Is it possible to su without being prompted for a password?

Question

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?

Answer 1

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 echo
log_user 0
set timeout 2
if [catch "spawn su root" reason] {
    send_user "Failed to spawn su: $reason\n"
    exit 1
}
expect "Password:"
send "$PASSWORD\r"
expect "\r\n"
#set timeout 5
log_user 1
expect {
    "# " {send_user "\nsu successful.\n"}
    timeout {send_user "\nsu timed out.\n"; exit 1}
    "incorrect" {send_user "\nsu failed.\n"; exit 1}
}

Answer 2

It's an incredibly bad idea, but if you really want to do it you can use sudo's NOPASSWD option:

In /etc/sudoers: 
# User privilege specification
johndoe  ALL=(ALL) NOPASSWD:ALL

Then user johndoe can do sudo su and wind up with a root prompt without ever being asked for a password.

I reiterate, though, that this is a REALLY BAD IDEA in most circumstances. Sudo, preferably /without/ the nopasswd flag is significantly safer.