compact(1) General Commands Manual compact(1)Name
compact, uncompact, ccat - compress and uncompress files, and cat them
Syntax
compact [name...]
uncompact [name...]
ccat [file...]
Description
The command compresses the named files using an adaptive Huffman code. If no file names are given, the standard input is compacted to the
standard output. The command operates as an on-line algorithm. Each time a byte is read, it is encoded immediately according to the cur-
rent prefix code. This code is an optimal Huffman code for the set of frequencies seen so far. It is unnecessary to prepend a decoding
tree to the compressed file since the encoder and the decoder start in the same state and stay synchronized. Furthermore, and can operate
as filters. In particular,
... | compact | uncompact | ...
operates as a (very slow) no-op.
When an argument file is given, it is compacted and the resulting file is placed in file.C; file is unlinked. The first two bytes of the
compacted file code the fact that the file is compacted. This code is used to prohibit recompaction.
The amount of compression to be expected depends on the type of file being compressed. Typical values of compression are: Text (38%), Pas-
cal Source (43%), C Source (36%) and Binary (19%). These values are the percentages of file bytes reduced.
The command restores the original file from a file compressed by If no file names are given, the standard input is uncompacted to the stan-
dard output.
The command cats the original file from a file compressed by without uncompressing the file.
The command is present only for compatibility. In general, the command runs faster and gives better compression.
Restrictions
The last segment of the file name must contain fewer than thirteen characters to allow space for the appended '.C'.
Files
compacted file created by compact, removed by uncompact
See Alsocompress(1)compact(1)
Check Out this Related Man Page
COMPRESS(1) BSD General Commands Manual COMPRESS(1)NAME
compress, uncompress, -- compress and expand data
SYNOPSIS
compress [-cfv] [-b bits] [file ...]
uncompress [-cfv] [file ...]
DESCRIPTION
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 cod-
ing (as used in the historical command pack), or adaptive Huffman coding (as used in the historical command compact), and takes less time to
compute.
DIAGNOSTICS
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.
SEE ALSO 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.
STANDARDS
The compress and uncompress utilities conform to IEEE Std 1003.1-2001 (``POSIX.1'').
HISTORY
The compress command appeared in 4.3BSD.
BSD May 17, 2002 BSD