Okay, so this is embarrassing, but I seem to have lost the admin group on my machine, though I would have sworn I hadn't gone anywhere near it. I'm running Mint Maya.
Trying to innocently
sudo this morning (to
update-alternatives) I got the response:
kate is not in the sudoers file. This incident will be reported.
I can get to a root shell from the boot menu, thankfully, and
/etc/sudoers looks fine, but it refers to admin and
/etc/group doesn't contain it. (I can only assume it used to; I never had any problem with sudo before today.) Trying to recreate an admin group with
groupadd admin (as root) gives me this:
groupadd: cannot lock /etc/group; try again later.
I can't find any
.lock files in
/etc, and I've run out of Google leads, so I'm really at a loss.
UPDATE: The locking issue may be because the boot root shell is read-only.