added kill command instructions

08 Understanding APIs.md

@@ -36,6 +36,22 @@ If you get a response of `command not found` or equivalent you will need to inst
 sudo apt-get update || sudo apt-get install curl
 ```
 
+### Port in Use
+
+Sometimes when a nodejs script crashes it keeps the port open which means you will get an error if you try to run your API.
+```
+Error: listen EADDRINUSE :::8080
+```
+To fix this you will need to find the PID (process ID) of the script currently using the port and kill it before running your API again.
+
+To see what process is running on a given port (Ubuntu) we can use the [lsof](https://linux.die.net/man/8/lsof) (list open files). The `-i` lists all files of whose internet address matches the address specified (in our case we are looking for any files using the port 8080). Once we have the PID we can pass it to the [kill](http://manpages.ubuntu.com/manpages/xenial/man1/kill.1.html) tool.
+```
+$ lsof -i :8080
+  COMMAND   PID    USER   FD   TYPE   DEVICE SIZE/OFF NODE NAME
+  node    30135 johndoe   12u  IPv6 33466259      0t0  TCP *:8080 (LISTEN)
+$ kill 30135
+```
+
 #### 1.4 Test Your Knowledge
 
 We will practice using cURL using the Web API provided by [GitHub](https://github.com). Your first task is to make sure you have a valid login.