Re: 'wait' in command group in pipeline fails to recognize child process

[Martin Jambon]

On 6/22/21 1:37 PM, Greg Wooledge wrote:
> On Tue, Jun 22, 2021 at 01:12:10PM -0700, Martin Jambon wrote:
> > On 6/22/21 4:31 AM, Greg Wooledge wrote:
> > > A pipeline creates two or more subshells, one for each command in the
> > > pipeline.  Therefore, your wait command is running in a different
> > > process than the one which created the sleep background job.
> > >
> > > The curly braces are irrelevant here.
> > >
> > > unicorn:~$ sleep 1 & wait "$!"|cat
> > > [1] 1290127
> > > bash: wait: pid 1290127 is not a child of this shell
> >
> > Thank you! Now I know that a subshell is not a shell, $$ being the process
> > ID of the shell, not of the subshell.
>
> It's a forked child process of bash, and as such, it's still a shell.
> Specifically, it's bash.
>
> There's some formal definition that you can ignore.  The real definition
> of a subshell is "the direct result of a fork()".  It inherits all of
> the shell variables, functions, aliases, and so on that the parent
> shell process had at the time of the fork.
>
> This is different from a direct call to "bash", which would not inherit
> shell variables and so on -- only environment variables.

Great, then I would change the documentation of '$$'. Currently it says 
this:

  ($$) Expands to the process ID of the shell. In a () subshell, it 
expands to the process ID of the invoking shell, not the subshell.

This first mentions just "the shell". I suggest calling it the "root 
shell" instead, i.e. the process where "exec bash" took place. I would 
also mention pipelines here, since these are more commonly used than () 
subshells. I don't know if there are other ways of creating subshells. 
If that's all, I think it would be valuable to mention those two cases 
rather than just one.

Well, I suppose "root shell" could be misunderstood as a shell run by 
the 'root' user but this doesn't make much sense in this context. I 
can't find a better name. Here's how we might change the description of 
'$$':

  ($$) Expands to the process ID of the root shell. In a pipeline or in 
a () subshell, it expands to the process ID of the root shell, not the 
subshell.

I also changed "invoking shell" to "root shell" because the invoking 
shell or parent shell is not necessarily the same as the root shell e.g.

$ echo $$; (echo $$; (echo $$))
688918
688918
688918

> In the pipeline, there are two subshells created.  One of them runs
> the command wait "$!", and the other runs the command cat.
>
> The special parameter $! is inherited from the parent shell, and contains
> the PID of the sleep process that the parent shell created.  But the
> child subshell can't wait for that process, because it belongs to someone
> else.  Hence the error message.

👍 Yup. I think the error message is clear. No complaint about that.