Csh setenv and `` don't like each other!

[Jonathan I. Kamens]

In article <11744 at jarthur.Claremont.EDU>, dfoster at jarthur.Claremont.EDU (Derek R. Foster) writes:
|> > alias test 'echo "Test OK"'  <- Define an alias...
|> > test                         <- It works correctly
|> TEST OK
|> > echo `test`                  <- Even in backquotes
|> TEST OK
|> 
|> > setenv A `test`              <- But not after setenv. WHY?
|> `test`: Ambiguous              <- What on earth does "Ambiguous" mean here?
|>                                <- (Why on earth do UNIX system programmers
|>                                <- fear self-explanatory error messages so
|>                                <- much?)

  At some point in the past, this question was brought up in the context of
why this doesn't work:

	kill `echo 255 256`
	`echo 255 256`: Ambiguous.

but this does:

	/bin/kill `echo 255 256`

Someone who knows far more about the internals of csh than I do (I believe it
was Doug Gwyn, although my memory may be failing here) posted a very good
explanation of what's going wrong here.  I was hoping that whoever posted that
explanation last time would post it again, because I didn't save it, but it's
been five days and that hasn't happened, so I'll take a stab at trying to
explain what's going on (since the article is going to expire from my spool in
a day or so :-).

  First of all, I should point out that the shell *is* correct to complain
about

	setenv A `test`

because the setenv command takes two arguments, and you're passing it three
("A", "Test", and "OK").  The problem is that, because of csh brain-damage
when parsing arguments to builtin functions, it doesn't quite get the correct
error message.  Things work when you do

	setenv A "`test`"

or at least they do for me:

	% alias test 'echo "Test OK"'
	% setenv A "`test`"
	% echo $A
	Test OK
	%

  The "ambiguous" error is telling you that csh is expecting to see one
argument at that point in the command line, and it's seeing multiple arguments
instead.  The problem, I believe, is that the backquote substitution takes
place too place for the code that checks for "too many arguments" (the correct
error) to notice it.

|>                                <- but for another file....
|> > echo "`source /hmc2/hmc_1993/dfoster/project2/host_supports`"
|>  jove vi tex  msg
|> 
|>                                <- it breaks, and the program is never run.
|> > setenv A "`source /hmc2/hmc_1993/dfoster/project2/host_supports`"
|> `source /hmc2/hmc_1993/dfoster/project2/host_supports`: Ambiguous.

  What is in the file /hmc2/hmc_1993/dfoster/project2/host_supports?  I can't
seem to duplicate this problem:

	% cat > /tmp/host_supports
	echo "jove vi tex  msg"
	% echo "`source /tmp/host_supports`"
	jove vi tex  msg
	% setenv A "`source /tmp/host_supports`"
	% echo $A
	jove vi tex msg

Without more information, it's difficult to tell you why you're having this
problem or how to solve it.

-- 
Jonathan Kamens			              USnail:
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