Script to count number of incoming packets per second?

Currently, I am trying to write a script that will run upon startup (Debian Linux), and count the number of incoming packets per second every ten seconds. The goal of this is to study different ways to mitigate DoS attacks, but I don't really know how to setup this base function of the script. I have done quite a bit of research into the best way to accomplish this, but really cannot find what I'm looking for. Since I'm a beginner in Linux scripting, can someone describe the process to accomplish this in layman's terms? Thank you so much!

Try this :

#!/bin/bash

old_packets=0

while true; do
    packets=$(awk '$1 == "eth0:"{print $3}' /proc/net/dev)
    if ((old_packets)); then
        clear
        echo "$(bc <<< "($packets - $old_packets) / 10") packets/seconds"
    fi
    old_packets=$packets
    sleep 10
done

where eth0 is the targeted interface.

Edit:

from comments, if you want to disable eth0 if the rate exceed 20 packets/seconds :

#!/bin/bash

old_packets=0

while true; do
    packets=$(awk '$1 == "eth0:"{print $3}' /proc/net/dev)
    if ((old_packets)); then
        clear
        # the variable rate contains the packets/seconds
        rate=$(bc <<< "($packets - $old_packets) / 10")
        echo "$rate packets/seconds"
        if ((rate>20)); then
            ip link set eth0 down
        fi
    fi
    old_packets=$packets
    sleep 10
done

If you want to run this script at startup, you can edit your /etc/rc.local file, it's executed at the end of boot process.