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COMPRESS(1)									      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  own-
       ership  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  regu-
       lar  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 com-
       mand 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 compres-
       sion or uncompression.

       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
       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.

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)

					      local				      COMPRESS(1)
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