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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 [ " \-acdfhlLnNrtvV19 " ]
8 .RB [ \-S\ suffix ]
9 [
10 .I "name \&..."
11 ]
12 .ll -8
13 .br
14 .B gunzip
15 .RB [ " \-acfhlLnNrtvV " ]
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 .B "\&.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 .I 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 .I gzip, zip, compress, compress -H
99 or
100 .I pack.
101 The detection of the input format is automatic. When using
102 the first two formats,
103 .I gunzip
104 checks a 32 bit CRC. For
105 .I pack, gunzip
106 checks the uncompressed length. The standard
107 .I compress
108 format was not designed to allow consistency checks. However
109 .I gunzip
110 is sometimes able to detect a bad .Z file. If you get an error
111 when uncompressing a .Z file, do not assume that the .Z file is
112 correct simply because the standard
113 .I uncompress
114 does not complain. This generally means that the standard
115 .I uncompress
116 does not check its input, and happily generates garbage output.
117 The SCO compress -H format (lzh compression method) does not include a CRC
118 but also allows some consistency checks.
119 .PP
120 Files created by
121 .I zip
122 can be uncompressed by gzip only if they have a single member compressed
123 with the 'deflation' method. This feature is only intended to help
124 conversion of tar.zip files to the tar.gz format. To extract a
125 .I zip
126 file with a single member, use a command like
127 .I "gunzip <foo.zip"
128 or
129 .IR "gunzip -S .zip foo.zip" .
130 To extract zip files
131 with several members, use
132 .I unzip
133 instead of
134 .I gunzip.
135 .PP
136 .I zcat
137 is identical to
138 .I gunzip
139 .B \-c.
140 (On some systems,
141 .I zcat
142 may be installed as
143 .I gzcat
144 to preserve the original link to
145 .I compress.)
146 .I zcat
147 uncompresses either a list of files on the command line or its
148 standard input and writes the uncompressed data on standard output.
149 .I zcat
150 will uncompress files that have the correct magic number whether
151 they have a
152 .B "\&.gz"
153 suffix or not.
154 .PP
155 .I Gzip
156 uses the Lempel-Ziv algorithm used in
157 .I zip
158 and PKZIP.
159 The amount of compression obtained depends on the size of the
160 input and the distribution of common substrings.
161 Typically, text such as source code or English
162 is reduced by 60\-70%.
163 Compression is generally much better than that achieved by
164 LZW (as used in
165 .IR compress ),
166 Huffman coding (as used in
167 .IR pack ),
168 or adaptive Huffman coding
169 .RI ( compact ).
170 .PP
171 Compression is always performed, even if the compressed file is
172 slightly larger than the original. The worst case expansion is
173 a few bytes for the gzip file header, plus 5 bytes every 32K block,
174 or an expansion ratio of 0.015% for large files. Note that the actual
175 number of used disk blocks almost never increases.
176 .I gzip
177 preserves the mode, ownership and timestamps of files when compressing
178 or decompressing.
179 .PP
180 The
181 .I gzip
182 file format is specified in P. Deutsch, \s-1GZIP\s0 file format
183 specification version 4.3, <ftp://ftp.isi.edu/in-notes/rfc1952.txt>,
184 Internet RFC 1952 (May 1996). The
185 .I zip
186 deflation format is specified in P. Deutsch, \s-1DEFLATE\s0 Compressed
187 Data Format Specification version 1.3,
188 <ftp://ftp.isi.edu/in-notes/rfc1951.txt>, Internet RFC 1951 (May 1996).
190 .SH OPTIONS
191 .TP
192 .B \-a --ascii
193 Ascii text mode: convert end-of-lines using local conventions. This option
194 is supported only on some non-Unix systems. For MSDOS, CR LF is converted
195 to LF when compressing, and LF is converted to CR LF when decompressing.
196 .TP
197 .B \-c --stdout --to-stdout
198 Write output on standard output; keep original files unchanged.
199 If there are several input files, the output consists of a sequence of
200 independently compressed members. To obtain better compression,
201 concatenate all input files before compressing them.
202 .TP
203 .B \-d --decompress --uncompress
204 Decompress.
205 .TP
206 .B \-f --force
207 Force compression or decompression even if the file has multiple links
208 or the corresponding file already exists, or if the compressed data
209 is read from or written to a terminal. If the input data is not in
210 a format recognized by
211 .I gzip,
212 and if the option --stdout is also given, copy the input data without change
213 to the standard output: let
214 .I zcat
215 behave as
216 .I cat.
217 If
218 .B \-f
219 is not given,
220 and when not running in the background,
221 .I gzip
222 prompts to verify whether an existing file should be overwritten.
