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
