This paper considers straight line path following control of wheel-less planar snake robots using sliding mode techniques. We first derive the Poincaré representation of the equations of motion of the robot using the techniques of differential geometry. Furthermore, we use partial feedback linearization to linearize the directly actuated part of the system dynamics. Subsequently, we propose an analytical solution to the robust path following control problem in two steps. In the first step, we use sliding mode techniques to design a robust tracking controller for the joints of the robot to track a desired gait pattern. In the second step, we stabilize an appropriately defined sliding manifold for the underactuated configuration variables of the robot, thereby guaranteeing convergence of the robot to the desired straight path. The paper presents simulation results which validate the theoretical results.