How to permanently delete ALL older kernels

This script will remove ALL versions but two, the currently active and the most recent of the remaining installed versions:

#/bin/bash
keep=2
ls /boot/ | grep vmlinuz | sed 's@vmlinuz-@linux-image-@g' | grep -v $(uname -r) | sort -Vr | tail -n +$keep | while read I
do
    aptitude purge -y $I
done
update-grub

you can specify how many kernels to keep by adjusting the keep variable, if you set it to 1, only the active kernel will be left installed.

Or you can do it in one line:

ls /boot/ | grep vmlinuz | sed 's@vmlinuz-@linux-image-@g' | grep -v $(uname -r) | sort -Vr | tail -n +2 | xargs -r sudo aptitude purge -y

that you can use in crontab.