Alexander Garland, 23, was in Kens Kebabs in Guildhall Walk, and had got food with his friend.
They then spoke to victim Edwin Randell, who was out with his friend in the early hours of January 2 last year.
Rob Harding, prosecuting, said something had happened that caused ‘friction’ and Garland tried to get the victim to come outside the shop.
Portsmouth Crown Court heard Garland was pacing around outside the shop and gesturing for Randell to come outside and fight.
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When the victim did not come out, Garland went back into the kebab shop and punched Randell, breaking his jaw. The court heard both men were then ejected from the kebab house.
Garland, of Broadmeadows Lane, Waterlooville, admitted the attack to police and later admitting causing grievous bodily harm in court.
Guy Russell, mitigating, said: ‘He doesn’t usually drink. On this occasion it was his birthday, the day before.
‘Foolishly he was persuaded to go out.
‘He finds drinking in large crowds very difficult to do.
‘This evening he drank far too much for his own good.
‘He thought that Mr Randell had offered to him to go outside.
‘He actually tried to apologise and see if Mr Randell was alright at the end of the fracas.’
Sentencing, judge Sarah Munro handed Garland a 12-month community order with 20 days of rehabilitation activities.
Addressing the defendant in the dock, judge Munro said: ‘Because he didn’t come out, you went back into the shop.
‘You punched him once to the jaw causing a fracture.
‘You gave yourself up to the police and admitted the offence in interview, to your credit.’
The judge said Garland had been ‘in drink and drugs’ to boost his confidence.
Mr Russell said: ‘What one sees in the dock is not one would normally get most of the time. He’s very pleasant and appears hard-working when he can get a job.’
It comes as Kens in Guildhall Walk lost its late-night licence.