I have a system that runs an up-to-date Ubuntu 14.04 LTS, with Xfce as the window manager and the default login/session manager. (LightDM with the Unity greeter? It certainly looks very similar to this.)

What I want to do is to ensure that whenever I press the power button briefly (to trigger an ACPI shutdown signal), the system immediately begins a clean shutdown.

In a logged-in session, I simply went into Applications > Settings > Power Manager (the Xfce Power Manager) > General and set When power button is pressed to Shutdown. This works nicely.

In a console session, without having made any changes to that effect (that I remember), when I briefly press the power button the system initiates a clean shutdown. That's what I want.

However, if I do the same thing on the login screen, then I am prompted for what I want to do. I don't want any kind of confirmation. (Yes, I am aware of the drawbacks of that, and am willing to live with them. No need to educate me about that.)

For that final step towards my ultimate goal: How do I configure the system such that also at the login screen, if I press the power button briefly then the system initiates a clean shutdown?

GUI or terminal-based solutions are equally welcome as answers. (For GUI-based solutions, please keep in mind the fact that I'm not running the Unity desktop manager.)

How to disable shutdown/reboot from lightdm in 14.04? seems to be basically the very opposite of what I want, and most of the other potentially useful questions I found are either dated or seem to only affect already logged-in sessions (which I have already solved). That said, if this really is a duplicate of some other question here, feel free to close as such.

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