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When I try to switch to another user from the command line using "su", I get an authentication failure, as shown below:

su - otheruser

Password: *****

su: Authentication failure

Yet I know that the password is correct because I can successfully log in to the desk top as the other user from the Ubuntu login screen.

Does anyone know what could be causing this problem?

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See this page for what you should use for a rootshell: RootSudo

This page also will explain how to get a terminal as another user. Short version: sudo -i -u <username>

There's an Ubuntu SO-based site here:

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You need to be root to do "su". You can "sudo su - user" or you can "sudo su -" then do "su - user". Enter the password of the user you are now when you do the sudo. ex. "whoami"

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Don't use su on Ubuntu. – Broam Oct 13 '10 at 17:59
My question is why is it there if I should not use it? Ok, how about this one: "sudo -u <username> <command>" ? – SuperJames Oct 14 '10 at 13:46
Please see my answer for why - the official justification is listed there. – Broam Oct 14 '10 at 19:19
sudo -i is proper if you need a rootshell. See – Broam Oct 14 '10 at 19:22

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