How to get the process id of a bash subprocess on command line

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.

Answer 2

Unfortunately there's no easy way to do this prior to bash version 4, when $BASHPID was introduced. One thing you can do is to write a tiny program that prints its parent PID:

int main() {     printf("%d\n", getppid());     return 0; } 

If you compile that as ppid and put it in your path, you can call it, eg:

$ (echo $$; ppid) 2139 29519 $ (x=$(ppid); echo $x) 29521 

One oddness I noticed, however, is that if you write

$ (ppid) 

it doesn't seem to actually run it in a subshell -- you need at least two commands inside the parentheses for bash to actually run them in a subshell.

Answer 3

You can do :

$ ( your_action ) &
[1] 44012

And find subprocess' PID like that :

$ echo "The sub PID : $!"
The Sub PID : 44012

$! returns the last job in background's PID. (see this manual)

Answer 4

Use homebrew to install pgrep on the Mac: brew install pgrep

Check out Link to install Homebrew.