What does $? mean in Ruby?

Question

I just read the comparison of PHP, Perl, Ruby and Python

http://hyperpolyglot.org/scripting

and saw the following code sample.

files = `ls -l /tmp`
unless $?.success?
  raise "ls failed"
end

files = %x(ls)
unless $?.success?
   raise "ls failed"
end

I was wondering what $? stands for....

Answer 1

$? means exit status of last executed child process

Answer 2

$? contains the last executed command's exit code. Which can be accessed as $?.exitstatus