src/Pure/System/system_channel.scala

 
   private def mk_fifo(): String =
   {
     val i = Fifo_Channel.next_fifo()
     val script =
-      "FIFO=\"/tmp/isabelle-fifo-${PPID}-$$" + i + "\"\n" +
+      "FIFO=\"${TMPDIR:-/tmp}/isabelle-fifo-${PPID}-$$" + i + "\"\n" +
       "echo -n \"$FIFO\"\n" +
       "mkfifo -m 600 \"$FIFO\"\n"
     val result = Isabelle_System.bash(script)
     if (result.rc == 0) result.out else error(result.err)
   }