The annoying port 8080 is already in use message and not knowing which process is occupying this port sucks. I knew it was lsof that can help me. But how exactly? TL;DR: ps -p `lsof -i :8080 -t` -o command= (on Mac OSX)

I just wanted to start the react-native app via react-native run-ios. It didn't start the first time. Because of my built-in laziness I just ran the command again. Ran it again, and read the output, just than

Port 8080 already in use, packager is either not running or not running correctly

Ah the port is in use, that's why react-native didn't come up. There was this handy linux command lsof (list open files). A simple web search later and I found a great article on it, which showed the solution lsof -i :8080 where 8080 is the port. When you run this you might get something like this:

> lsof -i :8080
node    178 wk     13u  IPv4 0xaa0005c2e1d30003      0t0  TCP *:http-alt (LISTEN)

This is a nicely formatted table. It first lists node which is the command that was run and is still running and occupying the port 8080.

Which program is running?

In order to find out the full command was run the PID (process identifier) will help us. Fortunately we got it in the line above, see the second column, it's 28161. By adding -t to the lsof command we can receive only the process ID.

> lsof -i :8080 -t

Now we can pass the PID into ps (process status) and we will get all info about the process running, like so:

> ps -p 178
  PID TTY           TIME CMD
  178 ttys006    0:01.00 node /Users/wk/cosmowiki/node_modules/.bin/http-server dist

In the last column we can see the command that was actually run. So let's call ps so that it will JUST list the command that is running and occupying port 8080. We do that by adding -o command which tells ps to just show the command. This will leave an useless headline above the command. We can remove it by passing an empty headline, which ps will ignore, we do that by using the parameter we pass to -o as a key-value pair and leave the value (the headline) empty. We use -o command=, looks strange but works :).

> ps -p `lsof -i :8080 -t` -o command=
node /Users/wk/cosmowiki/node_modules/.bin/http-server dist

This results in just the command that I had run and which occupies port 8080.

Kill the running process

Now I know why and I know what I have to stop, if I want to stop the process right now, I can call

> kill `lsof -i :8080 -t`

Some words about lsof

Just play around with it, and you will see there are many open files, and on a unix style system most of the things are files, that's why a line count of a lsof run will also show you quite a big number of lines.

> lsof | wc -l