[mkdirp]

> The process ID of the last executed background command in Bash is available as $!.

After a quick search it doesn't seem like this is an option for anything but the last executed background process. So if you have the typical `diff <(left) <(right)`, there is no easy way to get the pid of `<(left)`.

I suppose you could output the pid to a file, and use it from there, though I haven't tested if that would work.