reduces the size of the named files using adaptive Lempel-Ziv coding,
in deflate mode.
If invoked as
the compress mode of compression is chosen;
for more information.
Each file is renamed to the same name plus the extension
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
If compression would not reduce the size of a file,
the file is ignored (unless
utility restores compressed files to their original form, renaming the
files by removing the extension (or by using the stored name if the
flag is specified).
It has the ability to restore files compressed by both
recognising the following extensions:
Extensions ending in
are not removed when decompressing, instead they are converted to
command is equivalent in functionality to
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, 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
flags, described below.
The options are as follows:
Use the deflate scheme, with compression factor of
is the fastest, but provides a poorer level of compression.
provides the best level of compression, but is relatively slow.
The default is
This option implies
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
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
and if the option
is also given, copy the input data without change
to the standard output: let
Use the deflate scheme, which reportedly provides better compression rates
Print a short help message.
Print the license.
List information for the specified compressed files.
The following information is listed:
Size of the compressed file.
Size of the file when uncompressed.
Ratio of the difference between the compressed and uncompressed
sizes to the uncompressed size.
Name the file will be saved as when uncompressing.
option is specified, the following additional information is printed:
Name of the method used to compress the file.
(cyclic redundancy code)
of the uncompressed file.
Date and time corresponding to the last data modification time
(mtime) of the compressed file (if the
option 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.
When compressing, do not store the original file name and time stamp
will descend into specified directories.
Set the suffix for compressed files.
Test the integrity of each file leaving any files intact.
Display the program version
(RCS IDs of the source files)
Print the percentage reduction of each file and other information.
uses a modified Lempel-Ziv algorithm
Common substrings are replaced by pointers to previous strings,
and are found using a hash table.
Unique substrings are emitted as a string of literal bytes,
and compressed as Huffman trees.
When code 512 is reached, the algorithm switches to 10-bit codes and
continues to use more bits until the
limit specified by the
flag is reached.
must be between 9 and 16
(the default is 16).
limit is reached,
periodically checks the compression ratio.
If it is increasing,
continues to use the existing code dictionary.
However, if the compression ratio decreases,
discards the table of substrings and rebuilds it from scratch.
This allows the algorithm to adapt to the next
of the file.
flag is omitted for
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
per code, and the distribution of common substrings.
Typically, text such as source code or English is reduced by 60 \- 70% 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.
utilities exit with 0 on success; 1 if an error occurred;
or 2 if a warning occurred.
Options which are passed to