The current $i$ in a coil varies with time as shown in the figure. The variation of induced emf with time would be:

Induced emf is given by:
$$ϵ=-L\frac{di}{dt}$$
In a triangular $i$ -$t$ graph, the slope $\frac{di}{dt}$ is constant in each segment (positive then negative).Therefore, the induced emf $ϵ$ will be a constant negative value where the slope is positive, and a constant positive value where the slope is negative. This results in a step/square wave function.