Which of the following special shell variables is used to process number of the last background job? A. $! B. $# C. $0 D. $* E. None of the above

The correct answer is: A. $!

The special shell variable $! is used to refer to the process ID of the last background job. The process ID is a unique number assigned to each process running on a computer. You can use the $! variable to refer to the last background job in a number of ways. For example, you can use it to kill the job, to suspend it, or to get its output.

The other options are incorrect.

• Option B, $#, is the number of positional parameters. Positional parameters are the arguments that are passed to a script when it is executed.
• Option C, $0, is the name of the script that is being executed.
• Option D, $*, is a special shell variable that expands to a list of all the positional parameters.
• Option E is incorrect because it is not a valid shell variable.