compress man page on Ultrix

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compress(1)							   compress(1)

       compress, uncompress, zcat - compress and expand data

       compress [ options ] [ name ...	]
       uncompress [ options ] [ name ...  ]
       zcat [ name ...	]

       The  command 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 modi‐
       fication times.	If no files are specified, the standard input is  com‐
       pressed	to  the	 standard output.  Compressed files can be restored to
       their original form using or

       The command uses the modified Lempel-Ziv algorithm.  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).  The bits must be between 9 and 16.  The default  can  be
       changed in the source to allow to be run on a smaller machine.

       After  the  bits limit is attained, periodically checks the compression
       ratio.  If the ratio is increasing, continues to use the existing  code
       dictionary.   However, if the compression ratio decreases, discards the
       table of substrings and rebuilds it  from  scratch.   This  allows  the
       algorithm to adapt to the next block of the file.

       How  much each file is compressed 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 cod‐
       ing or adaptive Huffman coding, and takes less time to compute.

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

       -b   The -b flag is omitted for since the bits parameter specified dur‐
	    ing compression is encoded within the output along with  a	number
	    that  ensures that neither decompression of random data nor recom‐
	    pression of compressed data is attempted.

       -c   The cat option.  Makes compress/uncompress write to	 the  standard
	    output without changing any files.	Neither -c nor alter files.

       -f   Forces  compression	 of  name, even if it does not actually shrink
	    name, or if the corresponding name.Z file already exists.  If  the
	    -f option is omitted, the user is asked whether an existing name.Z
	    file should be overwritten unless is run in the background under

       -q   Quiet, not as verbose.

       -v   Displays the percent reduction of each file.

       -V   Prints version and options.

       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
       The 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 compressed already.  Rename the file and  try

       file: filename too long to tack on .Z
       The  file cannot be compressed because its name is longer than 12 char‐
       acters.	Rename and try again.

       file already exists; do you wish to overwrite (y or n)?
       Type y if you want the output file to be replaced; type	n  if  you  do

       uncompress: corrupt input
       A  SIGSEGV  violation  was  detected which usually means that the input
       file is corrupted.

       Compression: xx.xx%
       Percent of the input saved by compression.  (For the -v option only.)

       -- not a regular file: unchanged
       If the input file is not a regular file (for example, a directory),  it
       remains unchanged.

       -- has xx other links: unchanged
       The  input file has links; it is left unchanged.	 See for more informa‐

       -- file unchanged
       No saving is achieved by compression.  The input remains unchanged.

       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).

				      VAX			   compress(1)

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