NAME
     su - substitute user identity

SYNOPSIS
     su [-fKlm] [-a auth-type] [-c login-class] [login [argument ...]]

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 obtain-
     ing 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, or if the system is not configured for Ker-
     beros.  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.

     The options are as follows:

     -a      Specify an authentication type.

     -c      Specify a login class.  You may only override the default class
             if you're switching to or from root.

     -f      If the invoked shell is csh(1),  this option prevents it from
             reading the ``.cshrc'' file.  (The [f] option may be passed as a
             shell argument after the login name, so this option is redundant
             and obsolescent.)

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

     -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 environ-
             ment.  The invoked shell is the target login's, and su will
             change directory to the target login's home directory.

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

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

     Any arguments after the login name are passed to the shell.  This feature
     may be used to execute commands as another user without starting up an
     interactive shell, which may be especially useful in the rc(8) script.

     Only users in group 0 (normally ``wheel'') 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.

EXAMPLES
     su daemon /usr/contrib/lib/shell-script arguments
     su news -c 'cd /var/spool/news; du -s * | mail usenet'

SEE ALSO
     csh(1),  kerberos(1),  kinit(1),  group(5),  login.conf(5),  passwd(5),
     environ(7),  login(8),  sh(1)

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).

HISTORY
     A su command appeared in Version 7 AT&T UNIX.

BUGS
     There is no direct way to force a particular shell to be used.

     The login name is not optional for root if there are shell arguments.