NAME

       compress, uncompress.real - compress and expand data

SYNOPSIS

       compress [ -f ] [ -k ] [ -v ] [ -c ] [ -V ] [ -r ] [ -b bits ] [ -- ] [ name ...  ]
       uncompress.real [ -f ] [ -k ] [ -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 -k option makes compress/uncompress keep the input files instead of automatically removing them.

       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.  When compressing, any files already compressed will be ignored, and when decompressing, any
       files already decompressed will be ignored.

       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.

       -- may be used to halt option parsing and force all remaining arguments to be treated as paths.

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.real 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.real: 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 (64 KiB or less, as exhibited by the DEC PDP
       series, the Intel 80286, etc.)

SEE ALSO

       pack(1), compact(1)

ncompress                                          2024-05-21                                        COMPRESS(1)