Blender rendering from the command line.

animation_config.py

import sys
import bpy
import addon_utils

argv = sys.argv
argv = argv[argv.index("--") + 1:]  # get all args after "--"
    
C = bpy.context
bpy.context.scene.cycles.device = 'GPU'
cycles_prefs = C.user_preferences.addons['cycles'].preferences
cycles_prefs.compute_device_type = "CUDA"

# Don't step on the work of the other threads
bpy.data.scenes["Scene"].render.use_overwrite = False
bpy.data.scenes["Scene"].render.use_placeholder = True

# Turn off all GPUs
for device in cycles_prefs.devices:
	device.use = False
  
# And then choose only the one we want
cycles_prefs.devices[int(argv[0])].use = True

bpy.ops.render.render(animation=True)

image_config.py

import sys
import bpy

C = bpy.context
bpy.context.scene.cycles.device = 'GPU'
cycles_prefs = C.user_preferences.addons['cycles'].preferences
cycles_prefs.compute_device_type = "CUDA"

for device in cycles_prefs.devices:
    device.use = True

render.sh

# When rendering an animation, you should use a blender process for each GPU on the system. If you have 4 GPUs, for example:
blender -b myanimation.blend -noaudio -nojoystick --use-extension 1 -E CYCLES -s 0 -e 250 -o ~/myanimation -P animation_config.py -- 1
blender -b myanimation.blend -noaudio -nojoystick --use-extension 1 -E CYCLES -s 0 -e 250 -o ~/myanimation -P animation_config.py -- 2
blender -b myanimation.blend -noaudio -nojoystick --use-extension 1 -E CYCLES -s 0 -e 250 -o ~/myanimation -P animation_config.py -- 3
blender -b myanimation.blend -noaudio -nojoystick --use-extension 1 -E CYCLES -s 0 -e 250 -o ~/myanimation -P animation_config.py -- 4

# The number at the end of each line is passed as an argument to the script, and it chooses which GPU to use for that process.
# The 0th device is the CPU, which is why these start at 1.

# To render just a single frame, it's pretty straightforward:
blender -b myanimation.blend -noaudio -nojoystick --use-extension 1 -E CYCLES -o ~/myanimation -P image_config.py -f 1

#NOTE: option order matters, -f must go at the end of that command.