LINUX:编写一个shell脚本,并利用函数实现数列求和运算

#!/bin/sh
fun()
{
sum=0
for i in `seq $1 $2`
do
sum=`expr $i + $sum`
done
echo $sum
}
echo "Please input 2 number:"
read a b
expr $a + 0 1>/dev/null 2>&1
if [[ -ne 0 ]];then
echo "wrong number"
exit 1;
fi
expr $b + 0 1>/dev/null 2>&1
if [[ -ne 0 ]];then
echo "wrong number"
exit 1;
fi
if [[ $a -ge $b ]];then
echo "wrong number"
exit 1;
fi
fun $a $