Merge tag 'upstream/1.5'

[debian/gzip] / gzip.doc

GZIP(1)                                                                GZIP(1)

1mNAME0m
       gzip, gunzip, zcat - compress or expand files

1mSYNOPSIS0m
       1mgzip 22m[ 1m-acdfhlLnNrtvV19 22m] [1m-S suffix22m] [ 4mname24m 4m...24m  ]
       1mgunzip 22m[ 1m-acfhlLnNrtvV 22m] [1m-S suffix22m] [ 4mname24m 4m...24m  ]
       1mzcat 22m[ 1m-fhLV 22m] [ 4mname24m 4m...24m  ]

1mDESCRIPTION0m
       4mGzip24m  reduces  the  size  of  the  named  files using Lempel-Ziv coding
       (LZ77).  Whenever possible, each file  is  replaced  by  one  with  the
       extension 1m.gz, 22mwhile keeping the same ownership modes, access and modi-
       fication times.  (The default extension is 1m-gz 22mfor VMS,  1mz  22mfor  MSDOS,
       OS/2  FAT, Windows NT FAT and Atari.)  If no files are specified, or if
       a file name is "-", the standard input is compressed  to  the  standard
       output.  4mGzip24m will only attempt to compress regular files.  In particu-
       lar, it will ignore symbolic links.

       If the compressed file name is too long for its file system, 4mgzip24m trun-
       cates  it.   4mGzip24m  attempts to truncate only the parts of the file name
       longer than 3 characters.  (A part is delimited by dots.) If  the  name
       consists  of  small  parts  only,  the longest parts are truncated. For
       example, if file names are limited to 14 characters, gzip.msdos.exe  is
       compressed to gzi.msd.exe.gz.  Names are not truncated on systems which
       do not have a limit on file name length.

       By default, 4mgzip24m keeps the original file name and timestamp in the com-
       pressed  file.  These  are used when decompressing the file with the 1m-N0m
       option. This is useful when the compressed file name was  truncated  or
       when the time stamp was not preserved after a file transfer.

       Compressed  files  can be restored to their original form using 4mgzip24m 4m-d0m
       or 4mgunzip24m or 4mzcat.24m  If the original name saved in the  compressed  file
       is not suitable for its file system, a new name is constructed from the
       original one to make it legal.

       4mgunzip24m takes a list of files on its command line and replaces each file
       whose  name ends with .gz, -gz, .z, -z, or _z (ignoring case) and which
       begins with the correct magic number with an uncompressed file  without
       the  original extension.  4mgunzip24m also recognizes the special extensions
       1m.tgz 22mand 1m.taz 22mas shorthands for 1m.tar.gz 22mand 1m.tar.Z 22mrespectively.   When
       compressing, 4mgzip24m uses the 1m.tgz 22mextension if necessary instead of trun-
       cating a file with a 1m.tar 22mextension.

       4mgunzip24m can currently decompress files created by 4mgzip,24m  4mzip,24m  4mcompress,0m
       4mcompress24m  4m-H24m  or 4mpack.24m  The detection of the input format is automatic.
       When using the first two formats, 4mgunzip24m checks a 32 bit CRC. For 4mpack,0m
       4mgunzip24m checks the uncompressed length. The standard 4mcompress24m format was
       not designed to allow consistency checks. However 4mgunzip24m  is  sometimes
       able  to detect a bad .Z file. If you get an error when uncompressing a
       .Z file, do not assume that the .Z file is correct simply  because  the
       standard  4muncompress24m  does  not complain. This generally means that the
       standard 4muncompress24m does not check its  input,  and  happily  generates
       garbage  output.   The  SCO compress -H format (lzh compression method)
       does not include a CRC but also allows some consistency checks.

       Files created by 4mzip24m can be uncompressed by gzip only if  they  have  a
       single  member  compressed with the 'deflation' method. This feature is
       only intended to help conversion of tar.zip files to the tar.gz format.
       To  extract  a 4mzip24m file with a single member, use a command like 4mgunzip0m
       4m<foo.zip24m or 4mgunzip24m 4m-S24m 4m.zip24m 4mfoo.zip24m.  To extract zip files with  several
       members, use 4munzip24m instead of 4mgunzip.0m

       4mzcat24m  is  identical  to  4mgunzip24m  1m-c.   22m(On  some  systems,  4mzcat24m may be
       installed as 4mgzcat24m to preserve the original link  to  4mcompress.)24m   4mzcat0m
       uncompresses either a list of files on the command line or its standard
       input and writes the uncompressed data on standard output.   4mzcat24m  will
       uncompress files that have the correct magic number whether they have a
       1m.gz 22msuffix or not.

