compress.man

和Unix的compress/uncompress兼容的压缩/解压算法16位程序

COMPRESS 1 local

NAME compress, uncompress, zcat - compress and expand data

SYNOPSIS

     compress [-cCdDf?hkKvV][-b maxbits][-Iinpath][-Ooutpath][filenames...]
     uncompress [-fCcvVkK?h][-Iinpath][-Ooutpath][filenames...]
     zcat [-CvV?h][-Iinpath][-Ooutpath][filenames...]

     Argument Processing..case is significant:
     MUST use '-' for switch character, all flags are optional.

     [options]
     -V => print Version
     -d => do_decomp default = off
     -v => verbose
     -f => force overwrite of output file default = off
     -n => no header: useful to uncompress old files
     -c => cat all output to stdout default = off
     -C => generate output compatible with compress 2.0
     -k => %s input file, default = keep
     -K => %s output file on error, default = kill
     -b maxbits  => default = 16  bits
     -I pathname => infile path  = none
     -O pathname => outfile path = none
     -? -h => help, print full usage message

DESCRIPTION

This manual refers to compress v4.10 don release. The compression and
decompression routines are modified from the current version 4.0 joe release.
However, the compressed output is cross compatible.

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.Compressed files can be restored to their original form using
uncompress or zcat.

The -f option will force compression of name. This is useful for compressing
an entire directory, even if some of the files do not actually shrink.
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.

The -c option makes compress/uncompress write to the standard output;
no files are changed.  The nondestructive behavior of zcat is identical to
that of uncompress -c.

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.

If the -V option is specified, the current version and compile options
are printed on stderr.

CAVEATS


FILES


RETURN VALUE

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.

SEE ALSO
        pack(1), compact(1)

DIAGNOSTICS
        Usage: compress [\-dfvcV] [\-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 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: unchanged
                When the input file is not a regular file,
                (e.g. a directory), it is left unaltered.
        -- has other links: unchanged
                The input file has links; it is left unchanged.  See
                ln "(1)" for more information.
        -- file unchanged
                No savings is achieved by compression.  The input remains
                virgin.

BUGS

Although compressed files are compatible between machines with large memory,
-b 12 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.)

MISCELLANEOUS


NOTES