Re: receiving SIGHCHLD if job control is disabled - expected behavior?

[Chet Ramey]

On 10/1/21 4:24 AM, Vladimir Marek wrote:
> Hello,
>
> The code in question is
>
> set +m
> echo $BASH_VERSION
> echo $SHELLOPTS
> trap 'echo =====SIGCHLD=====' 18
> sleep 1
> echo done
>
>
> Bash 5 output:
> 5.0.17(1)-release
> braceexpand:hashall:interactive-comments
> =====SIGCHLD=====
> done
>
>
> Bash 4 output:
> 4.4.19(1)-release
> braceexpand:hashall:interactive-comments
> done
>
>
>
> I was trying to find a relevant note in changelog and I found this in CHANGES:
>
> n. The SIGCHLD trap is run once for each exiting child process even if job
>     control is not enabled when the shell is in Posix mode.
>
> and this in CWRU/changelog:
>
>          - waitchld: run SIGCHLD trap for each child exited even if job control
>            is not enabled when in Posix mode. Prompted by an austin-group-l
>            discussion
>
> I was trying to find the discussion and the closest thing I found was
>
> https://www.mail-archive.com/austin-group-l@opengroup.org/msg00898.html

That's the one.

A SIGCHLD is generated when a child process dies, whether or not the shell
currently has job control active, since monitor mode is basically about
process groups. You can trap any signal, including SIGCHLD, so why not
make it as reliable as you can?

It's useful to be able to guarantee that you'll get a SIGCHLD trap for
each child exiting, to do exactly the sort of counting that was the subject
of that discussion, so I made that work whether or not monitor mode is
enabled.

The documentation, even though it talks about this feature in the JOB
CONTROL section, never made any distinction.

> As a side note, another interesting bit I found is that if I replace the
> 'sleep 1' with 'read' I will not get SIGCHLD in bash 5.

Well, of course (?). `read' is a builtin, no child process is created, and
so the shell doesn't get a SIGCHLD.

--
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/