# What a Filesystem Is Let's build the picture first - once you have it, paths and folders stop being a maze and become a map you can read at a glance. Forget commands for a minute; we're installing one mental model. ## The disk is just numbered storage **What it actually is.** Down at the hardware, your disk (or SSD) has no idea what a "file" or a "folder" is. It's a huge wall of numbered boxes, each holding a chunk of bytes - box 0, box 1, box 2, on and on, millions of them. That's all the hardware offers: "give me a number, I'll give you back what's stored there." ```text THE RAW DISK (what the hardware actually is) ┌────┬────┬────┬────┬────┬────┬────┬────┬────┐ │ 0 │ 1 │ 2 │ 3 │ 4 │ 5 │ 6 │ 7 │ … │ each box = a chunk of bytes └────┴────┴────┴────┴────┴────┴────┴────┴────┘ no names, no folders, no order ``` **Why people get this wrong.** Most people imagine their files are literally *inside* folders, the way papers sit inside a physical drawer, neatly grouped on the disk. They aren't. A photo might be scattered across boxes 4,201 and 88,015 and 502,330. The tidy folders you see are not a property of the disk - they're a story the operating system tells *on top of* those numbered boxes. 📝 **Terminology.** A *filesystem* is exactly that story: the system the OS uses to turn dumb numbered storage into named files organized in folders, and to remember which boxes belong to which file. (If "the OS" is fuzzy, [What an Operating System Is](/guides/what-an-operating-system-is) covers the manager-in-the- middle idea.) ## The OS lays a tree on top **What it actually is.** The filesystem organizes everything into a **tree**: folders that contain files and other folders, branching down from a single starting point. A *folder* (also called a *directory*) is just a named container; a *file* is a named blob of bytes. That's the whole structure. ```mermaid flowchart TD root["/ (root)"] --> home[home/] root --> etc["etc/ (system config)"] root --> usr[usr/] home --> ada["ada/ (your home folder)"] ada --> notes[notes.txt] ada --> projects[projects/] projects --> budget[budget.xlsx] usr --> bin["bin/ (installed programs)"] ``` Notice there's exactly one box at the very top that everything hangs off. That's the **root**. 📝 **Terminology.** *Directory* and *folder* mean the same thing. "Folder" is the friendly desktop word; "directory" is the word the terminal and most docs use. We'll use them interchangeably. 💡 **Key point.** The disk is a flat wall of numbered boxes. The folders-and-files *tree* is a structure the OS imposes on top to make that storage usable by humans. Everything else in this guide is about navigating that tree. ## The root: the top of the tree **What it actually is.** Every filesystem has a single starting point that contains everything else, directly or indirectly. On macOS and Linux it's written as a lone forward slash: `/`. On Windows each drive has its own root, written with a drive letter and a backslash: `C:\`, `D:\`, and so on. Let's actually look at the root on a Mac or Linux machine. (We'll use the terminal here; if it's unfamiliar, [The Terminal and Shell](/guides/the-terminal-and-shell) introduces it gently.) ```console $ ls / bin etc home lib tmp usr var ``` *What just happened:* `ls` means "list what's in this folder," and `/` is the root. So you asked "what sits at the very top of the tree?" and the OS listed the top-level folders. Everything on the machine lives somewhere underneath one of these. ⚠️ **Gotcha - Windows is different here.** Windows doesn't have one single root. Each storage device gets its own letter, so there's a `C:\` tree (usually your main drive), and a USB stick might show up as `E:\` with its *own* separate tree. The equivalent peek: ```console C:\> dir C:\ Directory of C:\ Program Files Users Windows ``` *What just happened:* `dir` is the Windows version of `ls`, and `C:\` is the root of the C drive. Same idea as `/` - the top of one tree - but Windows has one tree per drive, not one tree for the whole machine. ## A path is an address **What it actually is.