If you want to kill a process running on port number 8080 then first you need to find the 8080 port process identification number(PID) and then kill it. Run the following command to find 8080 port number PID:
sudo lsof -t -i:8080
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So you can now easily kill your PID using following command:
sudo kill -9 <PID>
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You can use one command to to kill a process on a specific port using the following command:
sudo kill -9 $(sudo lsof -t -i:8080)
For more you can see the following link How to kill a process on a specific port on linux