shell 脚本中的运算

#!/bin/bash
read -p "please Insert two integer values: " a b
if [ ! -n "$a" -o ! -n "$b" ]
then
echo "please insert two values!"
exit 1
fi
expr 1 + $a &>/dev/null  #整数加法
A=$?
expr 1 + $b &>/dev/null
B=$?
 
if [ "$A" -ne 0 -o "$B" -ne 0 ]
then
echo "Please insert two int!"
exit 1
fi
echo "$a-$b=$(($a-$b))"
echo "$a+$b=$(($a+$b))"
echo "$a"x"$b"="$(($a*$b))"
echo "$a"/"$b"="$(($a/$b))"
echo "$a%$b=$(($a%$b))"    #求余
let r=$a**$b              #乘幂
echo "$a"**"$b"=$r