Is there a keyboard command for 'show desktop'?

Solution 1:

By default, it's set to Ctrl+Alt+D. You can set your own shortcut by going to System → Settings → Keyboard Shortcuts and changing "Hide all normal windows and set focus to the desktop" located under "window management":

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Solution 2:

I tried both Super+d and Ctrl+Alt+d to see the default for Ubuntu 14.04 and neither worked. Then again, I didn't bother to change the default setting. Try Ctrl+Super+d, thats what worked for me.

btw, the Super key for any computer with Windows automatically installed is the key that looks like the Windows logo.