I needed a little tool to do ifdefs, for conditional inclusion of text; I didn't want the full panoply of the C preprocessor. I like this. Test-driven development in awk.

ifdef.awk

#!/usr/bin/awk -f

# Experimental implementation of ifdef in awk

BEGIN { 
	depth = 0;
	defined = "";
	printing = 1;
}
$1 ~ /#DEFINE/ {
	defined = defined ":" $2;
	next;
}
$1 ~ /#IFDEF/ {
	if (printing) {
		printing = match( defined, $2);
	} 
	if (!printing) {
		depth ++;
    }
	next;
}
$1 ~ /#ENDIF/ {
	depth --;
	if (printing == 0 && depth < 1) {
		printing = 1;
	}
	next;
}
{
	if (printing) {
		print($0);
	}
}

ifdef.test

#DEFINE test1
#DEFINE test3

1 This should print anyway.
#IFDEF test1
2 This should print because test1 is defined.
#ENDIF
3 This should print anyway.
#IFDEF test1
4 This should print because test1 is defined.
#IFDEF test2
5 This should NOT print because test2 is not defined.
#IFDEF test3
6 This should NOT print even though test3 is defined, because still in scope of test2.
#ENDIF
#ENDIF
7 This should print because out of scope of test2.
#ENDIF
8 This should print anyway.
#IFDEF test3 
9 this should print because test3 is defined.
#IFDEF test2
10 This should not print because test2 is not defined.
#ENDIF
11 This should print because out of scope of test2.
#ENDIF
12 This should print anyway.

Summary: Lines 1, 2, 3, 4, 7, 8, 9, 11 and 12 should print; all
others should not.