/

the root of the tree

The root directory is the top of the tree. It has no parent and no name of its own — just the slash. Every file on the system is somewhere beneath it, and a second disk or a USB stick does not get a letter of its own: it is attached to a directory inside this tree.

One tree, one root. Every absolute path begins with this slash.

Holds

Everything

All other directories hang off it

On disk

Yes

The root filesystem, mounted first at boot

Who writes

root

Ordinary users cannot create files here

Parent

None

.. in / leads back to / itself

Why it exists

Every directory at the top of the tree is there for a reason. This is the problem / solves.

One tree instead of drive letters

Windows gives each disk a letter. Linux gives the whole system a single tree, and each disk or partition is mounted onto a directory in it. A program never needs to know which disk a file is on.

The first thing mounted

At boot the kernel mounts the root filesystem before anything else. It must contain enough to bring the rest of the system up — which is why the essential directories live directly under it.

A fixed starting point

Because every absolute path starts at /, a path such as /etc/hosts means the same file whoever types it and wherever they are standing.

What you find inside

Not everything, but the entries worth recognising. Each is marked as a directory, a file, a link or a device.

usrdirectoryInstalled programs and libraries — most of the operating system.
etcdirectorySystem-wide configuration, as text files.
homedirectoryOne directory for each user.
vardirectoryLogs, caches and other data that changes as the system runs.
binlinkA link to usr/bin, kept so old paths still work.
procdirectoryA view of the running kernel, not stored on any disk.

At the terminal

Four commands that show what this directory is doing on a real machine.

ls /

List the top level of the tree — the directories described in this section.

cd /

Go to the root. From here every path can be written without the leading slash.

df -h /

Show which disk holds the root filesystem, and how full it is.

findmnt

Show every filesystem mounted into the tree, and the directory each is attached to.

Handle with care

What goes wrong here, and what someone securing or attacking the machine looks at.

Keep your files out of it

Nothing of yours belongs directly in /. Personal files go in your home directory, and a full root filesystem can stop the system from booting or logging in.

Mind a path that starts with /

A command run as root against / acts on the whole system. Read a recursive command such as rm -r or chmod -R twice before pressing Enter.

Read alongside

/usrInstalled programs, libraries and their documentation — most of the operating system./etcSystem-wide configuration, kept as plain text files./homeOne directory per user, for their own files and personal settings.