|COMPRESS(1)||General Commands Manual||COMPRESS(1)|
compressutility reduces the size of the named files using adaptive Lempel-Ziv coding, in compress mode. If invoked as
-g, the deflate mode of compression is chosen; see gzip(1) for more information. Each file is renamed to the same name plus the extension “.Z”. As many of the modification time, access time, file flags, file mode, user ID, and group ID as allowed by permissions are retained in the new file. If compression would not reduce the size of a file, the file is ignored (unless
-fis used). The
uncompressutility restores compressed files to their original form, renaming the files by removing the extension (or by using the stored name if the
-Nflag is specified). It has the ability to restore files compressed by both
compressand gzip(1), recognising the following extensions: “.Z”, “-Z”, “_Z”, “.gz”, “-gz”, “_gz”, “.tgz”, “-tgz”, “_tgz”, “.taz”, “-taz”, and “_taz”. Extensions ending in “tgz” and “taz” are not removed when decompressing, instead they are converted to “tar”. The
zcatcommand is equivalent in functionality to
-c. If renaming the files would cause files to be overwritten and the standard input device is a terminal, the user is prompted (on the standard error output) for confirmation. If prompting is not possible or confirmation is not received, the files are not overwritten. If no files are specified, the standard input is compressed or uncompressed to the standard output. If either the input or output files are not regular files, the checks for reduction in size and file overwriting are not performed, the input file is not removed, and the attributes of the input file are not retained. By default, when compressing using the deflate scheme (
-g), the original file name and time stamp are stored in the compressed file. When uncompressing, this information is not used. Instead, the uncompressed file inherits the time stamp of the compressed version and the uncompressed file name is generated from the name of the compressed file as described above. These defaults may be overridden by the
-nflags, described below. The options are as follows:
-9. Compression factor
-1is the fastest, but provides a poorer level of compression. Compression factor
-9provides the best level of compression, but is relatively slow. The default is
-6. This option implies
compressand if the option
-cis also given, copy the input data without change to the standard output: let
zcatbehave as cat(1).
-voption is specified, the following additional information is printed:
-noption is specified, the time stamp stored in the compressed file is printed instead).
-g) is used.
compresswill descend into specified directories.
compressuses a modified Lempel-Ziv algorithm (LZW). 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
-bflag is reached. bits must be between 9 and 16 (the default is 16). After the bits limit is reached,
compressperiodically checks the compression ratio. If it is increasing,
compresscontinues to use the existing code dictionary. However, if the compression ratio decreases,
compressdiscards the table of substrings and rebuilds it from scratch. This allows the algorithm to adapt to the next “block” of the file. The
-bflag is omitted for
uncompresssince 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% using
compress. Compression is generally much better than that achieved by Huffman coding (as used in the historical command pack), or adaptive Huffman coding (as used in the historical command compact), and takes less time to compute.
compressutility exits with one of the following values:
-fwas not specified and compression would have resulted in a size increase.
zcatutilities exit 0 on success, and >0 if an error occurs. gzexe(1), gzip(1), zdiff(1), zforce(1), zmore(1), znew(1), compress(3) Welch, Terry A., A Technique for High Performance Data Compression, IEEE Computer, 17:6, pp. 8-19, June, 1984.
zcatutilities are compliant with the X/Open System Interfaces option of the IEEE Std 1003.1-2008 (“POSIX.1”) specification. The
-b) of 14 to “achieve portability to all systems”.
compresscommand appeared in 4.3BSD. Deflate compression support was added in OpenBSD 2.1.
|March 17, 2014||OpenBSD-current|