The default user ID of the root user in Linux is 0.
Linux identifies users internally with numeric user IDs, or UIDs. UID 0 is reserved for the superuser, conventionally named root. The kernel uses this numeric identity when checking permissions, so the privilege belongs to UID 0 even if an account has a different username. A root-capable process can ordinarily read, modify, or delete system resources that ordinary users cannot.
A common mix-up is confusing the root user with the root directory. The root user is the privileged account, while the root directory is written `/` and forms the top of the filesystem tree. The directory `/root` is normally the root user’s home directory. Modern distributions often discourage logging in directly as root; Ubuntu, for example, commonly grants administrative access through `sudo`, while the underlying elevated process still uses UID 0. Linux user namespaces can also map an apparent UID 0 inside a container to an unprivileged UID on the host.