I recently installed Ubuntu 12.10.
As i started the terminal and ran the command su which we usually use to perform root operations and to get the # symbol.
After running the command it asks me to enter password, I entered the password which Ubuntu had asked me during installation.
I entered the password many times , but it gives me a message Authentication Failure
What should i do ???
Short answer: in Ubuntu the root account is disabled, so you cannot log in as root or use su - to become root.
Instead (as the user who was created during installation) use sudo <command-you-want-to-issue, after which you are prompted for your password and the command is executed with root priviledges.
Please read more about the ideas behind this in the question Differences between su and sudo...
The Ubuntu installer never asks you to enter a root password, it only asks for a user password. This is in contrast with many other distros, for example Debian, which will ask you for both.
The Ubuntu installer only asks for a user password. It grants special permission for this user so that it will be able to run any and all programs with sudo.
In Ubuntu the root user is disabled by default, whenever you need to run something as root you should run it with sudo. This is not a limitation, as you are still able to do all the same things as when you used su, just slightly differently.
