How do I log every method that’s called in a Ruby program?

This is definitely possible — in fact, there’s even a method for it! Just add this somewhere in your code before the point that you want to start logging things:

set_trace_func proc { |event, file, line, id, binding, classname|
  printf "%8s %s:%-2d %10s %8s\n", event, file, line, id, classname
}

The secret sauce you want comes from Kernel#set_trace_func, as noted above:

• set_trace_func(proc) => proc
• set_trace_func(nil) => nil

Establishes proc as the handler for tracing, or disables tracing if the parameter is nil. proc takes up to six parameters: an event name, a filename, a line number, an object id, a binding, and the name of a class. proc is invoked whenever an event occurs. Events are: c-call (call a C-language routine), c-return (return from a C-language routine), call (call a Ruby method), class (start a class or module definition), end (finish a class or module definition), line (execute code on a new line), raise (raise an exception), and return (return from a Ruby method). Tracing is disabled within the context of proc.

Here’s a handy example:

class Test
  def test
    a = 1
    b = 2
  end
end

set_trace_func proc { |event, file, line, id, binding, classname|
  printf "%8s %s:%-2d %10s %8s\n", event, file, line, id, classname
}

t = Test.new
t.test

(Note: don’t try this in irb unless you want a huge scrolling screen of text.) The resulting output is:

    line test.rb:11               false
  c-call test.rb:11        new    Class
  c-call test.rb:11 initialize   Object
c-return test.rb:11 initialize   Object
c-return test.rb:11        new    Class
    line test.rb:12               false
    call test.rb:2        test     Test
    line test.rb:3        test     Test
    line test.rb:4        test     Test
  return test.rb:4        test     Test

You can play around with the formatting string above to get just the results you want to log (for example, it sounds like you’re only interested in call events).