Hide all windows & show the desktop by command.

You can use wmctrl as follows:

wmctrl  is  a  command  that  can  be  used  to  interact with an X Window manager that is compatible with the EWMH/NetWM specification.  wmctrl can query the window manager for information, and it can request  that  certain window management actions be taken.

Fisrt Install wmctrl:
sudo apt-get install wmctrl

From It's manpage:

-k  ( on  |  off  )
              Turn on or off the window manager's "show the desktop" mode (if the window manager implements this feature).

So, Run following command to "show the desktop"
wmctrl -k on