- NetBSD Manual Pages
COMPRESS(1) NetBSD General Commands Manual COMPRESS(1)
Powered by man-cgi (2021-06-01).
Maintained for NetBSD
by Kimmo Suominen.
Based on man-cgi by Panagiotis Christias.
compress, uncompress -- compress and expand data
compress [-cdfv] [-b bits] [file ...]
uncompress [-cdfv] [file ...]
compress 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
uncompress 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 stan-
dard input device is a terminal, the user is prompted (on the standard
error output) for confirmation. If prompting is not possible or confir-
mation is not received, the files are not overwritten.
If no files are specified, the standard input is compressed or uncom-
pressed 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 out-
put. No files are modified.
-d Force decompression.
-f Force compression of file, even if it is not actually reduced in
size. Additionally, files are overwritten without prompting for
-v Print the percentage reduction of each file.
compress 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 com-
pression 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 num-
ber 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. Typ-
ically, text such as source code or English is reduced by 50-60%. Com-
pression 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 utility exits 0 on success, and >0 if an error occurs.
Welch, Terry A., "A Technique for High Performance Data Compression",
IEEE Computer, 17:6, pp. 8-19, June, 1984.
The compress command appeared in 4.3BSD.
NetBSD 8.1 January 23, 2003 NetBSD 8.1