This library provides debugging functionality to Ruby.

To add a debugger to your code, start by requiring debug in your program:

def say(word)
  require 'debug'
  puts word

This will cause Ruby to interrupt execution and show a prompt when the say method is run.

Once you’re inside the prompt, you can start debugging your program.

(rdb:1) p word

Getting help

You can get help at any time by pressing h.

(rdb:1) h
Debugger help v.-0.002b
  b[reak] [file:|class:]<line|method>
  b[reak] [class.]<line|method>
                             set breakpoint to some position
  wat[ch] <expression>       set watchpoint to some expression
  cat[ch] (<exception>|off)  set catchpoint to an exception
  b[reak]                    list breakpoints
  cat[ch]                    show catchpoint
  del[ete][ nnn]             delete some or all breakpoints
  disp[lay] <expression>     add expression into display expression list
  undisp[lay][ nnn]          delete one particular or all display expressions
  c[ont]                     run until program ends or hit breakpoint
  s[tep][ nnn]               step (into methods) one line or till line nnn
  n[ext][ nnn]               go over one line or till line nnn
  w[here]                    display frames
  f[rame]                    alias for where
  l[ist][ (-|nn-mm)]         list program, - lists backwards
                             nn-mm lists given lines
  up[ nn]                    move to higher frame
  down[ nn]                  move to lower frame
  fin[ish]                   return to outer frame
  tr[ace] (on|off)           set trace mode of current thread
  tr[ace] (on|off) all       set trace mode of all threads
  q[uit]                     exit from debugger
  v[ar] g[lobal]             show global variables
  v[ar] l[ocal]              show local variables
  v[ar] i[nstance] <object>  show instance variables of object
  v[ar] c[onst] <object>     show constants of object
  m[ethod] i[nstance] <obj>  show methods of object
  m[ethod] <class|module>    show instance methods of class or module
  th[read] l[ist]            list all threads
  th[read] c[ur[rent]]       show current thread
  th[read] [sw[itch]] <nnn>  switch thread context to nnn
  th[read] stop <nnn>        stop thread nnn
  th[read] resume <nnn>      resume thread nnn
  p expression               evaluate expression and print its value
  h[elp]                     print this help
  <everything else>          evaluate


The following is a list of common functionalities that the debugger provides.

Navigating through your code

In general, a debugger is used to find bugs in your program, which often means pausing execution and inspecting variables at some point in time.

Let’s look at an example:

def my_method(foo)
  require 'debug'
  foo = get_foo if foo.nil?
  raise if foo.nil?

When you run this program, the debugger will kick in just before the foo assignment.

(rdb:1) p foo

In this example, it’d be interesting to move to the next line and inspect the value of foo again. You can do that by pressing n:

(rdb:1) n # goes to next line
(rdb:1) p foo

You now know that the original value of foo was nil, and that it still was nil after calling get_foo.

Other useful commands for navigating through your code are:


Runs the program until it either exists or encounters another breakpoint. You usually press c when you are finished debugging your program and want to resume its execution.


Steps into method definition. In the previous example, s would take you inside the method definition of get_foo.


Restart the program.


Quit the program.

Inspecting variables

You can use the debugger to easily inspect both local and global variables. We’ve seen how to inspect local variables before:

(rdb:1) p my_arg

You can also pretty print the result of variables or expressions:

(rdb:1) pp %w{a very long long array containing many words}

You can list all local variables with +v l+:

(rdb:1) v l
  foo => "hello"

Similarly, you can show all global variables with +v g+:

(rdb:1) v g
  all global variables

Finally, you can omit p if you simply want to evaluate a variable or expression

(rdb:1) 5**2

Going beyond basics

Ruby Debug provides more advanced functionalities like switching between threads, setting breakpoints and watch expressions, and more. The full list of commands is available at any time by pressing h.

Staying out of trouble

Make sure you remove every instance of +require ‘debug’+ before shipping your code. Failing to do so may result in your program hanging unpredictably.

Debug is not available in safe mode.


MUTEX = Thread::Mutex.new # :nodoc:


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