config.py

import bpy

# https://blender.stackexchange.com/questions/5281/blender-sets-compute-device-cuda-but-doesnt-use-it-for-actual-render-on-ec2
bpy.context.user_preferences.addons['cycles'].preferences.compute_device_type = 'CUDA'
bpy.context.user_preferences.addons['cycles'].preferences.devices[0].use = True

bpy.context.scene.cycles.device = 'GPU'

bpy.data.scenes["Scene"].render.filepath = "/tmp/output.png"
bpy.ops.render.render(write_still=True)

rendering.sh

BLEND="/tmp/foo.blend"
/home/ec2-user/blender --background "$BLEND" \
	--use-extension 1 \
	-noaudio -nojoystick \
	-E CYCLES -t 0 \
	-P "config.py"