From Scratch Programming Class

Session 1: The command line — your first cd, ls, python

Phase 4 — Intermediate Python · Session 1 of 9

What we’re learning today

Until now, you’ve run Python by clicking the green button in Thonny. Today we learn another way: the command line. You open a terminal window, type a command, and the computer does what you said. By the end of class, you’ll be running Python scripts from the terminal — the way every professional programmer in the world does it.

You’ll need to remember from last time

• Thonny — the editor where you’ve been writing and running Python.
• The shell at the bottom of Thonny — where output appeared when your programs ran.
• A few of your own .py files from earlier sessions — we’ll run them today.

Part A: Open a terminal

Find the Terminal Emulator on your computer. On Linux Mint XFCE, it’s usually:

• Applications menu → System → Terminal Emulator, OR
• Right-click on the desktop → Open Terminal Here

A black (or sometimes white) window opens. There’s a blinking cursor and some text — probably your username and a $ or % symbol. That’s the prompt. It’s the terminal saying “type a command.”

What this is

The terminal lets you talk to your computer without using a mouse. You type a command, press Enter, the computer does something, and the result shows up in the same window.

The terminal is also called:

• The command line (because you type one line at a time)
• The shell (because it’s a “shell” around the operating system)
• The console
• bash (the specific shell program most Linuxes use)

These all mean roughly the same thing in our class.

Your first command — pwd

Type pwd and press Enter. (Stands for “print working directory.”)

$ pwd
/home/your_username

It tells you where you are in the file system. By default, you start in your “home” directory.

The $ is the prompt; you don’t type it. Just the pwd. Throughout this chapter, lines starting with $ are commands you type; the rest is what the terminal prints back.

ls — what’s here?

Type ls (LS — for “list”):

$ ls
Desktop  Documents  Downloads  Music  Pictures  Public  Videos

You see the things in your home directory — folders and files.

Try ls -la:

$ ls -la
total 96
drwxr-xr-x 18 you you  4096 Apr 25 09:00 .
drwxr-xr-x  3 root root 4096 Apr  1 12:00 ..
drwxr-xr-x  2 you you  4096 Apr 25 09:00 Desktop
...

The -la is a flag — extra options. -l means “long format” (more details). -a means “include hidden files” (files starting with .).

Most commands have flags. You don’t need to memorize them; you’ll learn the ones you use.

cd — go somewhere else

cd means “change directory.” Try:

$ cd Desktop
$ pwd
/home/your_username/Desktop

You moved to the Desktop folder. Now ls shows what’s on the desktop.

To go back up to your home directory:

$ cd ..
$ pwd
/home/your_username

The .. means “the parent folder.” It’s a special name that always means “one level up.”

To jump straight back home from anywhere:

$ cd
$ pwd
/home/your_username

cd with no argument always takes you home.

Try this on your own

Spend a few minutes navigating around. Try:

• cd Documents then ls
• cd Pictures then ls
• cd .. to go back
• cd ../Music to go up and over

Get comfortable moving around. The terminal is just a way of walking through your folders.

Checkpoint: You’ve used pwd to see where you are, ls to list files, and cd to navigate to a different folder. This is the natural stop point if class is cut short.

Part B: Running Python from the terminal

Time for the actual point of all this — running Python without the green button.

Find one of your old programs

You have lots of .py files saved from previous sessions — hangman, the contact book, the text adventure. Pick one.

In the terminal, navigate to the folder where that file is. For example, if your hangman is in Documents/python:

$ cd Documents/python
$ ls
hangman.py  notes.txt  words.txt

You see your file.

Run it

Type:

$ python hangman.py

(On some systems it’s python3 instead of python. Try python first; if “command not found”, use python3.)

The program runs — same as if you’d hit the green button in Thonny. Same input, same output, same everything. But now you’re running it from the terminal.

When the program ends, you’re back at the $ prompt, ready for another command.

Why this matters

Three reasons:

1. No editor required. You can run a Python script anywhere, anytime, without opening Thonny.
2. Other tools work the same way. Most professional tools (Git, Node.js, web servers, you name it) are command-line first. Learning the terminal opens up all of them.
3. It’s faster. Once you’re used to it, running a script with python myscript.py is faster than clicking through menus.

Try a few

Pick three different .py files you’ve written. Navigate to each and run it from the terminal:

$ cd Documents/python
$ python hangman.py
[play hangman]
$ python adventure.py
[explore the adventure]
$ python contacts.py
[manage contacts]

This is your new daily workflow.

Stretch — useful CLI commands

Some other commands you’ll use over and over:

Try cat words.txt in your hangman folder — your word list prints out.

Try mkdir test_folder then cd test_folder then ls.

Be careful with rm. It’s permanent — there’s no trash can. If you delete a file by accident, it’s gone. Always look at what you’re deleting first.

Stretch — python -i

python -i myscript.py runs your script and then drops you into an interactive shell with all your variables still defined. Useful for poking at your program after it runs.

Extension — make your script feel like a real command

Add this line at the very top of one of your scripts:

#!/usr/bin/env python3

(Yes, that exact text — including the #!.)

Then in the terminal:

$ chmod +x myscript.py
$ ./myscript.py

Now you can run it like a normal command. The first line tells your system “use Python to run this.” The chmod +x makes the file executable. The ./ says “run this from the current folder.”

This is how real programmers package their tools. Phase 4’s later sessions will use this pattern.

Wrap-up

Before we leave, share with the room:

• For the kids who ran one of your earlier programs from the terminal — did it feel different from clicking the green button?
• Did anyone get lost in the file system? How did you find your way back?
• For the stretch commands — anyone use mkdir or cat? What for?

You learned today the most-used tool in all of professional programming. Every developer uses the terminal every day. You don’t have to like it; you do have to be comfortable with it.

The terminal is also how you’ll use Git in a few sessions — Git is a command-line tool. Today’s cd and ls and python are the foundation; Git’s commands will feel natural once these do.

If you missed this session

Open a terminal on your computer (Applications → Terminal, or right-click desktop → Open Terminal). Then:

1. Type pwd and press Enter. See where you are.
2. Type ls. See what’s in this folder.
3. Type cd Documents (or wherever your .py files are). Then ls again.
4. Type python yourfile.py to run a Python file from the terminal.

About 20-30 minutes of practice. The more you use it, the faster it gets.

If you get stuck, ask your buddy at the start of next class.

Stretch and extension ideas

• Tab completion — start typing a folder or file name, press Tab. The terminal completes it for you. Saves a lot of typing.
• Up arrow — recalls previous commands. Useful for re-running something.
• history — shows your command history.
• Pipes — cat words.txt | wc -l counts the lines in the file. The | pipes the output of one command into another. Powerful and confusing; mention if curious.
• Output redirection — python myscript.py > output.txt saves the output to a file instead of printing it.

What’s next

Next week we’ll learn how to split a Python program into multiple files — because real programs don’t fit in one file. We’ll learn how to import your own files, just like you’ve been importing random and turtle.