Iíll be back by the weekend, unless there is a train strike.
Iíll be back by the weekend, if the train drivers arenít on strike.
Does unless suggest that the action is unlikely to happen,
whether If suggests that the action may happen ?
'Unless' can be used instead of 'if..not' (not if) when we refer to exceptional circumstances which whould change a situation.(like your examples).
Both have the same meaning, but they aren't always used interchangeably.
see this example:
He will be happy if you don't go there.
(You can't change it to 'He will be happy unless you go there.)
We don't use unless to refer to something negative that would be the main cause of the situation that we are talking about.
And what about: "He won't be happy unless you go there" ? It doesn't have the same meaning as "He will be happy if you don't go there", does it?
According to all my grammar books (Practical English Usage and other books),
when the subordinate clause becomes a reason of the content of the major clause, it should be started with if...not (not unless). I also wanted to differentiate between unless and if..not, and this is what I've found out!
In conclusion, in the sentence that 'He won't be happy if you don't go there.', if-clause is a reason of why he won't be happy. Therefore, it is correct, but you can't use unless here instead of if..not. However, those conjunctions have the same meaning. I mean, you just can't say 'He will be happy unless you go there.'.