174 lines
5.5 KiB
Groff
174 lines
5.5 KiB
Groff
.\" Copyright (c) 1986, 1990, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" This code is derived from software contributed to Berkeley by
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.\" James A. Woods, derived from original work by Spencer Thomas
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.\" and Joseph Orost.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" from: @(#)compress.1 8.2 (Berkeley) 4/18/94
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.\" $Id: compress.1,v 1.3 1994/06/13 14:48:22 cgd Exp $
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.\"
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.Dd April 18, 1994
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.Dt COMPRESS 1
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.Os BSD 4.3
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.Sh NAME
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.Nm compress ,
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.Nm uncompress ,
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.Nm zcat
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.Nd compress and expand data
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.Sh SYNOPSIS
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.Nm compress
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.Op Fl cfv
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.Op Fl b Ar bits
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.Op Ar
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.Nm uncompress
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.Op Fl cfv
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.Op Ar
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.Nm zcat
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.Op Ar
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.Sh DESCRIPTION
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.Nm Compress
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reduces the size of the named files using adaptive Lempel-Ziv coding.
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Each
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.Ar file
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is renamed to the same name plus the extension
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.Dq .Z .
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As many of the modification time, access time, file flags, file mode,
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user ID, and group ID as allowed by permissions are retained in the
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new file.
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If compression would not reduce the size of a
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.Ar file ,
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the file is ignored.
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.Pp
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.Nm Uncompress
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restores the compressed files to their original form, renaming the
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files by deleting the
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.Dq .Z
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extension.
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.Pp
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.Nm Zcat
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is an alias for
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.Dq "uncompress -c" .
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.Pp
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If renaming the files would cause files to be overwritten and the standard
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input device is a terminal, the user is prompted (on the standard error
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output) for confirmation.
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If prompting is not possible or confirmation is not received, the files
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are not overwritten.
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.Pp
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If no files are specified, the standard input is compressed or uncompressed
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to the standard output.
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If either the input and output files are not regular files, the checks for
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reduction in size and file overwriting are not performed, the input file is
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not removed, and the attributes of the input file are not retained.
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.Pp
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The options are as follows:
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.Bl -tag -width Ds
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.It Fl b
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Specify the
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.Ar bits
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code limit (see below).
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.It Fl c
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Compressed or uncompressed output is written to the standard output.
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No files are modified.
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.It Fl f
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Force compression of
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.Ar file ,
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even if it is not actually reduced in size.
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Additionally, files are overwritten without prompting for confirmation.
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.It Fl v
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Print the percentage reduction of each file.
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.El
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.Pp
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.Nm Compress
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uses a modified Lempel-Ziv algorithm.
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Common substrings in the file are first replaced by 9-bit codes 257 and up.
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When code 512 is reached, the algorithm switches to 10-bit codes and
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continues to use more bits until the
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limit specified by the
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.Fl b
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flag is reached (the default is 16).
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.Ar Bits
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must be between 9 and 16.
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.Pp
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After the
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.Ar bits
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limit is reached,
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.Nm compress
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periodically checks the compression ratio.
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If it is increasing,
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.Nm compress
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continues to use the existing code dictionary.
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However, if the compression ratio decreases,
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.Nm compress
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discards the table of substrings and rebuilds it from scratch. This allows
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the algorithm to adapt to the next "block" of the file.
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.Pp
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The
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.Fl b
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flag is omitted for
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.Ar uncompress
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since the
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.Ar bits
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parameter specified during compression
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is encoded within the output, along with
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a magic number to ensure that neither decompression of random data nor
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recompression of compressed data is attempted.
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.Pp
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.ne 8
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The amount of compression obtained depends on the size of the
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input, the number of
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.Ar bits
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per code, and the distribution of common substrings.
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Typically, text such as source code or English is reduced by 50\-60%.
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Compression is generally much better than that achieved by Huffman
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coding (as used in the historical command pack), or adaptive Huffman
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coding (as used in the historical command compact), and takes less
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time to compute.
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.Pp
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The
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.Nm compress
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utility exits 0 on success, and >0 if an error occurs.
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.Sh SEE ALSO
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.Rs
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.%A Welch, Terry A.
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.%D June, 1984
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.%T "A Technique for High Performance Data Compression"
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.%J "IEEE Computer"
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.%V 17:6
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.%P pp. 8-19
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.Re
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.Sh HISTORY
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The
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.Nm
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command appeared in
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.Bx 4.3 .
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