debian-unattended/custom/generate_ssh_cluster.sh

#!/bin/sh

# This if statement evaulates if the script is launched using sudo AND not
# a `sudo su` logged user
if [ -n "$SUDO_USER" ] && [ "$SUDO_COMMAND" != "/usr/bin/su" ]; then

    echo "Creating SSH key..."
    su -c 'ssh-keygen -t rsa -b 4096 -q -N "" -f ~/.ssh/id_rsa' "$SUDO_USER"

    echo "Adding SSH key to authorized..."
    su -c 'cat ~/.ssh/id_rsa.pub >> ~/.ssh/authorized_keys' "$SUDO_USER"

else

    echo "Creating SSH key..."
    ssh-keygen -t rsa -b 4096 -q -N "" -f ~/.ssh/id_rsa

    echo "Adding SSH key to authorized..."
    cat ~/.ssh/id_rsa.pub >> ~/.ssh/authorized_keys

fi