This tutorial will explain how to show desktop from command line in ubuntu.You can use xdotool to do this.
xdotool lets you programatically (or manually) simulate keyboard input and mouse activity, move and resize windows, etc. It does this using X11's XTEST extension and other Xlib functions.
Install xdotool using the following command
sudo apt-get install xdotool
Once you install this tool you can use the following command to show desktop
xdotool key ctrl+alt+d