<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def cmd_nsh(self, args):
        '''nsh shell commands'''
        usage = "Usage: nsh <start|stop|set>"
        if len(args) < 1:
            print(usage)
            return
        if args[0] == "start":
            self.mpstate.functions.input_handler = self.send
            self.started = True
            self.mpstate.rl.set_prompt("")
        elif args[0] == "stop":
            self.stop()
        elif args[0] == "set":
            self.serial_settings.command(args[1:])
        else:
            print(usage)