How to find the PID of a process using a specific port

To find out what processes are using a specific port, use lsof.

lsof -i :PORT

Examples

List all processes using the 5432 port (commonly used by postgres)

$ lsof -i :5432
COMMAND  PID   USER   FD   TYPE             DEVICE SIZE/OFF NODE NAME
postgres 777    1ma    7u  IPv6 0x0123456789abcdef 0t0  TCP localhost:postgresql (LISTEN)
postgres 777    1ma    8u  IPv4 0x1123456789abcdef 0t0  TCP localhost:postgresql (LISTEN)

List all processes using the 4000 port (this blog in development)

$ lsof -i :4000
COMMAND   PID   USER   FD   TYPE             DEVICE SIZE/OFF NODE NAME
ruby    59182    1ma   10u  IPv4 0x2123456789abcdef      0t0  TCP localhost:terabase (LISTEN)

Alternatives

netstat can be used with grep to look for the port, but not possible in macOS as netstat won't output the process ID.

ps -ef can be used with grep too, but we'll need some bash sorcery to keep the first line of the ps output (the headers line).