  We prove shock formation results for the compressible Euler equations and
related systems of conservation laws in one space dimension, or three
dimensions with spherical symmetry. We establish an $L^\infty$ bound for $C^1$
solutions of the one-D Euler equations, and use this to improve recent shock
formation results of the authors. We prove analogous shock formation results
for one-D MHD with orthogonal magnetic field, and for compressible flow in a
variable area duct, which has as a special case spherically symmetric three
dimensional flow on the exterior of a ball
