ebuild-writing/eapi: Add section for EAPI 9

[ulm]

This is based on the EAPI cheat sheet, plus some additions.

[sbraz]

Just learned about both of these. What's the point of ${|command;}? The example from the manpage doesn't seem very helpful to me: https://www.gnu.org/software/bash/manual/bash.html#:~:text=while%20this%20construct%20does%20not%20require%20any%20output%20to%20expand%20to

[Flowdalic]

If commandis a shell function, then this avoids the overhead of forking.

[sbraz]

@Flowdalic you mean for the ${ command; } form, what about the second one, the one that "expands to the value of the REPLY shell variable after command executes"?

[ulm]

I have expanded the section quite a bit (with some bits and pieces from the Bash manual), but I would be grateful if someone could come up with a real example that could be used in an ebuild.

The ${| REPLY=12345; } example in the Bash manual is indeed pointless because it is equivalent to writing 12345 as a constant.

[Flowdalic]

@Flowdalic you mean for the ${ command; } form, what about the second one, the one that "expands to the value of the REPLY shell variable after command executes"?

Right, I missed that this is about the piped variant. Sorry.

I assume one use case of {| command } is command also being required to print some status output. But that's just an educated guess.

[ulm]

I assume one use case of {| command } is command also being required to print some status output. But that's just an educated guess.

Yes. However, in ebuild context one would use einfo or elog for status output, which don't use stdout but stderr. So I still don't see where it could be useful for us.

[ulm]

If there are no further comments, then I'm going to merge this tomorrow.