What is the meaning of $$ in Ruby?

$$ is the process ID. It's named that way to follow Perl, which in turn followed the Bourne shell; both of them also use $$ in the same way.

This seems like a good reference for stuff like this...has a lot of symboly goodness.

It's the process ID of the Ruby interpreter. $ is a prefix for global variables, see here for a list.

pipe = IO.popen("")
if pipe
  STDERR.puts "In parent, child pid is #{pipe.pid}"
else
  STDERR.puts "In child, pid is #{$$}"
end

This example shows how the process ID can be used. Process ID is part of 'Execution Environment Variables' (part of predifined variables).

It's the process ID of the current Ruby process. Read-only