How do I install handbrake cli?
The best way to install anything on OSX is (IMHO) with Homebrew. You can install Homebrew with this command:

/usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"


Then you can install stuff (like handbrakeCLI) like this:

brew install handbrake


Then you can run it straight away from any command prompt with:

handbrakeCLI ...