1.\" $OpenBSD: gzip.1,v 1.7 2011/09/22 10:41:04 deraadt Exp $ 2.\" 3.\" Copyright (c) 1986, 1990, 1993 4.\" The Regents of the University of California. All rights reserved. 5.\" 6.\" This code is derived from software contributed to Berkeley by 7.\" James A. Woods, derived from original work by Spencer Thomas 8.\" and Joseph Orost. 9.\" 10.\" Redistribution and use in source and binary forms, with or without 11.\" modification, are permitted provided that the following conditions 12.\" are met: 13.\" 1. Redistributions of source code must retain the above copyright 14.\" notice, this list of conditions and the following disclaimer. 15.\" 2. Redistributions in binary form must reproduce the above copyright 16.\" notice, this list of conditions and the following disclaimer in the 17.\" documentation and/or other materials provided with the distribution. 18.\" 3. Neither the name of the University nor the names of its contributors 19.\" may be used to endorse or promote products derived from this software 20.\" without specific prior written permission. 21.\" 22.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 23.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 24.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 25.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 26.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 27.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 28.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 29.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 30.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 31.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 32.\" SUCH DAMAGE. 33.\" 34.\" @(#)compress.1 8.2 (Berkeley) 4/18/94 35.\" 36.Dd $Mdocdate: September 22 2011 $ 37.Dt GZIP 1 38.Os 39.Sh NAME 40.Nm gzip , 41.Nm gunzip , 42.Nm gzcat 43.Nd compress and expand data (deflate mode) 44.Sh SYNOPSIS 45.Nm gzip 46.Op Fl 123456789cdfghLlNnOqrtVv 47.Op Fl b Ar bits 48.Op Fl o Ar filename 49.Op Fl S Ar suffix 50.Op Ar 51.Nm gunzip 52.Op Fl cfhlNnqrtVv 53.Op Fl o Ar filename 54.Op Ar 55.Nm gzcat 56.Op Fl fghqr 57.Op Ar 58.Sh DESCRIPTION 59The 60.Nm 61utility 62reduces the size of the named files using adaptive Lempel-Ziv coding, 63in deflate mode. 64If invoked as 65.Nm gzip -O , 66the compress mode of compression is chosen; 67see 68.Xr compress 1 69for more information. 70Each file is renamed to the same name plus the extension 71.Dq .gz . 72As many of the modification time, access time, file flags, file mode, 73user ID, and group ID as allowed by permissions are retained in the 74new file. 75If compression would not reduce the size of a file, 76the file is ignored (unless 77.Fl f 78is used). 79.Pp 80The 81.Nm gunzip 82utility restores compressed files to their original form, renaming the 83files by removing the extension (or by using the stored name if the 84.Fl N 85flag is specified). 86It has the ability to restore files compressed by both 87.Nm 88and 89.Xr compress 1 , 90recognising the following extensions: 91.Dq .Z , 92.Dq -Z , 93.Dq _Z , 94.Dq .gz , 95.Dq -gz , 96.Dq _gz , 97.Dq .tgz , 98.Dq -tgz , 99.Dq _tgz , 100.Dq .taz , 101.Dq -taz , 102and 103.Dq _taz . 104Extensions ending in 105.Dq tgz 106and 107.Dq taz 108are not removed when decompressing, instead they are converted to 109.Dq tar . 110.Pp 111The 112.Nm gzcat 113command is equivalent in functionality to 114.Nm gunzip 115.Fl c . 116.Pp 117If renaming the files would cause files to be overwritten and the standard 118input device is a terminal, the user is prompted (on the standard error 119output) for confirmation. 120If prompting is not possible or confirmation is not received, the files 121are not overwritten. 122.Pp 123If no files are specified, the standard input is compressed or uncompressed 124to the standard output. 125If either the input or output files are not regular files, the checks for 126reduction in size and file overwriting are not performed, the input file is 127not removed, and the attributes of the input file are not retained. 128.Pp 129By default, when compressing, the original file name and time stamp 130are stored in the compressed file. 131When uncompressing, this information is not used. 132Instead, the uncompressed file inherits the time stamp of the 133compressed version and the uncompressed file name is generated from 134the name of the compressed file as described above. 135These defaults may be overridden by the 136.Fl N 137and 138.Fl n 139flags, described below. 140.Pp 141The options are as follows: 142.Bl -tag -width Ds 143.It Fl 1...9 144Use the deflate scheme, with compression factor of 145.Fl 1 146to 147.Fl 9 . 148Compression factor 149.Fl 1 150is the fastest, but provides a poorer level of compression. 151Compression factor 152.Fl 9 153provides the best level of compression, but is relatively slow. 154The default is 155.Fl 6 . 156This option implies 157.Fl g . 158.It Fl b Ar bits 159Specify the 160.Ar bits 161code limit 162.Pq see below . 163.It Fl c 164Compressed or uncompressed output is written to the standard output. 165No files are modified (force 166.Nm gzcat 167mode). 