xvfb.py complains if command points to a directory.

testing/xvfb.py

Index: testing/xvfb.py
diff --git a/testing/xvfb.py b/testing/xvfb.py
index a5620e7cde4072d7bd8b5f6bef54b27af767d9e1..3ca9aa8aa154902b87d410a4b93ce223015b27a8 100755
--- a/testing/xvfb.py
+++ b/testing/xvfb.py
@@ -6,6 +6,7 @@
 """Runs tests with Xvfb and Openbox on Linux and normally on other platforms."""
 
 import os
+import os.path
 import platform
 import signal
 import subprocess
@@ -80,10 +81,19 @@ def run_executable(cmd, env):
 
 
 def main():
+  USAGE = 'Usage: xvfb.py [command args...]'
   if len(sys.argv) < 2:
-    print >> sys.stderr, (
-        'Usage: xvfb.py [command args...]')
+    print >> sys.stderr, USAGE
     return 2
+
+  # If the user still thinks the first argument is the execution directory then
+  # print a friendly error message and quit.
+  if os.path.isdir(sys.argv[1]):
+    print >> sys.stderr, (
+        'Invalid command: \"%s\" is a directory' % sys.argv[1])
+    print >> sys.stderr, USAGE
+    return 3
+
   return run_executable(sys.argv[1:], os.environ.copy())