How to Use Ruby's Built-In Debugger from the Terminal

March 12, 2024

Ruby’s debug gem is part of the standard library since Ruby 2.6. No external gems needed.

Getting Started

Require the gem and set breakpoints:

require 'debug'

def calculate(x, y)
  debugger  # Execution stops here
  result = x * y + 5
  puts result
end

Basic Commands

Command	Action
`continue` / `c`	Resume execution until next breakpoint
`step` / `s`	Execute next line, stepping into method calls
`next` / `n`	Execute next line, stepping over method calls
`break <line>`	Set a new breakpoint at a specific line
`p <variable>`	Print the current value of a variable
`quit`	Exit the debugger

Example Session

$ ruby my_script.rb
Debugger stopped.
[1, 12] in my_script.rb
   11:
   12:   debugger  # Execution stops here
=> 13:   result = x * y + 5
   14:   puts result
   15: end

(rdbg) p x
10
(rdbg) p y
2
(rdbg) next
[1, 14] in my_script.rb
   13:   result = x * y + 5
=> 14:   puts result
   15: end
(rdbg) p result
25
(rdbg) continue
25

Advanced Features

Conditional breakpoints: Stop only when certain conditions are met
Catchpoints: Trigger breakpoints on specific exceptions
Remote debugging: Debug applications running on different machines

More info: github.com/ruby/debug

#ruby #debugging #tools