[darkr]

If you're using Xscreensaver instead of xlock:

    #!/usr/bin/env bash
    set -e
    while true; do
        sleep 20m
        xscreensaver-command -activate
        sleep 20s
        xscreensaver-command -deactivate
    done

This presumes that xscreensaver is configured to have a delay before it is locked (and thus requiring a password to unlock - as would happen if we used `xscreensaver-command -lock`).