read network interfaces

#!/bin/bash


echo
echo


DEVS=($(cat /proc/net/dev | tail -n+3 | cut -d: -f1 | tr -d " "))

for i in "${DEVS[@]}"
do
  
  ip link show $i | grep -q "state DOWN" # check interface status
  if (( $? == 1 ))
  then
    # interface is up
    echo "$i:"
    ipv4=$(ip addr show $i | grep -o -P "inet ([0-9\.]+)" | cut -d' ' -f2)
    ipv6=$(ip addr show $i | grep "inet6 " | sed -n 's/.*inet6 \(.*\)\/.*/\1/p')
    if [[ "$ipv4" != "" ]]
    then
      echo "	IPv4 $ipv4"
    fi
    if [[ "$ipv6" != "" ]]
    then
      echo "	IPv6 $ipv6"
    fi
  else
    # interface is down
    #echo "$i (is down)"
    do=nothing
  fi
  
done

DEFAULT_ROUTE=$(ip route | grep default | sed -n 's/default via \(.*\) dev.*/\1/p')
DEFAULT_ROUTE_DEV=$(ip route | grep default | sed -n 's/default via .* dev \([^ ]*\) .*/\1/p')

echo "default route: $DEFAULT_ROUTE (over $DEFAULT_ROUTE_DEV)"


echo
echo

have a look at http://askubuntu.com/questions/265072/ubuntu-linux-12-10-message-of-the-day to see how to put it into the message-of-the-day in ubuntu systems

call the script in /etc/rc.local to adjust the /etc/issue file on system start:

/usr/local/bin/show_network_config.sh >> /etc/issue