This is an ultra simple shell script to send an email notification if an Ubuntu machine needs a reboot for packages to be upgraded.

reboot-required.sh

#!/bin/bash
# Simple script to send an email when a server restart is required.

# Requires mailutils package and will send the email to the local user by default.

if [ ! -f /var/run/reboot-required ]; then
    # No reboot required
    exit 0
else
    # Reboot required, so notify by email
    USER=$(whoami)
    PACKAGE=`cat /var/run/reboot-required.pkgs`
    /usr/bin/mail -s "Reboot required on $HOSTNAME" $USER<<EOF
Some packages require a reboot for their upgrades to be applied on $HOSTNAME:

$PACKAGE
EOF
    exit 0
fi