How do I debug Ruby scripts?

Question

I copied the following Ruby code from the Internet and made a few changes.

#insert code here

But it doesn't work!

Please help. What can I do to debug the program by myself?

Answer 1

There is many debuggers with different features, based on which you make choice. My priorities was satisfied with pry-moves which was:

• quickly comprehensible info on how to use
• intuitive steps (like easy stepping into blocks)
• "stepping back" (pry-moves partially satisfies the need)

Answer 2

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.

Answer 3

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.

Answer 4

1. You can print your variables out along the way
2. Turn on the -w (warnings) flag
3. Use a tool such as ruby-debug

Answer 5

If you are using RubyMine, debugging ruby scripts is simple and straightforward.

Suppose you have a Ruby script hello_world.rb

1. Set breakpoints

Set a breakpoint at line 6 as below.

2. Start debugging

Now you can just start the debugger to run the script:

3. Inspect variables, etc.

Then when the execution hits a breakpoint, you'll be able to inspect variables, etc.

Further information for your reference

1. If you would like to use RubyMine to do remote debugging, you can do so.
2. If you would like to use RubyMine to remote debug rails running inside a docker, it is also straightforward.