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
.Pf ( Fl 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
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 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
.Pf ( Fl 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 program version
(RCS IDs of the source files)
and exit.
-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.
The
compress,
uncompress,
and
zcat
utilities exit with 0 on success; 1 if an error occurred;
or 2 if a warning occurred.
The
compress,
uncompress,
and
zcat
utilities are compliant with the
specification.
The
compress
flags
[-123456789dghLlNnOqrtV],
uncompress
flags
[-hlNnqrt],
and the
zcat
flags
[-fghqr]
are extensions to that specification.
HISTORY
The
compress
command appeared in
4.3BSD.
Deflate compression support was added in
OpenBSD 2.1.