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1 .TH GZIP 1 local
2 .SH NAME
3 gzip, gunzip, zcat \- compress or expand files
4 .SH SYNOPSIS
5 .ll +8
6 .B gzip
7 .RB [ " \-acdfhklLnNrtvV19 " ]
8 .RB [ \-S\ suffix ]
9 [
10 .I "name \&..."
11 ]
12 .ll -8
13 .br
14 .B gunzip
15 .RB [ " \-acfhklLnNrtvV " ]
16 .RB [ \-S\ suffix ]
17 [
18 .I "name \&..."
19 ]
20 .br
21 .B zcat
22 .RB [ " \-fhLV " ]
23 [
24 .I "name \&..."
25 ]
26 .SH DESCRIPTION
27 .I Gzip
28 reduces the size of the named files using Lempel-Ziv coding (LZ77).
29 Whenever possible,
30 each file is replaced by one with the extension
31 .BR "\&.gz" ,
32 while keeping the same ownership modes, access and modification times.
33 (The default extension is
34 .B "\-gz"
35 for VMS,
36 .B "z"
37 for MSDOS, OS/2 FAT, Windows NT FAT and Atari.)
38 If no files are specified, or if a file name is "-", the standard input is
39 compressed to the standard output.
40 .I Gzip
41 will only attempt to compress regular files.
42 In particular, it will ignore symbolic links.
43 .PP
44 If the compressed file name is too long for its file system,
45 .I gzip
46 truncates it.
47 .I Gzip
48 attempts to truncate only the parts of the file name longer than 3 characters.
49 (A part is delimited by dots.) If the name consists of small parts only,
50 the longest parts are truncated. For example, if file names are limited
51 to 14 characters, gzip.msdos.exe is compressed to gzi.msd.exe.gz.
52 Names are not truncated on systems which do not have a limit on file name
53 length.
54 .PP
55 By default,
56 .I gzip
57 keeps the original file name and timestamp in the compressed file. These
58 are used when decompressing the file with the
59 .B \-N
60 option. This is useful when the compressed file name was truncated or
61 when the time stamp was not preserved after a file transfer.
62 .PP
63 Compressed files can be restored to their original form using
64 .I gzip -d
65 or
66 .I gunzip
67 or
68 .IR zcat .
69 If the original name saved in the compressed file is not suitable for its
70 file system, a new name is constructed from the original one to make it
71 legal.
72 .PP
73 .I gunzip
74 takes a list of files on its command line and replaces each
75 file whose name ends with .gz, -gz, .z, -z, or _z (ignoring case)
76 and which begins with the correct magic number with an uncompressed
77 file without the original extension.
78 .I gunzip
79 also recognizes the special extensions
80 .B "\&.tgz"
81 and
82 .B "\&.taz"
83 as shorthands for
84 .B "\&.tar.gz"
85 and
86 .B "\&.tar.Z"
87 respectively.
88 When compressing,
89 .I gzip
90 uses the
91 .B "\&.tgz"
92 extension if necessary instead of truncating a file with a
93 .B "\&.tar"
94 extension.
95 .PP
96 .I gunzip
97 can currently decompress files created by
98 .IR gzip ,
99 .IR zip ,
100 .IR compress ,
101 .I "compress -H"
102 or
103 .IR pack .
104 The detection of the input format is automatic. When using
105 the first two formats,
106 .I gunzip
107 checks a 32 bit CRC. For
108 .I pack
109 and
110 .I gunzip
111 checks the uncompressed length. The standard
112 .I compress
113 format was not designed to allow consistency checks. However
114 .I gunzip
115 is sometimes able to detect a bad .Z file. If you get an error
116 when uncompressing a .Z file, do not assume that the .Z file is
117 correct simply because the standard
118 .I uncompress
119 does not complain. This generally means that the standard
120 .I uncompress
121 does not check its input, and happily generates garbage output.
122 The SCO compress -H format (lzh compression method) does not include a CRC
123 but also allows some consistency checks.
124 .PP
125 Files created by
126 .I zip
127 can be uncompressed by gzip only if they have a single member compressed
128 with the 'deflation' method. This feature is only intended to help
129 conversion of tar.zip files to the tar.gz format. To extract a
130 .I zip
131 file with a single member, use a command like
132 .I "gunzip <foo.zip"
133 or
134 .IR "gunzip -S .zip foo.zip" .
