Schematic illustration of a fast-mode shock propagating through a self-gravitating molecular cloud with hourglass-shaped magnetic-field lines. The upper panels show a two-dimensional slice in the x–z-plane, in which the cloud is flattened along the z-direction. The grayscale shading represents the gas density. The blue arrows indicate the velocity field, the magenta curves show the magnetic-field lines, and the orange line represents the shock front. (a) Longitudinal slice of the filament, corresponding to cut R1 in Figure 4. (b) Nearly perpendicular slice of the filament, corresponding to cut R2. The velocity v* denotes the velocity in the shock rest frame.