Check whether linux server needs reboot and return for how long has the reboot been necessary in PRTG's format

checkreboot.sh

#!/bin/bash

#You can install and also update this script by running the follwoing command
# sudo curl https://gist.githubusercontent.com/jvacek/18aaf9858d8ab8016297bee5e71f59ef/raw/checkreboot.sh -L --create-dirs -o /var/prtg/scripts/checkreboot.sh; sudo chmod a+rx -R /var/prtg/

#Create a new sensor for your machine, select SSH script, and select checkreboot.sh from the script dropdown.

#The value returned is in seconds. 0 seconds means no file is present.
#The sensor will be set to warning when the file is present.
#If you want the sensor to go down when a reboot has been required for more than 24h, set the upper error limit of the valu channel on the sensor to 86400.
#This is handy in case you have unattended-upgrades configured with reboots and want to make sure that updates apply as they should.

if [ -f /var/run/reboot-required ]; then
        age=$(( $(date +%s) - $(date -r /var/run/reboot-required +%s) ))
        echo "1:$age:Restart Required"
else
        echo "0:0:No restart required"
fi