COMPRESS(1) MS-DOS Programmer's Manual COMPRESS(1) NAME compress, uncompress, zcat - compress or expand data SYNOPSIS compress [-cdfivV] [-b bits] [name ...] uncompress [-cfivV] [name ...] zcat [-iV] [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 or XZ, while keeping the same 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 uncompress or zcat. The -c option makes compress/uncompress write to the standard output; no files are changed. The nondestructive behavior of zcat is identical to that of uncompress -c. The -d (decompress) option makes compress restore its input files to their normal form. Uncompress is identical to compress with the -d option specified. The -f option will force compression of "name". This is useful for compressing an entire directory, even if some of the files do not actually shrink. If -f is not given, the user is prompted as to whether an existing file should be overwritten. The -i (image mode) option suppresses the transformation of text lines from MS-DOS (CR-LF delimited) form to UNIX (LF delimited) form during compression, and suppresses the reverse transformation during decompression. The -v (verbose) option causes a message to be printed, yielding the percentage of reduction for each file compressed. 1 COMPRESS(1) MS-DOS Programmer's Manual COMPRESS(1) The -V option causes the current version and compile options to be printed on stderr. 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 is the maximum for which the program was built). "Bits" must be between 9 and the lesser of 16, and the limit imposed at compile-time. The MS-DOS version of compress comes in two sizes. One has a 12-bit limit, and will run in a machine with 128K bytes of available user memory. The other has a 16-bit limit, and requires about 450K bytes to run. 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 2 COMPRESS(1) MS-DOS Programmer's Manual COMPRESS(1) better than that achieved by Huffman coding (as used in SQ), and takes less time to compute. 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 SQ(1) DIAGNOSTICS Usage: compress [-cdfivV] [-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 xx suffix -- no change The file is assumed to be already compressed because the last two characters of its extension are ".Z" or "XZ". Rename the file and try again. fn: part of filename extension will be replaced by XZ File name, fn, contains at least two characters in the "extension" field. The second and third will be replaced by "XZ" in the compressed file's name. fn already exists; do you wish to overwrite fn? Respond "y" if you want the output file, fn, to be replaced; "n" if not. Compression: xx.xx% Percentage of the input saved by compression. (Relevant only for -v.) -- file unchanged No savings is achieved by compression. The 3 COMPRESS(1) MS-DOS Programmer's Manual COMPRESS(1) 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, or the small MS-DOS version, for example). MS-DOS version 2 does not permit a program to determine the name used to call it. As a result, the aliases, uncompress and zcat, cannot be used. They can be used under MS-DOS version 3, though the actual file name for uncompress will be "uncompre.exe". MS-DOS does not support UNIX-style file links. As a result, even though compress, uncompress and zcat are all the same program, it (they) will have to be stored three times, once under each of the three names, in order to use them under MS-DOS version 3. As explained in the previous paragraph, this is not an option under MS-DOS version 2. 4 !