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diff --git a/usr.bin/compress/compress.1 b/usr.bin/compress/compress.1 new file mode 100644 index 0000000..84bc47c --- /dev/null +++ b/usr.bin/compress/compress.1 @@ -0,0 +1,171 @@ +.\" $NetBSD: compress.1,v 1.16 2017/07/04 06:58:55 wiz Exp $ +.\" +.\" 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. 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 +.\" +.Dd January 23, 2003 +.Dt COMPRESS 1 +.Os +.Sh NAME +.Nm compress , +.\".Nm uncompress , +.Nm uncompress +.\".Nm zcat +.Nd compress and expand data +.Sh SYNOPSIS +.Nm +.Op Fl cdfv +.Op Fl b Ar bits +.Op Ar +.Nm uncompress +.Op Fl cdfv +.Op Ar +.\".Nm zcat +.\".Op Ar +.Sh DESCRIPTION +.Nm +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 Ds +.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 d +Force decompression. +.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 +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 +.Ar 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 EXIT STATUS +.Ex -std compress +.Sh SEE ALSO +.Xr zcat 1 +.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 . |