Shell program to print Fibonacci series

!/bin/bash
echo " Enter the lenght of series "
read n
t1=0
t2=1
s=`expr $t1 + $t2`
echo " $s "
for((i=0;i<n;i++))
        do
                t1=`expr $t2`
                t2=`expr $s`
                s=`expr $t1 + $t2`
                echo " $s "
        done