In Linux, processes and ports have a symbiotic relationship. One cannot exist nor function without the other. Processes tend to share ports and some processes rely on a specific port to meet an operating system objective.
This article guide gives you a walkthrough on how to free a port used by one process in order to use it on another process.
How to List a Port with its Associated Process
If you can list a targeted port and a process PID, you can kill the process associated with it. There are several tools you can use to list actively used ports in Linux.
A more flexible recommendation or approach is through the network statistics (netstat) command or ss command (faster than netstat). It is a default installation in most Linux distributions but you can install it with the following command if it’s not present.
Install Netstat and SS Command in Linux
$ sudo apt-get install net-tools iproute [On Debian/Ubuntu & Mint] $ sudo dnf install net-tools iproute [On CentOS/RHEL/Fedora and Rocky Linux/AlmaLinux] $ pacman -S netstat-nat iproute [On Arch Linux] $ emerge sys-apps/net-tools iproute [On Gentoo] $ sudo dnf install net-tools iproute [On Fedora] $ sudo zypper install net-tools iproute [On openSUSE]
To list a port and find out if it is locked to a particular process or service, adhere to the following syntax rule.
$ sudo netstat -ltnp | grep -w ':[targeted_port]' OR $ ss -ltnp | grep -w ':[targeted_port]'
For example, processes or services listening to port 80 can be listed in the following manner.
$ sudo netstat -ltnp | grep -w ':80' OR $ ss -ltnp | grep -w ':80'
Under the command string “netstat -ltnp”, the option
-p points to 1520 which is the PID of the process you wish to kill. Take note of it.
How to Create Dummy Process in Linux
So as not to mess with the ecosystem of your Linux operating system, we will create some dummy processes and use them to demonstrate the objective of this article. We will use the “socat” command-line utility to create our demo processes.
Let’s have three processes under SCTP, TCP, and UDP protocols respectively. We are going to bind these new processes to a random port like 9999.
The following command will generate a temporary process with a PID (Process ID) of 8695.
$ socat sctp-listen:9999,bind=127.0.0.1 stdout &
The following command will generate a temporary process with a PID (Process ID) of 9080.
$ socat tcp-listen:9999,bind=127.0.0.1 stdout &
The following command will generate a temporary process with a PID (Process ID) of 9270.
$ socat udp-listen:9999,bind=127.0.0.1 stdout &
How to Kill a Process with its PID (Process ID)
"kill -9" command, we can get rid of a process that is holding hostage a port number we want to use on another prioritized process. In this case, we created three processes with PIDs 8695, 9080, and 9270.
To kill the process under PID 8695, I ran the following command:
$ kill -9 8695
We can even kill more than one process at a go. To kill the created processes with PIDs 9080 and 9270, use a command similar to this one:
$ kill -9 9080 9270
Whether you are killing one process or more than one process at the same time, the execution of the kill command should lead to an empty terminal instance as depicted above.
To confirm that the processes are no longer active, we will try to re-run the kill command:
$ kill -9 8695
An error message will indicate that the process is no longer active. Also, the two recently killed processes  9080 and  9270 are showcased as no longer active.
The rules of killing a running process are simple. First, identify the port you want to use through the “netstat” or “ss” command and then take note of the PID attached to the process. Finally, Use the “kill -9” command together with the identified PID to get rid of that process.