Linux 'netstat' command finds out which process is listing upon a port. Obviously, you usually need it when something doesn't allow to start/restart an application or docker.

If you type the following command, you will get the list:
netstat -tulpn

Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp6       0      0 :::9100                 :::*                    LISTEN      1168/./node_exporte
tcp6       0      0 :::9090                 :::*                    LISTEN      1272/./prometheus
tcp6       0      0 :::9093                 :::*                    LISTEN      1324/./alertmanager
tcp6       0      0 :::9094                 :::*                    LISTEN      1324/./alertmanager
udp6       0      0 :::9094                 :::*                                1324/./alertmanager

To close the process, enter the command like this:
kill -TERM 1168

1168 is the value from the first column of the netstat command.