The command line

In Explorer or in Finder you click on icons to open folders and move files. You can do the same things with typed commands as well: you write a line, press Enter, and the computer answers with a line of text. This window is called the command line, or, after the window it runs in, the terminal. The program inside it that reads and carries out your commands is called the shell: PowerShell on Windows and zsh on the Mac.

Why type instead of click?

For single files the mouse is often faster. The command line, however, has three advantages that count when you program. Many tools exist only there, Python itself included, and the easiest way to start a program you have written is with a command. Commands can be written down, repeated and passed on to others, which hardly works for sequences of clicks. And on servers, where your programs will later run, there are often no windows at all, only the command line.

Opening the command line

• Windows: Press the Windows key, type PowerShell and open Windows PowerShell. On Windows 11 you also find it in Terminal, which starts with a PowerShell.
• Mac: Press Cmd+Space, type Terminal and press Enter. You also find the program in Finder under Applications → Utilities.

A window opens with little more than a line that waits for your input. This line is called the prompt.

The prompt tells you where you are

On Windows the prompt ends with > and on the Mac with %. Before that stands the folder you are currently working in, the current directory. On Windows this is, for example, PS C:\Users\anna>, on the Mac anna@MacBook ~ %. The tilde ~ is short for your home folder, which on the Mac is /Users/anna.

The current directory matters because every relative path refers to it. After opening the command line you are always in your home folder.

How a command is built

A command starts with its name, followed, separated by spaces, by its arguments: what the command should act on. Try out the first two commands. pwd stands for print working directory and prints the current directory:

pwd

ls stands for list and shows what is in the current directory:

ls

If you give ls a folder as an argument, it shows that folder's content without you having to change into it:

ls Documents

The output in PowerShell looks more detailed than on the Mac, because it also gives the type, date and size of every entry. The first column has a d (directory) for every folder.

Two helpers that save you a lot of typing

• The up arrow key brings back the last command, and pressing it several times brings back the ones before. This lets you repeat a command or fix a typo without writing everything again.
• The Tab key completes names: type ls Doc and press Tab, and the shell writes the rest of Documents for you. PowerShell puts .\ in front, which means “in the current folder” and changes nothing.

With clear you tidy up the window when it gets too full; your files are not affected by it.