NAME
compress,
uncompress —
compress and expand data
SYNOPSIS
compress |
[-cdfv]
[-b bits]
[file ...] |
uncompress |
[-cdfv]
[file ...] |
DESCRIPTION
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 ignored.
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 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 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.
-
-
- -d
- Force decompression.
-
-
- -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.
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 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.
EXIT STATUS
The
compress utility exits 0 on success, and >0
if an error occurs.
SEE ALSO
zcat(1)
Welch, Terry A., A
Technique for High Performance Data Compression, IEEE
Computer, 17:6, pp.
8-19, June, 1984.
HISTORY
The
compress command appeared in
4.3BSD.