Police said three men armed with a handgun and a knife went into Bastwell's on Whalley New Road, Blackburn, on Monday evening.
One man forced the shopkeeper to hand over the till's contents - about £700 - before attacking him with the knife.
The 32-year-old was taken to the Royal Blackburn Hospital with lacerations to his head. Police are hunting the gang.
A Lancashire Constabulary spokesman said they were last seen crossing a footbridge leading to Wesley Street.
The gunman is described as having a slim build and was wearing dark clothing, a hooded top and a Halloween mask.
The knifeman was also wearing dark clothing, a hooded top and a scarf concealing his face.
Police said the third was wearing a light-coloured or green balaclava.
Det Sgt Paul Ryan, leading the investigation, said: "This was a very serious incident and very traumatic for the victim, who was subjected to needless violence and was in the shop alone at the time."
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