Linux Program Has Exited but Port is Still Occupied

After the program is closed, the port is already in use. Using

netstat -npla | grep {port_number}

you can see the occupied port and process ID, but using kill prompts that the process cannot be found.

1. Forcefully kill the process occupying the port

      sudo fuser -k -n tcp 80

2. Close a specific occupied port based on PID

      netstat -tunlp | grep 80

      lsof -i:80

      sudo kill -9 PID