1.\" $OpenBSD: compress.1,v 1.49 2022/10/22 14:41:27 millert Exp $ 2.\" $NetBSD: compress.1,v 1.5 1995/03/26 09:44:34 glass Exp $ 3.\" 4.\" Copyright (c) 1986, 1990, 1993 5.\" The Regents of the University of California. All rights reserved. 6.\" 7.\" This code is derived from software contributed to Berkeley by 8.\" James A. Woods, derived from original work by Spencer Thomas 9.\" and Joseph Orost. 10.\" 11.\" Redistribution and use in source and binary forms, with or without 12.\" modification, are permitted provided that the following conditions 13.\" are met: 14.\" 1. Redistributions of source code must retain the above copyright 15.\" notice, this list of conditions and the following disclaimer. 16.\" 2. Redistributions in binary form must reproduce the above copyright 17.\" notice, this list of conditions and the following disclaimer in the 18.\" documentation and/or other materials provided with the distribution. 19.\" 3. Neither the name of the University nor the names of its contributors 20.\" may be used to endorse or promote products derived from this software 21.\" without specific prior written permission. 22.\" 23.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 24.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 25.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 26.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 27.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 28.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 29.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 30.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 31.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 32.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 33.\" SUCH DAMAGE. 34.\" 35.\" @(#)compress.1 8.2 (Berkeley) 4/18/94 36.\" 37.Dd $Mdocdate: October 22 2022 $ 38.Dt COMPRESS 1 39.Os 40.Sh NAME 41.Nm compress , 42.Nm uncompress , 43.Nm zcat 44.Nd compress and expand data (compress mode) 45.Sh SYNOPSIS 46.Nm compress 47.Op Fl 123456789cdfghlNnOqrtv 48.Op Fl b Ar bits 49.Op Fl o Ar filename 50.Op Fl S Ar suffix 51.Op Ar 52.Nm uncompress 53.Op Fl cfhlNnqrtv 54.Op Fl o Ar filename 55.Op Ar 56.Nm zcat 57.Op Fl fghqr 58.Op Ar 59.Sh DESCRIPTION 60The 61.Nm 62utility 63reduces the size of the named files using adaptive Lempel-Ziv coding, 64in compress mode. 65If invoked as 66.Nm compress Fl g , 67the deflate mode of compression is chosen; 68see 69.Xr gzip 1 70for more information. 71Each file is renamed to the same name plus the extension 72.Dq .Z . 73As many of the modification time, access time, file flags, file mode, 74user ID, and group ID as allowed by permissions are retained in the 75new file. 76If compression would not reduce the size of a file, 77the file is ignored (unless 78.Fl f 79is used). 80.Pp 81The 82.Nm uncompress 83utility restores compressed files to their original form, renaming the 84files by removing the extension (or by using the stored name if the 85.Fl N 86flag is specified). 87It has the ability to restore files compressed by 88.Nm , 89.Xr gzip 1 , 90and 91.Xr zip 1 , 92recognising the following extensions: 93.Dq .Z , 94.Dq -Z , 95.Dq _Z , 96.Dq .gz , 97.Dq -gz , 98.Dq _gz , 99.Dq .tgz , 100.Dq -tgz , 101.Dq _tgz , 102.Dq .taz , 103.Dq -taz , 104and 105.Dq _taz . 106The 107.Fl S 108option can be used to support other file extensions. 109Extensions ending in 110.Dq tgz 111and 112.Dq taz 113are not removed when decompressing, instead they are converted to 114.Dq tar . 115Files in zip format are only supported if they contain a single member 116either compressed with the deflate scheme or stored uncompressed. 117.Pp 118The 119.Nm zcat 120command is equivalent in functionality to 121.Nm uncompress 122.Fl c . 123.Pp 124If renaming the files would cause files to be overwritten and the standard 125input device is a terminal, the user is prompted (on the standard error 126output) for confirmation. 127If prompting is not possible or confirmation is not received, the files 128are not overwritten. 129.Pp 130If no files are specified, the standard input is compressed or uncompressed 131to the standard output. 132If either the input or output files are not regular files, the checks for 133reduction in size and file overwriting are not performed, the input file is 134not removed, and the attributes of the input file are not retained. 135.Pp 136By default, when compressing using the deflate scheme 137.Pf ( Fl g ) , 138the original file name and time stamp are stored in the compressed file. 139When uncompressing, this information is not used. 140Instead, the uncompressed file inherits the time stamp of the 141compressed version and the uncompressed file name is generated from 142the name of the compressed file as described above. 143These defaults may be overridden by the 144.Fl N 145and 146.Fl n 147flags, described below. 148.Pp 149The options are as follows: 150.Bl -tag -width Ds 151.It Fl 1...9 152Use the deflate scheme, with compression factor of 153.Fl 1 154to 155.Fl 9 . 156Compression factor 157.Fl 1 158is the fastest, but provides a poorer level of compression. 159Compression factor 160.Fl 9 161provides the best level of compression, but is relatively slow. 162The default is 163.Fl 6 . 164This option implies 165.Fl g . 166.It Fl b Ar bits 167Specify the 168.Ar bits 169code limit 170.Pq see below . 171.It Fl c 172Compressed or uncompressed output is written to the standard output. 173No files are modified (force 174.Nm zcat 175mode). 176.It Fl d 177Decompress the source files instead of compressing them (force 178.Nm uncompress 179mode). 