Ruby: how to step through ruby code [closed]

What you're looking for is a debugger. With ruby 1.8, the ruby-debug gem provides the canonical one, and in ruby 1.9, the debugger gem provides a version of its successor that is well-maintained.

Start your program with the gem loaded, for example ruby -rdebugger yourfile.rb, and you'll have access to the debugger.

The debugger kicks in at a breakpoint, which is a signal to suspend execution, and give you a chance to poke around. Typically, the easiest way to set a breakpoint is to add a source line at the point in execution you're interested in. Inserting a call to debugger, does this. Lets debug this function that asks for a user name, but restricts it to doctors. It's not letting "Dr. Bob" log in:

def get_user
  debugger  # This is where we break.
  name = ""
  begin
    print "What is your name?: "
    name = gets
  end until name.match /\ADr /
  puts "Welcome, #{name}."
  name
end

user = get_user

Lets launch it:

ruby -rdebugger login.rb

The program starts executing, then the debugger breaks us at line four. It shows us the next statement which is to be executed next. We can get a more complete listing of our surroundings by typing l.

(rdb:1) l
[-1, 8] in login.rb
   1  
   2  def get_user
   3    debugger
=> 4    name = ""
   5    begin
   6      print "What is your name?: "
   7      name = gets 
   8    end until name.match /\ADr /

You can examine the result of expressions by using the p command. p name will show us that the name variable is an empty string at this point of execution.

n steps, which executes the next line, and returns control to the debugger. s will allow you to step into a method call. c continues execution until the program terminates, or hits another breakpoint. In this case, we'd probably want to use the n command to trace the execution of this method. Just these handful of commands describes the vast majority of my debugging sessions.

The debugger has has interactive help which can be accessed with help. A more conversational tutorial on the web is: Debugging with ruby-debug