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This page is dedicated to sparse and casual tricks with some common GFOSS tools.
This command works with relatively recent Gdal versions:
gdal_merge.py -o multiband.tif -separate band1.tif band2.tif band3.tif
Just use the builtin computing region facility:
g.region rast=original_map -p g.region res=<new> r.mapcalc "new_map=original_map"
A similar trick can be used to cut a portion of the original map, apply a mask, etc.