Once on one server it was necessary to monitor the integrity of the file in which the changes were made, and if someone performed a system update, then this file could change and corrections had to be made again.
Usually I use a Zabbix agent and a Zabbix server for such purposes, but this server was single and it makes no sense to install a Zabbix server on it, so I wrote a simple script:
if md5sum -c /scripts/md5/filename.md5; then echo "OK" else (echo "Subject:File changed"; echo "File /dir/filename.txt changed on $HOSTNAME!";) | sendmail firstname.lastname@example.org md5sum /dir/filename.txt > /scripts/md5/filename.md5 fi
Add to /etc/crontab (for example, to check every 2 hours):
1 */2 * * * root /scripts/md5/script.sh >/dev/null 2>&1
As a result, if the file is changed, the script will send a notification to the specified email, and the new hash will be recorded in filename.md5, that is, the notification will be sent to the email only once after the file has been changed.