SU(1)

NAME
su - substitute user identity

SYNOPSIS
su [-Kflm] [-c class] [login [args]]

DESCRIPTION
Su requests the Kerberos password for login (or for “login.root”, if no login is provided), and switches to that user and group ID after obtaining a Kerberos ticket granting ticket. A shell is then executed. Su will resort to the local password file to find the password for login if there is a Kerberos error. If su is executed by root, no password is requested and a shell with the appropriate user ID is executed; no additional Kerberos tickets are obtained.

By default, the environment is unmodified with the exception of USER, HOME, and SHELL. HOME and SHELL are set to the target login's default values. USER is set to the target login, unless the target login has a user ID of 0, in which case it is unmodified. The invoked shell is the target login's. This is the traditional behavior of su. Resource limits and session priority applicable to the original user's login class (See login.conf(5)) are also normally retained unless the target login as a user ID of 0.

The options are as follows:

-K  Do not attempt to use Kerberos to authenticate the user.

-f  If the invoked shell is csh(1), this option prevents it from reading the “.cshrc” file.

-l  Simulate a full login. The environment is discarded except for HOME, SHELL, PATH, TERM, and USER. HOME and SHELL are modified as above. USER is set to the target login. PATH is set to “/bin:/usr/bin”. TERM is imported from your current environment. Environment variables may be set or overridden from the login class capabilities database according to the class of the target login. The invoked shell is the target login's, and su will change directory to the target login's home directory. Resource limits and session priority are modified to that for the target account's login class.

-m  Leave the environment unmodified. The invoked shell is your login shell, and no directory changes are made. As a security precaution, if the target user's shell is a non-standard shell (as defined by getusershell(3)) and the caller's real uid is non-zero, su will fail.

-c class
    Use the settings of the specified login class. Only allowed for the super-user.

The -l and -m options are mutually exclusive; the last one specified overrides any previous ones.

If the optional args are provided on the command line, they are passed to the login shell of the target login.

Only users who are a member of group 0 (normally “wheel”) can su to “root”. If group 0 is missing or empty, any user can su to “root”.

By default (unless the prompt is reset by a startup file) the super-user prompt is set to “#” to remind one of its awesome power.

SEE ALSO
csh(1), kerberos(1), kinit(1), login(1), sh(1), group(5), login.conf(5), passwd(5), environ(7)

ENVIRONMENT
Environment variables used by su:

HOME
    Default home directory of real user ID unless modified as specified above.

PATH
    Default search path of real user ID unless modified as specified above.

TERM
    Provides terminal type which may be retained for the substituted user ID.

USER
    The user ID is always the effective ID (the target user ID) after an su unless the user ID is 0 (root).

EXAMPLES
su man -c catman
    Runs the command catman as user man. You will be asked for man's password unless your real UID is 0.
su man -c 'catman /usr/share/man /usr/local/man /usr/X11R6/man'
    Same as above, but the target command constitutes of more than a single word and hence is quoted for use with the -c option being passed to the shell. (Most shells expect the argument to -c to be a single word).
su -c staff man -c 'catman /usr/share/man /usr/local/man /usr/X11R6/man'
    Same as above, but the target command is run with the resource limits of the login class “staff”. Note: in this example, the first -c option applies to su while the second is an argument to the shell being invoked.
su -l foo
    Pretend a login for user foo.

HISTORY
A su command appeared in AT&T UNIX.