Question about job control in non-interactive shells

Question about job control in non-interactive shells

[Michael Welsh Duggan]

I am trying to determine why:

  dash -c "sleep 5 & kill %1"

results in:

  dash: 1: kill: No such process

whereas from an interactive dash

  sleep 5 & kill %1

seems to do exactly what it should.  Any hints?

-- 
Michael Welsh Duggan
(mwd@cert.org)

Re: Question about job control in non-interactive shells

[Jonathan Nieder]

Hi Michael,

Michael Welsh Duggan wrote:

> I am trying to determine why:
>
>   dash -c "sleep 5 & kill %1"
>
> results in:
>
>   dash: 1: kill: No such process

You are probably looking for the -m option.

Thanks for writing, and hope that helps,
Jonathan

Re: Question about job control in non-interactive shells

[Jilles Tjoelker]

On Mon, Jan 09, 2012 at 10:37:45AM -0600, Jonathan Nieder wrote:
> Michael Welsh Duggan wrote:
> > I am trying to determine why:

> >   dash -c "sleep 5 & kill %1"

> > results in:

> >   dash: 1: kill: No such process

> You are probably looking for the -m option.

The cause is that the -m option ("job control") enables running commands
in separate process groups, and dash follows literally what POSIX says
about kill %job: a background process group should be signaled; however,
there is no background process group. Some shells signal one or more
processes they know are part of the job in this case, but dash calls
kill() on a process group that is guaranteed not to exist.

-- 
Jilles Tjoelker

Re: Question about job control in non-interactive shells

[Herbert Xu]

Jilles Tjoelker <jilles@stack.nl> wrote:
> On Mon, Jan 09, 2012 at 10:37:45AM -0600, Jonathan Nieder wrote:
>> Michael Welsh Duggan wrote:
>> > I am trying to determine why:
> 
>> >   dash -c "sleep 5 & kill %1"
> 
>> > results in:
> 
>> >   dash: 1: kill: No such process
> 
>> You are probably looking for the -m option.
> 
> The cause is that the -m option ("job control") enables running commands
> in separate process groups, and dash follows literally what POSIX says
> about kill %job: a background process group should be signaled; however,
> there is no background process group. Some shells signal one or more
> processes they know are part of the job in this case, but dash calls
> kill() on a process group that is guaranteed not to exist.

Right.  And you don't even need %1:

	dash -c "sleep 5 & kill $!"

works just fine.

Cheers,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt