SUMMARY: Getting PID of a Sub-Shell

I got a number of responses. Two pointed to this,

 
http://www.unix.com/shell-programming-scripting/27767-ksh-how-does-line-child-sub-process-get-its-own-pid.html


Which verifies that others have had the same problem, but
does not provide a good workaround.

Someone else noted that this type of behavior is documented
in the manpage for PPID,

     PPID  This variable is set by the shell to the decimal  pro-
           cess  ID  of  the process that invoked the shell. In a
           subshell, PPID will be set to the same value  as  that
           of  the parent of the current shell. For example, echo
           $PPID and (echo $PPID) would produce the same value.

Which lends weight to the idea that it is a feature, not a bug.

I got a note from self-appointed list-cop saying it was off topic.
This was actually urgent even if I did not state it in the message.
I wanted to get the script in production before COB today. Yes, it
turned out not to be Solaris specific, but I had initially done an
(incorrect) test on another OS that made it look like it had
different behavior. Sorry on that account.

OTOH, I got two posts where people wanted to find out if I had
an answer and to please post a summary.

One response just said, "$!", which is a workaround when you
are running asynchronously, but can be a lot of overhead depending
on how you want to do it. Similarly, pulling the info out using
ptree(1), is a little clunky and not reliable if your parent
process is no longer around (you'd presumably be parsing output
of "ptree $$" which doesn't work after $$ is dead).

Finally, the best response mentioned that "$$" is set at an exec(2),
but not at later fork(2)s. So combining that with PPID, the trick
I'm going to use is,

	echo "Parent: $$"
	(
		MYPID=$(ksh -c 'echo $PPID')
		echo "Subshell: $MYPID"
	)

That is, run a command that tells you its parent's PID, which is the
process that called it.

Thanks.

On 2/28/2008 at 2:36 PM, "Crist Clark" <Crist.Clark@globalstar.com>
wrote:
> I've got a ksh script where I'd like to learn the PID of
> a sub-shell within the sub-shell code.
> 
> If you have code like,
> 
> 	$ cat pidtest.sh
> 	/bin/echo "$$"
> 	( /bin/echo "$$" )
> 	$ ksh pidtest.sh
>       29586
>       29586
> 
> You'll see the same number even though the second echo runs
> in a sub-shell with a different PID. If I run the script
> in truss(1) I do see that it is indeed happening as I expect
> it would, but the "$$" does not get changed when the sub-shell
> gets fork(2)ed off.
> 
> Bug or feature?
> 
> Anyway, anyone know of a way to work around it? How do I get
> the PID of that sub-shell from code within the sub-shell?


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