path: root/gzip.doc

GZIP(1)			    General Commands Manual		       GZIP(1)

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

1mSYNOPSIS0m
       1mgzip 22m[ 1m-acdfhklLnNrtvV19 22m] [1m-S suffix22m] [ 4mname24m 4m...24m  ]
       1mgunzip 22m[ 1m-acfhklLnNrtvV 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.gz22m, while 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 4mzcat24m.  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 4mgzip24m,	 4mzip24m,  4mcompress24m,
       4mcompress24m  4m-H24m  or 4mpack24m.  The detection of the input format is automatic.
       When using the first two formats, 4mgunzip24m checks a 32 bit CRC. For  4mpack0m
       and 4mgunzip24m checks the uncompressed length. The standard 4mcompress24m format
       was not designed to allow consistency checks. However 4mgunzip24m  is  some-
       times  able  to	detect	a bad .Z file. If you get an error when uncom-
       pressing 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 gen-
       erates  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 4mgunzip24m.

       4mzcat24m  is  identical	 to  4mgunzip24m  1m-c22m.   (On  some	systems,  4mzcat24m may be
       installed as 4mgzcat24m to preserve the original link  to  4mcompress24m.)   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.

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 4mgzip24m, and  if	the  option
	      --stdout	is  also  given, copy the input data without change to
	      the standard output: let 4mzcat24m behave	 as  4mcat24m.	 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-k --keep0m
	      Keep (don't delete) input files during compression or decompres-
	      sion.

       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.

       1m--synchronous0m
	      Use  synchronous	output.	 With this option, 4mgzip24m is less likely
	      to lose data during a system crash, but it can  be  considerably
	      slower.

       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).

       1m--rsyncable0m
	      When you synchronize a compressed file  between  two  computers,
	      this  option  allows  rsync  to  transfer	 only  files that were
	      changed in the archive instead of the entire archive.  Normally,
	      after  a change is made to any file in the archive, the compres-
	      sion algorithm can generate a new version of  the	 archive  that
	      does  not	 match	the  previous  version of the archive. In this
	      case, rsync transfers the entire new version of the  archive  to
	      the  remote computer.  With this option, rsync can transfer only
	      the changed files as well as a small amount of metadata that  is
	      required	to  update  the archive structure in the area that was
	      changed.

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  obsolescent	 environment  variable	1mGZIP 22mcan hold a set of default
       options for 4mgzip24m.  These options are interpreted first and can be over-
       written	by  explicit command line parameters.  As this can cause prob-
       lems when using scripts, this feature is	 supported  only  for  options
       that  are  reasonably likely to not cause too much harm, and 4mgzip24m warns
       if it is used.  This feature will be removed in	a  future  release  of
       4mgzip24m.

       You can use an alias or script instead.	For example, if 4mgzip24m is in the
       directory 1m/usr/bin 22myou can prepend 1m$HOME/bin 22mto your 1mPATH 22mand create an
       executable script 1m$HOME/bin/gzip 22mcontaining the following:

	     #! /bin/sh
	     export PATH=/usr/bin
	     exec gzip -9 "$@"

       On  VMS,	 the name of the obsolescent 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)

       The 4mgzip24m file format is specified in P. Deutsch, GZIP file format spec-
       ification version 4.3, 1m<http://www.ietf.org/rfc/rfc1952.txt>22m,  Internet
       RFC  1952  (May	1996).	 The  4mzip24m	deflation format is specified in P.
       Deutsch, DEFLATE Compressed  Data  Format  Specification	 version  1.3,
       1m<http://www.ietf.org/rfc/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 [-cdfhklLnNrtvV19] [-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 -- unchanged
	      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 can use the --quiet option to sup-
       press the warning.

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 4mgzip24m.

1mCOPYRIGHT NOTICE0m
       Copyright (C) 1998-1999, 2001-2002, 2012, 2015-2016 Free Software Foun-
       dation, 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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