Index: svnmerge.py
===================================================================
--- svnmerge.py	(revision 32538)
+++ svnmerge.py	(working copy)
@@ -191,10 +191,11 @@
         pass
 
     # Parse the output of stty -a
-    out = os.popen("stty -a").read()
-    m = re.search(r"columns (\d+);", out)
-    if m:
-        return int(m.group(1))
+    if os.isatty(1):
+        out = os.popen("stty -a").read()
+        m = re.search(r"columns (\d+);", out)
+        if m:
+            return int(m.group(1))
 
     # sensible default
     return 80