#!/bin/bash

# get subnet
subnet=$(ip a | grep "inet " | tail -1 | awk '{print $2}')

# get router/gateway
router=$(ip route show | head -1 | awk '{print $3}')

# get size of network portion of address in bytes
sz=$(echo $subnet | awk -F / '{print $2}')
bytes=$(("$sz" / 8))
prefix=$(echo "$subnet" | cut -d. -f1-$bytes)      # e.g., 192.168.0

# get IP address to be set
IP=$(hostname -I | awk '{print $1}')             # current IP
echo -n "Keep IP address?—$IP [yn]> "
read -r ans
if [ "$ans" == "n" ]; then
  echo -n "Enter new IP address: "
  read -r IP
  # check if specified IP is properly formatted
  if [[ ! $IP =~ ^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$ ]]; then
    echo Invalid IP
  fi
  # check if specified IP works for local network
  if [[ ! $IP =~ ^$prefix ]]; then
    echo "ERROR: Specified IP not usable for local network"
    exit
  fi
fi

# check if specified IP is properly formatted
if [[ ! $IP =~ ^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$ ]]; then
  echo Invalid IP
fi

# fetch the UUID
UUID=$(nmcli connection show | tail -1 | awk '{print $4}')
if [[ "$UUID" == "ethernet" ]]; then
 # This is the other format of nmcli connection show
 UUID=$(nmcli connection show | head -2 | tail -1 | awk '{print $3}')
fi

# run commands to set up the permanent IP address
nmcli connection modify "$UUID" IPv4.address "$IP"/"$sz"
nmcli connection modify "$UUID" IPv4.gateway "$router"
nmcli connection modify "$UUID" IPv4.method manual
nmcli connection up "$UUID"