Who is the root user?

Question

What is the root user on Linux? What does it do for the system and why is it important?

Answer 1

The root (also known as UID 0, the superuser, and the admin) user on Linux is the full administrator of any given Linux system. It essentially has the power to make any change that it wants, save for those things that are protected by the kernel itself.

The root user itself is an artifact from the time of Unix being a fully-fledged multi-user system, with root being the global administrator that runs everything. As time went on, this practice became more and more forbidden, both to prevent security flaws and to prevent accidental deletion of important things. For this reason, you can consider root and sudo to be more of a confirmation step to a modern Linux admin; you use it when you're ready to make a real change to the system in such a way where something bad could happen.

For example, most important system files on Linux are owned by root and can only be changed by root in order to prevent random (unprivileged) users from actually changing and breaking the system as a whole. Such files include /etc/passwd (a record of all users on a Linux system), /etc/shadow (a list of all passwords on the active Linux system), and the majority of /bin (essential system programs required for the OS to function).

Similarly, the root user is typically needed to make any changes to the functionality of a Linux system -- for example, starting/stopping services, rebooting the machine, and installing/removing software.

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For safety reasons, the root user is typically only "virtual," meaning a user can't typically log in to the root account directly. Instead, any user with appropriate privileges can request to be temporarily escalated to a session impersonating the root user, in order to make changes to the system at large.