Identifying ports already in use
$ lsof -i -n -P | grep TCP | grep 4000
beam.smp 28569 mnishiguchi 49u IPv4 0x4d3d8b73de14af89 0t0 TCP *:4000 (LISTEN)
$ kill -9 28569
$ lsof -i -n -P | grep TCP | grep 4000
beam.smp 28569 mnishiguchi 49u IPv4 0x4d3d8b73de14af89 0t0 TCP *:4000 (LISTEN)
$ kill -9 28569