Find (and kill) process locking port 9000 on Mac

1. You can try netstat

   netstat -vanp tcp | grep 9000
   

2. For macOS El Capitan and newer (or if your netstat doesn't support -p), use lsof

   lsof -i tcp:9000
   sudo lsof -i :9000
   

3. Kill:

   kill -9 <PID>
   

PLEASE NOTE: -9 kills the process immediately, and gives it no chance of cleaning up after itself. This may cause problems. Consider using -15 (TERM) or -3 (QUIT) for a softer termination which allows the process to clean up after itself.