Re: Interrupting scripts with asynchronous subprocesses

[Chet Ramey]

On 6/29/24 10:45 PM, Yuri Kanivetsky wrote:
> Hi,
>
> I've noticed a strange behavior:
>
> a.sh:
>
>      trap - INT
>      while true; do
>          echo -n .
>          sleep 2
>      done
>
> b.sh:
>
>      bash a.sh &
>      wait
>
> c.sh:
>
>      {
>          trap - INT
>          while true; do
>              echo -n .
>              sleep 2
>          done
>      } &
>      wait
>
> If I interrupt (Ctrl-C) b.sh it leaves a.sh running. That doesn't
> happen with c.sh.

b.sh runs a.sh asynchronously, so the shell started to run a.sh ignores
SIGINT and SIGQUIT. a.sh is started with SIGINT ignored, so cannot reset
the disposition to SIG_DFL. That's not the case with c.sh.

https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_11 
:

"If job control is disabled (see the description of set -m) when the shell
executes an asynchronous list, the commands in the list shall inherit from
the shell a signal action of ignored (SIG_IGN) for the SIGINT and SIGQUIT
signals."

https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_28 
:

"Signals that were ignored on entry to a non-interactive shell cannot be
trapped or reset, although no error need be reported when attempting to do
so."

But (and there's always a but with POSIX), POSIX decided that under some
circumstances, setting the disposition to SIG_DFL in an asynchronous
command should be allowed:

https://www.austingroupbugs.net/view.php?id=751

so c.sh should allow it.

There was a discussion about this after bash-5.2 came out:

https://lists.gnu.org/archive/html/bug-bash/2023-01/msg00044.html

that resulted in some behavior changes.

--
``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/

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