bash how to add numbers

Bash provides a lot of methods how to add numbers. First of all is use $(())

echo $((2+2+1))

Output: 5

If we have variable which has integer value, we can use "$var":

var=5
echo $((6+$var))

Output: 11

We can assign to variable counted numbers:

var=$((3+6))

The value of "var" is 9 now.

Another arithmetic expansion:
var=$((num1 + num2))
var=$(($num + $num2))
var=$((num1 + 10 + 20))
var=$[num1+num2]

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