COMPRESS(1)                                           General Commands Manual                                           COMPRESS(1)

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 speci‐
       fied, 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 contin‐
       ues to use the existing code dictionary.  However, if the compression ratio decreases, compress discards the table  of  sub‐
       strings 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  com‐
       pressed 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 bet‐
       ter  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  compres‐
               sion 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 archi‐
       tectures 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)

Usage: compress [-dfhvcVr] [-b maxbits] [--] [file ...]
       -d   If given, decompression is done instead.
       -c   Write output on stdout, don't remove original.
       -b   Parameter limits the max number of bits/code.
       -f   Forces output file to be generated, even if one already.
            exists, and even if no space is saved by compressing.
            If -f is not used, the user will be prompted if stdin is.
            a tty, otherwise, the output file will not be overwritten.
       -h   This help output.
       -v   Write compression statistics.
       -V   Output version and compile options.
       -r   Recursive. If a filename is a directory, descend
            into it and compress everything in it.