code: def casperjs_command():
      
      method = app_settings['CAPTURE_METHOD']
      cmd = app_settings['%s_CMD' % method.upper()]
      sys_path = os.getenv('PATH', '').split(':')
      if cmd is None:
          for binpath in sys_path:
              cmd = os.path.join(binpath, method)
              if os.path.exists(cmd):
                  break
      cmd = [cmd]
  
      if app_settings['TEST_CAPTURE_SCRIPT']:
          try:
              proc = subprocess.Popen(cmd + ['--version'], **casperjs_command_kwargs())
              proc.communicate()
              status = proc.returncode
              assert status == 0
          except OSError:
              msg = "%s binary cannot be found in PATH (%s)" % (method, sys_path)
              raise ImproperlyConfigured(msg)
          except AssertionError:
              msg = "%s returned status code %s" % (method, status)
              raise ImproperlyConfigured(msg)
  
      # Add extra CLI arguments
      cmd += app_settings['CLI_ARGS']
  
      # Concatenate with capture script
      app_path = os.path.dirname(__file__)
  
      capture = app_settings['CAPTURE_SCRIPT']
      if capture.startswith('./'):
          capture = os.path.join(app_path, 'scripts', capture)
  
      assert os.path.exists(capture), 'Cannot find %s' % capture
      return cmd + [capture]
docs: Determine which capture engine is specified. Possible options:
      - casperjs
      - phantomjs
      Based on this value, locate the binary of the capture engine.
  
      If setting <engine>_CMD is not defined, then
      look up for ``<engine>`` in shell PATH and
      build the whole capture command.