NAME

       compress, uncompress.real - compress and expand data

SYNOPSIS

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

DESCRIPTION

       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.

       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.

DIAGNOSTICS

       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 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 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.
       -- file unchanged
               No savings is achieved by compression.  The input remains virgin.

BUGS

       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.

SEE ALSO

       pack(1), compact(1)

                                              local                                   COMPRESS(1)