diff --git a/scsh/scsh.scm b/scsh/scsh.scm
index d405737..04238ce 100644
--- a/scsh/scsh.scm
+++ b/scsh/scsh.scm
@@ -444,7 +444,8 @@
 	     (close-fdes (open-fdes fname oflags #o600))
 	     fname)
 	   (if (null? maybe-prefix) '()
-	       (list (string-append (car maybe-prefix) ".~a"))))))
+	       (list (string-append (constant-format-string (car maybe-prefix))
+				    ".~a"))))))
 
 (define *temp-file-template*
   (make-fluid (string-append "/tmp/" (number->string (pid)) ".~a")))
@@ -463,6 +464,23 @@
 		  (loop (+ i 1)))))))))
 
 
+;; Double tildes in S. 
+;; Using the return value as a format string will output exactly S.
+(define (constant-format-string s)	; Ugly code. Would be much clearer
+  (let* ((len (string-length s))	; if written with string SRFI.
+	 (tilde? (lambda (s i) (char=? #\~ (string-ref s i))))
+	 (newlen (do ((i (- len 1) (- i 1))
+		      (ans 0 (+ ans (if (tilde? s i) 2 1))))
+		     ((< i 0) ans)))
+	 (fs (make-string newlen)))
+    (let lp ((i 0) (j 0))
+      (cond ((< i len)
+	     (let ((j (cond ((tilde? s i) (string-set! fs j #\~) (+ j 1))
+			    (else j))))
+	       (string-set! fs j (string-ref s i))
+	       (lp (+ i 1) (+ j 1))))))
+    fs))
+
 
 ;;; Roughly equivalent to (pipe).
 ;;; Returns two file ports [iport oport] open on a temp file.