Having been blessed with an unreliable internet, I always have had the need to change the default route when one goes down. ip provides you the required tooling to do that. I had been doing it so often, that I ended up writing a function to do this:

route_to () {
    interface=$1
    route=$(ip route | grep $interface | sed -r 's/\.0\/[[:digit:]]{2,3} /.1 /' | cut -d ' ' -f -4)
    if [ -n "$route" ]; then
        sudo ip route del default &> /dev/null
        sudo ip route replace default via $route
    else
        echo "No routes match: $interface" >&2
    fi
}

With this in available in my shell, I can simply do route_to wlan0 and my system will start using wlan0 as the default interface.

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