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.Dd March 4, 2021 33.Dt COMPRESS 1 34.Os 35.Sh NAME 36.Nm compress , 37.Nm uncompress 38.Nd compress and expand data 39.Sh SYNOPSIS 40.Nm 41.Op Fl fv 42.Op Fl b Ar bits 43.Op Ar 44.Nm 45.Fl c 46.Op Fl b Ar bits 47.Op Ar file 48.Nm uncompress 49.Op Fl f 50.Op Ar 51.Nm uncompress 52.Fl c 53.Op Ar file 54.Sh DESCRIPTION 55The 56.Nm 57utility reduces the size of files using adaptive Lempel-Ziv coding. 58Each 59.Ar file 60is renamed to the same name plus the extension 61.Pa .Z . 62A 63.Ar file 64argument with a 65.Pa .Z 66extension will be ignored except it will cause an 67error exit after other arguments are processed. 68If compression would not reduce the size of a 69.Ar file , 70the file is ignored. 71.Pp 72The 73.Nm uncompress 74utility restores compressed files to their original form, renaming the 75files by deleting the 76.Pa .Z 77extensions. 78A file specification need not include the file's 79.Pa .Z 80extension. 81If a file's name in its file system does not have a 82.Pa .Z 83extension, it will not be uncompressed and it will cause 84an error exit after other arguments are processed. 85.Pp 86If renaming the files would cause files to be overwritten and the standard 87input device is a terminal, the user is prompted (on the standard error 88output) for confirmation. 89If prompting is not possible or confirmation is not received, the files 90are not overwritten. 91.Pp 92As many of the modification time, access time, file flags, file mode, 93user ID, and group ID as allowed by permissions are retained in the 94new file. 95.Pp 96If no files are specified or a 97.Ar file 98argument is a single dash 99.Pq Sq Fl , 100the standard input is compressed or uncompressed to the standard output. 101If either the input and output files are not regular files, the checks for 102reduction in size and file overwriting are not performed, the input file is 103not removed, and the attributes of the input file are not retained 104in the output file. 105.Pp 106The options are as follows: 107.Bl -tag -width ".Fl b Ar bits" 108.It Fl b Ar bits 109The code size (see below) is limited to 110.Ar bits , 111which must be in the range 9..16. 112The default is 16. 113.It Fl c 114Compressed or uncompressed output is written to the standard output. 115No files are modified. 116The 117.Fl v 118option is ignored. 119Compression is attempted even if the results will be larger than the 120original. 121.It Fl f 122Files are overwritten without prompting for confirmation. 123Also, for 124.Nm compress , 125files are compressed even if they are not actually reduced in size. 126.It Fl v 127Print the percentage reduction of each file. 128Ignored by 129.Nm uncompress 130or if the 131.Fl c 132option is also used. 133.El 134.Pp 135The 136.Nm 137utility uses a modified Lempel-Ziv algorithm. 138Common substrings in the file are first replaced by 9-bit codes 257 and up. 139When code 512 is reached, the algorithm switches to 10-bit codes and 140continues to use more bits until the 141limit specified by the 142.Fl b 143option or its default is reached. 144.Pp 145After the limit is reached, 146.Nm 147periodically checks the compression ratio. 148If it is increasing, 149.Nm 150continues to use the existing code dictionary. 151However, if the compression ratio decreases, 152.Nm 153discards the table of substrings and rebuilds it from scratch. 154This allows 155the algorithm to adapt to the next "block" of the file. 156.Pp 157The 158.Fl b 159option is unavailable for 160.Nm uncompress 161since the 162.Ar bits 163parameter specified during compression 164is encoded within the output, along with 165a magic number to ensure that neither decompression of random data nor 166recompression of compressed data is attempted. 167.Pp 168The amount of compression obtained depends on the size of the 169input, the number of 170.Ar bits 171per code, and the distribution of common substrings. 172Typically, text such as source code or English is reduced by 50\-60%. 173Compression is generally much better than that achieved by Huffman 174coding (as used in the historical command pack), or adaptive Huffman 175coding (as used in the historical command compact), and takes less 176time to compute. 177.Pp 178If 179.Ar file 180is a soft or hard link 181.Nm 182will replace it with a compressed copy of the file pointed to by the link. 183The link's target file is left uncompressed. 184.Sh EXIT STATUS 185.Ex -std compress uncompress 186.Pp 187The 188.Nm compress 189utility exits 2 if attempting to compress a file would not reduce its size 190and the 191.Fl f 192option was not specified and if no other error occurs. 193.Sh EXAMPLES 194Create a file 195.Pa test_file 196with a single line of text: 197.Bd -literal -offset indent 198echo "This is a test" > test_file 199.Ed 200.Pp 201Try to reduce the size of the file using a 10-bit code and show the exit status: 202.Bd -literal -offset indent 203$ compress -b 10 test_file 204$ echo $? 2052 206.Ed 207.Pp 208Try to compress the file and show compression percentage: 209.Bd -literal -offset indent 210$ compress -v test_file 211test_file: file would grow; left unmodified 212.Ed 213.Pp 214Same as above but forcing compression: 215.Bd -literal -offset indent 216$ compress -f -v test_file 217test_file.Z: 79% expansion 218.Ed 219.Pp 220Compress and uncompress the string 221.Ql hello 222on the fly: 223.Bd -literal -offset indent 224$ echo "hello" | compress | uncompress 225hello 226.Ed 227.Sh SEE ALSO 228.Xr gunzip 1 , 229.Xr gzexe 1 , 230.Xr gzip 1 , 231.Xr zcat 1 , 232.Xr zmore 1 , 233.Xr znew 1 234.Rs 235.%A Welch, Terry A. 236.%D June, 1984 237.%T "A Technique for High Performance Data Compression" 238.%J "IEEE Computer" 239.%V 17:6 240.%P pp. 8-19 241.Re 242.Sh STANDARDS 243The 244.Nm compress 245and 246.Nm uncompress 247utilities conform to 248.St -p1003.1-2001 . 249.Sh HISTORY 250The 251.Nm 252command appeared in 253.Bx 4.3 . 254.Sh BUGS 255The program does not handle links well and has no link-handling options. 256.Pp 257Some of these might be considered otherwise-undocumented features. 258.Pp 259.Nm compress : 260If the utility does not compress a file because doing so would not 261reduce its size, and a file of the same name except with an 262.Pa .Z 263extension exists, the named file is not really ignored as stated above; 264it causes a prompt to confirm the overwriting of the file with the extension. 265If the operation is confirmed, that file is deleted. 266.Pp 267.Nm uncompress : 268If an empty file is compressed (using 269.Fl f ) , 270the resulting 271.Pa .Z 272file is also empty. 273That seems right, but if 274.Nm uncompress 275is then used on that file, an error will occur. 276.Pp 277Both utilities: If a 278.Sq Fl 279argument is used and the utility prompts the user, the standard input 280is taken as the user's reply to the prompt. 281.Pp 282Both utilities: 283If the specified file does not exist, but a similarly-named one with (for 284.Nm compress ) 285or without (for 286.Nm uncompress ) 287a 288.Pa .Z 289extension does exist, the utility will waste the user's time by not 290immediately emitting an error message about the missing file and 291continuing. 292Instead, it first asks for confirmation to overwrite 293the existing file and then does not overwrite it. 294