ppm3d --version (return code: 0)
ppm3d: Using libpbm from Netpbm Version: Netpbm 10.0
ppm3d: BSD defined
ppm3d: RGB_ENV='RGBDEF'
ppm3d: RGBENV= 'RGBDEF' (env vbl is unset)
ppm3d -h (return code: 1)
usage: ppm3d leftppmfile rightppmfile [horizontal_offset]