       4mGzip24m uses the Lempel-Ziv algorithm used in 4mzip24m and PKZIP.   The  amount
       of  compression  obtained depends on the size of the input and the dis-
       tribution of common substrings.  Typically, text such as source code or
       English  is  reduced  by  60-70%.  Compression is generally much better
       than that achieved by LZW (as used in  4mcompress24m),  Huffman  coding  (as
       used in 4mpack24m), or adaptive Huffman coding (4mcompact24m).

       Compression  is  always  performed,  even  if  the  compressed  file is
       slightly larger than the original. The worst case expansion  is  a  few
       bytes  for  the  gzip  file header, plus 5 bytes every 32K block, or an
       expansion ratio of 0.015% for large files. Note that the actual  number
       of  used  disk blocks almost never increases.  4mgzip24m preserves the mode,
       ownership and timestamps of files when compressing or decompressing.

       The 4mgzip24m file format is specified in P. Deutsch, GZIP file format spec-
       ification version 4.3, <ftp://ftp.isi.edu/in-notes/rfc1952.txt>, Inter-
       net RFC 1952 (May 1996).  The 4mzip24m deflation format is specified  in  P.
       Deutsch,  DEFLATE  Compressed  Data  Format  Specification version 1.3,
       <ftp://ftp.isi.edu/in-notes/rfc1951.txt>, Internet RFC 1951 (May 1996).

1mOPTIONS0m
       1m-a --ascii0m
              Ascii text mode: convert end-of-lines using  local  conventions.
              This  option  is  supported  only  on some non-Unix systems. For
              MSDOS, CR LF is converted to LF when compressing, and LF is con-
              verted to CR LF when decompressing.

       1m-c --stdout --to-stdout0m
              Write  output on standard output; keep original files unchanged.
              If there are several input  files,  the  output  consists  of  a
              sequence  of  independently compressed members. To obtain better
              compression, concatenate  all  input  files  before  compressing
              them.

       1m-d --decompress --uncompress0m
              Decompress.

       1m-f --force0m
              Force compression or decompression even if the file has multiple
              links or the corresponding file already exists, or if  the  com-
              pressed data is read from or written to a terminal. If the input
              data is not in a format recognized by 4mgzip,24m and  if  the  option
              --stdout  is  also  given, copy the input data without change to
              the standard output: let 4mzcat24m behave  as  4mcat.24m   If  1m-f  22mis  not
              given,  and  when not running in the background, 4mgzip24m prompts to
              verify whether an existing file should be overwritten.

       1m-h --help0m
              Display a help screen and quit.

       1m-l --list0m
              For each compressed file, list the following fields:

                  compressed size: size of the compressed file
                  uncompressed size: size of the uncompressed file
                  ratio: compression ratio (0.0% if unknown)
                  uncompressed_name: name of the uncompressed file

              The uncompressed size is given as -1 for files not in gzip  for-
              mat,  such  as compressed .Z files. To get the uncompressed size
              for such a file, you can use:

                  zcat file.Z | wc -c

              In combination with the --verbose option, the  following  fields
              are also displayed:

                  method: compression method
                  crc: the 32-bit CRC of the uncompressed data
                  date & time: time stamp for the uncompressed file

              The  compression  methods  currently supported are deflate, com-
              press, lzh (SCO compress -H) and pack.   The  crc  is  given  as
              ffffffff for a file not in gzip format.

              With  --name,  the  uncompressed name,  date and time  are those
              stored within the compress file if present.

              With --verbose, the size totals and compression  ratio  for  all
              files  is  also  displayed,  unless some sizes are unknown. With
              --quiet, the title and totals lines are not displayed.

       1m-L --license0m
              Display the 4mgzip24m license and quit.

