sudo bash
i got
Sorry, sudo must be setuid root

I got this error a couple of times, usually because I ran a recursive chmod command on a directory which had sudo symlinked inside.After this sudo or other affected commands which must be setuid root won't run.
The solution:
Find the affected command by which sudo or which... where ... is the affected command.
Then:
su
chown root:root /usr/bin/sudo
chmod 4111 /usr/bin/sudo
or whichever the path to the affected command.

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