COMPRESS

Section: User Commands (1)
Updated: local
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NAME

compress, uncompress, zcat - compress and uncompress files

SYNOPSIS

compress [ -c ] [ -C ] [ -d ] [ -f ] [ -v ] [ -b bits ] [ filename ... ]
uncompress [ -c ] [ -f ] [ -v ] [ -V ] [ filename ... ]
zcat [ filename ... ]

DESCRIPTION

Compresses the specified files or standard input. Each file is replaced by a file with the extension .Z, but only if the file got smaller. If no files are specified, the compression is applied to the standard input and is written to standard output regardless of the results. Compressed files can be restored to their original form by specifying the -d option, or by running uncompress (linked to compress), on the .Z files or the standard input.

If the output file exists, it will not be overwritten unless the -f flag is given. If -f is not specified and compress is run in the foreground, the user is prompted as to whether the file should be overwritten.

If the -f flag is given, all files specified are replaced with .Z files - even if the file didn't get smaller.

When file names are given, the ownership (if run by root), modes, accessed and modified times are maintained between the file and its .Z version. In this respect, compress can be used for archival purposes, yet can still be used with make(1) after uncompression.

The -c option causes the results of the compress/uncompress operation to be written to stdout; no files are changed. The zcat program is the same as specifying -c to uncompress (all files are unpacked and written to stdout).

Compress uses the modified Lempel-Ziv algorithm described 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 bits limit as 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 reached, compress periodically checks the compression ratio. If it is increasing, compress continues to use the codes that were previously found in the file. 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. The -C (compatibility) flag prevents subdivision of the file into blocks; this produces an output file that old versions of compress can read.

A two byte magic number is prepended to the file to ensure that neither uncompression of random text nor recompression of compressed text are attempted. In addition, the bits specified during compress is written to the file so that the -b flag can be omitted for uncompress.

The amount of compression obtained depends on the size of the input file, the amount of bits per code, and the distribution of character substrings. Typically, text files, such as C programs, are 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.

If the -v (verbose) flag is given, then after each file is compressed, a message is printed giving the percentage of the input file that has been saved by compression.

If the -V (version) flag is given, the program's version number is printed.

The exit status is normally 0; if the last file gets bigger after compression, the exit status is 2; if an error occurs, the exit status is 1.

DIAGNOSTICS

Usage: compress [-cCdfvV] [-b maxbits] [file ...] Invalid options were specified on the command line. Missing maxbits Maxbits must follow -b. Unknown flag: 'x'; Invalid flags were specified on the command line. file: not in compressed format The specified file has not been compressed. file: compressed with xx bits, can only handle yy bits The specified file was compressed by a compress program that could handle more bits than the current compress program. Recompress the file with a smaller bits. file: already has .Z suffix -- no change Cannot compress a file that has a ".Z" suffix. mv(1) the file to a different name and try again. file: filename too long to tack on .Z The specified file cannot be compressed because its filename is longer than 12 characters. mv(1) the file to a different name and try again. This message does not occur on 4.2BSD 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 you want it to be left alone. file: This message fragment is written during the processing of a file. Compression: xx.xx% This message fragment gives the percentage of the input file that has been saved by compression. -- not a regular file: unchanged This message fragment is written when the input file is not a regular file. The input file is left unchanged. -- has xx other links: unchanged This message fragment is written when the input file has links. The input file is left unchanged. See ln(1) for more information. -- file unchanged This message fragment is written when no savings are achieved by compression. The input file is left unchanged. -- replaced with file This message fragment is written when a file has been sucessfully compressed/uncompressed.