/var/run/reboot-required

file /var/run/reboot-required
stat /var/run/reboot-required
ls /var/run/reboot-required

#!/bin/bash
if [ -f /var/run/reboot-required ]; then
bash
  echo 'reboot required'
fi

alias rr='if [ -f /var/run/reboot-required ]; then echo "reboot required"; else echo "No reboot needed"; fi'

The file /var/run/reboot-required.pkgs will list the packages that require the reboot.