Files, folders, and the terminal

A file is one saved thing: a song, a photo, a page of writing. Its name usually ends with a dot and a few letters, like .jpg or .py. That ending is called the extension, and it hints at what's inside. Python programs live in files ending in .py. The ending really is part of the name, so notes.py and notes.txt can sit side by side as two separate files.

A folder is a container for files. Folders can hold other folders, like boxes packed inside boxes. Some tools say directory instead of folder; it means the same thing.

A path is a file's address. It lists the folders you'd open, one after another, to reach the file. Sam's Windows laptop keeps a song at C:\Users\Sam\Music\song.mp3. Read it left to right: drive C, then the Users folder, then Sam's folder, then Music, and there's song.mp3. Windows puts a backslash (\) between the names. Macs and Linux use a forward slash (/), so on Sam's Mac the same kind of address reads /Users/sam/Music/song.mp3.

An address that starts from the outermost box (the drive C: on Windows, or the lone / on a Mac or Linux) is called absolute. It works from anywhere, like a full postal address with the country on it. A shorter address like python-course/hello.py is relative. It's like "second door on the left": it only makes sense once you know where you're standing.

The folder you're standing in right now is called the working directory. Two dots, .., mean "the folder one level up from here". A squiggle, ~ (called a tilde), means "my home folder": the folder that belongs to you when you log in. When you later type python hello.py, Python looks for hello.py in the folder you're standing in, and nowhere else.

A file is a named block of data stored on disk: a photo, a document, a Python source file. The part of its name after the last dot, the extension (.py, .txt, .jpg), tells people and programs what kind of content to expect. The extension is part of the name, so notes.py and notes.txt are two different files. A folder (also called a directory) holds files and other folders, and the whole disk forms a tree of folders with one top.

A path is the address of one file or folder: the folders you pass through from a starting point, joined by a separator. Windows uses \ and starts from a drive letter, as in C:\Users\Sam\python-course\hello.py. macOS and Linux use / and start from a single top folder written /, as in /Users/sam/python-course/hello.py on a Mac and /home/sam/python-course/hello.py on Linux. A path that starts at the top of the tree is absolute: it names the same place from anywhere.

A path such as python-course/hello.py has no top of its own. It is relative, read starting from the working directory: the folder a terminal, or a running program, is currently in. Three shorthands appear everywhere: . is the working directory itself, .. is the folder that contains it, and ~ is your home folder, the personal folder your account starts in. So the same relative path can name different files depending on the working directory, while an absolute path cannot. When you type python hello.py, hello.py is a relative path, which is why the working directory decides whether Python finds it.

A terminal is a window where you give your computer instructions by typing instead of clicking. Each instruction is called a command. You type it, press Enter, and the computer does it. Any reply is printed as text right below. Then a short marker called the prompt appears, and the terminal waits for your next command. The program inside the window that reads what you type is called a shell. Windows uses one called PowerShell, inside an app called Windows Terminal. A Mac has an app simply called Terminal. Linux desktops have one too, usually with "Terminal" in its name. The details differ; the idea is the same everywhere.

A command reads like a short order: a verb first, then the thing it works on. Take mkdir python-course. The verb is mkdir, short for "make directory", which means make a folder. python-course is the name of the new folder.

A terminal is always standing in one folder, its working directory (the folder that short, relative addresses start from). A newly opened terminal stands in your home folder. Four commands do nearly everything you'll need at first:

• pwd asks "where am I?" It prints the full address of the folder you're in. The letters stand for "print working directory".
• ls asks "what's in here?" It lists the files and folders.
• mkdir makes a new folder. You stay where you were.
• cd means "change directory". It moves you into another folder. cd .. moves you up one level, and cd ~ takes you home.

Two surprises. First, when a command works, it often prints nothing at all, and an empty reply usually means success. Second, a space splits a command into separate words, so cd My Projects looks like two folder names to the shell. Put quotes around a name with a space in it, or better, name your folders without spaces.

A terminal is a window for working with the computer by typing. It runs a shell, a program that reads one line you type (a command), runs it, prints any result, then shows a prompt and waits for the next line. The default shell is PowerShell on Windows (inside the Windows Terminal app), zsh on macOS, and usually bash on Linux. A command starts with its name; the words after it, separated by spaces, say what to act on. In mkdir python-course, mkdir is the command and python-course is the folder it works on.

Four commands cover everything this course needs early on, and they work in all three shells. pwd prints the working directory as an absolute path. ls lists what's in the working directory. mkdir name makes a new folder there, without moving into it. cd path changes the working directory to path, absolute or relative; cd .. moves up one level and cd ~ goes home. A new terminal starts in your home folder. Many commands print nothing when they succeed, so an empty reply is normal. Because a space separates the words of a command, a folder name containing a space must be wrapped in quotes (cd "My Projects"), which is one reason programmers name folders without spaces.

A command answers you in two ways. The first is the text it prints, which is its output. Error messages count as output too; they're there for a person to read.

The second answer is quieter. When any program finishes, it hands the computer one whole number, called its exit status (people also say exit code). It works like a thumbs-up or thumbs-down the program gives on its way out. The rule almost every program follows: 0 means "it worked", and any other number means "something went wrong". Which nonzero number came back can hint at what went wrong, and each program documents its own numbers.

You don't see the exit status unless you ask. On a Mac or Linux, typing echo $? right after a command prints its number. ($? is the shell's name for "the last exit status", and echo just prints it.) Windows PowerShell does it a little differently. There, $? prints True or False: did the last command work or not? A second name, $LASTEXITCODE, holds the actual number from the last program you ran.

Python plays by the same rule. A Python program that gets all the way to its end hands back 0. One that stops with an error report hands back 1. Now suppose you ask Python to run a file that isn't in the folder you're standing in. Then Python can't even start your program. It prints a message ending in No such file or directory and hands back 2. Why have a number at all when there's a message? Because other programs read the number. A tool that checks your work looks for 0 before moving on, and the number means the same thing however the message is worded.

A command reports back in two ways. The first is output: text it prints for you to read, including error messages. The second is its exit status (also called an exit code): a whole number every program hands to the operating system when it ends. By a convention nearly every program follows, 0 means success and any other number means some kind of failure; the program's own documentation says what a particular nonzero value means. The exit status isn't printed, so you ask for it. In zsh and bash, echo $? prints the last command's exit status. PowerShell splits the idea in two: $? is True or False for whether the last command succeeded, and $LASTEXITCODE holds the number the last program (rather than a built-in command such as cd) returned.

Python follows the same convention. A program that runs to its last line exits with 0. A program that stops with an error, either kind from the previous section, exits with 1. If you ask Python to run a file that isn't in the working directory, the interpreter itself prints a message ending in [Errno 2] No such file or directory and exits with 2, before reading any of your code. Editors, test runners, and automated build systems all branch on that number rather than on the wording of a message.