Teaching Cyber Security (Part 3)

This is the 3^rd post of a series for teaching cyber-security in a coding-club. Read [part 2]

Introduction

The shell is sometimes called terminal, command line interface or CLI. It's the place where you can send commands to the Kernel. Or the place where you can send commands behind the curtain.

This technology works with text commands typed line by line. You should have access to a shell which looks something like this:

source: linuxcommand.org

Ok, so let's get our current username:

1. Click on the CLI
2. Type: whoami
3. Press the return key

Easy right? You've learned your first command:

Command: whoami

Description: The whoami utility displays your current user ID as a name.
The little brain icon will indicate a definition which you need to learn.

It's common to make typos

When typing commands in the CLI, it's common to make typos. Don't be afraid of some weird output in case you've typed something wrong, even the best developers make mistakes.

After typing the whoami command, you will have got:

user@machine:~/home$
user

The first line, where you typed the command, is called the command prompt. In the second line, we've got an output after executing that command by pressing the return key.

Basically, this is how the CLI works:

Source: codio.com

One important question to ask is how does the CLI recognize the whoami command?

How does the CLI recognize commands

Pay attention to the 2nd step of the program execution flow:

After hitting the return key, the CLI takes the whoami command and looks for a program with the same name somewhere in the computer:

command = program name

To better understand this, try to type any word into the command prompt and hit the return key, we'll use anyword as an example:

user@machine:~/home$  anyword
-bash: anyword: command not found 

Did you get similar output?

The command line programs

The CLI recognizes several program names depending on the computer and the operating system that lives in the computer. A typicall shell or terminal window has an installed software called: bash.

Bash

Bash is the name of the software that makes the interaction between the user and the terminal window possible. Bash has a vast catalog of program names, each of them performs different actions.

Ready to learn another bash command?

We already know how to get the current user with the whoami command, but what about getting the current working directory?

In other words, where we are right now inside the computer files and folders structure?

Where we are right now inside the computer?

Similar to the Mac's Finder or the Windows Document Explorer, the command line interface provides an alternative for navigating, displaying, editing, copying and moving files and folders across your computer's storage.

We will learn about it in [Part 4]