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Check If A Port Is In Use

The lsof command is used to list open files. This includes listing network connections. This means I can check if a particular port is in use and what process is using that port. For instance, I can check if my rails application is currently running on port 3000.

$ lsof -i TCP:3000
COMMAND   PID       USER   FD   TYPE             DEVICE SIZE/OFF NODE NAME
ruby    13821 jbranchaud   12u  IPv6 0xdf2e9fd346cc12b5      0t0  TCP localhost:hbci (LISTEN)
ruby    13821 jbranchaud   13u  IPv4 0xdf2e9fd33ca74d65      0t0  TCP localhost:hbci (LISTEN)

I can see that a ruby process (my rails app) is using port 3000. The PID and a number of other details are included.

See more details with man lsof.

h/t Mike Chau