Debian 9: su command not working

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Problem :

I have just set up a new PC with a fresh debian 9.0.1 iso. The system is running fine but the su command does not work.

user@debian:~$ su
Password: 
su: Authentication failure

I reinstall system several times and test all install options (graphical install, expert install, lvm, standart partition …) but I always encounter this issue with the su command.

I am sure of my password, I choose ben as root password for testing purpose.
I am also sure of my keyboard layout.

I am totally blocked since the sudo command is not configured by default on debian 9.

How can I fix that?

Solution :

Had the same thing; solved it like this:

In your grub menu, select your Debian install and press ‘e’ to edit the kernel arguments.

Find the line starting with linux and add init=/bin/bash to the end of the line. Then start Debian with F10. You will see a terminal.

Mount the filesystem as read-write with:
mount -o remount,rw /

Change the password with the passwd command: 
passwd root

Your root account might be locked. You can verify this via:

sudo passwd -S root

If in the output line root is followed by an L, the root account is locked.

You can set a new password for root via:

sudo passwd root

Afterwards, su should work.

Besides that, it is often recommended to use sudo -i or sudo su instead of su. For further information, have a look at the Debian Wiki: https://wiki.debian.org/sudo

When you exec su command you must provide the password of target user. In your case:

su

this is the root password. To exec the operation with your password you should use the command:

sudo su -

P.S. Use dash after su command to get the environment of target user

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