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