Manual Pages for UNIX Darwin command on man compress

COMPRESS(1) BSD General Commands Manual COMPRESS(1)

NAME

ccoommpprreessss, uunnccoommpprreessss - compress and expand data

SYNOPSIS

ccoommpprreessss [-ccffvv] [-bb bits] [file ...]

uunnccoommpprreessss [-ccffvv] [file ...]

DESCRIPTION

The ccoommpprreessss utility reduces the size of the named files using adaptive

Lempel-Ziv coding. Each file is renamed to the same name plus the exten-

sion ``.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 uunnccoommpprreessss 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 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 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:

-bb Specify the bits code limit (see below).

-cc Compressed or uncompressed output is written to the standard out-

put. No files are modified.

-ff Force compression of file, even if it is not actually reduced in

size. Additionally, files are overwritten without prompting for confirmation.

-vv Print the percentage reduction of each file.

The ccoommpprreessss utility uses a modified Lempel-Ziv algorithm. Common sub-

strings 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 -bb flag is reached (the

default is 16). Bits must be between 9 and 16.

After the bits limit is reached, ccoommpprreessss periodically checks the com-

pression ratio. If it is increasing, ccoommpprreessss continues to use the

existing code dictionary. However, if the compression ratio decreases,

ccoommpprreessss discards the table of substrings and rebuilds it from scratch. This allows the algorithm to adapt to the next "block" of the file.

The -bb flag is omitted for uunnccoommpprreessss 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. DIAGNOSTICS The ccoommpprreessss and uunnccoommpprreessss utilities exit 0 on success, and >0 if an error occurs.

The ccoommpprreessss utility exits 2 if attempting to compress the file would not

reduce its size and the -ff option was not specified.