Re: Shell script terminates when spawning and killing a background shell process

[Robert Elz <kre%munnari.OZ.AU@localhost>]

    Date:        Sat, 18 Jul 2026 13:53:10 +0200
    From:        Gerhard Rieger <gerhard%rieger-priv.org@localhost>
    Message-ID:  <71de6a8f-aa10-4540-a53c-af67694f6194%rieger-priv.org@localhost>

  | I have reduced the case to the following ksh script that terminates when 
  | it kills its background process, this happens on NetBSD 9.3 and NetBSD 
  | 10.1; when you don't think this script should print "SURVIVED"
  | this issue might be a feature though...

Not a feature...

  | #! /bin/ksh
  |
  | /bin/ksh -c "sleep 2" </dev/null &

This seems to be causing the problem, replace that ksh with sh bash
(or probably any other shell) and it all works, regardless of
which shell is running the outer script.   While it is running
ksh as the background process, it is also irrelevant which shell
is running the outer script, that shell is killed.

The only rational explanation for this is that the ksh is killing
its parent when it is sent the SIGTERM.

Note that unlike most other shells, our ksh isn't smart enough to
recognise that with just "sleep 2" as a command, it can just exec
that, rather it forks, and runs the sleep as a child.

While it is doing that, it blocks SIGTERM.   When the kill command
is run, the signal is sent to that ksh (not the sleep), which achieves
nothing immediately.  Later after the sleep has finished, the SIGTERM is
unblocked in that ksh, and the ksh realises that it was supposed
to be killed.   At that point it kills its parent (maybe it kills
its own process group, which would include its parent in this case).

Our ksh is full of bugs, there are better ksh versions in pkgsrc if
you want one of those (even a pdksh, which probably has less bugs
that our version).   It kind of works, but no-one seems to interested
in ever fixing any of the bugs it does have, I'd simply avoid it.

kre