Robert Burton walked into the Costcutter store in Newmarket, Louth, just before it was due to close at 10pm and ordered the shopkeeper Nadarajah Kirubaharan to empty his pockets.
Gareth Gimson, prosecuting, said “Burton’s face was covered and he was carrying a large black-handled bread knife.”
Burton told the shopkeeper: “Take your money out of your pockets or I’ll stab you.”
Mr Gimson said: “Mr Kirubaharan recognised the defendant’s voice almost immediately.
“Because of the disguise and the weapon Mr Kirubaharan felt extremely scared.
“Burton repeatedly told him to empty his pockets and then began physically assaulting him.
“Mr Kirubaharan was pushed to the floor. He could feel the defendant pulling at his trouser pockets to get his money.
“There was a point where he managed to get the knife from Burton.”
Mr Gimson said that during the assault Mr Kirubaharan pleaded with Burton to stop but he continued with Burton pinning the shopkeeper to the ground by putting his foot on his head and delivering more blows.
The attack ended when Burton picked up the till drawer which contained around £300 and walked out of the store.
Mr Kirubaharan suffered facial injuries including a broken tooth and bruising to his left cheek.
Burton later left Louth but was traced to an address in Wales and arrested.
Burton ,28, of no fixed address, admitted charges of robbery and threatening a person with an offensive weapon as a result of the incident on April 18 this year.
Recorder William Harbage QC passing sentence, told Burton “This was an appalling offence. The shopkeeper speaks of being extremely scared.
“It must have been a very frightening experience for him.”
Mark Watson, in mitigation, said that Burton suffers from a long standing mental health disorder and may have taken too much medication.
Mr Watson said: “The knife was there to threaten as opposed to being used.
“In his interview he said he was sickened and disgusted by what he did. It may be that he was not thinking straight at the time.”
Mr Watson said that Burton has been receiving help for his mental health issues while on remand in custody and when released plans to move away from the Louth area.