I suspect su doesn't have any way to allow passing the password from the shell instead of being prompted for one, for security reasons (couldn't find anything in the MAN). Is this true?
Is it possible to su without being prompted for a password?
su
That's true, there is no way to have su(1) redirect its input from a place other than the tty.  But there are libraries and programs to simulate a terminal for automated scripting, for example, Expect.  Something similar to:stty echolog_user 0set timeout 2if [catch spawn su root reason] {    send_user Failed to spawn su: $reason\n    exit 1}expect Password:send $PASSWORD\rexpect \r\n#set timeout 5log_user 1expect {    #  {send_user \nsu successful.\n}    timeout {send_user \nsu timed out.\n; exit 1}    incorrect {send_user \nsu failed.\n; exit 1}}