The 28-year-old man suffered a head injury when he was assaulted by a man who demanded money from Suleman Newsagents in Kennedy Path, Townhead, Glasgow, around 07:40am today.
He was taken by ambulance to Glasgow Royal Infirmary, where his condition is described as "serious but stable".
The attacker, who police said was carrying a weapon, made off with a small sum of money and cigarettes and was last seen running towards St Mungo's Road.
He is white, around 6ft and in his 30s. He was wearing a black waterproof jacket with a hood, dark blue jeans with a white logo on the back and white trainers.
Detective Constable David Hardie said: "This was an unprovoked and particularly violent attack on an innocent member of the public who was simply doing his job.
"It is vital that we trace the man responsible, and I would therefore urge anyone who saw this incident, or anyone who perhaps saw the suspect in the area around the time of the robbery this morning, to get in touch."
Meanwhile, police in Edinburgh are investigating a series of jewellery robberies that happened in Longniddry, Gullane and North Berwick on Thursday and yesterday.
The houses were empty at the time of the break ins which happened in Douglas Road, Longniddry, High Street, North Berwick, and Whin Road, Gullane.
Inspector Stuart Reid said: "Anyone who recalls seeing anything or anyone suspicious between these times in the vicinity of these properties is asked to contact the police immediately.
"The homeowners are very distressed at the theft of their possessions and the damage to their properties."
Officers asked anyone with information to contact them on 101.