       1m-n --no-name0m
              When compressing, do not save the original file  name  and  time
              stamp by default. (The original name is always saved if the name
              had to be truncated.) When decompressing,  do  not  restore  the
              original  file name if present (remove only the 4mgzip24m suffix from
              the compressed file name) and do not restore the  original  time
              stamp if present (copy it from the compressed file). This option
              is the default when decompressing.

       1m-N --name0m
              When compressing, always save the original file  name  and  time
              stamp;  this  is  the  default.  When decompressing, restore the
              original file name and time stamp if  present.  This  option  is
              useful on systems which have a limit on file name length or when
              the time stamp has been lost after a file transfer.

       1m-q --quiet0m
              Suppress all warnings.

       1m-r --recursive0m
              Travel the directory structure recursively. If any of  the  file
              names  specified  on the command line are directories, 4mgzip24m will
              descend into the directory and compress all the files  it  finds
              there (or decompress them in the case of 4mgunzip24m ).

       1m-S .suf --suffix .suf0m
              When compressing, use suffix .suf instead of .gz.  Any non-empty
              suffix can be given, but suffixes other than .z and  .gz  should
              be  avoided  to  avoid  confusion  when files are transferred to
              other systems.

              When decompressing, add .suf to the beginning  of  the  list  of
              suffixes to try, when deriving an output file name from an input
              file name.

              4mpack24m(1)4m.0m

       1m-t --test0m
              Test. Check the compressed file integrity.

       1m-v --verbose0m
              Verbose. Display the name and percentage reduction for each file
              compressed or decompressed.

       1m-V --version0m
              Version. Display the version number and compilation options then
              quit.

       1m-# --fast --best0m
              Regulate the speed of compression using the specified  digit  4m#24m,
              where  1m-1  22mor  1m--fast  22mindicates  the fastest compression method
              (less compression) and 1m-9 22mor 1m--best 22mindicates the  slowest  com-
              pression  method  (best  compression).   The default compression
              level is 1m-6 22m(that is, biased towards high compression at expense
              of speed).

1mADVANCED USAGE0m
       Multiple  compressed  files  can  be concatenated. In this case, 4mgunzip0m
       will extract all members at once. For example:

             gzip -c file1  > foo.gz
             gzip -c file2 >> foo.gz

       Then

             gunzip -c foo

       is equivalent to

             cat file1 file2

       In case of damage to one member of a .gz file, other members can  still
       be  recovered  (if the damaged member is removed). However, you can get
       better compression by compressing all members at once:

             cat file1 file2 | gzip > foo.gz

       compresses better than

             gzip -c file1 file2 > foo.gz

       If you want to recompress concatenated files to get better compression,
       do:

             gzip -cd old.gz | gzip > new.gz

       If a compressed file consists of several members, the uncompressed size
       and CRC reported by the --list option applies to the last member  only.
       If you need the uncompressed size for all members, you can use:

             gzip -cd file.gz | wc -c

       If  you  wish  to create a single archive file with multiple members so
       that members can later be extracted independently, use an archiver such
       as  tar or zip. GNU tar supports the -z option to invoke gzip transpar-
       ently. gzip is designed as a complement to tar, not as a replacement.

1mENVIRONMENT0m
       The environment variable 1mGZIP 22mcan hold a set  of  default  options  for
       4mgzip.24m   These  options  are interpreted first and can be overwritten by
       explicit command line parameters. For example:
             for sh:    GZIP="-8v --name"; export GZIP
             for csh:   setenv GZIP "-8v --name"
             for MSDOS: set GZIP=-8v --name

       On Vax/VMS, the name of the environment variable is GZIP_OPT, to  avoid
       a conflict with the symbol set for invocation of the program.

1mSEE ALSO0m
       znew(1), zcmp(1), zmore(1), zforce(1), gzexe(1), zip(1), unzip(1), com-
       press(1), pack(1), compact(1)

       The 4mgzip24m file format is specified in P. Deutsch, GZIP file format spec-
       ification version 4.3, 1m<ftp://ftp.isi.edu/in-notes/rfc1952.txt>22m, Inter-
       net RFC 1952 (May 1996).  The 4mzip24m deflation format is specified  in  P.
       Deutsch,  DEFLATE  Compressed  Data  Format  Specification version 1.3,
       1m<ftp://ftp.isi.edu/in-notes/rfc1951.txt>22m, Internet RFC 1951 (May 1996).

