COMPRESS (1) Public-Domain Utilities COMPRESS (1) NAME compress, uncompress, zcat - compress and expand data SYNOPSIS pcompressq [ p-fq ] [ p-vq ] [ p-cq ] [ p-Vq ] [ p-bq pbitsq ] [ pname ...q ] puncompressq [ p-fq ] [ p-vq ] [ p-cq ] [ p-Vq ] [ pname ...q ] pzcatq [ p-Vq ] [ pname ...q ] DESCRIPTION pCompressq reduces the size of the named files using adaptive Lempel-Ziv coding. Whenever possible, each file is replaced by one with the extension p.Z,q while keeping the same ownership modes, access and modification times. If no files are specified, the standard input is compressed to the standard output. Compressed files can be restored to their original form using puncompressq or pzcat.q The p-fq option will force compression of pname.q This is useful for compressing an entire directory, even if some of the files do not actually shrink. If p-fq is not given and pcompressq is run in the foreground, the user is prompted as to whether an existing file should be overwritten. The p-cq option makes pcompress/uncompressq write to the standard output; no files are changed. The nondestructive behavior of pzcatq is identical to that of puncompressq p-c.q pCompressq uses the modified Lempel-Ziv algorithm popularized in "A Technique for High Performance Data Compression", Terry A. Welch, pIEEE Computer,q 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 p-bq flag is reached (default 16). pBitsq must be between 9 and 16. The default can be changed in the source to allow pcompressq to be run on a smaller machine. After the pbitsq limit is attained, pcompressq periodically checks the compression ratio. If it is increasing, pcompressq continues to use the existing code dictionary. However, if the compression ratio decreases, pcompressq 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 p-bq flag is omitted for puncompress,q since the pbitsq 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. Tue Feb 23 22:42:04 1993 Page 1 COMPRESS (1) Public-Domain Utilities COMPRESS (1) The amount of compression obtained depends on the size of the input, the number of pbitsq 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 ppackq), or adaptive Huffman coding q(pcompactq),pqpq and takes less time to compute. Under the p-vq option, a message is printed yielding the percentage of reduction for each file compressed. If the p-Vq option is specified, the current version and compile options are printed on stderr. 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 [-dfvcV] [-b maxbits] [file ...] Invalid options were specified on the command line. Missing maxbits Maxbits must follow p-bq.pqpq pfileq: not in compressed format The file specified to puncompressq has not been compressed. pfileq: compressed with pxxq bits, can only handle pyyq bits pFileq was compressed by a program that could deal with more pbitsq than the compress code on this machine. Recompress the file with smaller pbitsq. pfileq: already has .Z suffix -- no change The file is assumed to be already compressed. Rename the file and try again. pfileq: 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. pfileq 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: pxx.xx%q Percentage of the input saved by compression. (Relevant only for p-vq.)pqpq -- not a regular file: unchanged When the input file is not a regular file, (e.g. a directory), it is left unaltered. -- has pxxq other links: unchanged The input file has links; it is left unchanged. See Tue Feb 23 22:42:04 1993 Page 2 COMPRESS (1) Public-Domain Utilities COMPRESS (1) plnq(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, p-bq12pqpq 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.) This manual page does not actually describe the Atari ST/TOS version. COPYRIGHT NOTE This program is supplied with HERMES for your convenience. It is not covered by the HERMES license regulations, and is in the public domain, as far as I know. Tue Feb 23 22:42:04 1993 Page 3