171 lines
5.4 KiB
Groff
171 lines
5.4 KiB
Groff
.\" Copyright (c) 1986, 1990, 1993
|
|
.\" The Regents of the University of California. All rights reserved.
|
|
.\"
|
|
.\" This code is derived from software contributed to Berkeley by
|
|
.\" James A. Woods, derived from original work by Spencer Thomas
|
|
.\" and Joseph Orost.
|
|
.\"
|
|
.\" Redistribution and use in source and binary forms, with or without
|
|
.\" modification, are permitted provided that the following conditions
|
|
.\" are met:
|
|
.\" 1. Redistributions of source code must retain the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer.
|
|
.\" 2. Redistributions in binary form must reproduce the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer in the
|
|
.\" documentation and/or other materials provided with the distribution.
|
|
.\" 3. All advertising materials mentioning features or use of this software
|
|
.\" must display the following acknowledgement:
|
|
.\" This product includes software developed by the University of
|
|
.\" California, Berkeley and its contributors.
|
|
.\" 4. Neither the name of the University nor the names of its contributors
|
|
.\" may be used to endorse or promote products derived from this software
|
|
.\" without specific prior written permission.
|
|
.\"
|
|
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
|
|
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
|
|
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
|
|
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
|
|
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
|
|
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
|
|
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
|
|
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
|
|
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
|
|
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
|
|
.\" SUCH DAMAGE.
|
|
.\"
|
|
.\" @(#)compress.1 8.2 (Berkeley) 4/18/94
|
|
.\" $FreeBSD$
|
|
.\"
|
|
.Dd April 18, 1994
|
|
.Dt COMPRESS 1
|
|
.Os
|
|
.Sh NAME
|
|
.Nm compress ,
|
|
.Nm uncompress ,
|
|
.Nm zcat
|
|
.Nd compress and expand data
|
|
.Sh SYNOPSIS
|
|
.Nm
|
|
.Op Fl cfv
|
|
.Op Fl b Ar bits
|
|
.Op Ar
|
|
.Nm uncompress
|
|
.Op Fl cfv
|
|
.Op Ar
|
|
.Nm zcat
|
|
.Op Ar
|
|
.Sh DESCRIPTION
|
|
.Nm Compress
|
|
reduces the size of the named files using adaptive Lempel-Ziv coding.
|
|
Each
|
|
.Ar file
|
|
is renamed to the same name plus the extension
|
|
.Dq .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
|
|
.Ar file ,
|
|
the file is ignored.
|
|
.Pp
|
|
.Nm Uncompress
|
|
restores the compressed files to their original form, renaming the
|
|
files by deleting the
|
|
.Dq .Z
|
|
extension.
|
|
.Pp
|
|
.Nm Zcat
|
|
is an alias for
|
|
.Dq "uncompress -c" .
|
|
.Pp
|
|
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.
|
|
.Pp
|
|
If no files are specified, the standard input is compressed or uncompressed
|
|
to the standard output.
|
|
If either the input and 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.
|
|
.Pp
|
|
The options are as follows:
|
|
.Bl -tag -width indent
|
|
.It Fl b
|
|
Specify the
|
|
.Ar bits
|
|
code limit (see below).
|
|
.It Fl c
|
|
Compressed or uncompressed output is written to the standard output.
|
|
No files are modified.
|
|
.It Fl f
|
|
Force compression of
|
|
.Ar file ,
|
|
even if it is not actually reduced in size.
|
|
Additionally, files are overwritten without prompting for confirmation.
|
|
.It Fl v
|
|
Print the percentage reduction of each file.
|
|
.El
|
|
.Pp
|
|
.Nm Compress
|
|
uses a modified Lempel-Ziv algorithm.
|
|
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
|
|
.Fl b
|
|
flag is reached (the default is 16).
|
|
.Ar Bits
|
|
must be between 9 and 16.
|
|
.Pp
|
|
After the
|
|
.Ar bits
|
|
limit is reached,
|
|
.Nm
|
|
periodically checks the compression ratio.
|
|
If it is increasing,
|
|
.Nm
|
|
continues to use the existing code dictionary.
|
|
However, if the compression ratio decreases,
|
|
.Nm
|
|
discards the table of substrings and rebuilds it from scratch. This allows
|
|
the algorithm to adapt to the next "block" of the file.
|
|
.Pp
|
|
The
|
|
.Fl b
|
|
flag is omitted for
|
|
.Nm uncompress
|
|
since the
|
|
.Ar 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.
|
|
.Pp
|
|
The amount of compression obtained depends on the size of the
|
|
input, the number of
|
|
.Ar bits
|
|
per code, and the distribution of common substrings.
|
|
Typically, text such as source code or English is reduced by 50\-60%.
|
|
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.
|
|
.Sh DIAGNOSTICS
|
|
.Ex -std compress uncompress zcat
|
|
.Sh SEE ALSO
|
|
.Rs
|
|
.%A Welch, Terry A.
|
|
.%D June, 1984
|
|
.%T "A Technique for High Performance Data Compression"
|
|
.%J "IEEE Computer"
|
|
.%V 17:6
|
|
.%P pp. 8-19
|
|
.Re
|
|
.Sh HISTORY
|
|
The
|
|
.Nm
|
|
command appeared in
|
|
.Bx 4.3 .
|