1GZIP(1) General Commands Manual GZIP(1)
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6 gzip, gunzip, zcat - compress or expand files
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9 gzip [ -acdfhlLnNrtvV19 ] [-S suffix] [ name ... ]
10 gunzip [ -acfhlLnNrtvV ] [-S suffix] [ name ... ]
11 zcat [ -fhLV ] [ name ... ]
12
14 Gzip reduces the size of the named files using Lempel-Ziv coding
15 (LZ77). Whenever possible, each file is replaced by one with the
16 extension .gz, while keeping the same ownership modes, access and modi‐
17 fication times. (The default extension is -gz for VMS, z for MSDOS,
18 OS/2 FAT, Windows NT FAT and Atari.) If no files are specified, or if
19 a file name is "-", the standard input is compressed to the standard
20 output. Gzip will only attempt to compress regular files. In particu‐
21 lar, it will ignore symbolic links.
22 If the compressed file name is too long for its file system, gzip trun‐
24 cates it. Gzip attempts to truncate only the parts of the file name
25 longer than 3 characters. (A part is delimited by dots.) If the name
26 consists of small parts only, the longest parts are truncated. For
27 example, if file names are limited to 14 characters, gzip.msdos.exe is
28 compressed to gzi.msd.exe.gz. Names are not truncated on systems which
29 do not have a limit on file name length.
30 By default, gzip keeps the original file name and timestamp in the com‐
32 pressed file. These are used when decompressing the file with the -N
33 option. This is useful when the compressed file name was truncated or
34 when the time stamp was not preserved after a file transfer.
35 Compressed files can be restored to their original form using gzip -d
37 or gunzip or zcat. If the original name saved in the compressed file
38 is not suitable for its file system, a new name is constructed from the
39 original one to make it legal.
40 gunzip takes a list of files on its command line and replaces each file
42 whose name ends with .gz, -gz, .z, -z, or _z (ignoring case) and which
43 begins with the correct magic number with an uncompressed file without
44 the original extension. gunzip also recognizes the special extensions
45 .tgz and .taz as shorthands for .tar.gz and .tar.Z respectively. When
46 compressing, gzip uses the .tgz extension if necessary instead of trun‐
47 cating a file with a .tar extension.
48 gunzip can currently decompress files created by gzip, zip, compress,
50 compress -H or pack. The detection of the input format is automatic.
51 When using the first two formats, gunzip checks a 32 bit CRC. For pack,
52 gunzip checks the uncompressed length. The standard compress format was
53 not designed to allow consistency checks. However gunzip is sometimes
54 able to detect a bad .Z file. If you get an error when uncompressing a
55 .Z file, do not assume that the .Z file is correct simply because the
56 standard uncompress does not complain. This generally means that the
57 standard uncompress does not check its input, and happily generates
58 garbage output. The SCO compress -H format (lzh compression method)
59 does not include a CRC but also allows some consistency checks.
60 Files created by zip can be uncompressed by gzip only if they have a
62 single member compressed with the 'deflation' method. This feature is
63 only intended to help conversion of tar.zip files to the tar.gz format.
64 To extract a zip file with a single member, use a command like gunzip
65 <foo.zip or gunzip -S .zip foo.zip. To extract zip files with several
66 members, use unzip instead of gunzip.
67 zcat is identical to gunzip -c. (On some systems, zcat may be
69 installed as gzcat to preserve the original link to compress.) zcat
70 uncompresses either a list of files on the command line or its standard
71 input and writes the uncompressed data on standard output. zcat will
72 uncompress files that have the correct magic number whether they have a
73 .gz suffix or not.
74 Gzip uses the Lempel-Ziv algorithm used in zip and PKZIP. The amount
76 of compression obtained depends on the size of the input and the dis‐
77 tribution of common substrings. Typically, text such as source code or
78 English is reduced by 60-70%. Compression is generally much better
79 than that achieved by LZW (as used in compress), Huffman coding (as
80 used in pack), or adaptive Huffman coding (compact).
81 Compression is always performed, even if the compressed file is
83 slightly larger than the original. The worst case expansion is a few
84 bytes for the gzip file header, plus 5 bytes every 32K block, or an
85 expansion ratio of 0.015% for large files. Note that the actual number
86 of used disk blocks almost never increases. gzip preserves the mode,
87 ownership and timestamps of files when compressing or decompressing.
88 The gzip file format is specified in P. Deutsch, GZIP file format spec‐
90 ification version 4.3, <ftp://ftp.isi.edu/in-notes/rfc1952.txt>, Inter‐
91 net RFC 1952 (May 1996). The zip deflation format is specified in P.
92 Deutsch, DEFLATE Compressed Data Format Specification version 1.3,
93 <ftp://ftp.isi.edu/in-notes/rfc1951.txt>, Internet RFC 1951 (May 1996).
