COMPRESS(1)						      General Commands Manual						       COMPRESS(1)

NAME

       compress, uncompress, zcat - compress and expand data (version 4.1)

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 ownership modes, access and modification times.  If no files are specified, the standard input is com-
       pressed	to the standard output.  Compress will only attempt to compress regular 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 command 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  compila-
       tion, on stderr before doing any compression or uncompression.

       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.

       Under the -v option, a message is printed yielding the percentage of reduction for each file compressed.

       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 [-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 compress.c:compdir() for an explanation.

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