How to Debug Ruby Scripts

How to debug Ruby scripts

Use Pry (GitHub).

Install via:

$ gem install pry
$ pry

Then add:

require 'pry'; binding.pry

into your program.

As of pry 0.12.2 however, there are no navigation commands such as next, break, etc. Some other gems additionally provide this, see for example pry-byebug.

How to debug ruby code?

Ruby-debug is for 1.8+ and ruby-debug19 is for 1.9+.

ruby-debug is easy to learn and very useful. You can tell the application to run until a certain condition exists, then have it break, making it easy to locate nil values, or other conditions that occur sporadically.

From the command-line use rdebug appname and you'll end up at the debugger prompt. If you want to run to line 100 and stop you can enter c 100 and the debugger will set a temporary break-point, the program will run then stop there if it's in the execution path. Once it stops the temporary break-point will be cleared. If you always want to stop at line 100 you could do b 100 then c and the debugger will set a permanent break-point, continue, then stop when the break-point is reached. You can clear the breakpoints, set conditional ones that occur when certain conditions apply, etc. You can type n to step to the next instruction skipping over subroutine calls, or s to step into them. There are commands to display contents of variables in various ways, so read through the docs.

From inside rdebug you can drop into an IRB shell with your variables already populated so you can poke at things to see what happens. From inside either you can inspect or set values, helping with what-if adjustments. If you do that from within rdebug you can continue the program with the altered value(s) and see how it behaves.

IRB has its place, and it's great for trying things, but it's not a substitute for the debugger, just as the debugger can do some IRB-ish things, but won't replace it. Both tools are a good combination and beat the heck out of relying on print statements or dumping to a log file.


Pry has emerged as a great combination of IRB and a debugger, and is well worth investigating.

How to debug Ruby code on Visual Studio Code?

This is actually fairly easy once you follow the right steps.
First, you have to download Ruby Extension which is available on vs code marketplace or simply via Extension tab in VS code itself: just search for Ruby, install it, and reload VS Code [update: vscode can perfectly load new extensions without needing a reload].
secondly, you have to follow the debugging guide for this extension which is available on the GitHub link I provided or in vs code marketplace. Here's the section you would be most interested in, but you could also see the original wiki:

Install Ruby Dependencies

In this extension, we implement ruby debug ide protocol to allow VS
Code to communicate with ruby debug, it requires ruby-debug-ide to be
installed on your machine. This is also how RubyMine/NetBeans does by
default.

  • If you are using JRuby or Ruby v1.8.x (jruby, ruby_18, mingw_18), run

    gem install ruby-debug-ide

The latest version is 0.6.0. Make sure ruby-debug-base is installed together with ruby-debug-ide.

  • If you are using Ruby v1.9.x (ruby_19, mingw_19), run

    gem install ruby-debug-ide

The latest version is 0.6.0. Make sure ruby-debug-base19x is installed together with ruby-debug-ide.

  • If you are using Ruby v2.x

    gem install ruby-debug-ide -v 0.6.0 (or higher versions)
    gem install debase -v 0.2.1 (or higher versions)

Add VS Code config to your project

Go to the debugger view of VS Code and hit the gear icon. Choose Ruby
or Ruby Debugger from the prompt window, then you'll get the sample
launch config in .vscode/launch.json. The sample launch configurations
include debuggers for RSpec (complete, and active spec file) and
Cucumber runs. These examples expect that bundle install --binstubs
has been called. Detailed instruction for debugging Ruby
Scripts/Rails/etc

Read following instructions about how to debug ruby/rails/etc locally
or remotely

01 Debugger installation

02 Launching from VS Code

03 Attaching to a debugger

04 Running gem scripts

05 Example configurations

if you follow these steps, you will have every dependency installed in Step 1.
Step 2 helps you config your project workspace to start debuging codes written in ruby.
by finishing step 2, you should be able to start debugging.
here is a simple config that I use in my recent ruby project to simply debug the current open file. this is fully explained in the second step I linked

{
"name": "Debug Local File",
"type": "Ruby",
"request": "launch",
"cwd": "${workspaceRoot}",
"program": "${file}"
}

"program": "${file}" is the line that enables debugging the current open file.

How to use the debugger with Ruby 2.0?

The debugger gem can be used but it still has issues.

Install byebug which was written for Ruby 2.0 debugging.

For breakpoints, use the byebug command in your code instead of debugger.



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