Re: When kill does not work for a sudo process in a script?

[Seth David Schoen]

Peng Yu writes:

> Hi,
> 
> When I finish running ./main.sh, the forever.sh sudo process will not
> be terminated. But I tried to kill the forever.sh sudo process in
> main.sh. But if I remove the two "sudo" words in main.sh, then the
> forever.sh process can be terminated.
> 
> Does anybody know why the forever.sh sudo process will not be
> terminated? Thanks.

I was surprised by this behavior, but it turns out it's more about sudo
than about bash.  The sudo program has a complicated logic about when
it will or won't forward signals that it catches to the subprocess that
it's spawned.  In this case the relevant logic is one about not
forwarding signals from processes in the same process group as the
original sudo command.  See the section beginning

        /*
         * Do not forward signals sent by a process in the command's process
         * group, as we don't want the command to indirectly kill itself.
         * For example, this can happen with some versions of reboot that
         * call kill(-1, SIGTERM) to kill all other processes.
         */

in src/exec_pty.c and src/exec_nopty.c.

I've rarely had occasion to think about the precise circumstances under
which programs spawned by the same shell are or aren't in the same process
group (in particular, it seems like there's an issue that job control
works slightly differently behind the scenes in shell scripts than in
interactive shells, although I haven't delved into that), but in this case
it matters a lot because sudo is explicitly checking for this condition.

I was able to nullify this with "set -m" in bash, which uses a different
process group for every external program that a shell script spawns.
With the addition of "set -m", the program does what I think you expected.

-- 
Seth David Schoen <schoen@loyalty.org>      |  Qué empresa fácil no pensar
     http://www.loyalty.org/~schoen/        |  en un tigre, reflexioné.
                                            |        -- Borges, "El Zahir"