uncompress

COMPRESS(1)                  General Commands Manual                 COMPRESS(1)



NAME
       compress, uncompress, zcat - compress and expand data

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

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.  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
       or zcat.

       uncompress 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 write to the standard output; no
       files are changed.

       zcat is identical to uncompress -c.  zcat uncompresses either a list of
       files on the command line or its standard input and writes the
       uncompressed data on standard output.  zcat will uncompress files that
       have the correct magic number whether they have a .Z suffix or not.

       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, 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 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)




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