Today I'd like to expand a bit more on the basic calculations in Unix scripts.
Use Parenthesis to simplify math expressions
In Basic Arithmetic Operations post I've shown you how expression evaluation can be used to calculate simple math expressions in your Unix shell:
ubuntu$ START=1 ubuntu$ FINISH=10 ubuntu$ ELAPSED=$(($FINISH - $START)) ubuntu$ echo $ELAPSED 9
Although this approach looks clean enough, there's a way to simplify it even further if you put everything in parenthesis. In other words, the same result (ELAPSED receiving a correct value of FINISH value minus START value) can be achieved this way:
ubuntu$ ((ELAPSED=FINISH-START)) ubuntu$ echo $ELAPSED 9
It's a matter of preference, and I used to always do such calculations the former way shown, but now that one of the readers of this blog pointed out the latter way of using it, I think I might change my habits – I actually like this way much better.