==== after reboot ====
<code>
@reboot /path/to/start_file_server.sh
</code>

five minutes after reboot
<code>
@reboot sleep 300 && /path/to/start_file_server.sh
</code>

It works for normal users (does not have to be root).

Ref:-
  * Go to the man page ["man 5 crontab", https://manpages.debian.org/bullseye/cron/crontab.5.en.html ] -> search for reboot

{{tag>["run a command as a user after machine starts"]}}