Debugging with pry and byebug in Ruby - Step-by-Step Execution

Concept Flow - Debugging with pry and byebug

Start Program

↓

Hit binding.pry or byebug

↓

Pause Execution

↓

Interact: Inspect Variables, Step, Continue

↓

Resume or Exit Debugger

↓

Program Continues or Ends

The program runs until it hits a debugger call, then pauses allowing you to inspect and control execution before continuing.

Execution Sample

Ruby

require 'pry'
def add(a, b)
  binding.pry
  a + b
end

add(2, 3)

This code pauses execution inside the add method so you can inspect variables before returning the sum.

Execution Table

Step	Action	Code Line	Variables State	Debugger Prompt	Output
1	Start program	def add(a, b)	a: undefined, b: undefined	No
2	Call add(2, 3)	add(2, 3)	a: 2, b: 3	No
3	Hit binding.pry	binding.pry	a: 2, b: 3	Yes - pry prompt
4	Inspect variables	binding.pry	a: 2, b: 3	Yes - pry prompt	a=2, b=3
5	Step over to next line	a + b	a: 2, b: 3	Yes - pry prompt
6	Continue execution	a + b	a: 2, b: 3	No	5
7	Program ends

💡 Program ends after returning the sum and no more code to run.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 6	Final
a	undefined	2	2	2	2
b	undefined	3	3	3	3

Key Moments - 3 Insights

Why does the program pause at binding.pry or byebug?

The debugger call tells Ruby to stop running and open an interactive prompt so you can inspect and control the program (see execution_table step 3).

Can I see variable values before the debugger line?

No, variables get their values when the method is called. Before that, they are undefined (see variable_tracker start values).

How do I continue running the program after stopping at the debugger?

You type 'continue' or 'c' at the debugger prompt to resume normal execution (see execution_table step 6).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of variable 'a' at step 4?

A 2

B undefined

C 3

D nil

💡 Hint

Check the 'Variables State' column at step 4 in the execution_table.

At which step does the debugger prompt first appear?

A Step 2

B Step 5

C Step 3

D Step 6

💡 Hint

Look for 'Yes - pry prompt' in the 'Debugger Prompt' column.

If you remove 'binding.pry' from the code, what changes in the execution table?

A Variables become undefined

B Debugger prompt never appears

C Program never ends

D Output changes to nil

💡 Hint

Without the debugger call, the program runs straight through without pausing (see exit_note).

Concept Snapshot

Use binding.pry or byebug to pause Ruby code execution.
At the pause, interactively inspect variables and control flow.
Commands: 'next' to step, 'continue' to resume.
Remove debugger calls to run normally.
Great for finding bugs by watching code run step-by-step.

Full Transcript

This visual trace shows how Ruby's debugging tools pry and byebug pause program execution at a specific line. When the program hits binding.pry, it stops and opens a prompt where you can check variable values and step through code. Variables get their values when the method is called, so before that they are undefined. You continue running the program by typing 'continue' at the prompt. Removing the debugger call lets the program run without stopping. This helps find bugs by letting you watch what happens inside your code.