168.It Fl d 169Decompress the source files instead of compressing them (force 170.Nm gunzip 171mode). 172.It Fl f 173Force compression of 174.Ar file , 175even if it is not actually reduced in size. 176Additionally, files are overwritten without prompting for confirmation. 177If the input data is not in a format recognized by 178.Nm 179and if the option 180.Fl c 181is also given, copy the input data without change 182to the standard output: let 183.Nm gzcat 184behave as 185.Xr cat 1 . 186.It Fl g 187Use the deflate scheme, which reportedly provides better compression rates 188(the default). 189.It Fl h 190Print a short help message. 191.It Fl L 192In other compatible programs, this displays program license. 193.It Fl l 194List information for the specified compressed files. 195The following information is listed: 196.Bl -tag -width "compression ratio" 197.It compressed size 198Size of the compressed file. 199.It uncompressed size 200Size of the file when uncompressed. 201.It compression ratio 202Ratio of the difference between the compressed and uncompressed 203sizes to the uncompressed size. 204.It uncompressed name 205Name the file will be saved as when uncompressing. 206.El 207.Pp 208If the 209.Fl v 210option is specified, the following additional information is printed: 211.Bl -tag -width "compression method" 212.It compression method 213Name of the method used to compress the file. 214.It crc 21532-bit CRC 216.Pq cyclic redundancy code 217of the uncompressed file. 218.It "time stamp" 219Date and time corresponding to the last data modification time 220(mtime) of the compressed file (if the 221.Fl n 222option is specified, the time stamp stored in the compressed file 223is printed instead). 224.El 225.It Fl N 226When uncompressing or listing, use the time stamp and file name stored 227in the compressed file, if any, for the uncompressed version. 228.It Fl n 229When compressing, do not store the original file name and time stamp 230in the 231.Nm 232header. 233.It Fl O 234Use old compression method 235(force 236.Xr compress 1 237mode). 238.It Fl o Ar filename 239Set the output file name. 240.It Fl q 241Be quiet: suppress all messages. 242.It Fl r 243Recursive mode: 244.Nm 245will descend into specified directories. 246.It Fl S Ar suffix 247Set the suffix for compressed files. 248.It Fl t 249Test the integrity of each file leaving any files intact. 250.It Fl V 251In other compatible programs, this displays version information. 252.It Fl v 253Print the percentage reduction of each file and other information. 254.El 255.Pp 256.Nm 257uses a modified Lempel-Ziv algorithm 258.Pq LZW . 259Common substrings are replaced by pointers to previous strings, 260and are found using a hash table. 261Unique substrings are emitted as a string of literal bytes, 262and compressed as Huffman trees. 263When code 512 is reached, the algorithm switches to 10-bit codes and 264continues to use more bits until the 265limit specified by the 266.Fl b 267flag is reached. 268.Ar bits 269must be between 9 and 16 270.Pq the default is 16 . 271.Pp 272After the 273.Ar bits 274limit is reached, 275.Nm 276periodically checks the compression ratio. 277If it is increasing, 278.Nm 279continues to use the existing code dictionary. 280However, if the compression ratio decreases, 281.Nm 282discards the table of substrings and rebuilds it from scratch. 283This allows the algorithm to adapt to the next 284.Dq block 285of the file. 286.Pp 287The 288.Fl b 289flag is omitted for 290.Nm gunzip 291since the 292.Ar bits 293parameter specified during compression 294is encoded within the output, along with 295a magic number to ensure that neither decompression of random data nor 296recompression of compressed data is attempted. 297.Pp 298The amount of compression obtained depends on the size of the 299input, the number of 300.Ar bits 301per code, and the distribution of common substrings. 302Typically, text such as source code or English is reduced by 60 \- 70% using 303.Nm . 304Compression is generally much better than that achieved by Huffman 305coding (as used in the historical command pack), or adaptive Huffman 306coding (as used in the historical command compact), and takes less 307time to compute. 308.Pp 309The 310.Nm gzip , 311.Nm gunzip , 312and 313.Nm gzcat 314utilities exit with 0 on success; 1 if an error occurred; 315or 2 if a warning occurred. 316.Sh ENVIRONMENT 317.Bl -tag -width Ds 318.It Ev GZIP 319Options which are passed to 320.Nm , 321.Nm gunzip , 322and 323.Nm gzcat 324automatically. 325.El 326.Sh SEE ALSO 327.Xr compress 1 , 328.Xr gzexe 1 , 329.Xr gzsig 1 , 330.Xr zdiff 1 , 331.Xr zforce 1 , 332.Xr zmore 1 , 333.Xr znew 1 , 334.Xr compress 3 335.Pp 336.Bl -tag -width 12n -compact 337.It RFC 1950 338ZLIB Compressed Data Format Specification. 339.It RFC 1951 340DEFLATE Compressed Data Format Specification. 341.It RFC 1952 342GZIP File Format Specification. 343.El 344.Sh HISTORY 345.Nm gzip 346compatibility was added to 347.Xr compress 1 348in 349.Ox 3.4 . 350The 351.Sq g 352in this version of 353.Nm gzip 354stands for 355.Dq gratis . 356