四则运算

#!/bin/bash
	read -t 10 -p "please input two number ": a b
	expr $a + $b> /dev/null
if [ $? -ne 0 ];then
	echo "please input zhengshu" 
else
	echo  "$a+$b=$(($a+$b))"
	echo  "$a-$b=$(($a-$b))"
	echo  "$a/$b=$(($a/$b))"
	echo  "$a*$b=$(($a*$b))"
fi