Fixing a corrupted sudoers file

If you accidentally corrupt your /etc/sudoers file (usually by reading it without sudo, incorrectly interpreting that it is empty, and then writing into it with sudo, and finally being unable to correct it because you can no longer use sudo mode), there is a way out. You can use pkexec visudo, which will allow you to edit the sudoers file, provided you are authorized to run programs as root with PolicyKit. Find the default /etc/sudoers file online: https://wiki.debian.org/sudo. In case you are not authorized via PolicyKit, you can still edit the sudoers file using a live CD. Mount the computer hard drive at /mnt, say, and then sudo visudo -f /mnt/etc/sudoers. Courtesy of this AU question. Of course, man sudoers and man visudo will tell you more as well.