223 .TP
224 .B \-h --help
225 Display a help screen and quit.
226 .TP
227 .B \-l --list
228 For each compressed file, list the following fields:
230 compressed size: size of the compressed file
231 uncompressed size: size of the uncompressed file
232 ratio: compression ratio (0.0% if unknown)
233 uncompressed_name: name of the uncompressed file
235 The uncompressed size is given as -1 for files not in gzip format,
236 such as compressed .Z files. To get the uncompressed size for such a file,
237 you can use:
239 zcat file.Z | wc -c
241 In combination with the --verbose option, the following fields are also
242 displayed:
244 method: compression method
245 crc: the 32-bit CRC of the uncompressed data
246 date & time: time stamp for the uncompressed file
248 The compression methods currently supported are deflate, compress, lzh
249 (SCO compress -H) and pack. The crc is given as ffffffff for a file
250 not in gzip format.
252 With --name, the uncompressed name, date and time are
253 those stored within the compress file if present.
255 With --verbose, the size totals and compression ratio for all files
256 is also displayed, unless some sizes are unknown. With --quiet,
257 the title and totals lines are not displayed.
258 .TP
259 .B \-L --license
260 Display the
261 .I gzip
262 license and quit.
263 .TP
264 .B \-n --no-name
265 When compressing, do not save the original file name and time stamp by
266 default. (The original name is always saved if the name had to be
267 truncated.) When decompressing, do not restore the original file name
268 if present (remove only the
269 .I gzip
270 suffix from the compressed file name) and do not restore the original
271 time stamp if present (copy it from the compressed file). This option
272 is the default when decompressing.
273 .TP
274 .B \-N --name
275 When compressing, always save the original file name and time stamp; this
276 is the default. When decompressing, restore the original file name and
277 time stamp if present. This option is useful on systems which have
278 a limit on file name length or when the time stamp has been lost after
279 a file transfer.
280 .TP
281 .B \-q --quiet
282 Suppress all warnings.
283 .TP
284 .B \-r --recursive
285 Travel the directory structure recursively. If any of the file names
286 specified on the command line are directories,
287 .I gzip
288 will descend into the directory and compress all the files it finds there
289 (or decompress them in the case of
290 .I gunzip
291 ).
292 .TP
293 .B \-S .suf --suffix .suf
294 When compressing, use suffix .suf instead of .gz.
295 Any non-empty suffix can be given, but suffixes
296 other than .z and .gz should be avoided to avoid confusion when files
297 are transferred to other systems.
299 When decompressing, add .suf to the beginning of the list of
300 suffixes to try, when deriving an output file name from an input file name.
302 .IR pack "(1)".
303 .TP
304 .B \-t --test
305 Test. Check the compressed file integrity.
306 .TP
307 .B \-v --verbose
308 Verbose. Display the name and percentage reduction for each file compressed
309 or decompressed.
310 .TP
311 .B \-V --version
312 Version. Display the version number and compilation options then quit.
313 .TP
314 .B \-# --fast --best
315 Regulate the speed of compression using the specified digit
316 .IR # ,
317 where
318 .B \-1
319 or
320 .B \-\-fast
321 indicates the fastest compression method (less compression)
322 and
323 .B \-9
324 or
325 .B \-\-best
326 indicates the slowest compression method (best compression).
327 The default compression level is
328 .BR \-6
329 (that is, biased towards high compression at expense of speed).
330 .SH "ADVANCED USAGE"
331 Multiple compressed files can be concatenated. In this case,
332 .I gunzip
333 will extract all members at once. For example:
335 gzip -c file1 > foo.gz
336 gzip -c file2 >> foo.gz
338 Then
340 gunzip -c foo
342 is equivalent to
344 cat file1 file2
346 In case of damage to one member of a .gz file, other members can
347 still be recovered (if the damaged member is removed). However,
348 you can get better compression by compressing all members at once:
350 cat file1 file2 | gzip > foo.gz
352 compresses better than
354 gzip -c file1 file2 > foo.gz
356 If you want to recompress concatenated files to get better compression, do:
358 gzip -cd old.gz | gzip > new.gz
360 If a compressed file consists of several members, the uncompressed
361 size and CRC reported by the --list option applies to the last member
362 only. If you need the uncompressed size for all members, you can use:
364 gzip -cd file.gz | wc -c
366 If you wish to create a single archive file with multiple members so
367 that members can later be extracted independently, use an archiver
368 such as tar or zip. GNU tar supports the -z option to invoke gzip
369 transparently. gzip is designed as a complement to tar, not as a
370 replacement.
371 .SH "ENVIRONMENT"
372 The environment variable
373 .B GZIP
374 can hold a set of default options for
375 .I gzip.
376 These options are interpreted first and can be overwritten by
377 explicit command line parameters. For example:
378 for sh: GZIP="-8v --name"; export GZIP
379 for csh: setenv GZIP "-8v --name"
380 for MSDOS: set GZIP=-8v --name
382 On Vax/VMS, the name of the environment variable is GZIP_OPT, to
383 avoid a conflict with the symbol set for invocation of the program.
