How set External Webcam as a default webcam?

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http://askubuntu.com – I have HP 18 AIO Desktop, which has inbuilt web cam. How can I set an external Webcam (Logitech C170) as a default cam from command ? Is there any way, Please help me. Cheese webcam booth or other software can do this but I wan to do it on system based so that it will be work on all webcam related software. Thanks! Birendra (HowTos)