It happened at about 7.30pm on Wednesday 4 March 2015, when the man entered the shop in Middleton Shopping Centre.
He demanded money, but he was disturbed and left the shop empty handed towards the Dusty Miller pub.
He is described as white, about 6ft 3in tall, of slim build and was wearing a light grey hooded top with his hood up and the bottom of his face covered. He was carrying a large knife with an eight inch blade.
Detective Constable Dan Daly from Middleton CID, said: â€œI am appealing for anyone to get in touch if they were in the area at the time of the incident and saw a man matching this description.
â€œLuckily he was disturbed before he could steal any money from the shop, but the shop assistant was left traumatised after he threatened him with a large hunting knife which he pointed at him after he jumped over the counter.
â€œIf you are any information about the incident please call us on 0161 856 8701 or independent charity Crimestoppers on 0800 555 111.â€