gzip.doc

GNU 系统开发的基于Iz77语法的压缩 学习lz77压缩算法

GZIP(1)                  USER COMMANDS                    GZIP(1)

NAME
     gzip, gunzip, zcat - compress or expand files

SYNOPSIS
     gzip [ -cdfhLrtvV19 ] [ name ... ]
     gunzip [ -cfhLrtvV ] [ name ... ]
     zcat [ -hLV ] [ 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 .z, while keeping the same  ownership
     modes,  access and modification times.  (The extension is -z
     for VMS, z for MSDOS, OS/2  and  Atari.)  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 symbolic links.

     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 or .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.z or .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. (On some systems,  zcat  may
     be  installed  as  gzcat  to  preserve  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
     .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.

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 or decompression even if the file has
          multiple   links  or  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.

ADVANCED USAGE
     Multiple compressed files can be concatenated. In this case,
     gunzip will extract all members at once. For example:

           gzip -c file1  > foo.z
           gzip -c file2 >> foo.z Then
           gunzip -c foo

     is equivalent to

           cat file1 file2

     In case of damage to one member of a .z 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.z

     compresses better than

           gzip -c file1 file2 > foo.z

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

           zcat old.z | gzip > new.z

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="-8 -v"; export GZIP
           for csh:   setenv GZIP "-8 -v"
           for MSDOS: set GZIP=-8 -v

     On  Vax/VMS,  the  name  of  the  environment  variable   is
     GZIP_OPT,  to avoid a conflict with the symbol set for invo-
     cation 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 [-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
                     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 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,  (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
     The .z extension is already used by pack(1).  You  can  link
     gzip  to  pcat to get transparent decompression for programs
     expecting .z files to be in pack format.