How to detect and solve port occupancy in linux

Sometimes we may find that a program is occupying a port, and we cannot use this port.
We can use two commands to solve this problem.

Find the program

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netstat -anp | grep 4000
tcp6       0      0 :::4000                 :::*                    LISTEN      81970/hexo

Here we can find the pid 81970 before the program name.

Kill the program

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kill -9 81970

If the permissions are not enough, add sudo before the command.