shell 设置超时时间

等待20s后退出

a=
b=
while(true)
do
        if [ $a -eq $b ]
        then
           echo "20s ！！！"
           break
        else
                if [[ true ]]; then
                        sleep
                        let "a=a+1"
                        echo $a
                else
                        break
                fi
        fi
done