How to add values in a variable in Unix shell scripting?

Question

I have two variables called count1 and count7

count7=0
count7=$(($count7 + $count1))

This shows an error \"expression is not complete; more tok

Answer 1

In ksh ,bash ,sh:

$ count7=0                     
$ count1=5
$ 
$ (( count7 += count1 ))
$ echo $count7
$ 5