[Info-vax] What is a "real" Unix ?

[Arne Vajhøj]

On 9/4/2023 8:15 AM, Simon Clubley wrote:
> In that case, what is a "real" Unix ?
> 
> Is it something that implements a set of user-visible APIs and certain
> behaviour within its kernel (fork() semantics for example) ?
> 
> Is it something that came from a specific source code base and hence
> nothing else can never be called Unix no matter how compatible that other
> something is ?
> 
> If BSD is a Unix, then is System V also a Unix ?
> 
> If System V is a Unix, then why can't something else that also implements
> the same APIs and kernel behaviour also be a Unix ?
> 
> Or is Linux really a Unix after all (in every way that matters) and what's
> really going on here is just some out-of-touch BSD Unix elitism ?

It is possible to pick different criteria.

The historical criteria: if it is based on original
AT&T SysV or Berkeley BSD code then it is Unix.

The duck criteria: if the shell commands and programming
API's expected from Unix is there then it is Unix.

The legal criteria: if it has been certified as compliant
to Posix/SUS then it is Unix.

The majority criteria: if the majority of IT people
consider it Unix then it is Unix.

I don't think it makes much sense to discuss what criteria
is "best".

I believe applying them would give:

Commercial Unixes (AIX, Solaris, HP-UX etc.) : Y Y Y Y
*BSD (FreeBSD, NetBSD, OpenBSD etc.) : Y Y N Y
Linux : N Y N Y
OS X/macOS : Y Y Y N
z/OS, OpenVMS, Windows : N N(native)/Y(addon) Y(long time ago) N

Arne