    def read_lines(filename, selector)
      if selector
        IO.foreach(filename).select.with_index(1, &selector)
      else
        URI.open(filename, &:read)
      end
    end
The software constructs all or part of an OS command using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the intended OS command when it is sent to a downstream component.