94
97 -a --ascii
98 Ascii text mode: convert end-of-lines using local conventions.
99 This option is supported only on some non-Unix systems. For
100 MSDOS, CR LF is converted to LF when compressing, and LF is con‐
101 verted to CR LF when decompressing.
102 -c --stdout --to-stdout
104 Write output on standard output; keep original files unchanged.
105 If there are several input files, the output consists of a
106 sequence of independently compressed members. To obtain better
107 compression, concatenate all input files before compressing
108 them.
109 -d --decompress --uncompress
111 Decompress.
112 -f --force
114 Force compression or decompression even if the file has multiple
115 links or the corresponding file already exists, or if the com‐
116 pressed data is read from or written to a terminal. If the input
117 data is not in a format recognized by gzip, and if the option
118 --stdout is also given, copy the input data without change to
119 the standard output: let zcat behave as cat. If -f is not
120 given, and when not running in the background, gzip prompts to
121 verify whether an existing file should be overwritten.
122 -h --help
124 Display a help screen and quit.
125 -l --list
127 For each compressed file, list the following fields:
128 compressed size: size of the compressed file
130 uncompressed size: size of the uncompressed file
131 ratio: compression ratio (0.0% if unknown)
132 uncompressed_name: name of the uncompressed file
133 The uncompressed size is given as -1 for files not in gzip for‐
135 mat, such as compressed .Z files. To get the uncompressed size
136 for such a file, you can use:
137 zcat file.Z | wc -c
139 In combination with the --verbose option, the following fields
141 are also displayed:
142 method: compression method
144 crc: the 32-bit CRC of the uncompressed data
145 date & time: time stamp for the uncompressed file
146 The compression methods currently supported are deflate, com‐
148 press, lzh (SCO compress -H) and pack. The crc is given as
149 ffffffff for a file not in gzip format.
150 With --name, the uncompressed name, date and time are those
152 stored within the compress file if present.
153 With --verbose, the size totals and compression ratio for all
155 files is also displayed, unless some sizes are unknown. With
156 --quiet, the title and totals lines are not displayed.
157 -L --license
159 Display the gzip license and quit.
160 -n --no-name
162 When compressing, do not save the original file name and time
163 stamp by default. (The original name is always saved if the name
164 had to be truncated.) When decompressing, do not restore the
165 original file name if present (remove only the gzip suffix from
166 the compressed file name) and do not restore the original time
167 stamp if present (copy it from the compressed file). This option
168 is the default when decompressing.
169 -N --name
171 When compressing, always save the original file name and time
172 stamp; this is the default. When decompressing, restore the
173 original file name and time stamp if present. This option is
174 useful on systems which have a limit on file name length or when
175 the time stamp has been lost after a file transfer.
176 -q --quiet
178 Suppress all warnings.
179 -r --recursive
181 Travel the directory structure recursively. If any of the file
182 names specified on the command line are directories, gzip will
183 descend into the directory and compress all the files it finds
184 there (or decompress them in the case of gunzip ).
185 -S .suf --suffix .suf
187 When compressing, use suffix .suf instead of .gz. Any non-empty
188 suffix can be given, but suffixes other than .z and .gz should
189 be avoided to avoid confusion when files are transferred to
190 other systems.
191 When decompressing, add .suf to the beginning of the list of
193 suffixes to try, when deriving an output file name from an input
194 file name.
195 pack(1).
197 -t --test
199 Test. Check the compressed file integrity.
200 -v --verbose
202 Verbose. Display the name and percentage reduction for each file
203 compressed or decompressed.
204 -V --version
206 Version. Display the version number and compilation options then
207 quit.
208 -# --fast --best
210 Regulate the speed of compression using the specified digit #,
211 where -1 or --fast indicates the fastest compression method
212 (less compression) and -9 or --best indicates the slowest com‐
213 pression method (best compression). The default compression
214 level is -6 (that is, biased towards high compression at expense
215 of speed).
216
218 Multiple compressed files can be concatenated. In this case, gunzip
219 will extract all members at once. For example:
220 gzip -c file1 > foo.gz
222 gzip -c file2 >> foo.gz
223 Then
225 gunzip -c foo
227 is equivalent to
229 cat file1 file2
231 In case of damage to one member of a .gz file, other members can still
233 be recovered (if the damaged member is removed). However, you can get
234 better compression by compressing all members at once:
235 cat file1 file2 | gzip > foo.gz
237 compresses better than
239 gzip -c file1 file2 > foo.gz
241 If you want to recompress concatenated files to get better compression,
243 do:
244 gzip -cd old.gz | gzip > new.gz
246 If a compressed file consists of several members, the uncompressed size
248 and CRC reported by the --list option applies to the last member only.
249 If you need the uncompressed size for all members, you can use:
250 gzip -cd file.gz | wc -c
252 If you wish to create a single archive file with multiple members so
254 that members can later be extracted independently, use an archiver such
255 as tar or zip. GNU tar supports the -z option to invoke gzip transpar‐
256 ently. gzip is designed as a complement to tar, not as a replacement.
