How to debug a Ruby on Rails application?
I needed to debug a Ruby on Rails application which I just have started to develop. I put some logging commands to the code but it's not as good as using a full debugger for inspecting internal data at runtime.
For debugging a Rails application ruby-debug gem is needed. It can be installed by command sudo gem install ruby-debug in Linux or as a package from an installation repository.
How to start the debugger?
At first, the place in the application where the debugger should be started must be marked by debugger keyword.
Then the application must be started in the debugger - it's simple, just use --debugger option of the starting script, e.g. script/server --debugger.
How to use the debugger?
When the application executes debugger command it stops and a debugger session is opened in the terminal.
It supports these basic commands:
l (line) - print the source script
c (continue) - stop the debug session at continue executing the application
bt (backtrace) - print the execution stack (which functions were called)
n (next) - execute the next command
s (step) - execute the next command, step inside a function
h (help) - will list more available commands with details
Moreover it's possible to use normal ruby commands to print or inspect the current objects, using e.g. object.class, objec.inspect methods. And it's also possible to modify the current variables, just use var = value command.
This makes the debugger really powerful, it's far better than simple puts command!