Comment by chubot

Yup, job control is a huge mess. I think Bill Joy was able to modify the shell, the syscall interface, and the terminal driver at the same time to implement the hacky mechanism of job control. But a few years later that kind of crosscutting change would have been harder

One thing we learned from implementing job control in https://oils.pub is that the differing pipeline semantics of bash and zsh makes a difference

In bash, the last part of the pipeline is forked (unless shopt -s lastpipe)

In zsh, it isn't

    $ bash -c 'echo hi | read x; echo $x'  # no output
          
    $ zsh -c 'echo hi | read x; echo $x'
    hi

And then that affects this case:

    bash$ sleep 5 | read
    ^Z
    [1]+  Stopped                 sleep 5 | read


    zsh$ sleep 5 | read    # job control doesn't apply to this case in zsh
    ^Zzsh: job can't be suspended

So yeah the semantics of shell are not very well specified (which is one reason for OSH and YSH). I recall a bug running an Alpine Linux shell script where this difference matters -- if the last part is NOT forked, then the script doesn't run

I think there was almost a "double bug" -- the script relied on the `read` output being "lost", even though that was likely not the intended behavior