How to Kill a Process Running on Particular Port in Linux?

Have you ever tried to launch a web server, database, or application in Linux, only to be stopped by the frustrating “Address already in use” error? This happens when another process is already occupying the required port, preventing your application from running. Freeing up the port is crucial to ensure smooth system operation, especially when working with Apache, Nginx, Node.js, MySQL, PostgreSQL, Docker, and other services.

In this article, we'll learn how to find and kill a process running on a particular port in Linux using commands like lsof, ss, netstat, and fuser. We’ll cover:

• How to find the Process ID (PID) associated with a specific port
• Different methods to terminate the process safely and effectively
• How to verify that the port is free after termination
• Troubleshooting common issues like permission errors and auto-restarting services.

Common Scenarios where Port Conflicts Occur

• Running a web server (Apache, Nginx, or Node.js) and finding the port already in use.
• Starting a database (MySQL, PostgreSQL, MongoDB) and encountering a binding error.
• Deploying a containerized application where a process is still running on the host machine.

Identify the Process ID (PID) Using the Port Number

Before terminating a process occupying a specific port, the first step is to identify the Process ID (PID). A PID is a unique identifier assigned by the Linux kernel to each running process. Identifying the PID helps prevent accidental termination of unrelated processes.

1. Using lsof (List Open Files Command)

The lsof (List Open Files) command is one of the most powerful utilities to list open files and network connections. Since Linux treats everything as a file (including network sockets), lsof can help find which process is occupying a specific port.

Syntax:

lsof -i :<port_number>

• COMMAND: Name of the process.
• PID: Process ID (needed for termination).
• USER: Owner of the process.
• FD: File descriptor.
• TYPE: Type of connection (IPv4 or IPv6).
• NAME: The associated port.

2. Using ss (Socket Statistics Command)

ss (Socket Statistics) is a modern replacement for netstat, providing faster and more detailed network-related information.

Why Use ss Instead of netstat?

• Faster execution speed.
• More detailed output.
• Installed by default in most modern Linux distributions.

ss -tulnp | grep <port_number>

3. Using netstat (Older Method)

While netstat is deprecated in some Linux distributions, it can still be useful for finding process details.

ss -tulnp | grep <port_number>

• LISTEN: The process is actively listening for connections.
• PID: The process ID of the program using the port.
• fd: File descriptor.
• users: The process name (nc in this case).

Note: If you want to find the PID of processes running in the foreground or background refer the article Get Process ID of Linux 

Methods to Kill the Process Using Its PID

Once you have identified the PID, you can terminate it using one of the following commands.

1. Using kill (Graceful Termination)

The kill command is the standard way to terminate a process in Linux. By default, it sends the SIGTERM (signal 15), which gracefully stops the process, allowing it to save any unsaved data before exiting. For more details read this article How to Kill a Process in Linux

Syntax:

kill <PID>            # This method allows the process to shut down properly, saving any necessary data before exiting.

2. Using fuser to Kill a Process

The fuser command is a quick and efficient way to find and terminate processes associated with a specific port. Instead of searching for the PID manually, fuser directly lists and kills the process.

Syntax:

fuser -k -n tcp <port_number>

• -k kills the process associated with the specified port.
• -n tcp specifies the TCP protocol (use -n udp for UDP ports).

When to Use:

• When you want a one-liner command to kill a process using a port.
• When you don’t want to manually search for the PID.

3. Using lsof to Kill a Process

The lsof (List Open Files) command is widely used to check open connections and identify processes occupying a specific port. It can also be used in combination with kill to terminate a process instantly.

Syntax:

kill -9 $(lsof -t -i :<port_number>)

• lsof -t -i :8080 extracts the PID of the process using port 8080.
• kill -9 $(...) sends a SIGKILL signal to terminate it.

When to Use:

• When you need a quick and efficient method to find and kill a process.
• When you want to avoid manually searching for the PID.

Verify That the Port is Free

Once you have terminated the process occupying a particular port, it’s crucial to verify whether the port has been successfully freed. If another application or process has taken over the port, you may still encounter issues when attempting to start your service.

lsof -i :8080                # If there is no output, the port is now free.

• If there is no output, the port is now free and available for use.
• If a process is still using the port, the output will display the Process ID (PID) and related details.

Troubleshooting Common Issues

Even after following the steps to kill a process, you might encounter errors or unexpected behavior. Below are some common issues and their solutions.

1. Permission Denied: Use sudo when attempting to terminate a process, you may get a "Permission denied" error. This happens when trying to kill a system process or an application running under another user.

sudo kill -9 <PID>

2. Process Keeps Restarting

Some processes are managed by systemd and will restart automatically even after being killed.

sudo systemctl stop <service_name>

Examples:

Stop Apache:

sudo systemctl stop apache2

Stop Nginx:

sudo systemctl stop nginx

3. Process Not Found

If you try to kill a process, but it’s not listed in lsof or fuser, it might have already exited, or another service might be using the port.

Conclusion

Knowing how to find and kill processes using specific ports in Linux is essential for managing server applications, troubleshooting conflicts, and ensuring smooth system operation. Whether you use lsof, ss, fuser, or netstat, these commands help you quickly identify and terminate unwanted processes, freeing up ports for essential applications.