Updates basics_of_linux.md

# Basics of linux

## 1. Basic commands

This is a huge chapter. I could divide it up in many subchapters but I like to have it all at one place so I can just do `ctr-f`, and search for whatever I am looking for.

## The Shell - Bash

Bash is the standard shell on most Linux distros.

**pwd**

Every listening process of course has a PID, but unless you are root you can't might not see them all.

### Firewall - Iptables

Iptables is a firewall tool in linux. A firewall is basically a tool that scans incoming and/or outgoing traffic. You can add rules to the iptables to filter for certain traffic.