ubuntu etho0 up cron

sudo touch if_interface_up.sh

sudo chmod 777 if_interface_up.sh

sudo vi if_interface_up.sh

#!/bin/bash

while true
do
   result=`cat /sys/class/net/enp0/carrier`
   echo ${result}
   if [ ${result} == '0' ]; then
       `ifconfig enp0 up`
   else
       `sleep 1`
   fi
done

sudo vi /etc/rc.local