<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_shell(self, environ):
 """Find a command to run. """

        command = self.headings[self.default_heading].get('shell')
        if not command and os.name != 'nt':
            command = environ.get('SHELL', '')
        command = shlex.split(command) if command else None
        return command