MacOS – Problems with sudo privileges as admin in OSX

sudo always requires the executing user's password (and requires that you have specific permissions to do this, i.e. are one of the sudoers).

su requires the password of the target account (root by default, but root account has no password on OS X by default). If you use su instead, you can enter the destination account's password and execute a command using that user's privileges.

su -c some_cmd # as root
su username -c some_cmd # as username

^{This works by passing all arguments after the user name to the destination account's login shell. Shells usually support -c <commands> arguments. In GNU coreutils su, there's an actual -c command argument to su that can be placed before the user name.}

You can su to another user account (using the other user account's password), and sudo from there, provided that other account is a sudoer.

If you want neither to enter another account's password, nor give your regular account sudoers permissions, you're pretty much out of options unless you consider SSH with key authentication or something like that.

You'll have to boot into recovery mode to gain root (system-wide) access in order to repair

If you have a single-boot (Ubuntu is the only operating system on your computer), to get the boot menu to show, you have to hold down the Shift key during bootup.

From the boot menu, select recovery mode

After you select recovery mode and wait for all the boot-up processes to finish, you'll be presented with a few options. In this case, you want the Drop to root shell prompt option.

/etc/group is the file that defines the groups on the system

You can then use adduser username admin to readd your user to the admin group.

Alternatively you can use the vigr command to edit the /etc/group file safely

format example;

group-name:x:group-number:user1,user2
admin:x:110:username