back2roots/padua/padua.7z/padua/uucp/duucp-1.17/AU-117b4-man.lha/man/Compress

Text File | 1993-12-29 | 4.9 KB | 115 lines

     NAME
      compress, uncompress, zcat - compress and expand data

     SYYNOPSIS
      compress [ -f ] [ -v ] [ -c ] [ -V ] [ -b <bits> ] [ name ... ]
      uncompress [ -f ] [ -v ] [ -c ] [ -V ] [ name ... ]
      zcat [ -V ] [ name ... ]

      NOTE:  These three executables are exactly the same, simply named
      differently causing different default action.

     DESCRIPTION
      Compress reduces the size of the named files using adaptive
      Lempel-Ziv coding.  Whenever possible, each file is replaced by
      one with the 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.
      Compressed files can be restored to their original form using
      uncompress or zcat.

      As of 1.16, AmigaUUCP's compress will decompress up to 16 bit
      compression formats.    Default compression is 14 bits but may be
      set to 16 bits with the '-b 16' option.  The minimum supported
      compression bits is 12.

      The -f option will force compression of name. This is useful for
      compressing an entire directory, even if some of the files do not
      actually shrink.  If -f is not given and compress is run in the
      foreground, the user is prompted as to whether an existing file
      should be overwritten.

      The -c option makes compress/uncompress write to the standard
      output; no files are changed.  The nondestructive behavior of
      zcat is identical to that of uncompress -c.

      Compress uses the modified Lempel-Ziv algorithm popularized in "A
      Technique for High Performance Data Compression", Terry A. Welch,
      IEEE Computer, vol. 17, no. 6 (June 1984), pp. 8-19.    Common
      substrings in the file are first replaced by 9-bit codes 257 and
      up.  When code 512 is reached, the algorithm switches to 10-bit
      codes and continues to use more bits until the limit specified by
      the -b flag is reached (default 16).    Bits must be between 9 and
      16.  The default can be changed in the source to allow compress
      to be run on a smaller machine.

      After the bits limit is attained, compress periodically checks
      the compression ratio.  If it is increasing, compress continues
      to use the existing code dictionary.    However, if the compression
      ratio decreases, compress discards the table of substrings and
      rebuilds it from scratch.  This allows the algorithm to adapt to
      the next "block" of the file.

      Note that the -b flag is omitted for uncompress, since the bits
      parameter specified during compression is encoded within the
      output, along with a magic number to ensure that neither
      decompression of random data nor recompression of compressed data
      is attempted.

      The amount of compression obtained depends on the size of the
      input, the number of bits per code, and the distribution of
      common substrings.  Typically, text such as source code or
      English is reduced by 50-60%.  Compression is generally much
      better than that achieved by Huffman coding (as used in pack), or
      adaptive Huffman coding (compact), and takes less time to
      compute.

      Under the -v option, a message is printed yielding the percentage
      of reduction for each file compressed.

      If the -V option is specified, the current version and compile
      options are printed on stderr.

      Exit status is normally 0; if the last file is larger after
      (attempted) compression, the status is 2; if an error occurs,
      exit status is 1.

     DIAGNOSTICS
      Usage: compress [-dfvcV] [-b maxbits] [file ...]
          Invalid options were specified on the command line.
      Missing maxbits
          Maxbits must follow -b.
      file: not in compressed format
          The file specified to uncompress has not been
          compressed.
      file: compressed with xx bits, can only handle yy bits
          File was compressed by a program that could deal
          with more bits than the compress code on this
          machine.  Recompress the file with smaller bits.
      file: already has .Z suffix -- no change
          The file is assumed to be already compressed.
          Rename the file and try again.
      file: filename too long to tack on .Z
          The file cannot be compressed because its name is
          longer than 12 characters.  Rename and try again.
          This message does not occur on BSD systems.
      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.
      uncompress: corrupt input
          A SIGSEGV violation was detected which usually means
          that the input file has been corrupted.
      Compression: xx.xx%
          Percentage of the input saved by compression.
          (Relevant only for -v.)
      -- not a regular file: unchanged
          When the input file is not a regular file, (e.g. a
          directory), it is left unaltered.
      -- has xx other links: unchanged
          The input file has links; it is left unchanged.  See
          ln(1) for more information.
      -- file unchanged
          No savings is achieved by compression.  The input
          remains virgin.