257
259 The environment variable GZIP can hold a set of default options for
260 gzip. These options are interpreted first and can be overwritten by
261 explicit command line parameters. For example:
262 for sh: GZIP="-8v --name"; export GZIP
263 for csh: setenv GZIP "-8v --name"
264 for MSDOS: set GZIP=-8v --name
265 On Vax/VMS, the name of the environment variable is GZIP_OPT, to avoid
267 a conflict with the symbol set for invocation of the program.
268
270 znew(1), zcmp(1), zmore(1), zforce(1), gzexe(1), zip(1), unzip(1), com‐
271 press(1), pack(1), compact(1)
272 The gzip file format is specified in P. Deutsch, GZIP file format spec‐
274 ification version 4.3, <ftp://ftp.isi.edu/in-notes/rfc1952.txt>, Inter‐
275 net RFC 1952 (May 1996). The zip deflation format is specified in P.
276 Deutsch, DEFLATE Compressed Data Format Specification version 1.3,
277 <ftp://ftp.isi.edu/in-notes/rfc1951.txt>, Internet RFC 1951 (May 1996).
278
280 Exit status is normally 0; if an error occurs, exit status is 1. If a
281 warning occurs, exit status is 2.
282 Usage: gzip [-cdfhlLnNrtvV19] [-S suffix] [file ...]
284 Invalid options were specified on the command line.
285 file: not in gzip format
287 The file specified to gunzip has not been compressed.
288 file: Corrupt input. Use zcat to recover some data.
290 The compressed file has been damaged. The data up to the point
291 of failure can be recovered using
292 zcat file > recover
294 file: compressed with xx bits, can only handle yy bits
296 File was compressed (using LZW) by a program that could deal
297 with more bits than the decompress code on this machine. Recom‐
298 press the file with gzip, which compresses better and uses less
299 memory.
300 file: already has .gz suffix -- no change
302 The file is assumed to be already compressed. Rename the file
303 and try again.
304 file already exists; do you wish to overwrite (y or n)?
306 Respond "y" if you want the output file to be replaced; "n" if
307 not.
308 gunzip: corrupt input
310 A SIGSEGV violation was detected which usually means that the
311 input file has been corrupted.
312 xx.x% Percentage of the input saved by compression.
314 (Relevant only for -v and -l.)
315 -- not a regular file or directory: ignored
317 When the input file is not a regular file or directory, (e.g. a
318 symbolic link, socket, FIFO, device file), it is left unaltered.
319 -- has xx other links: unchanged
321 The input file has links; it is left unchanged. See ln(1) for
322 more information. Use the -f flag to force compression of multi‐
323 ply-linked files.
324
326 When writing compressed data to a tape, it is generally necessary to
327 pad the output with zeroes up to a block boundary. When the data is
328 read and the whole block is passed to gunzip for decompression, gunzip
329 detects that there is extra trailing garbage after the compressed data
330 and emits a warning by default. You have to use the --quiet option to
331 suppress the warning. This option can be set in the GZIP environment
332 variable as in:
333 for sh: GZIP="-q" tar -xfz --block-compress /dev/rst0
334 for csh: (setenv GZIP -q; tar -xfz --block-compr /dev/rst0
335 In the above example, gzip is invoked implicitly by the -z option of
337 GNU tar. Make sure that the same block size (-b option of tar) is used
338 for reading and writing compressed data on tapes. (This example
339 assumes you are using the GNU version of tar.)
340
342 The gzip format represents the input size modulo 2^32, so the --list
343 option reports incorrect uncompressed sizes and compression ratios for
344 uncompressed files 4 GB and larger. To work around this problem, you
345 can use the following command to discover a large uncompressed file's
346 true size:
347 zcat file.gz | wc -c
349 The --list option reports sizes as -1 and crc as ffffffff if the com‐
351 pressed file is on a non seekable media.
352 In some rare cases, the --best option gives worse compression than the
354 default compression level (-6). On some highly redundant files, com‐
355 press compresses better than gzip.
356
358 Copyright © 1998, 1999, 2001, 2002 Free Software Foundation, Inc.
359 Copyright © 1992, 1993 Jean-loup Gailly
360 Permission is granted to make and distribute verbatim copies of this
362 manual provided the copyright notice and this permission notice are
363 preserved on all copies.
364 Permission is granted to copy and distribute modified versions of this
366 manual under the conditions for verbatim copying, provided that the
367 entire resulting derived work is distributed under the terms of a per‐
368 mission notice identical to this one.
369 Permission is granted to copy and distribute translations of this man‐
371 ual into another language, under the above conditions for modified ver‐
372 sions, except that this permission notice may be stated in a transla‐
373 tion approved by the Foundation.
374 local GZIP(1)