How Your Computer Thinks: Command Line Essentials

Before You Start

You should know: - How to open the Terminal on Mac/Linux, or PowerShell on Windows. - That computers use a hierarchical system to store information (folders inside folders).

You will learn: - How to navigate your computer’s file system without using a mouse. - Why the command line is an essential environment for scientific computing. - The four core commands: pwd, ls, cd, and mkdir.

Introduction

In the previous chapter, you wrote Python code in a browser. That was a great sandbox, but real scientific computing happens locally—on your own machine, using your own files, processing your own satellite imagery or datasets.

However, when you run Python locally, Python is essentially blind. It doesn’t magically know where your files are stored. If you tell Python to “open temperature_data.csv,” it will only look in the exact folder it is currently standing in. If it can’t find it, it throws an error.

To control Python, you must first learn how to control your computer the way a programmer does: through the command line. The command line (also called the terminal, console, or shell) is a text-based interface used to talk directly to your operating system.

The Mental Model: You Are Always “Somewhere”

When you use a mouse, you open a window (like Finder on Mac or File Explorer on Windows) and look inside a folder.

In the terminal, you don’t “open” a folder—you step inside it.

Imagine your computer’s hard drive is a massive building with millions of rooms. The terminal is you, standing in one specific room at a time. This room is called your working directory.

1. Where am I? (pwd)

The first thing you need to know is which room you are standing in.

Open your terminal (Mac/Linux) or PowerShell (Windows). You will see a blinking cursor waiting for an instruction. Type the following and press Enter:

pwd

(On Windows PowerShell, pwd still works perfectly!)

pwd stands for Print Working Directory. Your computer will reply with something like:

/Users/yourname

or

C:\Users\yourname

This is your absolute path—the exact address of the room you are currently standing in.

2. What is in this room? (ls)

Now that you know where you are, you need to see what’s in the room with you. Type:

ls

(On Windows PowerShell, use ls or dir.)

ls stands for List. Your terminal will print out all the files and folders located in your current working directory. You should see familiar folders like Desktop, Documents, and Downloads.

3. Making a new room (mkdir)

We are going to create a dedicated folder for all your computing work. Instead of right-clicking and selecting “New Folder,” we’ll do it using text. Type:

mkdir coding_projects

mkdir stands for Make Directory. If it succeeds, the terminal won’t say anything at all—it will just give you a new blinking cursor. In the command line, no news is good news!

To verify it worked, run ls again. You should see coding_projects listed among your folders.

4. Walking into a room (cd)

You’ve built the room, but pwd will show you are still standing outside of it. To step inside the new folder, use the Change Directory command:

cd coding_projects

Run pwd again. Your location should now explicitly end in /coding_projects. Run ls. It should return absolutely nothing, because this new room is currently empty.

5. Walking backward (cd ..)

To step backward—out of your current folder and into the parent folder—you use a special shortcut: two periods.

cd ..

The .. symbolizes “the folder immediately above this one.” Run pwd and you’ll see you are safely back where you started.

Why This Matters for Geography

Imagine you download a massive 5GB array of LiDAR data to your Downloads folder. You write a Python script and save it to your Desktop.

If you open your terminal, navigate to the Desktop, and tell Python to run your script, Python will wake up standing on the Desktop. When your script says “open the LiDAR file,” Python looks around the Desktop, doesn’t see the LiDAR file, and crashes.

Understanding the command line ensures you always know exactly where you are standing, allowing you to feed the right data to your code.

Verify Your Work

  1. Open your terminal or PowerShell.
  2. Run pwd to confirm your starting location.
  3. Use mkdir to create a folder called geography_workspace.
  4. Use cd geography_workspace to step inside it.
  5. Provide pwd one last time to verify your path changed successfully.

If you can confidently move around your file system using only these commands, you are officially ready to install Python on your machine.