diff -r c42d65e11b6e -r 9a06e10f1a5c src/Pure/System/isabelle_system.scala
--- a/src/Pure/System/isabelle_system.scala	Sun Apr 12 11:23:36 2015 +0200
+++ b/src/Pure/System/isabelle_system.scala	Sun Apr 12 11:38:05 2015 +0200
@@ -257,8 +257,8 @@
   /* mkdirs */
 
   def mkdirs(path: Path): Unit =
-    if (bash("mkdir -p " + shell_path(path)).rc != 0)
-      error("Failed to create directory: " + quote(platform_path(path)))
+    if (path.is_dir || bash("mkdir -p " + shell_path(path)).rc == 0) ()
+    else error("Failed to create directory: " + quote(platform_path(path)))