Writing logs for OpenVPN in Ubuntu18

I've installed OpenVPN on Ubuntu18, and it works. I'd like to have a log containing only the times when users connect and disconnect. Found a hint how to do it at [https://github.com/furlongm/openvpn-monitor/issues/27]

use OpenVPN's hooks to have it log when users connect/disconnect.It's trivial to write a script that reads env vars and writes them toa log file. Just add that script to OpenVPN's config file as:client-connect /path/to/script client-disconnect /path/to/script

I understand how to edit the config file, but not the part about environment vars.