There are actually two mechanisms set up in Ubuntu for performing administrative tasks.
sudo is one of them, and it and its graphical frontends (like
gksudo) are not going to work until you fix the permissions problem.
The other mechanism is PolicyKit. Depending on exactly what you have done to your file permissions, it's likely that you can still recover using PolicyKit to run commands as
pkexec command will run any (nongraphical) command as
root (provided that the user invoking it is an administrator on the system).
pkexec is capable of running graphical commands (like
root, too, but this is quite nontrivial, as you have to set up configuration files for them describing how they are supposed to be run and what they are supposed to be allowed to do. Therefore, you're best off using the command-line to fix this problem...at least to the point where
sudo works again. (Then you can run
gksu nautilus to get a
root file browser, if you're more comfortable using that to edit file permissions recursively.)
I don't know exactly what you did to your permissions, so it's hard for me to give you a specific
pkexec command to run, to fix the problem. But you can rectify the specific problem that
sudo is currently complaining about. It says:
sudo: /usr/lib/sudo/sudoers.so must be only be writable by owner
other write permissions (while not modifying
owner write permissions) for that file:
pkexec chmod go-w /usr/lib/sudo/sudoers.so
Then you can see if
sudo works (by running some innocuous command like
If you need to perform other operations on file permissions from the command-line with
(By the way, here's another, related situation where it's handy to know about
If you have trouble getting
pkexec to work for this, please feel free to comment here, and I'll try to render further assistance. But you should know that even if this method does not work, you shouldn't have to reinstall Ubuntu.
Instead, you can fix the problem from an Ubuntu live CD/DVD or live USB system, because the live system will be functioning--
sudo will work on it--and you can mount your Ubuntu partition and change the permissions that way.
Or as a third option (as forestpiskie has suggested), you could use recovery mode.