When changes are made to `/etc/network/interfaces` on a server system without physical or kvm switch access, it comes in handy to have a script that resets your network configuration to the previous state and restarts the networking in case of network connectivity loss.
Here is a little bash script that checks if it can reach a certain server via icmp ping after a few seconds of applying the new configuration and resets `/etc/network/interfaces` to your backup file, followed by a `service networking restart` command, if it cant reach the outside world any more. Make sure to execute this command in a `screen` session, otherwise it might be interrupted by your ssh session being closed.

#!/bin/bash
## Simple /etc/network/interfaces test script


## VARIABLES

# Path to new /etc/network/interfaces configuration file:
new_interfaces="/etc/network/interfaces.new"

# Original /etc/network/interfaces configuration file:
current_interfaces="/etc/network/interfaces"

# How long to wait in seconds before restoring original config
test_delay=5

# Ping check host 
ping_host="8.8.8.8"


## MAIN

# Backup original /etc/network/interfaces
cp -v $current_interfaces $current_interfaces.bak

# Apply new configuration
cat $test_interfaces > current_interfaces 
service networking restart

# Wait a little
sleep $test_delay

# Test network connectivity via icmp ping. If it fails, restore $current_interfaces.bak file
if [ ! "$(ping -q -c1 $ping_host)" ]; then
  cat $current_interfaces.bak > $current_interfaces
  service networking restart
fi

# You can also just wait and then restore the old configuration if you do not want to trust the ping check. You might want to set a higher $test_delay for this case to be able to test your networking
# sleep $test_delay
# cat $current_interfaces.bak > $current_interfaces
# service networking restart

exit 0