Quick Guide Debuggers

Introduction

This page is a page to remind me of how to use the cli debuggers for C++, Rails and Python

Debugging Commands Across Rails (rdbg), GDB, and Python (pdb)

Below are three separate MediaWiki tables — one for each debugger — covering: • Set breakpoint before starting • Set breakpoint inside debugger • Continue • Step • Step out • Print current source • Print/Evaluate expression

GDB

Action	Command	Notes
Set breakpoint before starting	gdb --args ./program then break file.c:LINE	Or break main.
Set breakpoint in debugger	break file.c:LINE	Can also break on functions.
Continue	continue or c	Runs until next breakpoint.
Step	step or s	Steps into function calls.
Step out	finish	Executes until current function returns.
Print current source	list or l	Shows source around current line.
Print/Evaluate expression	print expr	Evaluates C/C++ expressions.

Rails (rdbg)

Action	Command	Notes
Set breakpoint before starting	rdbg -n --command "break file.rb:LINE" -- bin/rails runner script.rb	Use -n to stop at the first breakpoint.
Set breakpoint in debugger	break file.rb:LINE	Or break Class#method.
Continue	continue or c	Runs until next breakpoint.
Step	step or s	Steps into method calls.
Step out	finish	Runs until current frame returns.
Print current source	list or l	Shows surrounding lines.
Print/Evaluate expression	p expr or pp expr	Ruby evaluation inside debugger.

Python (pdb)

Action	Command	Notes
Set breakpoint before starting	python -m pdb script.py then b file.py:LINE	Or insert breakpoint() in code.
Set breakpoint in debugger	b file.py:LINE	Or b function_name.
Continue	continue or c	Runs until next breakpoint.
Step	step or s	Steps into function calls.
Step out	return or r	Runs until current function returns.
Print current source	list or l	Shows surrounding lines.
Print/Evaluate expression	p expr or pp expr	Evaluates Python expressions.