It has been quite a while since I used UNIX. I am developing
a security system and I was wondering if UNIX and/or LINUX
user ID's are case-sensitive.

i.e. can user 'daveb' and 'Daveb' exist on the same system with
completely different authorizations/priorities, etc.?

No, for example, sending mail to Neo, NEO, neO or neo, etc. will all reach the same mailbox. If mail was rejected because the case didn't match, lots of mail would be rejected

Having said that, the acounts in the password file are case sensitive. So Root is not the same as root, is not the same as ROOT. For example, if the password file says 'neo' you cannot su to Neo.
I would have to test to see if there were accounts "neo" and also "Neo" in the password file if mail would find it's way to the right mailbox...... anyone?

Neo

This is actually a difficult question. My answer is that I would never depend on being able to create two accounts like daveb and daveB. And as a system admininistrator I would prohibit it on all systems that I administer. But as a programmer, I would insure that programs work correctly iif two such accounts are encountered.

Neo mentions mail and that is correct. Internet mail must ignore case and so daveb@xyz.org and daveB@xyz.org are the same address. The mail problem would not be impossible to solve but it would be very hard. One of the accounts will need an external mail address that doesn't match the user name. This may require writing a custom delivery agent. I suspect that "mailx Neo" and "mailx neo" would both work. But if it doesn't work, I certainly would not want to call the vendor and complain. And in any case, "mailx neo@localhost" and "mailx Neo@localhost" could not both work.

I'm dating myself here, but terminals originally were not multicase. They were upper case only. Unix had support for that environment. It is mostly gone, but some versions still have various pieces left. On HP-UX, if you sign on as DAVEB, the system will assume that you meant daveb and that you are using a terminal that cannot handle lower case.

Because of these problems, many adduser programs will disallow this sort of thing. But if you manually edit the passwd file, you will probably succeed in getting it to work.

So don't do that, you will regret it. But don't assume that no one else has done it. And be aware that many programmers have assumed that no one has done it.

I realize that it would be possible to create a system that would
allow case-sensitive ID's, if only for the convenience of the system
administrator with 2 or more users wanting the same alphabetic
characters for their ID. I realize other systems have probably
already done this, but I was mainly interested in whether or not
UNIX/LINUX has had this capability added since I last used UNIX.

The system I have created does not interface directly with the
internet or email system, so there will not be any email conflicts.
The system processes files and allows private communications
over an intranet system.

In developing a new product, the developer tries to give their
product additional capabilities that set it apart from other entries.
This new capability for the system I have created may be rarely
used. But if it IS used once in a while, then I feel it was worth
while.