Essential System Administration, 3rd Edition

Becoming Superuser

On a Unix system, the superuser refers to a privileged account with unrestricted access to all files and commands. The username of this account is root. Many administrative tasks and their associated commands require superuser status.

There are two ways to become the superuser. The first is to log in as root directly. The second way is to execute the command su while logged in to another user account. The su command may be used to change one’s current account to that of a different user after entering the proper password. It takes the username corresponding to the desired account as its argument; root is the default when no argument is provided.

After you enter the su command (without arguments), the system prompts you for the root password. If you type the password correctly, you’ll get the normal root account prompt (by default, a number sign: #), indicating that you have successfully become superuser and that the rules normally restricting file access and command execution do not apply. For example:

$ su 
Password:   Not echoed
#

If you type the password incorrectly, you get an error message and return to the normal command prompt.

You may exit from the superuser account with exit or Ctrl-D. You may suspend the shell and place it in the background with the suspend command; you can return to it later using fg.

When you run su, the new shell inherits the environment from your current shell environment rather than creating the environment that root would get after ...