compress, uncompress.real - compress and expand data


   compress [ -f ] [ -v ] [ -c ] [ -V ] [ -r ] [ -b bits ] [ name ...  ]
   uncompress.real [ -f ] [ -v ] [ -c ] [ -V ] [ name ...  ]


   Note that the program that would normally be installed as uncompress is
   installed for Debian as uncompress.real.  This has been done  to  avoid
   conflicting with the more-commonly-used program with the same name that
   is part of the gzip package.

   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.   Compress  will  only  attempt  to
   compress  regular files.  In particular, it will ignore symbolic links.
   If a file has multiple hard links, compress will refuse to compress  it
   unless the -f flag is given.

   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.

   Compressed  files  can  be  restored  to  their  original  form   using

   uncompress.real  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  .Z.   The
   uncompressed file will have the mode, ownership and timestamps  of  the
   compressed file.

   The  -c  option  makes  compress/uncompress.real  write to the standard
   output; no files are changed.

   If the -r flag is specified, compress will operate recursively. If  any
   of  the  file  names  specified  on  the  command line are directories,
   compress will descend into the directory and compress all the files  it
   finds there.

   The  -V  flag  tells  each  of  these programs to print its version and
   patchlevel,  along  with  any  preprocessor  flags   specified   during
   compilation, on stderr before doing any compression or uncompression.

   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.real,  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.


   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.

   Usage: compress [-dfvcVr] [-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
   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 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
   -- 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.
   -- file unchanged
           No savings is  achieved  by  compression.   The  input  remains


   Although  compressed  files  are compatible between machines with large
   memory, -b12 should be used for file transfer to architectures  with  a
   small  process  data  space  (64KB or less, as exhibited by the DEC PDP
   series, the Intel 80286, etc.)

   Invoking compress with a -r flag will occasionally cause it to  produce
   spurious error warnings of the form

    "<filename>.Z already has .Z suffix - ignored"

   These    warnings    can    be    ignored.    See   the   comments   in
   compress42.c:compdir() in the source distribution for an explanation.


   pack(1), compact(1)

                                 local                         COMPRESS(1)


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