180.It Fl f 181Force compression of 182.Ar file , 183even if it is not actually reduced in size. 184Additionally, files are overwritten without prompting for confirmation. 185If the input data is not in a format recognized by 186.Nm 187and if the option 188.Fl c 189is also given, copy the input data without change 190to the standard output: let 191.Nm zcat 192behave as 193.Xr cat 1 . 194.It Fl g 195Use the deflate scheme, which reportedly provides better compression rates 196(force 197.Xr gzip 1 198mode). 199.It Fl h 200Print a short help message. 201.It Fl l 202List information for the specified compressed files. 203The following information is listed: 204.Bl -tag -width "compression ratio" 205.It compressed size 206Size of the compressed file. 207.It uncompressed size 208Size of the file when uncompressed. 209.It compression ratio 210Ratio of the difference between the compressed and uncompressed 211sizes to the uncompressed size. 212.It uncompressed name 213Name the file will be saved as when uncompressing. 214.El 215.Pp 216If the 217.Fl v 218option is specified, the following additional information is printed: 219.Bl -tag -width "compression method" 220.It compression method 221Name of the method used to compress the file. 222.It crc 22332-bit CRC 224.Pq cyclic redundancy code 225of the uncompressed file. 226.It "time stamp" 227Date and time corresponding to the last data modification time 228(mtime) of the compressed file (if the 229.Fl n 230option is specified, the time stamp stored in the compressed file 231is printed instead). 232.El 233.It Fl N 234When uncompressing or listing, use the time stamp and file name stored 235in the compressed file, if any, for the uncompressed version. 236This information is only available when the deflate scheme 237.Pf ( Fl g ) 238is used. 239.It Fl n 240When compressing, do not store the original file name and time stamp 241in the header of the compressed file. 242.It Fl O 243Use compress mode 244(the default). 245.It Fl o Ar filename 246Set the output file name. 247.It Fl q 248Be quiet: suppress all messages. 249.It Fl r 250Recursive mode: 251.Nm 252will descend into specified directories. 253.It Fl S Ar suffix 254When compressing, use the specified 255.Ar suffix 256as the extension when creating output files. 257When uncompressing, recognize file names with the specified 258.Ar suffix 259as compressed files. 260.It Fl t 261Test the integrity of each file leaving any files intact. 262.It Fl v 263Print the percentage reduction of each file and other information. 264.El 265.Pp 266.Nm 267uses a modified Lempel-Ziv algorithm 268.Pq LZW . 269Common substrings in the file are first replaced by 9-bit codes 257 and up. 270When code 512 is reached, the algorithm switches to 10-bit codes and 271continues to use more bits until the 272limit specified by the 273.Fl b 274flag is reached. 275.Ar bits 276must be between 9 and 16 277.Pq the default is 16 . 278.Pp 279After the 280.Ar bits 281limit is reached, 282.Nm 283periodically checks the compression ratio. 284If it is increasing, 285.Nm 286continues to use the existing code dictionary. 287However, if the compression ratio decreases, 288.Nm 289discards the table of substrings and rebuilds it from scratch. 290This allows the algorithm to adapt to the next 291.Dq block 292of the file. 293.Pp 294The 295.Fl b 296flag is omitted for 297.Nm uncompress 298since the 299.Ar bits 300parameter specified during compression 301is encoded within the output, along with 302a magic number to ensure that neither decompression of random data nor 303recompression of compressed data is attempted. 304.Pp 305The amount of compression obtained depends on the size of the 306input, the number of 307.Ar bits 308per code, and the distribution of common substrings. 309Typically, text such as source code or English is reduced by 50 \- 60% using 310.Nm . 311Compression is generally much better than that achieved by Huffman 312coding (as used in the historical command pack), or adaptive Huffman 313coding (as used in the historical command compact), and takes less 314time to compute. 315.Sh EXIT STATUS 316The 317.Nm compress 318utility exits with one of the following values: 319.Pp 320.Bl -tag -width Ds -offset indent -compact 321.It 0 322Success. 323.It 1 324An error occurred. 325.It 2 326At least one of the specified files was not compressed since 327.Fl f 328was not specified and compression would have resulted in a size 329increase. 330.It \*(Gt2 331An error occurred. 332.El 333.Pp 334.Ex -std uncompress zcat 335.Sh SEE ALSO 336.Xr gzexe 1 , 337.Xr gzip 1 , 338.Xr zdiff 1 , 339.Xr zforce 1 , 340.Xr zmore 1 , 341.Xr znew 1 , 342.Xr compress 3 343.Rs 344.%A Welch, Terry A. 345.%D June, 1984 346.%T "A Technique for High Performance Data Compression" 347.%J "IEEE Computer" 348.%V 17:6 349.%P pp. 8\-19 350.Re 351.Sh STANDARDS 352The 353.Nm , 354.Nm uncompress , 355and 356.Nm zcat 357utilities are compliant with the 358X/Open System Interfaces option of the 359.St -p1003.1-2008 360specification. 361.Pp 362The 363.Nm 364flags 365.Op Fl 123456789dghlNnOoqrSt , 366.Nm uncompress 367flags 368.Op Fl hlNnoqrt , 369and the 370.Nm zcat 371flags 372.Op Fl fghqr 373are extensions to that specification. 374.Pp 375.St -p1003.1-2008 376specifies a maximum bits limit 377.Pq Fl b 378of 14 to 379.Qq achieve portability to all systems . 380.Sh HISTORY 381The 382.Nm 383command appeared in 384.Bx 4.3 . 385Deflate compression support was added in 386.Ox 2.1 . 387