What is the command to open the shutdown dialog?

/usr/lib/indicator-session/gtk-logout-helper can be used to open the following dialogs:

• Log Out (/usr/lib/indicator-session/gtk-logout-helper --logout)

  

• Restart (/usr/lib/indicator-session/gtk-logout-helper --restart)

  

• Shutdown (/usr/lib/indicator-session/gtk-logout-helper --shutdown)

  

To prompt only the shutdown/restart dialog, run the following command:

gnome-session-quit --reboot

Details on binding this on Ctrl-Alt-Del here.

In Gnome Shell if I try /usr/lib/indicator-session/gtk-logout-helper --shutdown I obtain the dialog in the screenshot you provided, but when I click on Shut Down I receive another dialog (similar to the first). Only after a second confirm my system shuts down.

This method reaches the second dialog directly via command line?

gnome-session-quit --power-off