gzip.doc

gzip源码

User Commands						  GZIP(1)

NAME
     gzip, gunzip, zcat	- compress or expand files

SYNOPSIS
     gzip [ -acdfhlLnNrtvV19 ] [-S suffix] [ name ...  ]
     gunzip [ -acfhlLnNrtvV ] [-S suffix] [ name ...  ]
     zcat [ -fhLV ] [ name ...	]

DESCRIPTION
     Gzip  reduces  the	 size of the named files using Lempel-Ziv
     coding (LZ77).  Whenever possible,	each file is replaced  by
     one with the extension .gz, while keeping the same	ownership
     modes, access and modification times.  (The  default  exten-
     sion  is  -gz for VMS, z for 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 out-
     put.  Gzip	will only attempt to compress regular files.   In
     particular, it will ignore	symbolic links.

     If	the compressed file name is too	long for its file system,
     gzip truncates it.	 Gzip 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  com-
     pressed  to gzi.msd.exe.gz.  Names	are not	truncated on sys-
     tems which	do not have a limit on file name length.

     By	default, gzip keeps the	original file name and	timestamp
     in	 the  compressed  file.	These are used when decompressing
     the file with the -N option. This is useful  when	the  com-
     pressed  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 gzip	-d or gunzip or	zcat.  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.

     gunzip takes a  list  of  files  on  its  command	line  and
     replaces each file	whose name ends	with .gz, -gz, .z, -z, _z
     or	.Z and which begins with the correct magic number with an
     uncompressed  file	 without  the original extension.  gunzip
     also recognizes the special  extensions  .tgz  and	 .taz  as
     shorthands	 for  .tar.gz and .tar.Z respectively.	When com-
     pressing, gzip uses the .tgz extension if necessary  instead
     of	truncating a file with a .tar extension.

     gunzip  can currently decompress files created by gzip, zip,
     compress, compress	-H or pack.  The detection of  the  input
     format  is	 automatic.   When  using  the first two formats,

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User Commands						  GZIP(1)

     gunzip checks a 32	bit CRC.  For  pack,  gunzip  checks  the
     uncompressed  length.  The	 standard compress format was not
     designed to allow	consistency  checks.  However  gunzip  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 uncompress does not
     complain. This generally means that the standard  uncompress
     does not check its	input, and happily generates garbage out-
     put.  The SCO compress -H format  (lzh  compression  method)
     does  not	include	 a  CRC	 but also allows some consistency
     checks.

     Files created by zip 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	zip files
     with several members, use unzip instead of	gunzip.

     zcat is identical to gunzip -c.  (On some systems,	zcat  may
     be	 installed as gzcat to preserve	the original link to com-
     press.)  zcat uncompresses	either a list  of  files  on  the
     command  line  or	its  standard input and	writes the uncom-
     pressed data on standard output.  zcat will uncompress files
     that  have	 the correct magic number whether they have a .gz
     suffix or not.

     Gzip uses the Lempel-Ziv algorithm	used in	 zip  and  PKZIP.
     The  amount  of  compression obtained depends on the size of
     the input and the distribution of common substrings.   Typi-
     cally,  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 compress), Huffman coding (as
     used in pack), or adaptive	Huffman	coding (compact).

     Compression is always performed, even if the compressed file
     is	 slightly larger than the original. The	worst case expan-
     sion 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.  gzip preserves the mode,	ownership
     and timestamps of files when compressing or decompressing.

OPTIONS
     -a	--ascii
	  Ascii	text mode: convert end-of-lines	using local  con-
	  ventions.  This  option  is supported	only on	some non-
	  Unix systems.	For MSDOS, CR LF is converted to LF  when
	  compressing,	and  LF	is converted to	CR LF when decom-
	  pressing.

     -c	--stdout --to-stdout

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User Commands						  GZIP(1)

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

     -d	--decompress --uncompress
	  Decompress.

     -f	--force
	  Force	compression or decompression even if the file has
	  multiple   links  or	the  corresponding  file  already
	  exists, or if	the compressed data is read from or writ-
	  ten to a terminal. If	the input data is not in a format
	  recognized by	gzip, and if the option	--stdout is  also
	  given,  copy the input data without change to	the stan-
	  dard ouput: let zcat behave  as  cat.	  If  -f  is  not
	  given,  and  when  not  running in the background, gzip
	  prompts to verify whether an existing	 file  should  be
	  overwritten.

     -h	--help
	  Display a help screen	and quit.

     -l	--list
	  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 format, 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, compress, 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.

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User Commands						  GZIP(1)

	  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.

     -L	--license
	  Display the gzip license and quit.

     -n	--no-name
	  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 decom-
	  pressing, do not restore the original	file name if pre-
	  sent	(remove	 only the gzip 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.

     -N	--name
	  When compressing, always save	the  original  file  name
	  and  time  stamp; this is the	default. When decompress-
	  ing, 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.

     -q	--quiet
	  Suppress all warnings.

     -r	--recursive
	  Travel  the  directory structure recursively.	If any of
	  the file names specified on the command line are direc-
	  tories,  gzip	 will descend into the directory and com-
	  press	all the	files it finds there (or decompress  them
	  in the case of gunzip	).

     -S	.suf --suffix .suf
	  Use  suffix  .suf  instead  of  .gz.	Any suffix can be
	  given, but suffixes other than .z  and  .gz  should  be
	  avoided  to  avoid confusion when files are transferred
	  to other systems.  A null suffix forces gunzip to   try
	  decompression	 on all	given files regardless of suffix,
	  as in:

	      gunzip -S	"" *	   (*.*	for MSDOS)