In the following command and its output
$ echo $$
12942
What is 12942?
A.
the process ID of the echo command
B.
the process ID of the current shell
C.
the process ID of the last command executed
D.
the process ID of the last backgrounded command
Explanation/Reference:
see http://tldp.org/LDP/abs/html/internalvariables.html
$? Exit status of a command, function, or the script itself
$$ Process ID (PID) of the script/process itself
$_ Special variable set to final argument of previous command executed.
$! PID (process ID) of last job run in background