384 .SH "SEE ALSO"
385 znew(1), zcmp(1), zmore(1), zforce(1), gzexe(1), zip(1), unzip(1), compress(1),
386 pack(1), compact(1)
387 .PP
388 The
389 .I gzip
390 file format is specified in P. Deutsch, \s-1GZIP\s0 file format
391 specification version 4.3,
392 .BR <ftp://ftp.isi.edu/in-notes/rfc1952.txt> ,
393 Internet RFC 1952 (May 1996).
394 The
395 .I zip
396 deflation format is specified in P. Deutsch, \s-1DEFLATE\s0 Compressed
397 Data Format Specification version 1.3,
398 .BR <ftp://ftp.isi.edu/in-notes/rfc1951.txt> ,
399 Internet RFC 1951 (May 1996).
400 .SH "DIAGNOSTICS"
401 Exit status is normally 0;
402 if an error occurs, exit status is 1. If a warning occurs, exit status is 2.
403 .TP
404 Usage: gzip [-cdfhlLnNrtvV19] [-S suffix] [file ...]
405 Invalid options were specified on the command line.
406 .TP
407 \fIfile\fP\^: not in gzip format
408 The file specified to
409 .I gunzip
410 has not been compressed.
411 .TP
412 \fIfile\fP\^: Corrupt input. Use zcat to recover some data.
413 The compressed file has been damaged. The data up to the point of failure
414 can be recovered using
416 zcat \fIfile\fP > recover
417 .TP
418 \fIfile\fP\^: compressed with \fIxx\fP bits, can only handle \fIyy\fP bits
419 .I File
420 was compressed (using LZW) by a program that could deal with
421 more
422 .I bits
423 than the decompress code on this machine.
424 Recompress the file with gzip, which compresses better and uses
425 less memory.
426 .TP
427 \fIfile\fP\^: already has .gz suffix -- no change
428 The file is assumed to be already compressed.
429 Rename the file and try again.
430 .TP
431 \fIfile\fP already exists; do you wish to overwrite (y or n)?
432 Respond "y" if you want the output file to be replaced; "n" if not.
433 .TP
434 gunzip: corrupt input
435 A SIGSEGV violation was detected which usually means that the input file has
436 been corrupted.
437 .TP
438 \fIxx.x%\fP Percentage of the input saved by compression.
439 (Relevant only for
440 .BR \-v
441 and
442 .BR \-l \.)
443 .TP
444 -- not a regular file or directory: ignored
445 When the input file is not a regular file or directory,
446 (e.g. a symbolic link, socket, FIFO, device file), it is
447 left unaltered.
448 .TP
449 -- has \fIxx\fP other links: unchanged
450 The input file has links; it is left unchanged. See
451 .IR ln "(1)"
452 for more information. Use the
453 .B \-f
454 flag to force compression of multiply-linked files.
455 .SH CAVEATS
456 When writing compressed data to a tape, it is generally necessary to
457 pad the output with zeroes up to a block boundary. When the data is
458 read and the whole block is passed to
459 .I gunzip
460 for decompression,
461 .I gunzip
462 detects that there is extra trailing garbage after the compressed data
463 and emits a warning by default. You have to use the --quiet option to
464 suppress the warning. This option can be set in the
465 .B GZIP
466 environment variable as in:
467 for sh: GZIP="-q" tar -xfz --block-compress /dev/rst0
468 for csh: (setenv GZIP -q; tar -xfz --block-compr /dev/rst0
470 In the above example, gzip is invoked implicitly by the -z option of
471 GNU tar. Make sure that the same block size (-b option of tar) is used
472 for reading and writing compressed data on tapes. (This example
473 assumes you are using the GNU version of tar.)
474 .SH BUGS
475 The gzip format represents the input size modulo 2^32, so the
476 --list option reports incorrect uncompressed sizes and compression
477 ratios for uncompressed files 4 GB and larger. To work around this
478 problem, you can use the following command to discover a large
479 uncompressed file's true size:
481 zcat file.gz | wc -c
483 The --list option reports sizes as -1 and crc as ffffffff if the
484 compressed file is on a non seekable media.
486 In some rare cases, the --best option gives worse compression than
487 the default compression level (-6). On some highly redundant files,
488 .I compress
489 compresses better than
490 .I gzip.
491 .SH "COPYRIGHT NOTICE"
492 Copyright \(co 1998, 1999, 2001, 2002 Free Software Foundation, Inc.
493 .br
494 Copyright \(co 1992, 1993 Jean-loup Gailly
495 .PP
496 Permission is granted to make and distribute verbatim copies of
497 this manual provided the copyright notice and this permission notice
498 are preserved on all copies.
499 .ig
500 Permission is granted to process this file through troff and print the
501 results, provided the printed document carries copying permission
502 notice identical to this one except for the removal of this paragraph
503 (this paragraph not being relevant to the printed manual).
504 ..
505 .PP
506 Permission is granted to copy and distribute modified versions of this
507 manual under the conditions for verbatim copying, provided that the entire
508 resulting derived work is distributed under the terms of a permission
509 notice identical to this one.
510 .PP
511 Permission is granted to copy and distribute translations of this manual
512 into another language, under the above conditions for modified versions,
513 except that this permission notice may be stated in a translation approved
514 by the Foundation.