I have been studying for my Unix exam and I often saw exclamation mark variable - $!If I write echo $! on my Mac shell tells me something like You have mail inIs purpose of this to store whether I do or do not have any mails or is there something else?EDIT: Funny coincidence happened. Exactly when I wrote echo $! my Shell also notified me that I have some mail. So that's why I wrote that mail part. 
Exclamation mark variable `$!`
shell;environment variables;variable
$! is the PID of the most recent background command.See this excellent answer for other special parameters.