Files and folders

Everything you save on your computer ends up in a file: a photo, a term paper, a song and also a program that you write. A file is a sequence of bytes with a name, and the computer keeps track of where on the storage these bytes are. So that you can find your files again, you put them into folders. In the command line, folders are often called directories; both words mean the same thing.

Name and extension

The name of a file usually has two parts separated by a dot: hausarbeit.docx, urlaub.jpg, hallo.py. The part after the last dot is the extension. It tells the operating system what kind of content is in the file and which program should open it: .docx with Word, .jpg with an image viewer, .py with an editor for Python. The extension does not change the content; if you rename hallo.py to hallo.txt, you still have the same program, the computer just opens it differently.

Windows and macOS hide extensions by default. When you program, however, you need them, otherwise the file you meant to save as hallo.py is really called hallo.py.txt. So switch them on once:

• Windows: in Explorer under View → Show → File name extensions.
• Mac: in Finder under Finder → Settings → Advanced → Show all filename extensions.

Folders form a tree

A folder can contain files and further folders, and those can contain folders again. This creates a tree that starts at a topmost folder. On Windows that is a drive such as C:\, on the Mac the folder /, which is also called the root. Below it there is, among other things, Users with a folder for every person who uses the computer, and in it your home folder with your desktop, documents and downloads.

In Explorer and in Finder you only ever see one folder at a time, but the tree behind it is always the same.

The path: the address of a file

To name a file unambiguously, you write down the way that leads from the root through the folders to it. This way is called a path. Between the folders there is a separator, the backslash \ on Windows and the forward slash / on the Mac:

C:\Users\anna\Documents\uni\notizen.txt
/Users/anna/Documents/uni/notizen.txt

Both lines mean the same place in the tree, the first on a Windows computer, the second on a Mac. The last part is the name of the file, and everything before it is the folders it sits in.

Absolute and relative

A path that starts at the root is called absolute: it leads to the target from any place, like a complete postal address. Often, though, it is needlessly long. If you are already in the folder Documents, uni\notizen.txt is enough (on the Mac uni/notizen.txt). Such a path is called relative, because it applies from the folder you are currently in, just as the directions “second door on the left” are only right from the correct corridor.

Two shorthands come up again and again in relative paths:

From C:\Users\anna\Documents\uni, ..\notizen.txt therefore leads to a file notizen.txt directly in Documents. These are exactly the paths you will soon use to move through the command line.