1GZIP(1)                     General Commands Manual                    GZIP(1)
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NAME

6       gzip, gunzip, zcat - compress or expand files
7

SYNOPSIS

9       gzip [ -acdfhlLnNrtvV19 ] [-S suffix] [ name ...  ]
10       gunzip [ -acfhlLnNrtvV ] [-S suffix] [ name ...  ]
11       zcat [ -fhLV ] [ name ...  ]
12

DESCRIPTION

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
23       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
31       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
36       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
41       gunzip takes a list of files on its command line and replaces each file
42       whose  name  ends with .gz, -gz, .z, -z, _z or .Z and which begins with
43       the correct magic number with an uncompressed file without the original
44       extension.  gunzip also recognizes the special extensions .tgz and .taz
45       as shorthands for .tar.gz and .tar.Z respectively.   When  compressing,
46       gzip  uses the .tgz extension if necessary instead of truncating a file
47       with a .tar extension.
48
49       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
61       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
68       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
75       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
82       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
89       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
95

OPTIONS

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
103       -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
110       -d --decompress --uncompress
111              Decompress.
112
113       -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
123       -h --help
124              Display a help screen and quit.
125
126       -l --list
127              For each compressed file, list the following fields:
128
129                  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
134              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
138                  zcat file.Z | wc -c
139
140              In combination with the --verbose option, the  following  fields
141              are also displayed:
142
143                  method: compression method
144                  crc: the 32-bit CRC of the uncompressed data
145                  date & time: time stamp for the uncompressed file
146
147              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
151              With  --name,  the  uncompressed name,  date and time  are those
152              stored within the compress file if present.
153
154              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
158       -L --license
159              Display the gzip license and quit.
160
161       -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
170       -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
177       -q --quiet
178              Suppress all warnings.
179
180       -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
186       -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
192              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
196              pack(1).
197
198       -t --test
199              Test. Check the compressed file integrity.
200
201       -v --verbose
202              Verbose. Display the name and percentage reduction for each file
203              compressed or decompressed.
204
205       -V --version
206              Version. Display the version number and compilation options then
207              quit.
208
209       -# --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

ADVANCED USAGE

218       Multiple  compressed  files  can  be concatenated. In this case, gunzip
219       will extract all members at once. For example:
220
221             gzip -c file1  > foo.gz
222             gzip -c file2 >> foo.gz
223
224       Then
225
226             gunzip -c foo
227
228       is equivalent to
229
230             cat file1 file2
231
232       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
236             cat file1 file2 | gzip > foo.gz
237
238       compresses better than
239
240             gzip -c file1 file2 > foo.gz
241
242       If you want to recompress concatenated files to get better compression,
243       do:
244
245             gzip -cd old.gz | gzip > new.gz
246
247       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
251             gzip -cd file.gz | wc -c
252
253       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

ENVIRONMENT

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
266       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

SEE ALSO

270       znew(1), zcmp(1), zmore(1), zforce(1), gzexe(1), zip(1), unzip(1), com‐
271       press(1), pack(1), compact(1)
272
273       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

DIAGNOSTICS

280       Exit status is normally 0; if an error occurs, exit status is 1.  If  a
281       warning occurs, exit status is 2.
282
283       Usage: gzip [-cdfhlLnNrtvV19] [-S suffix] [file ...]
284              Invalid options were specified on the command line.
285
286       file: not in gzip format
287              The file specified to gunzip has not been compressed.
288
289       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
293                    zcat file > recover
294
295       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
301       file: already has .gz suffix -- no change
302              The  file  is assumed to be already compressed.  Rename the file
303              and try again.
304
305       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
309       gunzip: corrupt input
310              A  SIGSEGV  violation  was detected which usually means that the
311              input file has been corrupted.
312
313       xx.x% Percentage of the input saved by compression.
314              (Relevant only for -v and -l.)
315
316       -- 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
320       -- 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

CAVEATS

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
336       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

BUGS

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
348             zcat file.gz | wc -c
349
350       The --list option reports sizes as -1 and crc as ffffffff if  the  com‐
351       pressed file is on a non seekable media.
352
353       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
361       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
365       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
370       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
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377                                     local                             GZIP(1)
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