Outputs a source level execution trace of a Ruby program.

It does this by registering an event handler with Kernel#set_trace_func for processing incoming events. It also provides methods for filtering unwanted trace output (see Tracer.add_filter, Tracer.on, and


Consider the following Ruby script

class A
  def square(a)
    return a*a

a =

Running the above script using ruby -r tracer example.rb will output the following trace to STDOUT (Note you can also explicitly require tracer’`)

#0:<internal:lib/rubygems/custom_require>:38:Kernel:<: -
#0:example.rb:3::-: class A
#0:example.rb:3::C: class A
#0:example.rb:4::-:   def square(a)
#0:example.rb:7::E: end
#0:example.rb:9::-: a =
#0:example.rb:10::-: a.square(5)
#0:example.rb:4:A:>:   def square(a)
#0:example.rb:5:A:-:     return a*a
#0:example.rb:6:A:<:   end
 |  |         | |  |
 |  |         | |   ---------------------+ event
 |  |         |  ------------------------+ class
 |  |          --------------------------+ line
 |   ------------------------------------+ filename
  ---------------------------------------+ thread

Symbol table used for displaying incoming events:

  • +}+: call a C-language routine
  • +{+: return from a C-language routine
  • +>+: call a Ruby method
  • C: start a class or module definition
  • E: finish a class or module definition
  • -: execute code on a new line
  • +^+: raise an exception
  • +<+: return from a Ruby method

Tracer Reference