macOS command line utility to retrieve the CGWindowID of the specified window (useful for screencapture).
Via Homebrew:
brew install smokris/getwindowid/getwindowid
./GetWindowID <application-bundle-name> <window-title>
For example, to get the ID of the iOS/tvOS/watchOS Simulator:
./GetWindowID Simulator 'iPhone X - iOS 11.0'
…and to capture a screenshot of it:
screencapture -l$(./GetWindowID Simulator 'iPhone X - iOS 11.0') simulator.png
To get the ID of a window without a title, pass an empty string as the title:
./GetWindowID GLFW ''
To list all of an app's windows, pass --list as the title:
./GetWindowID Simulator --list