trace_var (Kernel)

@tsaleh @daksis I was talking about trace_var the other day, in case you're curious:

Controls tracing of assignments to global variables. The parameter +symbol_ identifies the variable (as either a string name or a symbol identifier). cmd (which may be a string or a Proc object) or block is executed whenever the variable is assigned. The block or Proc object receives the variable’s new value as a parameter. Also see Kernel::untrace_var.

Full article: http://apidock.com/ruby/Kernel/trace_var