--- a/mach
+++ b/mach
@@ -1,13 +1,19 @@
-#!/usr/bin/env python
+#!/bin/sh
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
+# The beginning of this script is both valid shell and valid python,
+# such that the script starts with the shell and is reexecuted with
+# the right python.
+'''which' python2.7 > /dev/null && exec python2.7 "$0" "$@" || exec python "$0" "$@"
+'''
+
 from __future__ import print_function, unicode_literals
 
 import os
 import sys
 
 def ancestors(path):
     while path:
         yield path