How I broke and later fixed root shell

I was playing with shell changes and accidentally changed root user shell to nonexistent one. How did it happen? From Ubuntu terminal I switched to root’s environment executing “sudo su -” and then I tried to change login shell for root user performing chsh command. Instead of specifying correct shell with path like “/bin/sh” I typed only “sh”. The system presented reply about the error I made, but in the same time it changed a record in the system file /etc/passwd from correct one to erroneous. On terminal window it looked as:

Later I logged in to my Linux box through ssh, tried to switch to super user and received this:

The following messages were logged in /var/log/auth.log file:

The problem was because the login shell for root user was broken. I opened /etc/passwd file in text editor

and found that line for root user account was looked like:

Definitely it was incorrect, correct shells’ locations are

and found that line for root user account was looked like:

I replaced the line in /etc/passwd for root user account to:

saved /etc/passwd file and rebooted my Linux box. When I logged in agent and “sudo su -” started to work as expected and /var/log/auth.log showed this: