compress, uncompress, -- compress and expand data
compress [-cfv] [-b bits] [file ...]
uncompress [-cfv] [file ...]
The compress utility reduces the size of the named files using adaptive
Lempel-Ziv coding. 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.
The uncompress utility restores the compressed files to their original
form, renaming the files by deleting the ``.Z'' extension.
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 or a file argument is a single dash (`-'), the
standard input is compressed or uncompressed to the standard output. If
either the input and 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.
The options are as follows:
-b Specify the bits code limit (see below).
-c Compressed or uncompressed output is written to the standard output.
No files are modified.
-f Force compression of file, even if it is not actually reduced in
size. Additionally, files are overwritten without prompting for
confirmation.
-v Print the percentage reduction of each file.
The compress utility uses a modified Lempel-Ziv algorithm. 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 (the
default is 16). Bits must be between 9 and 16.
After the bits limit is reached, 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.
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 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.
The compress and uncompress utilities exit 0 on success, and >0 if an
error occurs.
The compress utility exits 2 if attempting to compress the file would not
reduce its size and the -f option was not specified.
gunzip(1), gzexe(1), gzip(1), zcat(1), zmore(1), znew(1)
Welch, Terry A., "A Technique for High Performance Data Compression",
IEEE Computer, 17:6, pp. 8-19, June, 1984.
The compress and uncompress utilities conform to IEEE Std 1003.1-2001
(``POSIX.1'').
The compress command appeared in 4.3BSD.
FreeBSD 5.2.1 May 17, 2002 FreeBSD 5.2.1 [ Back ] |
