Interactive debugging with ruby-debugEdit

I installed ruby-debug (see "Installing ruby-debug 0.7.5") because I was having trouble getting RSpec specifications to run correctly under Ruby’s built-in debugger:

ruby -r debug spec/file_to_run_in_debugger_spec.rb

The problem was that all sorts of superfluous exceptions not related to the code under test were being thrown, making debugging painful and impractical; the same specs ran fine from the command line using the normal Ruby interpreter (ruby spec/file_to_run_in_debugger_spec.rb), using the spec tool (spec spec/file_to_run_in_debugger_spec.rb) or from a Rakefile (rake spec).

Before I got a chance to figure out why this was happening I discovered ruby-debug and thought I’d give it a go. Everything worked, so I didn’t bother exploring the question any further.

Launching a spec under ruby-debug

rdebug spec/parser_spec.rb

Getting help from inside the debugger

Getting a list of commands


Getting help on a specific command

For example, getting help on the list command:

h list

Seeing where you are in the source code



Turning on automatic listing on every stop

set autolist

Printing a backtrace



Setting a breakpoint on a failing spec (example)

One of my specs was failing like this:

NoMethodError in 'parsing with the parser should be able to mix comments and plain text'
undefined method `[]' for #<Walrus::WalrusGrammar::Comment:0x348258>

To run the spec under ruby-debug from the trunk directory:

rdebug spec/parser_spec.rb

To set a breakpoint just before the failing spec:

b 101

To continue execution up to the breakpoint:


To list currently set breakpoints


Stepping through code

Moving to next line

This doesn’t go into methods:


The full help for the next command is:

n[ext][ nnn]    go over one line or till line nnn

Stepping into the next line

This does go into methods:


The full help for the step command is:

s[tep][ nnn]    step (into methods) one line or till line nnn

To step out of the current method, you finish it:


Moving up and down the stack frames

This would normally be used in conjunction with bt (to get a list of frames).

Moving to the newest frame

f 0

Moving to the oldest frame

f -1

Inspecting objects

p object

Evaluating expressions

In fact, p is just a synonym for eval and can be used to evaluate any Ruby expression:

eval 1 + 2

You can turn on auto-evaluation as well which automatically tries to eval any command not recognized by ruby-debug:

set autoeval

This allows you to type things like:


Instead of:

eval myvar

Showing the instance variables of an object

Showing the instance variables of the current receiver:

var i self

Showing current local variables

var l

Showing global variables

var g