man : compress(1)
COMPRESS(1) OpenBSD Reference Manual COMPRESS(1)
compress, uncompress, zcat - compress and expand data (compress mode)
compress [-123456789cdfghLlNnOqrtVv] [-b bits] [-o filename] [-S suffix]
uncompress [-cfhlNnqrtVv] [-o filename] [file ...]
zcat [-fghqr] [file ...]
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 both compress and gzip(1), recognising the following
extensions: ``.Z'', ``-Z'', ``_Z'', ``.gz'', ``-gz'', ``_gz'', ``.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 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.
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
-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 Print the license.
-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
uncompressed name Name the file will be saved as when
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
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
-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).
Set the output file name.
-q Be quiet: suppress all messages.
-r Recursive mode: compress will descend into specified directories.
Set the suffix for compressed files.
-t Test the integrity of each file leaving any files intact.
-V Display the program version (RCS IDs of the source files) and
-v Print the percentage reduction of each file and other
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.
The compress utility exits with one of the following values:
1 An error occurred.
2 At least one of the specified files was not compressed
since -f was not specified and compression would have
resulted in a size increase.
>2 An error occurred.
The uncompress and zcat utilities exit with 0 on success; 1 if an error
occurred; or 2 if a warning occurred.
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.
The compress, uncompress, and zcat utilities are compliant with the
X/Open Portability Guide Issue 4 (``XPG4'') specification.
The compress flags [-123456789dghLlNnOoqrStV], uncompress flags
[-hlNnoqrt], and the zcat flags [-fghqr] are extensions to that
The compress command appeared in 4.3BSD. Deflate compression support was
added in OpenBSD 2.1.
OpenBSD 4.9 September 3, 2010 OpenBSD 4.9