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I have encountered many times trying to run a service that use an existing port.

For example, my jekyll serve:

jekyll 3.1.6 | Error:  Ad dress already in use - bind(2) for 127.0.0.1:4000

This is because my jekyll process somehow wasn’t killed properly, and then I tried to run another process using the same port.

To solve, kill the process that is using the port (eg 4000):

lsof -i :4000
kill -9 <PID>