In automobiles, are we using the car chassis as a ground terminal of battery? Then why we don't get shocked while touching it?
To get a shock requires a sufficient current through the body. That requires a few things.

Firstly under normal circumstances you need enough voltage to break through the skin barrier. How much voltage this is does vary depending on circumstances but the normal rule is under dry conditions voltages up to about 50 volts are considered safe to touch. 

Secondly (ignoring static electricity which does lead to shocks but they are too short in duration to be dangerous to the body and ignoring capacitie coupling which is not relevant at DC) current normally flows in loops. No part of the cars electrical system is deliberately connected to the earth, so when you touch the cars body and the earth at the same time there is really nowhere for current to go. The voltage between different parts of the cars chassis will normally be a small fraction of the (already low enough to be safe) battery voltage.