How do I get a stack trace using the ruby debugger?

http://apidock.com/ruby/Kernel/caller

caller(0)  # Returns the stack trace, omitting 0 initial entry.

That's a bug in debugger. If you are using Ruby >= 2.0, I suggest you use byebug. The issue is solved there since version 1.5.0. This is the bug report in the debugger repo and this is the workaround suggested there:

pp caller.drop_while {|e| e[/ruby-debug|\(eval\)|debugger|\<internal:prelude\>/] }

Pry gem does have a plugin pry-stack_explorer can display stack

Example: Moving around between frames

[8] pry(J)> show-stack

Showing all accessible frames in stack:
--
=> #0 [method]  c <Object#c()>
   #1 [block]   block in b <Object#b()>
   #2 [method]  b <Object#b()>
   #3 [method]  alphabet <Object#alphabet(y)>
   #4 [class]   <class:J>
   #5 [block]   block in <main>
   #6 [eval]    <main>
   #7 [top]     <main>
[9] pry(J)> frame 3

Frame number: 3/7
Frame type: method

From: /Users/john/ruby/projects/pry-stack_explorer/examples/example.rb @ line 10 in Object#alphabet:

     5:
     6: require 'pry-stack_explorer'
     7:
     8: def alphabet(y)
     9:   x = 20
 => 10:   b
    11: end
    12:
    13: def b
    14:   x = 30
    15:   proc {
[10] pry(J)> x
=> 20

Further, it has lot of other features that are missing from ruby debugger. so I would suggest you try pry & its plugins