William George Hemingsley, 49, of Wesley Street in Newtown, Powys, was warned by a judge that in drink, the knife could have killed his victim.
Hemingsley admitted a wounding charge and was jailed for two years at Mold Crown Court.
He was cleared by a jury of the more serious charge of wounding with intent to cause GBH.
Judge Niclas Parry said that Hemingsley was “clearly a man not to be crossed” even for the most minor of matters.
His character was summed up by a support worker who described the defendant as a man who had a short fuse and who was unpredictable.
The danger increased when he had taken “a considerable amount of alcohol”, which he had consumed that night.
The starting point for sentence was 18 months going up to three years.
His case was aggravated by his previous convictions.
“Your reaction to being angered was to reach for a knife and to lash out. That knife could have landed anywhere. You could have killed him,” Judge Parry warned.
It was the prosecution case that it was an unprovoked attack on an innocent man who was stabbed in the back.
Prosecuting barrister Matthew Dunford said that on January 27 this year, the defendant stabbed Christopher Barnes in the back “with a large knife” and was heard shouting aggressively.
At the time Hemingsley was living in supported accommodation in a one bedroomed flat at The Wallich in Newtown.
The prosecution alleged that both victim Christopher Barnes and the defendant Hemingsley were heavy drinkers.
Both had been drinking and at some stage Mr Barnes fell asleep. In the morning, Mr Barnes went out for some food and some more drink and he was joined by a female friend.
The three of them carried on drinking together and Mr Dunford said there did not appear that there were any problems.
At around 2.30 p.m, Mr Barnes and his friend left and as he went out into the hallway he felt a thud in his shoulder.
He looked back and saw the defendant standing there with his hand raised.
The defendant moved forward for a second time and Mr Barnes put his foot out to stop him.
Mr Dunford said that the friend Marie Griffiths saw the defendant holding a knife in his hand.
It was just the blade, with no handle attached which he used to stab Mr Barnes in the back.
A support worker heard a commotion and was able to get the knife from the defendant.
Outside the building, Mr Barnes heard the defendant shout ‘Come here and I’ll finish you off’, it was alleged.
Police officers arrived shortly afterwards and found that Mr Barnes was bleeding from around a hole in his shirt on his back.
They seized a non-serrated single edged blade that was about 23cm long.
Mr Barnes was taken by ambulance to the Royal Shrewsbury Hospital.
He was found to have a wound of approximately two cm over his right shoulder which was less than one cm deep.
The court heard the knife had the defendant’s blood on the handle and the victim’s blood on the blade. Defence barrister John Hedgecoe produced a psychiatric report said that his client had made several attempts to stop taking drugs and at the time of the incident he was drinking and not taking his medication.