How to use mogrify to crop an image

Just found the answer based on chorobas approach:

mogrify -chop 0x20+0+0 -gravity South image.jpg

This site helped.

Using -chop with -gravity should do the work:

mogrify image.jpg  -gravity south -chop x20

Nothe that gravity must precede the geometry settings to work. See ImageMagick for details