What is the Linux filesystem hierarchy and what is the root directory?

Linux arranges everything as one upside-down tree whose single root is /; every file and directory hangs somewhere beneath it.

* The Linux filesystem as one inverted tree rooted at /, branching into /bin, /etc, /usr, /var, /home and the rest. — Ppgardne, CC BY-SA 4.0, via Wikimedia Commons. *

There is exactly one top, written / and called root. From there directories branch downward, and crucially there's no second tree: unlike Windows, Linux has no drive letters. A second disk, a USB stick, or a network share doesn't become D: — it gets mounted onto a directory inside the existing tree (say /mnt/usb), so its contents appear as just another branch.

This "single unified namespace" is a defining Linux idea. A program never has to ask "which drive?" — every path from / is unambiguous, and storage can be rearranged underneath without programs noticing.

Why "inverted"? Real trees have one root at the bottom and spread up; this tree has its one root at the top and spreads down. Picture / as the trunk of an upside-down tree and every directory as a branch growing downward from it.

Gotcha: don't confuse / (the root directory, top of the filesystem) with /root (just the home directory of the root user). Different things that happen to share the word "root."

Go deeper:

• Filesystem Hierarchy Standard 3.0 — the standards-body source defining the single-rooted / tree and every top-level directory.