Judge Lord Uist told Graham Elrick, 26, at the High Court in Edinburgh: "Your victim must have been terrified."
The judge said he was "appalled" by CCTV footage of the robbery at the Key Stores convenience store in Aberdeen, which was shown in court. Advocate depute Douglas Fairley, QC, said that after the armed raid the victim, Kwok Fong, 28, had left his job as he was afraid to work there alone and at night, and had trouble sleeping.
Elrick, of no fixed abode, who has previous convictions for violence, admitted assaulting Mr Fong on October 21 last year and stealing cash, cigarettes and alcohol. Defence lawyers said he "deeply regretted" his actions.
Lord Uist told Elrick it was clear from his record he was "a career criminal".