四則運算

#!/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