Selecting the iOS Simulator device type with RubyMotion

I found that the easiest way to do this is to add the following to the end of your Rakefile

desc "Run simulator on iPhone"
task :iphone4 do
    exec 'bundle exec rake device_name="iPhone 4s"'
end

desc "Run simulator on iPhone"
task :iphone5 do
    exec 'bundle exec rake device_name="iPhone 5"'
end

desc "Run simulator on iPhone"
task :iphone6 do
    exec 'bundle exec rake device_name="iPhone 6"'
end

desc "Run simulator in iPad Retina" 
task :retina do
    exec 'bundle exec rake device_name="iPad Retina"'
end

desc "Run simulator on iPad Air" 
task :ipad do
    exec 'bundle exec rake device_name="iPad Air"'
end

Then you can run rake iphone5 in your terminal and it will open the simulator for that device.

Run /Applications/Xcode.app/Contents/Developer/usr/bin/simctl list (or /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/usr/bin/simctl list for older versions of Xcode.

show your simulators following is my simulator devices

== Devices == -- iOS 7.0 -- -- iOS 7.1 -- iPhone 5s (971DB3D4-7FF4-4005-A11D-11541ED79193) (Shutdown) -- iOS 8.0 -- iPhone 5s (EE64F798-6CB9-40B1-8B19-30727C3CA538) (Shutdown) iPhone 6 Plus (D9F2BEEE-D341-4080-8A49-24AB6FACD9D9) (Shutdown) iPhone 6 (81229508-4D35-4BEE-B616-FB99FDC6BCDD) (Booted) iPad 2 (F2484155-E4A2-44E9-A113-AAF4B9A83717) (Shutdown) Resizable iPhone (B762046B-1273-4638-B0ED-A7827A822BDD) (Shutdown) Resizable iPad (AACAB77A-12BD-43F3-A847-3D11575F3BF3) (Shutdown)

if you want run iPhone 5s as IOS 7.1 (You must set app.deployment_target = '7.1'),you can do it like
rake device_name="iPhone 5s (971DB3D4-7FF4-4005-A11D-11541ED79193)"