Apple has introduced an interesting tool with Xcode 6!
simctl
simclt allows you to control the simulators running
run xcrun simctl to get the list of available subcommands. Lots of new options to play around.
Now to do what I wanted. Here is the command to launch simulator
xcrun instruments -w "iPhone 5 (8.0 Simulator)"
-w is to provide device type and to get the list of available devices, just execute this
xcrun instruments -s
After launching the simulator you can control it using simctl
To install you app
xcrun simctl install booted <app path>
To launch the app
xcrun simctl launch booted <app identifier>
Just run this
open -a simulator