I know this question asked so many times before but none of the solutions worked for me. My OS is Ubuntu 16.04.5 (alongside Win10) when I type the correct pw it enters the login loop.

journalctl colored output: https://gist.github.com/ridvansumset/40e9223262361fa9590aa5e5bc5e10a6

The solutions I found on the web and tried are:

1- (at login screen) Ctrl+Alt+F3 then chown username:username .Xauthority then mv .Xauthority .Xauthority.bak

2- (at login screen) Ctrl+Alt+F3 then remove lightdm and reinstall it

3- Entering the recovery mode then choosing dpkg then relogin

4- sudo apt-get purge nvidia*

5- etc/shadow file already exists.

Nothing works for me :(. Can someone please help? It's urgent

OK! I figured out what my problem is and now I can login!!! Here how I solved it:

Press Ctrl+Alt+F3 to enter terminal then login with your username and password.

Firstly, run nano ~/bash_history. This will show you the commands you used before. (Notice that the command at the top of the list is the oldest one). Then, look at your commands line by line, try to figure out which one can cause this problem. Pass unimportant commands like cd or cat. Try to focus on commands that makes effect on your system or environment. If you find anything, undo what you did.

In my case, it was sudo nano etc/environment command. When I inspected that file, there wasn't even any typo. But still, I removed all the lines
that I added before to revert my changes. Now it includes only $PATH variable which is:
PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games"

Then I ran source etc/environment. After that I was able to login!

P.S: Never edit your etc/environment file. Instead use export command to set an environment variable. Besides, check if there is a typo in your ~/.profile and ~/.bash_profile files, as well.

Welcome to Hell. I faced this too, and all I had to blame are unsigned Nvidia modules, or similar thing. So, I assume this isn't the first time you boot into Ubuntu since the first setup? And you got Nvidia drivers as well?

First of all, there has to be issues reported in your logs from

$ journalctl

If so, add to your post.

As a workaround, disable Secure Boot in your BIOS setup, if it's on (I mean, it MUST be on, because usually it blocks unsigned gfx driver from loading to kernel. This was my issue, however, since I left Ubuntu long time ago, I have different issue, for ex., on Arch, where desktop environment literally won't load up). Write me back with your results

1. Problem description

Nvidia-396, which you have installed intently or unawarely auto-installed by other related package, such as swig, can not properly used in utuntu 16.04.

2. Solution

The best way to solve the problem would be ever find the miss-operation firstly. To do this, firstly, you need to check your command history by :

vi ~/.bash_history

and then search "sudo" keywords which indicate essential command, and find suspects. In my case, it is

 sudo install swig

Finally, revert it by :

sudo apt-get purge swig

CAUTION : PLEASE NEVER DO

sudo apt-get upgrade

It will install newest package of your whole system which will include nividia-396