gzip.hlp

GNU 系统开发的基于Iz77语法的压缩 学习lz77压缩算法

1 GZIP
NAME
     gzip, gunzip, zcat - compress or expand files

SYNOPSIS
     gzip [ -cdfhLrtvV19 ] [ name ... ]
     gunzip [ -cfhLrtvV ] [ name ... ]
     zcat [ -hLV ] [ name ... ]

2 DESCRIPTION
     Gzip reduces the size of the named  files  using  Lempel-Ziv
     coding  (LZ77).  Whenever possible, each file is replaced by
     one with an extension *-z, while keeping the same  ownership
     modes,  access  and  modification  times.   If  no files are
     specified, the standard input is compressed to the  standard
     output.  If the new file name is too long, gzip truncates it
     and keeps the original file name  in  the  compressed  file.
     Gzip  will  only attempt to compress regular files.  In par-
     ticular, it will ignore devices.

     Compressed files can be  restored  to  their  original  form
     using gzip -d or gunzip or zcat.

     gunzip takes a  list  of  files  on  its  command  line  and
     replaces  each  file whose name ends with -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 shorthand for .tar-z.

     gunzip can currently decompress files created by gzip,  zip,
     compress  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 compress format was not designed to  allow  con-
     sistency  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 gen-
     erally means that the standard uncompress does not check its
     input, and happily generates garbage output.

     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.z format. To extract zip files with
     several members, use unzip instead of gunzip.

     zcat is identical to gunzip -c. 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 -z 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.  gzip preserves the mode, ownership and timestamps of
     files when compressing or decompressing.

2 OPTIONS
     -c --stdout
          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
          Decompress.

     -f --force
          Force compression even if the corresponding file already
          exists. 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.

     -L --license
          Display the gzip license.

     -q --quiet
          Suppress all warnings.

     -r --recurse
          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
          compress  all  the  files it finds there (or decompress
          them in the case of gunzip ).

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

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

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

     -# --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
          indicates   the  slowest  compression  method  (optimal
          compression).  The default compression level is -5.

2 ENVIRONMENT
     The environment variable GZIP_OPT 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:
           GZIP_OPT == "-8 -v"

2 SEE ALSO
     compress, zip, unzip

2 DIAGNOSTICS

     Usage: gzip [-cdfhLrtvV19] [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
		     define /user sys$output file.recover
                     zcat file
     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 compresses better and uses less memory.
     file: already has z suffix -- no change
             The  file  is  assumed  to  be  already  compressed.
             Rename the file and try again or use zcat.
     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.)
     -- not a regular file or directory: ignored
             When the input file is not a regular file or  direc-
             tory, it is left unaltered.

2 CAVEATS
     On VMS:
     - upper case options need quotes: gzip "-V".
     - restoration of timestamps and version numbers is not supported
     - If a compressed file already exists, gzip -f overwrites it, it
       does not create a new version.
     - wildcards in file names are not supported.
     - multi-part gzip files are not supported.
     - gunzip does not preserver the input file format. You can use a
       separate utility to restore the original format.
     - gunzip and zcat can be used only if you have created the
       links to gzip as documented in makegzip.com.  Otherwise
       you must use explicit parameters ("gzip -c" or "gzip -dc").

2 BUGS
     On VMS, files in VFC record format are not correctly handled by
     the C runtime library (the linefeed character is suppressed).