Also, you might find this slow little utility for converting a bunch of stuff into strings. It will do the job nicely, and it accepts every Lisp type.

  (defun str (&rest strings)
    (apply 'concatenate 'string
  	 (loop for x in strings 
               collecting (format nil "~a" x))))

You can use it like this:

  (shell (str "dot -Tpng -O " fname))

You might also find that WITH-OPEN-FILE arguments can get a little unwieldy to type in the repl, specially if you want to overwrite files, so you can use this WITH-TEXT-FILE macro. For binary files you will just need to add (:element-type '(unsigned-byte 8))

  (defmacro with-text-file ((stream path) &body body)
    `(with-open-file (,stream ,path
   			    :direction :io
			    :if-exists :supersede
			    :if-does-not-exist :create)
       ,@body))