.TH compress 1 VAX.SH Namecompress, uncompress, zcat \- compress and expand data.SH Syntax.B compress[.I options] [.I "name \&..."].br.B uncompress[.I options] [.I "name \&..."].br.B zcat[.I "name \&..."].SH Description.NXR "compress command"The .PN compresscommandreduces the size of the named files using adaptive Lempel-Ziv coding.Whenever possible,each file is replaced by one with the extension.B "\&.Z,"while keeping the same ownership modes, access, and modification times.If no files are specified, the standard input is compressed to thestandard output.Compressed files can be restored to their original form using.PN uncompressor.PN zcat ..PPThe.PN compresscommanduses 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 andcontinues to use more bits until thelimit specified by the.B \-bflag is reached (default 16).The.I bitsmust be between 9 and 16.  The default can be changed in the source to allow.PN compressto be run on a smaller machine..PPAfter the.I bitslimit is attained,.PN compressperiodically checks the compression ratio.  If the ratio is increasing,.PN compresscontinues to use the existing code dictionary.  However,if the compression ratio decreases,.PN compressdiscards the table of substrings and rebuilds it from scratch.  This allowsthe algorithm to adapt to the next block of the file..PPHow much each file is compressed depends on the size of theinput, the number of.I bitsper code, and the distribution of common substrings.Typically, text such as source code or Englishis reduced by 50\-60%.Compression is generally much better than that achieved byHuffman coding or adaptive Huffman coding,and takes less time to compute..PPIf an error occurs, exit status is 1.  However,if the last file was not compressed because it became larger, the statusis 2.  Otherwise, the status is 0..SH Options.IP \-b 5The .B \-bflag is omitted for.PN uncompress ,since the .I bitsparameter specified during compressionis encoded within the output along witha number that ensures that neither decompression of random data norrecompression of compressed data is attempted. .IP \-cThe cat option.  Makes.I compress/uncompresswrite to the standard output without changing anyfiles.Neither.PN uncompress.B \-cnor.PN zcatalter files..IP \-fForces compression of.IR name ,even if it does not actually shrink.IR name ,or if the corresponding \fIname\fR.Zfile already exists.If the.B \-foption is omitted, the user is asked whether an existing.IR name \&.Zfile should be overwritten unless.PN compressis run in the background under.PN /bin/sh ..IP \-qQuiet, not as verbose..IP \-vDisplays the percent reduction of each file..IP \-VPrints version and options..SH Diagnostics.B "Usage: compress [\-fvc] [\-b maxbits] [file ...]".brInvalid options were specified on the command line..PP.B "Missing maxbits".brMaxbits must follow.BR \-b \..PP.IR file :.B "not in compressed format.brThe file specified to.I uncompresshas not been compressed..PP.IR file :.B "compressed with".I xx.B "bits, can only handle".I yy.B "bits".brThe.I filewas compressed by a program that could deal withmore .I bitsthan the compress code on this machine.Recompress the file with smaller.IR bits \..PP.IR file :.B "already has .Z suffix -- no change".brThe file is assumed to be compressed already.Rename the file and try again..PP.IR file :.B "filename too long to tack on .Z".brThe file cannot be compressed because its name is longer than12 characters.Rename and try again..PP.I file.B "already exists; do you wish to overwrite (y or n)?".brType y if you want the output file to be replaced; type n if you do not..PP.B "uncompress: corrupt input".brA SIGSEGV violation was detected which usually means that the input file iscorrupted..PP.B "Compression:".I "xx.xx%".brPercent of the input saved by compression.(For the.B \-v option only.).PP.B "-- not a regular file: unchanged".brIf the input file is not a regular file(for example, a directory), it remains unchanged..PP.B "-- has" .I xx .B "other links: unchanged".brThe input file has links; it is left unchanged.  See.MS ln 1for more information..PP.B "-- file unchanged".brNo saving is achieved bycompression.  The input remains unchanged..SH RestrictionsAlthough compressed files are compatible between machines with large memory,.BR \-b \12should be used for file transfer to architectures with a small process data space (64KB or less).