1mDIAGNOSTICS0m
       Exit status is normally 0; if an error occurs, exit status is 1.  If  a
       warning occurs, exit status is 2.

       Usage: gzip [-cdfhlLnNrtvV19] [-S suffix] [file ...]
              Invalid options were specified on the command line.

       4mfile24m: not in gzip format
              The file specified to 4mgunzip24m has not been compressed.

       4mfile24m: Corrupt input. Use zcat to recover some data.
              The  compressed  file has been damaged. The data up to the point
              of failure can be recovered using

                    zcat 4mfile24m > recover

       4mfile24m: compressed with 4mxx24m bits, can only handle 4myy24m bits
              4mFile24m was compressed (using LZW) by a  program  that  could  deal
              with more 4mbits24m than the decompress code on this machine.  Recom-
              press the file with gzip, which compresses better and uses  less
              memory.

       4mfile24m: already has .gz suffix -- no change
              The  file  is assumed to be already compressed.  Rename the file
              and try again.

       4mfile24m already exists; do you wish to overwrite (y or n)?
              Respond "y" if you want the output file to be replaced;  "n"  if
              not.

       gunzip: corrupt input
              A  SIGSEGV  violation  was detected which usually means that the
              input file has been corrupted.

       4mxx.x%24m Percentage of the input saved by compression.
              (Relevant only for 1m-v 22mand 1m-l22m.)

       -- not a regular file or directory: ignored
              When the input file is not a regular file or directory, (e.g.  a
              symbolic link, socket, FIFO, device file), it is left unaltered.

       -- has 4mxx24m other links: unchanged
              The  input  file has links; it is left unchanged.  See 4mln24m(1) for
              more information. Use the 1m-f 22mflag to force compression of multi-
              ply-linked files.

1mCAVEATS0m
       When  writing  compressed  data to a tape, it is generally necessary to
       pad the output with zeroes up to a block boundary.  When  the  data  is
       read  and the whole block is passed to 4mgunzip24m for decompression, 4mgunzip0m
       detects that there is extra trailing garbage after the compressed  data
       and  emits  a warning by default. You have to use the --quiet option to
       suppress the warning. This option can be set in  the  1mGZIP  22menvironment
       variable as in:
         for sh:  GZIP="-q"  tar -xfz --block-compress /dev/rst0
         for csh: (setenv GZIP -q; tar -xfz --block-compr /dev/rst0

       In  the  above  example, gzip is invoked implicitly by the -z option of
       GNU tar. Make sure that the same block size (-b option of tar) is  used
       for  reading  and  writing  compressed  data  on  tapes.  (This example
       assumes you are using the GNU version of tar.)

1mBUGS0m
       The gzip format represents the input size modulo 2^32,  so  the  --list
       option  reports incorrect uncompressed sizes and compression ratios for
       uncompressed files 4 GB and larger.  To work around this  problem,  you
       can  use  the following command to discover a large uncompressed file's
       true size:

             zcat file.gz | wc -c

       The --list option reports sizes as -1 and crc as ffffffff if  the  com-
       pressed file is on a non seekable media.

       In  some rare cases, the --best option gives worse compression than the
       default compression level (-6). On some highly  redundant  files,  4mcom-0m
       4mpress24m compresses better than 4mgzip.0m

1mCOPYRIGHT NOTICE0m
       Copyright (C) 1998, 1999, 2001, 2002 Free Software Foundation, Inc.
       Copyright (C) 1992, 1993 Jean-loup Gailly

       Permission  is  granted  to make and distribute verbatim copies of this
       manual provided the copyright notice and  this  permission  notice  are
       preserved on all copies.

       Permission  is granted to copy and distribute modified versions of this
       manual under the conditions for verbatim  copying,  provided  that  the
       entire  resulting derived work is distributed under the terms of a per-
       mission notice identical to this one.

       Permission is granted to copy and distribute translations of this  man-
       ual into another language, under the above conditions for modified ver-
       sions, except that this permission notice may be stated in  a  transla-
       tion approved by the Foundation.

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