How to debug Ruby scripts [closed]

Question

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.

1. In Ruby:

   ruby -rdebug myscript.rb 
   

   then,

   • b <line>: put break-point
   • and n(ext) or s(tep) and c(ontinue)
   • p(uts) for display

   (like perl debug)

2. In Rails: Launch the server with

   script/server --debugger
   

   and add debugger in the code.

As banister recommended: use pry! I can only agree on this.

pry is a much better repl than irb.

You need to add

require 'pry'

to your source file and then insert a breakpoint in your source code by adding

binding.pry

at the place where you want to have a look at the things (this is like triggering a breakpoint in a classic IDE environment)

Once your program hits the

binding.pry

line, you'll be thrown right into the pry repl, with all the context of your program right at hand, so that you can simply explore everything around, investigate all objects, change state, and even change code on the fly.

I believe you can not change the code of the method that you are currently in, so you can sadly not change the next line to be executed. But good ruby code tends to be single line anyway ;-)

Debugging by raising exceptions is far easier than squinting through print log statements, and for most bugs, its generally much faster than opening up an irb debugger like pry or byebug. Those tools should not always be your first step.

---

Debugging Ruby/Rails Quickly:

1. Fast Method: Raise an Exception then and .inspect its result

The fastest way to debug Ruby (especially Rails) code is to raise an exception along the execution path of your code while calling .inspect on the method or object (e.g. foo):

raise foo.inspect

In the above code, raise triggers an Exception that halts execution of your code, and returns an error message that conveniently contains .inspect information about the object/method (i.e. foo) on the line that you're trying to debug.

This technique is useful for quickly examining an object or method (e.g. is it nil?) and for immediately confirming whether a line of code is even getting executed at all within a given context.

2. Fallback: Use a ruby IRB debugger like byebug or pry

Only after you have information about the state of your codes execution flow should you consider moving to a ruby gem irb debugger like pry or byebug where you can delve more deeply into the state of objects within your execution path.

---

General Beginner Advice

When you are trying to debug a problem, good advice is to always: Read The !@#$ing Error Message (RTFM)

That means reading error messages carefully and completely before acting so that you understand what it's trying to tell you. When you debug, ask the following mental questions, in this order, when reading an error message:

1. What class does the error reference? (i.e. do I have the correct object class or is my object nil?)
2. What method does the error reference? (i.e. is their a type in the method; can I call this method on this type/class of object?)
3. Finally, using what I can infer from my last two questions, what lines of code should I investigate? (remember: the last line of code in the stack trace is not necessarily where the problem lies.)

In the stack trace pay particular attention to lines of code that come from your project (e.g. lines starting with app/... if you are using Rails). 99% of the time the problem is with your own code.

---

To illustrate why interpreting in this order is important...

E.g. a Ruby error message that confuses many beginners:

You execute code that at some point executes as such:

@foo = Foo.new

...

@foo.bar

and you get an error that states:

undefined method "bar" for Nil:nilClass

Beginners see this error and think the problem is that the method bar is undefined. It's not. In this error the real part that matters is:

for Nil:nilClass

for Nil:nilClass means that @foo is Nil! @foo is not a Foo instance variable! You have an object that is Nil. When you see this error, it's simply ruby trying to tell you that the method bar doesn't exist for objects of the class Nil. (well duh! since we are trying to use a method for an object of the class Foo not Nil).

Unfortunately, due to how this error is written (undefined method "bar" for Nil:nilClass) its easy to get tricked into thinking this error has to do with bar being undefined. When not read carefully this error causes beginners to mistakenly go digging into the details of the bar method on Foo, entirely missing the part of the error that hints that the object is of the wrong class (in this case: nil). It's a mistake that's easily avoided by reading error messages in their entirety.

Summary:

Always carefully read the entire error message before beginning any debugging. That means: Always check the class type of an object in an error message first, then its methods, before you begin sleuthing into any stacktrace or line of code where you think the error may be occurring. Those 5 seconds can save you 5 hours of frustration.

tl;dr: Don't squint at print logs: raise exceptions or use an irb debugger instead. Avoid rabbit holes by reading errors carefully before debugging.

1. Print out the variables whenever possible. (This is called printf debugging) You can do this by running

   STDERR.puts x.inspect
   

   or

   STDERR.puts "Variable x is #{x.inspect}"
   

   If you want to make this easier to type, then you may want to use the exemplor gem.

2. Turn warnings on. If you're running ruby then run it with the -w switch (eg ruby -w script.rb). If you're running it from irb, and you're using a version of ruby prior to 1.9.2, type $VERBOSE = true at the start of your session. If you misspell an instance variable, once warnings are on you'll get

   warning: instance variable @valeus not initialized

3. Understand the concept of a binary chop (the following quote is from Practices of an Agile Developer)

   Divide the problem space in half, and see which half contains the problem. Then divide that half in half again, and repeat.

4. If you're successful with a binary chop, you may find that there's a single line that doesn't do what you expect it to do. For example

   [1, 2, 3].include?([1,2])
   

   gives a value of false, even though you'd think it'd return true. In that case, you may want to look at the documentation. Web sites for documentation include ruby-doc.org, or APIdock. In the latter case, you'd type include? next to the magnifying glass near the top right corner, choose the include? which has Array underneath it (if you don't know what class [1, 2, 3] is, type [1, 2, 3].class in irb), and you get to include? (Array), which describes what it does.

   However, if the documentation doesn't help, you're more likely to get a good answer if you can ask a question on how a specific line isn't doing what it should, rather than why an entire script isn't doing what it should.