Mac-to-Ubuntu and missing your ⌘ key?
I've found a useful configuration option for if you're using Ubuntu on a Windows-layout keyboard and you're used to Apple's keyboards and ⌘ (Command)-shortcuts: System → Preferences → Keyboard → Layouts → Options… → Alt/Win key behavior → Control is mapped to Alt keys, Alt is mapped to Win keys
With this option enabled, the Alt keys (in the Mac ⌘ position on the keyboard) function as Control keys, and the Windows keys (in the Mac Alt/Option position) function as Alt.
Having just discovered this, I find it quite handy for the years of muscle memory of ⌘W and other such shortcuts. (No, I haven't switched to Ubuntu; I just frequently find myself using it for software compatibility testing and so on.)