A UNIX command that allows a user to run an application
using different credentials from those employed for the current logon session.
Su is the UNIX equivalent of the Runas command on Microsoft Windows platforms, though Su is actually the older of the two. Su stands for “super user” and allows users to become another user (even root, if they have the credentials) to perform tasks without logging off from their currently logged on account. When you use Su, you supply a password for the second user, unless you are already logged on as root. Once the password is verified,
a new shell is opened using the credentials of the second and allowing you to run commands using these credentials. By pressing Ctrl+D, you can exit the shell and return to your current user session.
Su allows administrators to log on using their ordinary user account and then “su” whenever they need to perform an administrative task requiring root privileges.