.PU.TH COMPRESS 1 local.SH NAMEcompress, uncompress, zcat \- compress and expand data.SH SYNOPSIS.ll +8.B compress[.B \-f] [.B \-v] [.B \-c] [.B \-V] [.B \-b.I bits] [.I "name \&..."].ll -8.br.B uncompress[.B \-f] [.B \-v] [.B \-c] [.B \-V] [.I "name \&..."].br.B zcat[.B \-V] [.I "name \&..."].SH DESCRIPTION.I Compressreduces 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.I uncompressor.I zcat..PPThe.B \-foption will force compression of.I name.This is useful for compressing an entire directory,even if some of the files do not actually shrink.If.B \-fis not given and.I compressis run in the foreground,the user is prompted as to whether an existing file should be overwritten..PPThe.B \-coption makes.I compress/uncompresswrite to the standard output; no files are changed.The nondestructive behavior of.I zcatis identical to that of.I uncompress.B \-c..PP.I Compressuses the modified Lempel-Ziv algorithm popularized in"A Technique for High Performance Data Compression",Terry A. Welch,.I "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 andcontinues to use more bits until thelimit specified by the.B \-bflag is reached (default 16)..I Bitsmust be between 9 and 16.  The default can be changed in the source to allow.I compressto be run on a smaller machine..PPAfter the.I bitslimit is attained,.I compressperiodically checks the compression ratio.  If it is increasing,.I compresscontinues to use the existing code dictionary.  However,if the compression ratio decreases,.I compressdiscards the table of substrings and rebuilds it from scratch.  This allowsthe algorithm to adapt to the next "block" of the file..PPNote that the.B \-bflag is omitted for.I uncompress,since the .I bitsparameter specified during compressionis encoded within the output, along witha magic number to ensure that neither decompression of random data norrecompression of compressed data is attempted. .PP.ne 8The amount of compression obtained 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 (as used in.IR pack ),or adaptive Huffman coding.RI ( compact ),and takes less time to compute..PPUnder the.B \-voption,a message is printed yielding the percentage ofreduction for each file compressed..PPIf the.B \-Voption is specified, the current version and compile options are printed onstderr..PPExit 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..SH "SEE ALSO"pack(1), compact(1).SH "DIAGNOSTICS"Usage: compress [\-dfvcV] [\-b maxbits] [file ...].in +8Invalid options were specified on the command line..in -8Missing maxbits.in +8Maxbits must follow.BR \-b \..in -8.IR file :not in compressed format.in +8The file specified to.I uncompresshas not been compressed..in -8.IR file :compressed with .I xxbits, can only handle .I yybits.in +8.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 \..in -8.IR file :already has .Z suffix -- no change.in +8The file is assumed to be already compressed.Rename the file and try again..in -8.IR file :filename too long to tack on .Z.in +8The file cannot be compressed because its name is longer than12 characters.Rename and try again.This message does not occur on BSD systems..in -8.I filealready exists; do you wish to overwrite (y or n)?.in +8Respond "y" if you want the output file to be replaced; "n" if not..in -8uncompress: corrupt input.in +8A SIGSEGV violation was detected which usually means that the input file hasbeen corrupted..in -8Compression: .I "xx.xx%".in +8Percentage of the input saved by compression.(Relevant only for.BR \-v \.).in -8-- not a regular file: unchanged.in +8When the input file is not a regular file,(e.g. a directory), it isleft unaltered..in -8-- has .I xx other links: unchanged.in +8The input file has links; it is left unchanged.  See.IR ln "(1)"for more information..in -8-- file unchanged.in +8No savings is achieved bycompression.  The input remains virgin..in -8.SH "BUGS"Although 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, as exhibited by the DEC PDPseries, the Intel 80286, etc.)