How to restart laptop usb ports in the Terminal or by other method without restart?

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http://askubuntu.com – The usb ports of my laptop fail for some reason from time to time. This concerns apparently only the external mouse and keyboard connected by usb, while connecting an external hard disk to the same ports shows no problem with these. All gets back to normal after restart. How could I get the same effect without restart? In fact I should have edited this question so as to emphasize the basic problem: why this happens? (But now I got answers to the specific workaround.) But please feel free to address this broader issue: why mouse and keyboard fail and other devices don't and how to resolve th (HowTos)