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.

-, -l, --login
make the shell a login shell
-c, --commmand=COMMAND
pass a single COMMAND to the shell with -c
-f, --fast
pass -f to the shell (for csh or tcsh)
-m, --preserve-environment
do not reset environment variables
same as -m
-s, --shell=SHELL
run SHELL if /etc/shells allows it
display this help and exit
output version information and exit
A mere - implies -l. If USER not given, assume root.