Unix/Linux Go Back    


OpenDarwin 7.2.1 - man page for zcat (opendarwin section 1)

Linux & Unix Commands - Search Man Pages
Man Page or Keyword Search:   man
Select Man Page Set:       apropos Keyword Search (sections above)


GZIP(1) 										  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 possi-
       ble, each file is replaced by one with the extension .gz, while keeping the same ownership
       modes, access and modification times.  (The default extension 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 output.  Gzip will only attempt to com-
       press 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  trun-
       cated.  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 systems 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,  gunzip
       checks  a  32  bit CRC. For pack, gunzip checks the uncompressed length. The standard com-
       press 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  com-
       plain.  This  generally	means  that the standard uncompress 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  zip can be uncompressed by gzip only if they have a single member com-
       pressed 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  pre-
       serve  the  original  link  to compress.)  zcat uncompresses either a list of files on the
       command line or its standard input and writes the uncompressed data  on	standard  output.
       zcat will uncompress files that have the correct magic number whether they have a .gz suf-
       fix 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.  Typ-
       ically, text such as source code or English is reduced by 60-70%.  Compression  is  gener-
       ally  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 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.  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 conventions. This option is  sup-
	      ported only on some non-Unix systems. For MSDOS, CR LF is converted to LF when com-
	      pressing, and LF is converted to CR LF when decompressing.

       -c --stdout --to-stdout
	      Write output on standard output; keep original files unchanged.  If there are  sev-
	      eral  input  files,  the	output consists of a sequence of independently compressed
	      members. To obtain better compression, concatenate all input files before compress-
	      ing them.

       -d --decompress --uncompress
	      Decompress.

       -f --force
	      Force  compression or decompression even if the file has multiple links or the cor-
	      responding file already exists, or if the compressed data is read from  or  written
	      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  standard
	      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  com-
	      pressed .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 com-
	      press -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 dis-
	      played, 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 present (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 decom-
	      pressing.

       -N --name
	      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.

       -q --quiet
	      Suppress all warnings.

       -r --recursive
	      Travel  the  directory structure recursively. If any of the file names specified on
	      the command line are directories, gzip will descend into the directory and compress
	      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)

	      Previous versions of gzip used the .z suffix. This was changed to avoid a  conflict
	      with pack(1).

       -t --test
	      Test. Check the compressed file integrity.

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

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

       -# --fast --best
	      Regulate the speed of compression using the specified digit #, where -1  or  --fast
	      indicates  the fastest compression method (less compression) and -9 or --best indi-
	      cates the slowest compression method (best compression).	The  default  compression
	      level is -6 (that is, biased towards high compression at expense of speed).

ADVANCED USAGE
       Multiple  compressed files can be concatenated. In this case, gunzip will extract all mem-
       bers 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 transparently. gzip is designed as a complement to tar, not as	a
       replacement.

ENVIRONMENT
       The  environment  variable GZIP can hold a set of default options for gzip.  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.

SEE ALSO
       znew(1), zcmp(1), zmore(1), zforce(1), gzexe(1), zip(1), unzip(1),  compress(1),  pack(1),
       compact(1)

DIAGNOSTICS
       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.
       file: not in gzip format
	       The file specified to gunzip has not been compressed.
       file: 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 file > recover
       file: compressed with xx bits, can only handle yy bits
	       File  was  compressed (using LZW) by a program that could deal with more bits than
	       the decompress code on this machine.  Recompress the file with  gzip,  which  com-
	       presses better and uses less memory.
       file: already has .gz suffix -- no change
	       The file is assumed to be already compressed.  Rename the file and try again.
       file 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.
       xx.x%
	       Percentage of the input saved by compression.  (Relevant only for -v and -l.)
       -- 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 xx other links: unchanged
	       The  input  file has links; it is left unchanged.  See ln(1) for more information.
	       Use the -f flag to force compression of multiply-linked files.

CAVEATS
       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 gun-
       zip for decompression, gunzip detects that there is extra trailing garbage after the  com-
       pressed	data  and  emits a warning by default. You have to use the --quiet option to sup-
       press the warning. This option can be set in the GZIP environment 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.)

BUGS
       The --list option reports incorrect sizes if they exceed 2 gigabytes.  The  --list  option
       reports sizes as -1 and crc as ffffffff if the compressed 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, compress compresses better than gzip.

					      local					  GZIP(1)
Unix & Linux Commands & Man Pages : ©2000 - 2018 Unix and Linux Forums


All times are GMT -4. The time now is 11:17 PM.