12.7. Executing an action immediately: the Execute Function

We've been showing you how to use Action factories in the Command function. You can also execute an Action returned by a factory (or actually, any Action) at the time the SConscript file is read by using the Execute function. For example, if we need to make sure that a directory exists before we build any targets,

      Execute(Mkdir('/tmp/my_temp_directory'))
    

Notice that this will create the directory while the SConscript file is being read:

      % scons
      scons: Reading SConscript files ...
      Mkdir("/tmp/my_temp_directory")
      scons: done reading SConscript files.
      scons: Building targets ...
      scons: `.' is up to date.
      scons: done building targets.
    

If you're familiar with Python, you may wonder why you would want to use this instead of just calling the native Python os.mkdir() function. The advantage here is that the Mkdir action will behave appropriately if the user specifies the SCons -n or -q options--that is, it will print the action but not actually make the directory when -n is specified, or make the directory but not print the action when -q is specified.

The Execute function returns the exit status or return value of the underlying action being executed. It will also print an error message if the action fails and returns a non-zero value. SCons will not, however, actually stop the build if the action fails. If you want the build to stop in response to a failure in an action called by Execute, you must do so by explicitly checking the return value and calling the Exit function (or a Python equivalent):

    if Execute(Mkdir('/tmp/my_temp_directory')):
        # A problem occurred while making the temp directory.
        Exit(1)