How to suppress su authentication failure warning?

Question

I created an account "diag" and set it as expired (usermod --expiredate 1). Then I have a script which I want to run to log into it, so in the script I use the following command:

su -s /bin/bash - diag

This outputs like so:

Your account has expired; please contact your system administrator
su: Authentication failure
(Ignored)
diag@computer:~$

And then I'm able to use the account, as expected.

I want to suppress the first three lines, the warning about the expired account. I tried adding 2> /dev/null to the end of the command, but that suppresses all the output from bash; I just get a blank response, and I can type commands into it and see the result from them, but I see no bash prompt. I tried adding just > /dev/null and that does nothing.

So I've deduced that apparently su is piping all its output over stderr. How can I get su to just suppress those first three lines, but otherwise act normally as if the account weren't expired?

Answer 1

As far as I can tell, user account expiration and lock is functionally the same thing; login attempts will fail, SSH attempts will fail, etc. At least for my purposes they are the same.

So, in my script, I now detect whether the user is expired, and if so, lock the user and set the expiration to 'never'.

It looks something like this:

# Takes 1 parameter, the username to check
# Returns 0 if the user is expired, or 1 if it is not expired.
function userExpired()
{
    # These day values are just numbers, the number of days since the epoch.
    local expireDay=`grep $1 /etc/shadow | cut -d: -f8`

    # If diag has no expiration date, it will be empty.
    if [[ "$expireDay" -eq "" ]] ; then
        return 1
    fi

    local today=`perl -e 'print int(time/(60*60*24))'`
    local daysUntilExpire=`echo $expireDay - $today | bc`

    [[ $daysUntilExpire -lt 0 ]]
}

# Check if the diag user is expired and if so, lock and unexpire it.
userExpired diag && {
    usermod --lock --expiredate -1 diag
}

I wrote the userExpired function from code found in this forum post (modified to fit my code style and better variable naming).

Answer 2

Redirected stderr to stdout

su -s /bin/bash - diag 2>&1 >/dev/null

---

This one I haven't tried but expect should work (will update soon once home)

discard=$(su -s /bin/bash - diag)