Run Grid

$ java -jar selenium-server-standalone-2.53.1.jar -role hub -hubConfig hub_config.json

note Selenium Grid 3.4.0 is failed to establish sessions:

Run Appium

$ node . --nodeconfig ~/GitHub/ruby_lib/grid/config.json

# or

$ appium --nodeconfig ~/GitHub/ruby_lib/grid/config.json

Run Ruby Script

$ rake ios # for example