Introduction to the command-line interface

For readers at home: this chapter is covered in the Your new friend: Command Line video.

It’s exciting, right?! You’ll write your first line of code in just a few minutes! :)

Let us introduce you to your first new friend: the command line!

The following steps will show you how to use the black window all hackers use. It might look a bit scary at first but really it’s just a prompt waiting for commands from you.

Note Please note that throughout this book we use the terms ‘directory’ and ‘folder’ interchangeably but they are one and the same thing.

What is the command line?

The window, which is usually called the command line or command-line interface, is a text-based application for viewing, handling, and manipulating files on your computer. It’s much like Windows Explorer or Finder on the Mac, but without the graphical interface. Other names for the command line are: cmd, CLI, prompt, console or terminal.

Open the command-line interface

To start some experiments we need to open our command-line interface first.

It’s probably under Applications → Accessories → Terminal, or Applications → System → Terminal, but that may depend on your system. If it’s not there, you can try to Google it. :)

You now should see a white or black window that is waiting for your commands.

The command-line Prompt

If you’re on Linux, you probably see a $, like this:

command-line

$

Each command will be prepended by a $ and one space, but you should not type it. Your computer will do it for you. :)

Just a small note: in your case there may be something like ola@Olas-PC:~ $ before the prompt sign, and this is 100% OK.

The part up to and including the $ or the > is called the command line prompt, or prompt for short. It prompts you to input something there.

In the tutorial, when we want you to type in a command, we will include the $ or >, and occasionally more to the left. Ignore the left part and only type in the command, which starts after the prompt.

Your first command (YAY!)

Let’s start by typing this command:

command-line

$ whoami

And then hit enter. This is our result:

command-line

$ whoami
olasitarska

As you can see, the computer has just printed your username. Neat, huh? :)

Try to type each command; do not copy-paste. You’ll remember more this way!

Basics

Each operating system has a slightly different set of commands for the command line, so make sure to follow instructions for your operating system.

If you make a typo, you can use the left and right arrow keys to move your cursor, backspace and delete to edit the command. Most command lines don’t support using the mouse to move the cursor.

Let’s try this, shall we?

Current directory

It’d be nice to know where are we now, right? Let’s see. Type this command and hit enter:

command-line

$ pwd
/Users/olasitarska

Note: ‘pwd’ stands for ‘print working directory’.

You’ll probably see something similar on your machine. Once you open the command line you usually start at your user’s home directory.

Learn more about a command

Many commands you can type at the command prompt have built-in help that you can display and read! For example, to learn more about the current directory command:

macOS and Linux have a man command, which gives you help on commands. Try man pwd and see what it says, or put man before other commands to see their help. The output of man is normally paged. Use the space bar to move to the next page, and q to quit looking at the help.

List files and directories

So what’s in it? It’d be cool to find out. Let’s see:

command-line

$ ls
Applications
Desktop
Downloads
Music
...

Change current directory

Now, let’s go to our Desktop directory:

command-line

$ cd Desktop

Note that the directory name “Desktop” might be translated to the language of your Linux account. If that’s the case, you’ll need to replace Desktop with the translated name; for example, Schreibtisch for German.

Check if it’s really changed:

command-line

$ pwd
/Users/olasitarska/Desktop

Here it is!

PRO tip: if you type cd D and then hit tab on your keyboard, the command line will automatically fill in the rest of the name so you can navigate faster. If there is more than one folder starting with “D”, hit the tab key twice to get a list of options.

Create directory

How about creating a practice directory on your desktop? You can do it this way:

command-line

$ mkdir practice

command-line

$ cd practice
$ mkdir test
$ ls
test

Congrats! :)

Clean up

We don’t want to leave a mess, so let’s remove everything we did until that point.

First, we need to get back to Desktop:

command-line

$ cd ..

Using .. with the cd command will change your current directory to the parent directory (that is, the directory that contains your current directory).

Check where you are:

command-line

$ pwd
/Users/olasitarska/Desktop

Now time to delete the practice directory:

Attention: Deleting files using del, rmdir or rm is irrecoverable, meaning the deleted files will be gone forever! So be very careful with this command.

command-line

$ rm -r practice

Done! To be sure it’s actually deleted, let’s check it:

command-line

$ ls

Exit

That’s it for now! You can safely close the command line now. Let’s do it the hacker way, alright? :)

command-line

$ exit

Cool, huh? :)

Summary

Here is a summary of some useful commands:

These are just a very few of the commands you can run in your command line, but you’re not going to use anything more than that today.

If you’re curious, ss64.com contains a complete reference of commands for all operating systems.

Ready?

Let’s dive into Python!