The command su ( substitute user identity) is used on Unix derivatives (eg, AIX, BSD, Linux, and Solaris) to change the user context and thus run programs under a different user ID. It is one of the essential system administration tools of Unix.
The syntax is su username. Without calling the user name su replaced after a password prompt to the user root. su - or su-l Simulate a full login process, i.e., the new user's complete profile be read and set the environment variables again.
The command already appears in the earliest versions of Unix from AT & T, in version 7, however, still completely without documented options. Today, every Unix comes with su.
To prevent abuse, most systems record all successful and unsuccessful login attempts su to in the system log files.
Use the exit command returns you to the original user and his rights.
Under Mac OS X, the root account is disabled by default, instead of the su command sudo is recommended that starts not have its own subshell, but only executes the mitangegebenen command under administrator privileges.
Most flavors of BSD and Mac OS X limit the use of su to members of the groups wheel and admin. Based on a decision of the GNU Project founder Richard Stallman this property was not included in the GNU version of the command, the programmed David MacKenzie. Stallman argued that such a restriction cements the power of a few system administrators.