Few days I ago, I had a funny situation. Colleague of mine accidentally executed:
sudo chown -R www-data:www-data /*
so instead for current directory ./* he changed owner for entire root file system. Sure, he tried to change back owner to the root:root , but root did not have any permissions anymore, since /bin is owned by another user.
Fortunately, this kind of mess on Linux systems we can fix very quickly using Live CD. So reboot your system and run Live CD. Continue reading Pro tip: How to repair an Ubuntu system with a Live CD