That's not a question of certification or extensions, I'm talking about being able to write portable scripts. Due to the interaction between its definition and its extensions, echo is a prime example of being impossible to use portably (except in the very restricted case of only literal strings without escapes which don't start with a -).
> Ultimately, each platform has quirks, and it is up to the developer to port and test their script or application to a platform and make any necessary changes.
That is not humanly feasible and that's why specifications exists. You can't "port and test" your script to a platform which doesn't even exist yet, but if you follow the specification and the platform implements it (assuming it does so correctly) your scripts will run.
Ok, I think I understand the confusion now. You seem to be of the impression that shell scripts are like bytecode executed in a virtual machine. That is the only way I know of that you could write an application for a platform that doesn't exist and expect it to work. But even for that to work, it would need to be the same VM, and bytecode generated by & for it, or there's still no guarantee it will work.
Of course, you can already write a shell script for a particular shell, distribute that shell to that system, and depend on the shell to properly execute your script [by using internal functions only]. But that defeats the whole purpose of following a standard like POSIX, or caring at all how any given platform's 'echo' program works.
Bottom line, though: two independent implementations of a standard provide no guarantee they will work together. Practice over a couple decades shows this to be the case.
> Ok, I think I understand the confusion now. You seem to be of the impression that shell scripts are like bytecode executed in a virtual machine.
What in bloody hell are you talking about?
> Of course, you can already write a shell script for a particular shell
Which is irrelevant to my comment as that's not what portability means.
> But that defeats the whole purpose of following a standard like POSIX
Exactly.
> Bottom line, though: two independent implementations of a standard provide no guarantee they will work together.
If following a standard can't ensure your program can work on two different implementations of the standard, you don't have a standard you have decorated toilet paper.
Which is more or less what the "commands and utilities" part of POSIX is.
> Ultimately, each platform has quirks, and it is up to the developer to port and test their script or application to a platform and make any necessary changes.
That is not humanly feasible and that's why specifications exists. You can't "port and test" your script to a platform which doesn't even exist yet, but if you follow the specification and the platform implements it (assuming it does so correctly) your scripts will run.