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165 lines
4.5 KiB
Plaintext
165 lines
4.5 KiB
Plaintext
.\" This module is believed to contain source code proprietary to AT&T.
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.\" Use and redistribution is subject to the Berkeley Software License
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.\" Agreement and your Software Agreement with AT&T (Western Electric).
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.\"
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.\" @(#)tt12 8.1 (Berkeley) 6/8/93
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.\" Copyright (C) Caldera International Inc. 2001-2002. All rights reserved.
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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 are
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.\" met:
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.\"
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.\" Redistributions of source code and documentation must retain the above
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.\" copyright notice, this list of conditions and the following
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.\" disclaimer.
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.\"
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.\" 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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.\"
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.\" All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\"
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.\" This product includes software developed or owned by Caldera
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.\" International, Inc. Neither the name of Caldera International, Inc.
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.\" nor the names of other contributors may be used to endorse or promote
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.\" products derived from this software without specific prior written
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.\" permission.
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.\"
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.\" USE OF THE SOFTWARE PROVIDED FOR UNDER THIS LICENSE BY CALDERA
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.\" INTERNATIONAL, INC. AND CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
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.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
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.\" DISCLAIMED. IN NO EVENT SHALL CALDERA INTERNATIONAL, INC. BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
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.\" BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
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.\" WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE
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.\" OR OTHERWISE) RISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN
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.\" IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.NH
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Conditionals
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.PP
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Suppose we want the
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.BD .SH
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macro to leave two extra inches of space just before section 1,
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but nowhere else.
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The cleanest way to do that is to test inside the
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.BD .SH
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macro
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whether
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the section number is 1,
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and add some space if it is.
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The
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.BD .if
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command provides the conditional test
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that we can add
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just before the heading line is output:
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.P1 4
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^if \e\en(SH=1 ^sp 2i \e" first section only
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.P2
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.PP
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The condition after the
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.BD .if
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can be any arithmetic or logical expression.
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If the condition is logically true, or arithmetically greater than zero,
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the rest of the line is treated as if
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it were text _
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here a command.
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If the condition is false, or zero or negative,
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the rest of the line is skipped.
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.PP
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It is possible to do more than one command if a condition is true.
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Suppose several operations are to be done before section 1.
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One possibility is to define a macro
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.BD .S1
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and invoke it
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if we are about to do section 1
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(as determined by an
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.BD .if ).
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.P1
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^de S1
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--- processing for section 1 ---
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^^
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^de SH
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^^^
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^if \e\en(SH=1 ^S1
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^^^
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^^
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.P2
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.PP
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An alternate way is to use the
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extended form of the
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.BD .if ,
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like this:
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.P1
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^if \e\en(SH=1 \e{--- processing
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for section 1 ----\e}
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.P2
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The braces
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.BD \e{
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and
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.BD \e}
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must occur in the positions shown
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or you will get unexpected extra lines in your output.
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.UL troff
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also provides
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an `if-else' construction,
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which we will not go into here.
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.PP
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A condition can be negated by preceding it with
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.BD ! ;
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we get the same effect as above (but less clearly) by using
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.P1
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^if !\e\en(SH>1 ^S1
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.P2
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.PP
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There are a handful of
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other conditions that can be tested with
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.BD .if .
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For example, is the current page even or odd?
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.P1
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^if o ^tl 'odd page title''- % -'
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^if e ^tl '- % -''even page title'
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.P2
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gives facing pages different titles and page numbers on the
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outside edge when used inside an appropriate new page macro.
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.PP
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Two other conditions
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are
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.BD t
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and
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.BD n ,
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which tell you whether the formatter is
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.UL troff
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or
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.UL nroff .
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.P1
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^if t troff stuff ...
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^if n nroff stuff ...
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.P2
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.PP
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Finally, string comparisons may be made in an
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.BD .if :
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.P1
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^if 'string1'string2' stuff
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.P2
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does `stuff' if
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.ul
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string1
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is the same as
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.ul
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string2.
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The character separating the strings can be anything
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reasonable that is
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not contained in either string.
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The strings themselves can reference strings with
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.BD \e* ,
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arguments with
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.BD \e$ ,
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and so on.
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