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I tried to su to root so I could install lights, but I get an authentication error when I try:

user@host:~$ su
su: Authentication failure
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marked as duplicate by Eliah Kagan, Alaa Ali, Warren Hill, Andrea Corbellini, Kevin Bowen Aug 10 '14 at 22:39

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

Are you following a guide? In Ubuntu the root account is disabled and you may need sudo instead – Braiam Apr 11 '14 at 18:22
Just type -sudo su and than the password for your account. – bogdan.rusu Nov 28 '15 at 14:21

The root account is disabled by default in Ubuntu, so there is no root password, that's why su fails with an authentication error.

Use sudo to become root:

sudo -i  
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If su doesn't work, I do this (in bash):

user@host:~$ sudo bash
root@host:~# su

Voila! You are now root!

A shortcut for this would be sudo su. In this case given that you are a member of /etc/sudoers with all privileges, then you would only need your user's password.

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After sudo bash you are already root... – edwin Apr 11 '14 at 19:43
@edwin After sudo bash you are running bash as root, but '~' still points to /home/user or wherever your user's home directory is. So you are not quite root. A shortcut for this would be sudo su – e.thompsy Apr 21 '14 at 12:59
For all intends and purposes, you are already root... What's happening is that sudo is preserving some environment variables. Instead of sudo bash it's better to just use sudo -i. – edwin Apr 21 '14 at 16:50
@edwin In some cases these preserved environment variables matter a lot. So I would argue for most intents and purposes you are absolutely right. However, the OP has asked specifically about su to root. I was assuming they knew why they want to do that and that they have a good reason to do it. So I was adding an alternate path to becoming root to the discussion. And here is yet another way: sudo -i then su. But I would totally agree that in most cases just using sudo should be fine. That is what I usually do. Unless I need root. Then I use su. – e.thompsy Apr 23 '14 at 16:31
sudo -i already is enough. Seriously, you just sudo su or sudo -i, this is enough to become root (no need to "su" again)... – edwin Apr 23 '14 at 17:00

You are getting Authentication failure because you are trying to become root which is disabled by default in all version of Ubuntu. This can be easily circumvented in two ways:

  1. Enabling the root account. This can be archived by setting up a password.
  2. Instead of su using sudo -i or better yet, append to any command sudo in the way of:

    sudo apt-get update
    [sudo] password for braiam:

I wouldn't recommend enabling root, since it could rise a security concern, for example, if you use any service exposed to the web.

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Use sudo your_command in place of su.

sudo apt-get install "program to install"
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Open root with sudo -s and when it's in this mode type:


Then, choose password. This password will be for su command.

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