Question

I know in bash we can create subshells using round parenthesis ( and ). As per bash man page:

(list) list  is  executed  in  a  subshell environment 

Also to get the current process id we use:

echo $$

Now my question is how to get process id of a subshell created using ( and ) on command line?

If I use this:

echo $$; ( echo $$; ) 

I will get the parent shell’s process id printed twice on stdout since $$ gets expanded even before subshell is created. So how to really force the lazy expansion?

[Solution should work on Mac as well not just Linux]

Update:

Suggested linked answer doesn’t work since echo $BASHPID does not work on my Mac and returns blank.

Answer 1

Thanks to all of you for spending your valuable time in finding answer to my question here.

However I am now answering my own question since I’ve found a hack way to get this pid on bash ver < 4 (will work on all the versions though). Here is the command:

echo $$; ( F='/tmp/myps'; [ ! -f $F ] && echo 'echo $PPID' > $F; )

It prints:

5642
13715

Where 13715 is the pid of the subshell. To test this when I do:

echo $$; ( F='/tmp/myps'; [ ! -f $F ] && echo 'echo $PPID' > $F; bash $F; ps; )

I get this:

5642
13773
  PID   TT  STAT      TIME COMMAND
 5642 s001  S      0:02.07 -bash
13773 s001  S+     0:00.00 -bash

Telling me that 13773 is indeed the pid of the subshell.

Note: I reverted back to my original solution since as @ChrisDodd commented that echo $$; ( bash -c 'echo $PPID'; ) doesn’t work Linux. Above solution of mine works both on Mac and Linux.