(use posix utils)

(define (call-process-string command input)
  (receive
   (i o pid)
   (process command '())
   (display input o)
   (close-output-port o)
   (let ((output (read-all i)))
     (close-input-port i)
     output)))

(call-process-string "tac" "foo\nbar\nbaz\n")