Arduino yun Automatic On/Off control

I need following Help!
Now i buy a arduino yun
Is it's possible to do a automatic on/off based on present temperature,via online without human interface?
If it is possible plz tell me how to do?
Thanks in Advance...

As the arduino is not on it cannot read the temperature and turn on. You therefore need another system to apply power that starts the Arduino and it locks itself on.

You can use a LM35 temperature sensor feeding a comparitor. Once the required temperature is reached, it can turn on the power to the Arduino.