Add a process to an already executing process? Like && or so

If the shell that started the process still knows about it (try jobs), then the shell built in wait can wait for it to finish. Something like this might work:

$ jobs
[1] nohup a.sh
$ wait -f %1 && nohup b.sh &

If you have lost the association in the shell then the pidwait command might work in a similar way, although it probably can't capture the exit value of the first process, and may run the second one even fi the first fails, unlike what && does.

Only the parent process (or the child subreaper if any or init if the parent died), can retrieve the exit status of a process.

From the shell where you started the command, you can wait for it, and start b.sh in background if a.sh was successful with:

wait $pid && nohup b.sh &

In zsh or:

wait -f "$pid" && { nohup b.sh & }

In recent versions of bash (the -f for wait not to return when the process is only stopped; that's the default behaviour in zsh).

However the wait has to run in foreground. If run in background it would be in a child shell process and therefore would no longer be the parent of the process running nohup a.sh.

To retrieve the exit status of a process that is not your child one approach is to use a debugger and intercept the _exit(), exit_group() or equivalent system calls.

For instance, on Linux, one could do:

ret=$(strace -qqqa0 -e signal=none -e exit_group -p "$pid" 2>&1 | grep -Po '\(\K\d+')

The macos equivalent would likely involve a truss or dtrace command, but I don't have a machine to test that on.