Even though openHAB has a build-in backup system, as described in openHAB#Backup, I have found out that the backup for sure is not complete (at LEAST the addons folder is not backed up).
So I made a short story alternative backup (on Ubuntu LTS), before testing stuff etc:
openhab-tar-backup.sh
#!/bin/bash now=`date +"%Y_%d_%m"` cd /opt/openhab/ tar -cvzf /home/bnp/openhab-etc-$now.tgz /opt/openhab/*
Or, alternate as (with nice openHAB shutdown first):
openhab-tar-backup.sh
#!/bin/bash now=`date +"%Y_%d_%m"` docker stop openhab cd /opt/openhab/ tar -cvzf /home/bnp/openhab-etc-$now.tgz /etc/openhab2/* docker start openhab