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  subs         


        

Answer 1

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.