Files, Folders, and the Filesystem

You already use files and folders every day — you name a document, drop it in a folder, drag that folder into another one. None of that is new. What you may never have seen is the system underneath: how the operating system actually keeps track of every one of those files on the disk, names them, and finds the right one in an instant.

That whole organizing system has a name. The filesystem is the part of the operating system that decides how files are stored, named, and located on storage. From here on we'll call it the filesystem. Once you can see it, a lot of later technical talk — about paths, directories, and config files — stops being mysterious.

What a file really is

To you, a file is a document, a photo, a song. Underneath, every file is the same kind of thing: a named chunk of data sitting on storage — the disk you met in the last chapter. Whether it's a spreadsheet or a video, it's bytes on the disk with a name attached so the computer can find it again.

The name and the contents are separate. You can rename a file without changing a single byte inside it, and you can change everything inside while keeping the name. The filesystem's job is to keep that connection straight — this name points to that chunk of data, over here on the disk.

Folders as a tree

A folder — also called a directory, the word you'll see in technical contexts — is not itself a chunk of data like a file. It's a container: a way to group files, and other folders, under one name. The two words mean the same thing; this course will say folder, but expect directory everywhere later.

Because a folder can hold other folders, the whole thing forms a tree. There's one folder at the very top — on the servers this course leans toward, it's written simply as a slash, /, and called the root. Inside the root are more folders, inside those are more, and somewhere out at the tips of the branches sit your actual files.

Picture a filing cabinet, but a strange one: each drawer can hold not just papers but smaller cabinets, each of those holds smaller ones still, all the way down. The papers are your files; every cabinet and drawer is a folder. The whole nested arrangement is what we mean by the filesystem tree.

A path: the address of a file

If files are scattered across a deep tree of folders, the computer needs a precise way to say exactly which one you mean. That address is called a path: the full route from the top of the tree down to the file, written out folder by folder.

A path like /home/you/notes.txt reads left to right as a set of directions. Start at the root, /; go into the folder named home; inside it, the folder named you; and there sits the file notes.txt. Each slash is just a separator between one step and the next. You won't type paths in this course — but you'll see them constantly, and now you can read one.

Back to the cabinet: the path is the full label that tells you exactly which cabinet, which drawer, and which folder inside it to open. Give the same directions twice and you land on the same single file every time — which is the whole point of a path.

File types and extensions

Most file names end with a short tag after a dot — .txt, .jpg, .mp3. That tag is called the extension, and it's a hint about what kind of data the file holds: .txt for plain text, .jpg for a photo, .mp3 for audio. The operating system uses it to guess which app should open the file.

Here's the part worth getting right: the extension is only a label, not the contents. Rename song.mp3 to song.txt and the bytes inside don't change one bit — you've only changed the hint, and now the computer will try to open music as text and get confused. The real nature of a file is its actual data; the extension just advertises it.