Ruby debugger tutorial

It doesn't matter how easy a language is to use, it usually contains some bugs if it is more than a few lines long. To help deal with bugs, the standard distribution of Ruby includes a debugger.

In order to start the Ruby debugger, load the debug library using the command-line option -r debug. The debugger stops before the first line of executable code and asks for the input of user commands.

Usage Syntax:

Here is the usage syntax to use ruby debugger:

$ ruby -r debug filename[, ...]

Ruby Debugger Commands:

Here is a complete list of commands, which you can use while debugging your program. Here, it is not necessary to use complete keyword to give a command, part given inside [...] is option.

SN	Command with Description
1	b[reak] [< file| class>:]< line| method> Sets breakpoint to some position. Breakpoint is a place where you want to pause program execution for debugging purpose.
2	wat[ch] expression Sets watchpoints
3	cat[ch] (exception|off) Sets catchpoint to an exception.
4	b[reak] Displays breakpoints and watchpoints
5	del[ete] [n] Deletes breakpoints
6	disp[lay] expression Displays value of expression
7	undisp[lay] [ n] Removes display of n
8	c[ont] Continues execution
9	s[tep] [ n] Executes next n lines stepping into methods
10	n[ext] [ n] Executes next n lines stepping over methods
11	w[here] Displays stack frame
12	f[rame] Synonym for where
13	l[ist][<-| n- m>] Displays source lines from n to m
14	up [ n] Moves up n levels in the stack frame
15	down [ n] Moves down n levels in the stack frame
16	fin[ish] Finishes execution of the current method
17	tr[ace] [on|off] Toggles trace mode on and off
18	q[uit] Exits debugger
19	v[ar] g[lobal] Displays global variables
20	v[ar] l[ocal] Displays local variables
21	v[ar] i[instance] object Displays instance variables of object
22	v[ar] c[onst] object Displays constants of object
23	m[ethod] i[instance] object Displays instance methods of object
24	m[ethod] class| module Displays instance methods of the class or module
25	th[read] l[ist] Displays threads
26	th[read] c[ur[rent]] Displays current thread
27	th[read] n Stops specified thread
28	th[read] stop > Synonym for th[read] n
29	th[read] c[ur[rent]] n> Synonym for th[read] n
30	th[read] resume > Resumes thread n
31	p expression Evaluates the expression
32	h[elp] Displays help message
33	everything else Evaluates.

Example:

Consider the following file hello.rb, which needs to be debugged:

#!/usr/bin/env ruby
class Hello
   def initialize( hello )
      @hello = hello
   end
   def hello
      @hello
   end
end

salute = Hello.new( "Hello, Mac!" )
puts salute.hello

Here is one interactive session captured. Given commands are written in bold:

[root@ruby]# ruby -r debug hello.rb
Debug.rb
Emacs support available.

hello.rb:3:class Hello
(rdb:1) v l
  salute => nil
(rdb:1) b 10
Set breakpoint 1 at hello.rb:10
(rdb:1) c
Hello, Mac!
[root@ruby]#