COMPRESS(1)						      General Commands Manual						       COMPRESS(1)

NAME

       compress, uncompress, zcat - compress and expand data

SYNOPSIS

       compress [ -f ] [ -v ] [ -c ] [ -b bits ] [ name ...  ]
       uncompress [ -f ] [ -v ] [ -c ] [ name ...  ]
       zcat [ 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 com-
       pressed 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 overwrit-
       ten.

       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 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  sub-
       strings.   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.

DIAGNOSTICS

       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 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 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.

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.)

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

4.3 Berkeley Distribution					   May 11, 1986 						       COMPRESS(1)