.if

.if [!]condition anything

If condition is true, do anything. The presence of an ! negates the condition. If anything runs over more than one line, it must be delimited by \{ and \}.

Conditions

o	True if the page number is odd.
e	True if the page number is even.
n	True if the processor is nroff.
t	True if the processor is troff.
"str1"str2"	True if str1 is identical to str2. Often used to test the value of arguments passed to a macro.
expr	True if the value of expression expr is greater than zero.

Expressions

Expressions typically contain number register interpolations and can use any of the following operators:

+ -	Addition, subtraction
/ *	Multiplication, division
%	Modulo
< >	Less than, greater than
<= >=	Less than or equal, greater than or equal
= ==	Equal
!	Logical negation
&	Logical AND
:	Logical OR

Note: expressions are evaluated left to right; there is no operator precedence. Parentheses may be supplied to force a particular evaluation order.

Example

Inside a macro definition, set the spacing and print the second argument. (The extra backslashes are necessary in \\$2. One backslash is stripped off when the macro is first read, so the second one is needed for it to be evaluated correctly when the macro is executed.)

.if t .nr PD 0.5v  \" Set spacing between ms paragraphs
.if !"\\$2"" \{ \" If arg 2 is non null, print it in bold
\fB\\$2\fP\}