COMPRESS(1) BSD Reference Manual COMPRESS(1)NAME
compress, uncompress, zcat - compress and expand data (compress mode)
SYNOPSIScompress [-123456789cdfghLlNnOqrtVv] [-b bits] [-o filename] [-S suffix]
[file ...]
uncompress [-cfhlNnqrtv] [-o filename] [file ...]
zcat [-fghqr] [file ...]
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 de-
flate 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 (un-
less -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 both compress and gzip(1), recognising the following exten-
sions: ".Z", "-Z", "_Z", ".gz", "-gz", "_gz", ".cgz", "-cgz", "_cgz",
".ngz", "-ngz", "_ngz", ".tgz", "-tgz", "_tgz", ".taz", "-taz", and
"_taz". Extensions ending in "tgz" and "taz" are not removed when
decompressing, instead they are converted to "tar". The same goes for
"cgz" and "ngz" files, which are converted to "cpio".
The zcat command is equivalent in functionality to uncompress -c.
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 confirma-
tion is not received, the files are not overwritten.
If no files are specified, the standard input is compressed or un-
compressed 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 at-
tributes 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 un-
compressing, 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 op-
tion implies -g.
-b bits
Specify the bits code limit (see below).
-c Compressed or uncompressed output is written to the standard out-
put. 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 recognised by
compress and if the options -cd are also given, copy the input
data without change to the standard output: let zcat behave as
cat(1).
This option also turns off ignoring files based on their suffix.
-g Use the deflate scheme, which reportedly provides better compres-
sion rates (force gzip(1) mode).
-h Print a short help message.
-L Print the licence.
-l List information for the specified compressed files. The follow-
ing 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 un-
compressing.
If the -v option is specified, the following additional informa-
tion 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 print-
ed instead).
-N When uncompressing or listing, use the time stamp and file name
stored in the compressed file, if any, for the uncompressed ver-
sion. 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
Set the suffix for compressed files.
-t Test the integrity of each file leaving any files intact.
-V Display the programme version and exit.
-v Print the percentage reduction of each file and other informa-
tion.
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 ex-
isting 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 recompres-
sion 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. Typi-
cally, 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.
The compress, uncompress, and zcat utilities exit with 0 on success; 1 if
an error occurred; or 2 if a warning occurred.
SEE ALSOcpio(1), 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 utility is compliant with the IEEE Std 1003.2-1992 ("PO-
SIX.2") specification.
HISTORY
The compress command appeared in 4.3BSD. Deflate compression support was
added in OpenBSD 2.1. The cpio handling was added in MirOS #8.
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