Fix Sudo-ers file

By default, in Ubuntu, the first user account you created during the installation process is also the administrator of the system. Using this user account, he/she is able to perform administrative tasks with the “sudo” command. If you are not aware, “sudo” refers to Super User Do and all users in this group are geared with administrative rights to manage the system. However, if you accidentally removed yourself from this “sudo” group, you will not be able to do anything. Instead, the only thing you will see is:

loginname is not in the sudoers file. This incident will be reported

Read the original article here with illustrations.

In short its all about this:

Here’s the fix:

1. Reboot the computer. If Ubuntu is your primary and the only operating system in your computer, press the “Shift” button when it is booting up. This should make the Grub screen show up on your monitor. On the Grub screen, press the down button to the Recovery mode and press Enter.

2. On the next screen, select “Check all file system (will exit read only mode)”.

When you see the message “Finished, press Enter“, press Enter.

3. Next, select “Drop to root shell prompt”.

4. You will now be at the command line. Type:

usermod -a -G admin username

Replace the username with your own login name.

5. That’s it. Now type “exit” and select “Resume normal boot”.

If you have messed up the sudoers file use this:

The above steps assume that your /etc/sudoers file is still intact. If you have mishandled the /etc/sudoers file and cause it to be corrupted. Here is what you need to do:

1. Do the above steps until Step 3.

2. At the command line, type

sudo cp /etc/sudoers /etc/sudoers.backup
sudo nano /etc/sudoers

Enter the following lines to the file

# This file MUST be edited with the 'visudo' command as root.
# Please consider adding local content in /etc/sudoers.d/ instead of
# directly modifying this file.
# See the man page for details on how to write a sudoers file.
Defaults        env_reset
# Host alias specification
# User alias specification
# Cmnd alias specification
# User privilege specification
root    ALL=(ALL:ALL) ALL
is not in the sudoers file. This incident will be reported.
# Members of the admin group may gain root privileges
%admin ALL=(ALL) ALL
# Allow members of group sudo to execute any command
%sudo   ALL=(ALL:ALL) ALL
#includedir /etc/sudoers.d

Press “Ctrl + o” to save the file and “Ctrl + x” to exit.

3. Next, set the file permission of the sudoers file:

chmod 440 /etc/sudoers

4. Lastly, add yourself to the sudo group:

usermod -a -G admin damien

5. exit the shell prompt and resume normal boot.

You should be able to perform administrative task again.