weird if-then-else behavior in "csh"

[Chris Torek]

In article <540 at acf2.NYU.EDU> mckenney at acf2.NYU.EDU (Alan Michael McKenney) writes:
>... The only difference between the [csh if/then/else example that works
>and the one that does not] is that the second "if" in the second file is
>immediately followed by "(", rather than " (".  (In general, the "if"
>command seems to require spaces in many non-intuitive places.)

In general, the C shell's `parser' does not.  (Parse, that is.)

>    Is this a bug, a feature, or what?

How could it be anything but a bug?  It is not documented. :-)

>    And further, is there any good guide as to where spaces are required
>in "if" statements in "csh" (other than just putting them everywhere
>that they are legal)?

The trick is that, when skipping a false `if', the C shell uses a
completely different routine than when it is reading commands for
the purpose of actually executing them.  The latter routine breaks
things at spaces, parentheses, and other `special' characters, while
the former breaks only at spaces.  The false-if-skipping-code looks
for a line which, after being broken at spaces, starts with `if'
and ends with `then', with code like the following:

	count = 1;
	do {
		read line;
		break at spaces;
		if (strcmp(word[0], "endif") == 0)
			count--;
		else if (strcmp(word[0], "if") == 0 &&
		    strcmp(word[nwords-1], "then") == 0)
			count++;
	} while (count);

So:

	if (0) then		# enter above pseudo-code
		if then		# count now reaches 2
		if obviously&*csh[ would (never? like this` then # now 3
		endif this is bad	# now 2
		endif		# now 1
	endif			# count reaches 0
	echo this gets echoed.	# and we revert to the normal parser

But if you take out the first `if (0) then' you get an error (`if:
Expression syntax'); if you take out the first and second you get a
different error (`Unmatched `.').  If you put arguments after `endif'
inside a true `if' you get still another error.

The false-if-skipping-code recognises only `first word is "if",
last word is "then"', and breaks only at whitespace, so if you always
put a space after an `if' and one before a `then', you should never
have trouble.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris at mimsy.umd.edu	Path:	uunet!mimsy!chris