Fastest way to shutdown Ubuntu Mate 18.10

On Ubuntu, the command shutdown now allows you to shut down the system without being a root user. For a one-click solution in Mate, associate this command to a panel launcher.

Beware not to click this by accident, because it will heavily disrupt your work. But then, you asked a one-click solution.

For a two-click solution, Mate might still might offer a "traditional" shut down button you can add to the panel next to the 'Gear' icon. If not, then you still can create your own two-click shut-down launcher for mate-session-save --shutdown-dialog (gnome-session-quit --power-off for Gnome users).

How about a keyboard shortcut?

Go to your keyboard settings - for me, that's Settings ⇒ Devices ⇒ Keyboard(not sure what it is in Mate, but should be quite similar), scroll down to the bottom and click the + to add a new

Then, name it, the command you'll want is sudo poweroff (or sudo shutdown now does the same thing, really) - and set it to whatever keyboard shortcut you want. I recommend super+h, since that's not taken by default, and since super+l is lock- but obviously, you can do whatever you want.... including maping it to an F-key of your choice.

This works for me on Ubuntu 18.04

sudo poweroff

another option is to shutdown

shutdown –h 09:30

Power off the system.

     --help      Show this help
     --halt      Halt the machine
  -p --poweroff  Switch off the machine
     --reboot    Reboot the machine
  -f --force     Force immediate halt/power-off/reboot
  -w --wtmp-only Don't halt/power-off/reboot, just write wtmp record
  -d --no-wtmp   Don't write wtmp record
     --no-wall   Don't send wall message before halt/power-off/reboot