su - run a shell with substitute user and group IDs
su [OPTION]... [-] [USER [ARG]...]
Change the effective user id and group id to that of USER.
A mere - implies -l. If USER not given, assume root.
Written by David MacKenzie.
Report bugs to <bug-coreutils@gnu.org>.
Copyright (C) 2006 Free Software Foundation, Inc. This is free software. You may redistribute copies of it under the terms of the GNU General Public License <http://www.gnu.org/licenses/gpl.html>. There is NO WARRANTY, to the extent permitted by law.
The full documentation for su is maintained as a Texinfo manual. If the info and su programs are properly installed at your site, the command
info su
should give you access to the complete manual.