compress and expand data (compress
compress utility reduces the size of
the named files using adaptive Lempel-Ziv coding, in compress mode. If
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
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
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”.
zcat command is equivalent in
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
The options are as follows:
- Use the deflate scheme, with compression factor of
-9. Compression factor
-1is the fastest, but provides a poorer level of compression. Compression factor
-9provides the best level of compression, but is relatively slow. The default is
-6. This option implies
- Specify the bits code limit (see below).
- Compressed or uncompressed output is written to the standard output. No
files are modified (force
- Decompress the source files instead of compressing them (force
- 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
compressand if the option
-cis also given, copy the input data without change to the standard output: let
zcatbehave as cat(1).
- Use the deflate scheme, which reportedly provides better compression rates (force gzip(1) mode).
- Print a short help message.
- 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.
-voption is specified, the following additional information is printed:
- compression method
- Name of the method used to compress the file.
- 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
-noption is specified, the time stamp stored in the compressed file is printed instead).
- 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.
- When compressing, do not store the original file name and time stamp in the header of the compressed file.
- Use compress mode (the default).
- Set the output file name.
- Be quiet: suppress all messages.
- Recursive mode:
compresswill descend into specified directories.
- Set the suffix for compressed files.
- Test the integrity of each file leaving any files intact.
- 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.
-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
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.
compress utility exits with one of the
- An error occurred.
- At least one of the specified files was not compressed since
-fwas not specified and compression would have resulted in a size increase.
- An error occurred.
utilities exit 0 on success, and >0 if an error occurs.
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.
utilities are compliant with the X/Open System Interfaces option of the
IEEE Std 1003.1-2008 (“POSIX.1”)
-hlNnoqrt], and the
-fghqr] are extensions to that
IEEE Std 1003.1-2008
(“POSIX.1”) specifies a maximum bits limit
-b) of 14 to "achieve portability to all
compress command appeared in
4.3BSD. Deflate compression support was added in