src/Pure/System/isabelle_system.scala

--- a/src/Pure/System/isabelle_system.scala	Thu Mar 15 11:27:32 2018 +0100
+++ b/src/Pure/System/isabelle_system.scala	Thu Mar 15 11:44:07 2018 +0100
@@ -144,6 +144,10 @@
 
   def cygwin_root(): String = getenv_strict("CYGWIN_ROOT")
 
+  def isabelle_id(): String =
+    proper_string(getenv("ISABELLE_ID")) getOrElse
+      Mercurial.repository(Path.explode("~~")).id()
+
 
 
   /** file-system operations **/