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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 (toupdate-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.

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Try mount -w -o remount / to get the root filesystem out of readonly mode – Alan Curry Aug 31 '12 at 23:54
I've just tried the fsck recovery boot menu option which seemed to do just that, getting it out of readonly mode. Thank you. phew! – Kate McKenzie Sep 1 '12 at 0:15

Running fsck first mounted / in read/write mode, which allowed me to groupadd admin, usermod -a -G admim myself, and regain sudo privileges.

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