special built-in [Was: Re: "builtin jobs" does not output to stdout.]

[Robert Elz]

    Date:        Tue, 14 Feb 2023 11:04:48 -0500
    From:        Chet Ramey <chet.ramey@case.edu>
    Message-ID:  <8be447ca-5416-a64f-7e62-1e61102e897a@case.edu>

  | "Special builtin" is not a particularly useful concept

I disagree - most of the time the difference doesn't matter much,
but that the special built-ins cause the shell to exit if they fail
means that script writers don't need to do things like

        break || exit 99

just in case something is wrong with the script, and the break doesn't
happen in a loop, and so is an error.  With it being a special built-in
the script simply exits at that point, otherwise if the script writer
hasn't tested for the error, the script just keeps running to the next
command, which is fairly clear is not what was intended.

That's somewhat esoteric, as things like break/continue/ ... rarely
ever fail.

But for "eval" it matters, consider

        eval '| grep foo' || {
                # code to handle what happens when foo isn't there
        }

That's not likely as written, but could happen from

        eval " $command | grep foo" || ...

if "$command" turns out to be unset or null at this point due to some
bug or other.   That results in a syntax error from eval, which is a
special built-in failure, which should cause the shell to exit.   Allowing
it to continue with just a failure status means the script believes that
the grep failed.

That is, I don't really think that (from an earlier message):

  |  the only rationale for why those particular ones were chosen
  |  is "this is what the Bourne shell did."

or perhaps stated better, yes, that is more or less why they were chosen, but
that makes it seem like some of the other (bizarre) things that the Bourne
shell did (probably as a code space saving mechanism), whereas I suspect
that this division was probably more carefully considered from the start.

The special built-in commands are commands that could almost be reserved
words, and often would be, but were implemented as commands instead (and
probably are better that way).

kre