and the distribution of common substrings. 174Typically, text such as source code or English is reduced by 50\-60%. 175Compression is generally much better than that achieved by Huffman 176coding (as used in the historical command pack), or adaptive Huffman 177coding (as used in the historical command compact), and takes less 178time to compute. 179.Pp 180If 181.Ar file 182is a soft or hard link 183.Nm 184will replace it with a compressed copy of the file pointed to by the link. 185The link's target file is left uncompressed. 186.Sh EXIT STATUS 187.Ex -std compress uncompress 188.Pp 189The 190.Nm compress 191utility exits 2 if attempting to compress a file would not reduce its size 192and the 193.Fl f 194option was not specified and if no other error occurs. 195.Sh EXAMPLES 196Create a file 197.Pa test_file 198with a single line of text: 199.Bd -literal -offset indent 200echo "This is a test" > test_file 201.Ed 202.Pp 203Try to reduce the size of the file using a 10-bit code and show the exit status: 204.Bd -literal -offset indent 205$ compress -b 10 test_file 206$ echo $? 2072 208.Ed 209.Pp 210Try to compress the file and show compression percentage: 211.Bd -literal -offset indent 212$ compress -v test_file 213test_file: file would grow; left unmodified 214.Ed 215.Pp 216Same as above but forcing compression: 217.Bd -literal -offset indent 218$ compress -f -v test_file 219test_file.Z: 79% expansion 220.Ed 221.Pp 222Compress and uncompress the string 223.Ql hello 224on the fly: 225.Bd -literal -offset indent 226$ echo "hello" | compress | uncompress 227hello 228.Ed 229.Sh SEE ALSO 230.Xr gunzip 1 , 231.Xr gzexe 1 , 232.Xr gzip 1 , 233.Xr zcat 1 , 234.Xr zmore 1 , 235.Xr znew 1 236.Rs 237.%A Welch, Terry A. 238.%D June, 1984 239.%T "A Technique for High Performance Data Compression" 240.%J "IEEE Computer" 241.%V 17:6 242.%P pp. 8-19 243.Re 244.Sh STANDARDS 245The 246.Nm compress 247and 248.Nm uncompress 249utilities conform to 250.St -p1003.1-2001 . 251.Sh HISTORY 252The 253.Nm 254command appeared in 255.Bx 4.3 . 256.Sh BUGS 257The program does not handle links well and has no link-handling options. 258.Pp 259Some of these might be considered otherwise-undocumented features. 260.Pp 261.Nm compress : 262If the utility does not compress a file because doing so would not 263reduce its size, and a file of the same name except with an 264.Pa .Z 265extension exists, the named file is not really ignored as stated above; 266it causes a prompt to confirm the overwriting of the file with the extension. 267If the operation is confirmed, that file is deleted. 268.Pp 269.Nm uncompress : 270If an empty file is compressed (using 271.Fl f ) , 272the resulting 273.Pa .Z 274file is also empty. 275That seems right, but if 276.Nm uncompress 277is then used on that file, an error will occur. 278.Pp 279Both utilities: If a 280.Sq Fl 281argument is used and the utility prompts the user, the standard input 282is taken as the user's reply to the prompt. 283.Pp 284Both utilities: 285If the specified file does not exist, but a similarly-named one with (for 286.Nm compress ) 287or without (for 288.Nm uncompress ) 289a 290.Pa .Z 291extension does exist, the utility will waste the user's time by not 292immediately emitting an error message about the missing file and 293continuing. 294Instead, it first asks for confirmation to overwrite 295the existing file and then does not overwrite it. 296