1.\" $OpenBSD: gzip.1,v 1.14 2014/10/07 21:06:30 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: October 7 2014 $ 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 123456789cdfhLlNnOqrtVv 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 cfhLlNnqrtVv 53.Op Fl o Ar filename 54.Op Ar 55.Nm gzcat 56.Op Fl fhqr 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 . 156.It Fl b Ar bits 157Specify the 158.Ar bits 159code limit 160.Pq see below . 161.It Fl c 162Compressed or uncompressed output is written to the standard output. 163No files are modified (force 164.Nm gzcat 165mode). 166.It Fl d 167Decompress the source files instead of compressing them (force 168.Nm gunzip 169mode). 170.It Fl f 171Force compression of 172.Ar file , 173even if it is not actually reduced in size. 174Additionally, files are overwritten without prompting for confirmation. 175If the input data is not in a format recognized by 176.Nm 177and if the option 178.Fl c 179is also given, copy the input data without change 180to the standard output: let 181.Nm gzcat 182behave as 183.Xr cat 1 . 184.It Fl h 185Print a short help message. 186.It Fl L 187A no-op which exists for compatibility only. 188On GNU gzip, it displays the program's license. 189.It Fl l 190List information for the specified compressed files. 191The following information is listed: 192.Bl -tag -width "compression ratio" 193.It compressed size 194Size of the compressed file. 195.It uncompressed size 196Size of the file when uncompressed. 197.It compression ratio 198Ratio of the difference between the compressed and uncompressed 199sizes to the uncompressed size. 200.It uncompressed name 201Name the file will be saved as when uncompressing. 202.El 203.Pp 204If the 205.Fl v 206option is specified, the following additional information is printed: 207.Bl -tag -width "compression method" 208.It compression method 209Name of the method used to compress the file. 210.It crc 21132-bit CRC 212.Pq cyclic redundancy code 213of the uncompressed file. 214.It "time stamp" 215Date and time corresponding to the last data modification time 216(mtime) of the compressed file (if the 217.Fl n 218option is specified, the time stamp stored in the compressed file 219is printed instead). 220.El 221.It Fl N 222When uncompressing or listing, use the time stamp and file name stored 223in the compressed file, if any, for the uncompressed version. 224.It Fl n 225When compressing, do not store the original file name and time stamp 226in the 227.Nm 228header. 229.It Fl O 230Use old compression method 231(force 232.Xr compress 1 233mode). 234.It Fl o Ar filename 235Set the output file name. 236.It Fl q 237Be quiet: suppress all messages. 238.It Fl r 239Recursive mode: 240.Nm 241will descend into specified directories. 242.It Fl S Ar suffix 243Set the suffix for compressed files. 244.It Fl t 245Test the integrity of each file leaving any files intact. 246.It Fl V 247A no-op which exists for compatibility only. 248On GNU gzip, it displays version information. 249.It Fl v 250Print the percentage reduction of each file and other information. 251.El 252.Pp 253.Nm 254uses a modified Lempel-Ziv algorithm 255.Pq LZW . 256Common substrings are replaced by pointers to previous strings, 257and are found using a hash table. 258Unique substrings are emitted as a string of literal bytes, 259and compressed as Huffman trees. 260When code 512 is reached, the algorithm switches to 10-bit codes and 261continues to use more bits until the 262limit specified by the 263.Fl b 264flag is reached. 265.Ar bits 266must be between 9 and 16 267.Pq the default is 16 . 268.Pp 269After the 270.Ar bits 271limit is reached, 272.Nm 273periodically checks the compression ratio. 274If it is increasing, 275.Nm 276continues to use the existing code dictionary. 277However, if the compression ratio decreases, 278.Nm 279discards the table of substrings and rebuilds it from scratch. 280This allows the algorithm to adapt to the next 281.Dq block 282of the file. 283.Pp 284The 285.Fl b 286flag is omitted for 287.Nm gunzip 288since the 289.Ar bits 290parameter specified during compression 291is encoded within the output, along with 292a magic number to ensure that neither decompression of random data nor 293recompression of compressed data is attempted. 294.Pp 295The amount of compression obtained depends on the size of the 296input, the number of 297.Ar bits 298per code, and the distribution of common substrings. 299Typically, text such as source code or English is reduced by 60 \- 70% using 300.Nm . 301Compression is generally much better than that achieved by Huffman 302coding (as used in the historical command pack), or adaptive Huffman 303coding (as used in the historical command compact), and takes less 304time to compute. 305.Sh ENVIRONMENT 306.Bl -tag -width Ds 307.It Ev GZIP 308Options which are passed to 309.Nm , 310.Nm gunzip , 311and 312.Nm gzcat 313automatically. 314.El 315.Sh EXIT STATUS 316The 317.Nm gzip 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 334The 335.Nm gunzip 336and 337.Nm gzcat 338utilities exit 0 on success, 339and >0 if an error occurs. 340.Sh SEE ALSO 341.Xr compress 1 , 342.Xr gzexe 1 , 343.Xr zdiff 1 , 344.Xr zforce 1 , 345.Xr zmore 1 , 346.Xr znew 1 , 347.Xr compress 3 348.Sh STANDARDS 349.Rs 350.%A P. Deutsch 351.%A J-L. Gailly 352.%D May 1996 353.%R RFC 1950 354.%T ZLIB Compressed Data Format Specification version 3.3 355.Re 356.Pp 357.Rs 358.%A P. Deutsch 359.%D May 1996 360.%R RFC 1951 361.%T DEFLATE Compressed Data Format Specification version 1.3 362.Re 363.Pp 364.Rs 365.%A P. Deutsch 366.%D May 1996 367.%R RFC 1952 368.%T GZIP file format specification version 4.3 369.Re 370.Sh HISTORY 371.Nm gzip 372compatibility was added to 373.Xr compress 1 374in 375.Ox 3.4 . 376The 377.Sq g 378in this version of 379.Nm gzip 380stands for 381.Dq gratis . 382