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We can calculate slope and aspect from a DEM with r.slope.aspect. First we reset the current GRASS region settings to those of the input map:
g.region rast=elevation.dem -p
r.slope.aspect el=elevation.dem as=aspect_30m sl=slope_30m
d.rast aspect_30m
d.rast.leg slope_30m
Both maps are calculated in one step. Note that horizontal angles are counted counterclockwise from the East. Slopes are calculated by default in degrees. The d.rast.leg command adds a simple legend to the monitor.
There are additional modules which work with DEMs: depression areas can be filled with r.fill.dir, and flowlines calculated with r.flow. Watershed analysis can be done with r.watershed and, on massive grids, with r.terraflow.