popen behavior - interactive processes

[Allister MacLeod <listopad at mv3d dot com>]

How come (ice-9 popen) only does one-way pipes?  I'm interested in
using the guile interpreter as a replacement for expect/Tcl, so I'd
like to be able to set up a background process with ports for both
input and output (or possibly a bidirectional port, if such a thing
exists)

So, I've been able to get close to what I want by looking at popen.scm
and modifying it a little.. something like

(define (open-two-way)
  (define (pipe-nobuf) (let ((p (pipe))) (setvbuf (cdr p) _IONBF) p))
  (define (shutoff in out) (map close-port (list (cdr in) (car out))))
  (let ((c2p (pipe-nobuf)) (p2c (pipe-nobuf)))
    (let ((pid (primitive-fork)))
      (cond ((= pid 0)
             (set-batch-mode?! #t) ;blindly copied from popen.scm
	     (shutoff p2c c2p)
	     (let loop ((x (read-char (car p2c))))
	       (write-char (if (char=? x #\space) #\. x) (cdr c2p))
	       (loop (read-char (car p2c)))))
            (else
	     (shutoff c2p p2c)
	     (cons (cons (car c2p) (cdr p2c)) pid))))))

This works just fine..  I can feed in characters and they come out the
same except for spaces which turn into dots.  What I can't figure out
how to do is make the same thing work with a couple of move->fdes
calls and an execle instead of the loop (which, btw, is there a better
way to write that kind of loop whose argument is always the same
expression?)
I just keep ending up with zombies to feed to waitpid.  Maybe I'm
misunderstanding the way move->fdes works, but when I replace the loop
with : 

(move->fdes (car p2c) 0)
(move->fdes (cdr c2p) 1)
(apply execlp "/bin/sh" "/bin/sh" ())

sh seems to die before writing any characters to the port.  On the
other hand, when I substitute /bin/date for /bin/sh, I'm able to pull
characters from it.

I guess the real question is, am I even looking in the right place?
Is it conceivable that an interactive process can be spawned this way?
If not, is there some other way that's better?