src/Pure/ML/ml_process.scala

   val isabelle_tool = Isabelle_Tool("process", "raw ML process (batch mode)",
     Scala_Project.here,
     { args =>
       var dirs: List[Path] = Nil
       var eval_args: List[String] = Nil
-      var logic = Isabelle_System.getenv("ISABELLE_LOGIC")
+      var logic = Isabelle_System.default_logic()
       var modes: List[String] = Nil
       var options = Options.init()
 
       val getopts = Getopts("""
 Usage: isabelle process [OPTIONS]