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NAME

compress, uncompress, zcat — compress and expand data (compress mode)

SYNOPSIS

DESCRIPTION

The compress utility reduces the size of the named files using adaptive Lempel-Ziv coding, in compress mode. If invoked as compress -g, the deflate mode of compression is chosen; see gzip(1) for more information. 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 (unless -f is used).

The uncompress utility restores compressed files to their original form, renaming the files by removing the extension (or by using the stored name if the -N flag is specified). It has the ability to restore files compressed by compress, gzip(1), and zip(1), recognising the following extensions: “.Z”, “-Z”, “_Z”, “.gz”, “-gz”, “_gz”, “.tgz”, “-tgz”, “_tgz”, “.taz”, “-taz”, and “_taz”. The -S option can be used to support other file extensions. Extensions ending in “tgz” and “taz” are not removed when decompressing, instead they are converted to “tar”. Files in zip format are only supported if they contain a single member either compressed with the deflate scheme or stored uncompressed.

The zcat command is equivalent in functionality to uncompress -c.

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 or 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.

By default, when compressing using the deflate scheme (-g), the original file name and time stamp are stored in the compressed file. When uncompressing, this information is not used. Instead, the uncompressed file inherits the time stamp of the compressed version and the uncompressed file name is generated from the name of the compressed file as described above. These defaults may be overridden by the -N and -n flags, described below.

The options are as follows:

-1...9

Use the deflate scheme, with compression factor of -1 to -9. Compression factor -1 is the fastest, but provides a poorer level of compression. Compression factor -9 provides the best level of compression, but is relatively slow. The default is -6. This option implies -g.

-b bits

Specify the bits code limit (see below).

-c

Compressed or uncompressed output is written to the standard output. No files are modified (force zcat mode).

-d

Decompress the source files instead of compressing them (force uncompress mode).

-f

Force compression of file, even if it is not actually reduced in size. Additionally, files are overwritten without prompting for confirmation. If the input data is not in a format recognized by compress and if the option -c is also given, copy the input data without change to the standard output: let zcat behave as cat(1).

-g

Use the deflate scheme, which reportedly provides better compression rates (force gzip(1) mode).

-h

Print a short help message.

-l

List information for the specified compressed files. The following information is listed:

compressed size

Size of the compressed file.

uncompressed size

Size of the file when uncompressed.

compression ratio

Ratio of the difference between the compressed and uncompressed sizes to the uncompressed size.

uncompressed name

Name the file will be saved as when uncompressing.

If the -v option is specified, the following additional information is printed:

compression method

Name of the method used to compress the file.

crc

32-bit CRC (cyclic redundancy code) of the uncompressed file.

time stamp

Date and time corresponding to the last data modification time (mtime) of the compressed file (if the -n option is specified, the time stamp stored in the compressed file is printed instead).

-N

When uncompressing or listing, use the time stamp and file name stored in the compressed file, if any, for the uncompressed version. This information is only available when the deflate scheme (-g) is used.

-n

When compressing, do not store the original file name and time stamp in the header of the compressed file.

-O

Use compress mode (the default).

-o filename

Set the output file name.

-q

Be quiet: suppress all messages.

-r

Recursive mode: compress will descend into specified directories.

-S suffix

When compressing, use the specified suffix as the extension when creating output files. When uncompressing, recognize file names with the specified suffix as compressed files.

-t

Test the integrity of each file leaving any files intact.

-v

Print the percentage reduction of each file and other information.

compress uses a modified Lempel-Ziv algorithm (LZW). 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. bits must be between 9 and 16 (the default is 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% using compress. 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 with one of the following values:

The uncompress and zcat utilities exit 0 on success, and >0 if an error occurs.

SEE ALSO

gzexe(1), gzip(1), zdiff(1), zforce(1), zmore(1), znew(1), compress(3)

Welch, Terry A., A Technique for High Performance Data Compression, IEEE Computer, 17:6, pp. 8-19, June, 1984.

STANDARDS

The compress, uncompress, and zcat utilities are compliant with the X/Open System Interfaces option of the IEEE Std 1003.1-2008 (“POSIX.1”) specification.

The compress flags [-123456789dghlNnOoqrSt], uncompress flags [-hlNnoqrt], and the zcat flags [-fghqr] are extensions to that specification.

IEEE Std 1003.1-2008 (“POSIX.1”) specifies a maximum bits limit (-b) of 14 to "achieve portability to all systems".

HISTORY

The compress command appeared in 4.3BSD. Deflate compression support was added in OpenBSD 2.1.