135 To extract zip files
136 with several members, use
137 .I unzip
138 instead of
139 .IR gunzip .
140 .PP
141 .I zcat
142 is identical to
143 .I gunzip
144 .BR \-c .
145 (On some systems,
146 .I zcat
147 may be installed as
148 .I gzcat
149 to preserve the original link to
150 .IR compress .)
151 .I zcat
152 uncompresses either a list of files on the command line or its
153 standard input and writes the uncompressed data on standard output.
154 .I zcat
155 will uncompress files that have the correct magic number whether
156 they have a
157 .B "\&.gz"
158 suffix or not.
159 .PP
160 .I Gzip
161 uses the Lempel-Ziv algorithm used in
162 .I zip
163 and PKZIP.
164 The amount of compression obtained depends on the size of the
165 input and the distribution of common substrings.
166 Typically, text such as source code or English
167 is reduced by 60\-70%.
168 Compression is generally much better than that achieved by
169 LZW (as used in
170 .IR compress ),
171 Huffman coding (as used in
172 .IR pack ),
173 or adaptive Huffman coding
174 .RI ( compact ).
175 .PP
176 Compression is always performed, even if the compressed file is
177 slightly larger than the original. The worst case expansion is
178 a few bytes for the gzip file header, plus 5 bytes every 32K block,
179 or an expansion ratio of 0.015% for large files. Note that the actual
180 number of used disk blocks almost never increases.
181 .I gzip
182 preserves the mode, ownership and timestamps of files when compressing
183 or decompressing.
184 .SH OPTIONS
185 .TP
186 .B \-a --ascii
187 Ascii text mode: convert end-of-lines using local conventions. This option
188 is supported only on some non-Unix systems. For MSDOS, CR LF is converted
189 to LF when compressing, and LF is converted to CR LF when decompressing.
190 .TP
191 .B \-c --stdout --to-stdout
192 Write output on standard output; keep original files unchanged.
193 If there are several input files, the output consists of a sequence of
194 independently compressed members. To obtain better compression,
195 concatenate all input files before compressing them.
196 .TP
197 .B \-d --decompress --uncompress
198 Decompress.
199 .TP
200 .B \-f --force
201 Force compression or decompression even if the file has multiple links
202 or the corresponding file already exists, or if the compressed data
203 is read from or written to a terminal. If the input data is not in
204 a format recognized by
205 .IR gzip ,
206 and if the option --stdout is also given, copy the input data without change
207 to the standard output: let
208 .I zcat
209 behave as
210 .IR cat .
211 If
212 .B \-f
213 is not given,
214 and when not running in the background,
215 .I gzip
216 prompts to verify whether an existing file should be overwritten.
217 .TP
218 .B \-h --help
219 Display a help screen and quit.
220 .TP
221 .B \-k --keep
222 Keep (don't delete) input files during compression or decompression.
223 .TP
224 .B \-l --list
225 For each compressed file, list the following fields:
227 compressed size: size of the compressed file
228 uncompressed size: size of the uncompressed file
229 ratio: compression ratio (0.0% if unknown)
230 uncompressed_name: name of the uncompressed file
232 The uncompressed size is given as -1 for files not in gzip format,
233 such as compressed .Z files. To get the uncompressed size for such a file,
234 you can use:
236 zcat file.Z | wc -c
238 In combination with the --verbose option, the following fields are also
239 displayed:
241 method: compression method
242 crc: the 32-bit CRC of the uncompressed data
243 date & time: time stamp for the uncompressed file
245 The compression methods currently supported are deflate, compress, lzh
246 (SCO compress -H) and pack. The crc is given as ffffffff for a file
247 not in gzip format.
249 With --name, the uncompressed name, date and time are
250 those stored within the compress file if present.
252 With --verbose, the size totals and compression ratio for all files
253 is also displayed, unless some sizes are unknown. With --quiet,
254 the title and totals lines are not displayed.
255 .TP
256 .B \-L --license
257 Display the
258 .I gzip
259 license and quit.
260 .TP
261 .B \-n --no-name
262 When compressing, do not save the original file name and time stamp by
263 default. (The original name is always saved if the name had to be
264 truncated.) When decompressing, do not restore the original file name
265 if present (remove only the
266 .I gzip
267 suffix from the compressed file name) and do not restore the original
268 time stamp if present (copy it from the compressed file). This option
269 is the default when decompressing.
