The response of an inviscid shock to external pressure perturbations in a constant area duct is analyzed in terms of fundamental processes like perturbation propagation and its interaction with shock. The results of these elementary processes are formulated analytically and with a Riemann wave tracking method to enable the prediction of shock movement for both upstream and downstream perturbations. The predictions thus obtained are compared with the finite-volume based numerical simulations of the Euler equations. This study shows that the shock responds nonlinearly to perturbations and the nonlinearity has a cumulative effect. Contact surfaces generated during the interaction of normal shock with perturbations, which was ignored in previous investigations, are shown to be important in order to capture this cumulative nonlinearity. The nonlinearity alters the positive and negative duty cycles, which results in a net displacement of shock after responding to one full cycle of sinusoidal perturbation. This drift in shock location is pronounced at low supersonic Mach numbers (1.2-3) but is also present at higher Mach numbers. Furthermore, it is demonstrated that the duty cycle variations are higher for perturbations originating downstream of shock than those originating upstream. The variations in frequency and amplitude are found to merely scale the response and do not introduce any new physics. © 2018 Author(s).