Port collision is a problem that can happen when the port you are attempting to run your app on is already in use:

Port 4200 is already in use. Use '--port' to specify a different port.

This often happens if you accidentally leave open a previous instance of your application while trying to start another. On Unix systems, you can find and kill orphaned processes using the ps and grep commands:

ps aux | grep angular

This will return a process list you can consult to find your rogue Angular-CLI process:

nmcclay    88221   0.0  1.8  3398784 296516   ??  S     7:56PM   0:12.59 @angular/cli

Now that you have found your process, you can stop it with the kill command:

Kill 88221

Another common cause of this error can be running multiple different web applications locally. In this case, the best option is to simply change the default port used in the NPM start command of your package.json file:

 “start”: “ng serve --port 4500”,