LINUX Root Password Confusion

From time to time it is recommendable to change your password. When doing so, you meet the problem of sitting before some application that requires a password, thinking "Did I already change the password here too?".

Surely you know the situation. I met it when trying to upgrade my netbook's Lubuntu LINUX. The update-dialog was asking me for a password. Because the dialog did not proceed from receiving my new password, I decided that it is time to change the root password to my new password on this machine, canceled the software update and entered the following in a terminal window (the '$' is the UNIX prompt):

$ su
Password: oldPassword
$ passwd
Enter new UNIX password: newPassword
Retype new UNIX password: newPassword
passwd: password updated successfully

Starting the software updater then showed the same as before: the password was not accepted. I rebooted the machine to clear all "shadows" of the old password, but that did not help.

Then I searched the Internet for a way to change the root password on LINUX. I found some confusing discussions that Ubuntu has no root password any more, or that root access is done via sudo and the user-password is sufficient for this. But what about the software updater then?

I also tried this:

$ sudo passwd root
[sudo] password for xxx: oldPassword
Enter new UNIX password: newPassword
Retype new UNIX password: newPassword
passwd: password updated successfully

Did not help. The software updater denied the new password. I evaluated this also by calling the menu item "System" - "Network", there you have a button "Unlock" to get root access to network resources. Also "Synaptic" requires root permissions, and it rejected the new password, too.

Now I logged out and tried to log-in as root, to find out what the new root password has become. And it turned out to be the new one, so at least one of the methods above worked, most likely both!

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This was the moment when I understood that the password that I have to enter for the software updater, or sudo or su, is not the root password. It is my normal user-password. I hadn't yet changed my user password on that computer.

$ passwd
Changing password for xxx.
(current) UNIX password: oldPassword
Enter new UNIX password: newPassword
Retype new UNIX password: newPassword

After that, the software updater accepted the new password and started to work.

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That means I had changed the root password as normal user, using my user-password, not the root-password.
Normally you can not interact with a UNIX system without logging in, using your user-password. So why does the terminal window ask me for a password that I already had entered when logging into the system?
And why does it ask me for my own password instead of the root password when I want to get root by sudo or su? And why is my normal password sufficient to change the root password? Couldn't I change then anybodies password on this system?

Yes, because I am in the "sudo" group that has administrative permissions. Discussion and answers to some of these questions can be found here:

• https://help.ubuntu.com/community/RootSudo

Ubuntu has become a cooperative system where users are expected to behave properly.

After reading this I did

$ sudo passwd -dl root

to disable the root account again.