UNIX for Beginners Questions & Answers

#! /bin/bash
# file: whileloop.sh

arr1=(2 4 6 8)
arr2=(3 6 9 12)
arr3=()

indextotal=(${#arr1[@]})
i=0
sum=0
while [[ ${#arr1[@]} -eq ${#arr2[@]} ]] && [[ $i -le $indextotal ]] # ${#arr[@]} depicts the size of an array
do
    sum=$(( "${arr1[i]}"+"${arr2[i]}" )) 
    #echo $sum
    arr3=("${arr3[@]}" "$sum")
    i=$(( i+1 ))
done
echo "${arr3[@]}"

whileloop.sh: line 13: +  : syntax error: operand expected (error token is "+  ")
5 10 15 20

[[ $i -lt $indextotal ]]