Change the effective user id and group id to that of USER. No PAM hooks are run, and there will be no password prompt. This command is useful when run as the root user. If run as a non-root user without privilege to set user ID, the command will fail.

-, -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)

-g, --group=GROUP

specify the primary group

-G, --supp-group=GROUP

specify a supplemental group

-m, --preserve-environment

do not reset environment variables

-p

same as -m

-s, --shell=SHELL

run SHELL if /etc/shells allows it

--help

display this help and exit

--version

output version information and exit

A mere - implies -l. If USER not given, assume root.

info coreutils su

should give you access to the complete manual.