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
  • 3
    Are you following a guide? In Ubuntu the root account is disabled and you may need sudo instead
    – Braiam
    Apr 11, 2014 at 18:22
  • 6
    Just type -sudo su and than the password for your account. Nov 28, 2015 at 14:21
  • using su as mentioed by bogdan in the above comment worked for me.(I upvoted the comment)
    – ambassallo
    Jan 4, 2021 at 15:32

5 Answers 5


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  
  • 3
    thanks. I thoguht this error message is caused by wrong password Feb 8, 2017 at 0:56
  • 2
    in my case, setting the SUID bit solved the issue: sudo chmod +s /bin/su
    – CybeX
    May 8, 2017 at 1:19
  • What is -i and what's the difference with -s? @Seth
    – Shayan
    Jun 18, 2019 at 16:40
  • not working for me. Oct 24, 2019 at 5:39
  • @SarfarajSipai Can you elaborate more on what your situation is? What are you trying to do and why are you trying to do it? Thanks :)
    – Seth
    Oct 24, 2019 at 6:22

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.

  • 8
    After sudo bash you are already root...
    – edwin
    Apr 11, 2014 at 19:43
  • 7
    @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, 2014 at 12:59
  • 1
    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, 2014 at 16:50
  • 2
    @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, 2014 at 16:31
  • 2
    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, 2014 at 17:00

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

  1. Enabling the root account. This can be achieved by setting up a password.
  2. Instead of su use 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 raise a security concern, for example, if you use any service exposed to the web.


Open root with sudo -s and when it's in this mode type:


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


Use sudo your_command in place of su.

sudo apt-get install "program to install"

Not the answer you're looking for? Browse other questions tagged or ask your own question.