User management mechanisms

In the real world there are many user management mechanism implementations to consider. Some are native to UNIX or Linux, some are common in the Windows world, some are novel, and some are universal and agnostic.

It should go without saying that our first stop on any journey of investigating user management mechanisms is the Linux user system itself. Simple and universal, every Linux system of note ships with it. Of course, it can be replaced, but in practice it never is. This system carries the huge advantages of being always built in, very fast and secure, and well known by every UNIX admin anywhere. There is almost nothing to go wrong, nothing complex anywhere in the system. A few archaic components might linger on having been left over from the olden days that might be a little confusing if one finds it necessary to do manual configurations, but today almost no one manipulates these systems by hand anyway (although it is always good to know how to...