Ruby - Debugger

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.

Sr.No.	Command & 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]#

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