I want to accumulate the line size of a number of files contained in a folder. I have written the following script:

```
let a=0
let num=0
for i in folder/*
do
        num=`cat $i | wc -l`
        a=$a+$num
done
echo $a

```

What i am geting at the end of the script is 123+234+432+... and not the result of the arithmetic operation of addition.