RC Waveform output signal confusion
Both square and triangular waveforms are possible, as the asymptotic shapes in the limits of very high or very low frequencies into the RC filter. At middling frequencies, you get a an exponential shape.



The legend is the number of RC time constants that a single high or low input period, a half cycle, lasts.

You can see that for 0.2, the output amplitude is small. The voltage across the resistor doesn't change much during the high or low time, so the current is more or less constant. The output waveform approximates to a triangle, and will approximate more closely as the number of RCs per half cycle falls.

For 10 RCs per half cycle, the output is almost square. For 18, it's more nearly square again.

The 'classic' RC output we tend to draw is that for 3 RCs per half cycle. Clearly neither square nor triangular.