270 .TP
271 .B \-N --name
272 When compressing, always save the original file name and time stamp; this
273 is the default. When decompressing, restore the original file name and
274 time stamp if present. This option is useful on systems which have
275 a limit on file name length or when the time stamp has been lost after
276 a file transfer.
277 .TP
278 .B \-q --quiet
279 Suppress all warnings.
280 .TP
281 .B \-r --recursive
282 Travel the directory structure recursively. If any of the file names
283 specified on the command line are directories,
284 .I gzip
285 will descend into the directory and compress all the files it finds there
286 (or decompress them in the case of
287 .I gunzip
288 ).
289 .TP
290 .B \-S .suf --suffix .suf
291 When compressing, use suffix .suf instead of .gz.
292 Any non-empty suffix can be given, but suffixes
293 other than .z and .gz should be avoided to avoid confusion when files
294 are transferred to other systems.
296 When decompressing, add .suf to the beginning of the list of
297 suffixes to try, when deriving an output file name from an input file name.
298 .TP
299 .B \-t --test
300 Test. Check the compressed file integrity.
301 .TP
302 .B \-v --verbose
303 Verbose. Display the name and percentage reduction for each file compressed
304 or decompressed.
305 .TP
306 .B \-V --version
307 Version. Display the version number and compilation options then quit.
308 .TP
309 .B \-# --fast --best
310 Regulate the speed of compression using the specified digit
311 .IR # ,
312 where
313 .B \-1
314 or
315 .B \-\-fast
316 indicates the fastest compression method (less compression)
317 and
318 .B \-9
319 or
320 .B \-\-best
321 indicates the slowest compression method (best compression).
322 The default compression level is
323 .BR \-6
324 (that is, biased towards high compression at expense of speed).
325 .SH "ADVANCED USAGE"
326 Multiple compressed files can be concatenated. In this case,
327 .I gunzip
328 will extract all members at once. For example:
330 gzip -c file1 > foo.gz
331 gzip -c file2 >> foo.gz
333 Then
335 gunzip -c foo
337 is equivalent to
339 cat file1 file2
341 In case of damage to one member of a .gz file, other members can
342 still be recovered (if the damaged member is removed). However,
343 you can get better compression by compressing all members at once:
345 cat file1 file2 | gzip > foo.gz
347 compresses better than
349 gzip -c file1 file2 > foo.gz
351 If you want to recompress concatenated files to get better compression, do:
353 gzip -cd old.gz | gzip > new.gz
355 If a compressed file consists of several members, the uncompressed
356 size and CRC reported by the --list option applies to the last member
357 only. If you need the uncompressed size for all members, you can use:
359 gzip -cd file.gz | wc -c
361 If you wish to create a single archive file with multiple members so
362 that members can later be extracted independently, use an archiver
363 such as tar or zip. GNU tar supports the -z option to invoke gzip
364 transparently. gzip is designed as a complement to tar, not as a
365 replacement.
366 .SH "ENVIRONMENT"
367 The environment variable
368 .B GZIP
369 can hold a set of default options for
370 .IR gzip .
371 These options are interpreted first and can be overwritten by
372 explicit command line parameters. For example:
373 for sh: GZIP="-8v --name"; export GZIP
374 for csh: setenv GZIP "-8v --name"
375 for MSDOS: set GZIP=-8v --name
377 On Vax/VMS, the name of the environment variable is GZIP_OPT, to
378 avoid a conflict with the symbol set for invocation of the program.
379 .SH "SEE ALSO"
380 znew(1), zcmp(1), zmore(1), zforce(1), gzexe(1), zip(1), unzip(1), compress(1)
381 .PP
382 The
383 .I gzip
384 file format is specified in P. Deutsch, \s-1GZIP\s0 file format
385 specification version 4.3,
386 .BR <http://www.ietf.org/rfc/rfc1952.txt> ,
387 Internet RFC 1952 (May 1996).
388 The
389 .I zip
390 deflation format is specified in P. Deutsch, \s-1DEFLATE\s0 Compressed
391 Data Format Specification version 1.3,
392 .BR <http://www.ietf.org/rfc/rfc1951.txt> ,
393 Internet RFC 1951 (May 1996).
394 .SH "DIAGNOSTICS"
395 Exit status is normally 0;
396 if an error occurs, exit status is 1. If a warning occurs, exit status is 2.
