BSD 2.11 - man page for compress (bsd section 1)

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COMPRESS(1)									      COMPRESS(1)

       compress, uncompress, zcat - compress and expand data

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

       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  own-
       ership  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, even if it does not actually shrink or the
       corresponding name.Z file already  exists.   Except  when  run  in  the	background  under
       /bin/sh,  if  -f  is  not given the user is prompted as to whether an existing name.Z file
       should be overwritten.

       The -c (``cat'') 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 Per-
       formance 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

       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.

       The -v option causes the printing of the percentage reduction of each file.

       If an error occurs, exit status is 1, else if the last file was not compressed because  it
       became larger, the status is 2; else the status is 0.

       Usage: compress [-fvc] [-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 is cor-
       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  unal-
       -- has xx 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.

       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.)

       compress should be more flexible about the existence of the `.Z' suffix.

4.3 Berkeley Distribution		   May 11, 1986 			      COMPRESS(1)
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