/etc/shadow

Linux has a file for almost everything.

There is a place for users: /etc/passwd

A place for password hashes: /etc/shadow

A place for configuration: /etc/

A place for logs: /var/log/

A place for devices: /dev/

A place for processes: /proc/

Even the running system has a filesystem-shaped view of itself.

Linux has always had an interesting philosophy:
If you understand the system, you understand where things live.

But security makes the picture more complicated.

Knowing where something lives doesn't mean you can touch it.

You may know the file exists.

You may even know exactly what is inside it.

But chmod, ownership, capabilities, namespaces, SELinux, and the kernel itself may all answer the same question:
“Are you allowed?”

That is what makes Linux security fascinating.

The real boundary isn't the file.

It isn't the password.

It isn't even the command.

The boundary is privilege.

And once you start looking at Linux this way, the entire operating system begins to look like a map of permissions:

Who are you?
What can you see?
What can you execute?
What can you change?

And, perhaps most importantly...
What happens when you become root?

Welcome to /etc/shadow.

Where the interesting things are usually hidden just one permission away.