397 .TP
398 Usage: gzip [-cdfhklLnNrtvV19] [-S suffix] [file ...]
399 Invalid options were specified on the command line.
400 .TP
401 \fIfile\fP\^: not in gzip format
402 The file specified to
403 .I gunzip
404 has not been compressed.
405 .TP
406 \fIfile\fP\^: Corrupt input. Use zcat to recover some data.
407 The compressed file has been damaged. The data up to the point of failure
408 can be recovered using
410 zcat \fIfile\fP > recover
411 .TP
412 \fIfile\fP\^: compressed with \fIxx\fP bits, can only handle \fIyy\fP bits
413 .I File
414 was compressed (using LZW) by a program that could deal with
415 more
416 .I bits
417 than the decompress code on this machine.
418 Recompress the file with gzip, which compresses better and uses
419 less memory.
420 .TP
421 \fIfile\fP\^: already has .gz suffix -- no change
422 The file is assumed to be already compressed.
423 Rename the file and try again.
424 .TP
425 \fIfile\fP already exists; do you wish to overwrite (y or n)?
426 Respond "y" if you want the output file to be replaced; "n" if not.
427 .TP
428 gunzip: corrupt input
429 A SIGSEGV violation was detected which usually means that the input file has
430 been corrupted.
431 .TP
432 \fIxx.x%\fP Percentage of the input saved by compression.
433 (Relevant only for
434 .BR \-v
435 and
436 .BR \-l \.)
437 .TP
438 -- not a regular file or directory: ignored
439 When the input file is not a regular file or directory,
440 (e.g. a symbolic link, socket, FIFO, device file), it is
441 left unaltered.
442 .TP
443 -- has \fIxx\fP other links: unchanged
444 The input file has links; it is left unchanged. See
445 .IR ln "(1)"
446 for more information. Use the
447 .B \-f
448 flag to force compression of multiply-linked files.
449 .SH CAVEATS
450 When writing compressed data to a tape, it is generally necessary to
451 pad the output with zeroes up to a block boundary. When the data is
452 read and the whole block is passed to
453 .I gunzip
454 for decompression,
455 .I gunzip
456 detects that there is extra trailing garbage after the compressed data
457 and emits a warning by default. You have to use the --quiet option to
458 suppress the warning. This option can be set in the
459 .B GZIP
460 environment variable as in:
461 for sh: GZIP="-q" tar -xfz --block-compress /dev/rst0
462 for csh: (setenv GZIP -q; tar -xfz --block-compr /dev/rst0
464 In the above example, gzip is invoked implicitly by the -z option of
465 GNU tar. Make sure that the same block size (-b option of tar) is used
466 for reading and writing compressed data on tapes. (This example
467 assumes you are using the GNU version of tar.)
468 .SH BUGS
469 The gzip format represents the input size modulo 2^32, so the
470 --list option reports incorrect uncompressed sizes and compression
471 ratios for uncompressed files 4 GB and larger. To work around this
472 problem, you can use the following command to discover a large
473 uncompressed file's true size:
475 zcat file.gz | wc -c
477 The --list option reports sizes as -1 and crc as ffffffff if the
478 compressed file is on a non seekable media.
480 In some rare cases, the --best option gives worse compression than
481 the default compression level (-6). On some highly redundant files,
482 .I compress
483 compresses better than
484 .IR gzip .
485 .SH "COPYRIGHT NOTICE"
486 Copyright \(co 1998, 1999, 2001, 2002, 2012 Free Software Foundation, Inc.
487 .br
488 Copyright \(co 1992, 1993 Jean-loup Gailly
489 .PP
490 Permission is granted to make and distribute verbatim copies of
491 this manual provided the copyright notice and this permission notice
492 are preserved on all copies.
493 .ig
494 Permission is granted to process this file through troff and print the
495 results, provided the printed document carries copying permission
496 notice identical to this one except for the removal of this paragraph
497 (this paragraph not being relevant to the printed manual).
498 ..
499 .PP
500 Permission is granted to copy and distribute modified versions of this
501 manual under the conditions for verbatim copying, provided that the entire
502 resulting derived work is distributed under the terms of a permission
503 notice identical to this one.
504 .PP
505 Permission is granted to copy and distribute translations of this manual
506 into another language, under the above conditions for modified versions,
507 except that this permission notice may be stated in a translation approved
508 by the Foundation.