** A *path* is the route from somewhere to one specific file, written as a list of folder names separated by slashes - a street address (country, city, street, house number), except it's root, folder, subfolder, filename. Read this path left to right: ```text /home/ada/projects/budget.xlsx │ │ │ │ └── the file we want │ │ │ └── inside a folder called "projects" │ │ └── inside ada's home folder │ └── inside the "home" folder └── starting at the root ``` 📝 **Terminology.** macOS and Linux separate folders with a **forward slash** `/`. Windows traditionally uses a **backslash** `\`, so the same kind of path looks like `C:\Users\ada\projects\budget.xlsx`. Same concept, different separator - and this difference causes real confusion, which is why we name it now. ## Absolute vs relative paths This is the one distinction that clears up most "but I typed the right path!" frustration. **An absolute path** starts at the root and gives the complete address, so it means the same file no matter where you currently are. You can spot it because it begins with `/` (or `C:\` on Windows): ```console $ cat /home/ada/notes.txt Buy milk. Call the bank. Finish the budget. ``` *What just happened:* `cat` prints a file's contents. Because the path started at `/`, it's a full address - this finds the exact same `notes.txt` whether you run it from your home folder, the desktop, or anywhere else. **A relative path** starts from wherever you happen to be standing right now (your *current directory*) and gives directions from there. It does *not* begin with a slash: ```console $ cd /home/ada $ cat notes.txt Buy milk. Call the bank. Finish the budget. ``` *What just happened:* `cd` ("change directory") moved you into `/home/ada`. Now `notes.txt` - with no leading slash - means "`notes.txt` starting from where I am," which resolves to `/home/ada/notes.txt`. Same file, shorter address, because you were standing in the right place. 📝 **Terminology.** Two special relative names show up everywhere: `.` means "the folder I'm in right now," and `..` means "the folder one level up." So `cd ..` walks you up toward the root, and `../photos/cat.jpg` means "go up one, then into photos." ⚠️ **Gotcha.** A relative path that works in one place fails in another, and the error looks identical to a typo. If `cat notes.txt` says "no such file," you're probably not standing where you think you are. Run `pwd` ("print working directory") to ask the OS where you currently are: ```console $ pwd /home/ada/projects ``` *What just happened:* You asked "where am I?" and got the absolute path of your current folder. From `/home/ada/projects`, the relative name `notes.txt` points at `/home/ada/projects/notes.txt` - which doesn't exist. That's the whole mystery: the path was fine, your location wasn't. ## The home folder and `~` **What it actually is.** Your *home folder* is the one folder that belongs to you - where your documents, desktop, and personal settings live. Every user account gets its own. On Linux it's usually `/home/yourname`, on macOS `/Users/yourname`, on Windows `C:\Users\yourname`. Because you refer to your home folder constantly, Unix-style shells give it a one-character nickname: the tilde, `~`. ```console $ cd ~ $ pwd /home/ada ``` *What just happened:* `~` expanded to your home folder's full path, so `cd ~` took you home. `~/projects/budget.xlsx` is just shorthand for `/home/ada/projects/budget.xlsx`. Typing `cd` with no path at all usually does the same thing - drops you at home. 💡 **Key point.** `~` is not a folder named "tilde." It's a shorthand the shell replaces with the absolute path of *your* home folder. Two different users typing `~` get two different real paths. ## Recap 1. The **disk** is dumb numbered storage; the **filesystem** is the tree of folders and files the OS imposes on top of it. 2. The **root** is the single top of the tree - `/` on macOS/Linux, and one per drive (`C:\`, `D:\`) on Windows. 3. A **path** is an address built from folder names: forward slashes on Unix, backslashes on Windows. 4. **Absolute** paths start at the root and mean the same file everywhere; **relative** paths start from where you're standing - so `pwd` is your best friend when a path "should" work but doesn't. 5. Your **home folder** is your personal space, nicknamed `~` in the shell. Next, we'll look at why the tree won't always let you in - the rules of ownership and permission that decide who can read, change, or run each file.