File: README — Documentation for backtrace

bactrace_shortener

This shortens the backtraces of exceptions to make debugging more friendly. Exceptions which are thrown in Ruby apps which use rbenv and gems like Sinatra can be taller than one terminal screen, and each line can be long. See below for an example of the pain. Shortening these backtraces makes debugging more friendly.

I use this in many projects, so it's now packaged as a gem.

Usage

$ gem install backtrace_shortener

(or you can just take backtrace_shortener.rb and copy it to your project. It's short.)

In your code:

require "backtrace_shortener"
BacktraceShortener.monkey_patch_the_exception_class!

Features

With backtrace_shortener enabled, an unwieldy exception backtrace which previously looked like this:

RuntimeError: It's no good, I can't maneuver!
/Users/philc/api_server/server.rb:86:in `block in <class:ServerRoot>'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/sinatra-1.3.2/lib/sinatra/base.rb:1212:in `call'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/sinatra-1.3.2/lib/sinatra/base.rb:1212:in `block in compile!'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/sinatra-1.3.2/lib/sinatra/base.rb:785:in `[]'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/sinatra-1.3.2/lib/sinatra/base.rb:785:in `block (3 levels) in route!'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/sinatra-1.3.2/lib/sinatra/base.rb:801:in `route_eval'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/sinatra-1.3.2/lib/sinatra/base.rb:785:in `block (2 levels) in route!'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/sinatra-1.3.2/lib/sinatra/base.rb:822:in `block in process_route'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/sinatra-1.3.2/lib/sinatra/base.rb:820:in `catch'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/sinatra-1.3.2/lib/sinatra/base.rb:820:in `process_route'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/sinatra-1.3.2/lib/sinatra/base.rb:784:in `block in route!'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/sinatra-1.3.2/lib/sinatra/base.rb:783:in `each'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/sinatra-1.3.2/lib/sinatra/base.rb:783:in `route!'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/sinatra-1.3.2/lib/sinatra/base.rb:886:in `dispatch!'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/sinatra-1.3.2/lib/sinatra/base.rb:719:in `block in call!'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/sinatra-1.3.2/lib/sinatra/base.rb:871:in `block in invoke'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/sinatra-1.3.2/lib/sinatra/base.rb:871:in `catch'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/sinatra-1.3.2/lib/sinatra/base.rb:871:in `invoke'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/sinatra-1.3.2/lib/sinatra/base.rb:719:in `call!'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/sinatra-1.3.2/lib/sinatra/base.rb:705:in `call'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rack-protection-1.2.0/lib/rack/protection/xss_header.rb:22:in `call'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rack-protection-1.2.0/lib/rack/protection/path_traversal.rb:16:in `call'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rack-protection-1.2.0/lib/rack/protection/json_csrf.rb:17:in `call'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rack-protection-1.2.0/lib/rack/protection/base.rb:47:in `call'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rack-protection-1.2.0/lib/rack/protection/xss_header.rb:22:in `call'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rack-1.4.1/lib/rack/nulllogger.rb:9:in `call'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rack-1.4.1/lib/rack/head.rb:9:in `call'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/sinatra-1.3.2/lib/sinatra/base.rb:1334:in `block in call'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/sinatra-1.3.2/lib/sinatra/base.rb:1416:in `synchronize'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/sinatra-1.3.2/lib/sinatra/base.rb:1334:in `call'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rack-1.4.1/lib/rack/lint.rb:48:in `_call'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rack-1.4.1/lib/rack/lint.rb:36:in `call'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rack-1.4.1/lib/rack/showexceptions.rb:24:in `call'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rack-1.4.1/lib/rack/commonlogger.rb:20:in `call'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rack-1.4.1/lib/rack/chunked.rb:43:in `call'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rack-1.4.1/lib/rack/content_length.rb:14:in `call'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/thin-1.3.1/lib/thin/connection.rb:80:in `block in pre_process'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/thin-1.3.1/lib/thin/connection.rb:78:in `catch'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/thin-1.3.1/lib/thin/connection.rb:78:in `pre_process'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/thin-1.3.1/lib/thin/connection.rb:53:in `process'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/thin-1.3.1/lib/thin/connection.rb:38:in `receive_data'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/eventmachine-0.12.10/lib/eventmachine.rb:256:in `run_machine'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/eventmachine-0.12.10/lib/eventmachine.rb:256:in `run'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/thin-1.3.1/lib/thin/backends/base.rb:61:in `start'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/thin-1.3.1/lib/thin/server.rb:159:in `start'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rack-1.4.1/lib/rack/handler/thin.rb:13:in `run'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rack-1.4.1/lib/rack/server.rb:265:in `start'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rack-1.4.1/lib/rack/server.rb:137:in `start'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rack-1.4.1/bin/rackup:4:in `<top (required)>'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/bin/rackup:19:in `load'
/Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/bin/rackup:19:in `<main>'api_server

gets shortened to what you see below. Long method chains from gems get collapsed, and the RubyGems path (in my case, /Users/philc/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1) gets trimmed from each line.

  RuntimeError: It's no good, I can't maneuver!
  /Users/philc/html5player/api_server/app/server_root.rb:88:in `block in <class:ServerRoot>'
  <..>
  .../gems/sinatra-1.3.2/lib/sinatra/base.rb:705:in `call'
  <..>
  .../gems/rack-protection-1.2.0/lib/rack/protection/xss_header.rb:22:in `call'
  .../gems/rack-1.4.1/lib/rack/nulllogger.rb:9:in `call'
  .../gems/rack-1.4.1/lib/rack/head.rb:9:in `call'
  <..>
  .../gems/sinatra-1.3.2/lib/sinatra/base.rb:1334:in `call'
  <..>
  .../gems/rack-1.4.1/lib/rack/content_length.rb:14:in `call'
  <..>
  .../gems/thin-1.3.1/lib/thin/connection.rb:38:in `receive_data'
  .../gems/eventmachine-0.12.10/lib/eventmachine.rb:256:in `run_machine'
  .../gems/eventmachine-0.12.10/lib/eventmachine.rb:256:in `run'
  .../gems/thin-1.3.1/lib/thin/backends/base.rb:61:in `start'
  .../gems/thin-1.3.1/lib/thin/server.rb:159:in `start'
  <..>
  .../gems/rack-1.4.1/bin/rackup:4:in `<top (required)>'
  .../bin/rackup:19:in `load'
  .../bin/rackup:19:in `<main>

If you want to see even less or filter things specific to your application, add your own filters. For example, this filter rejects any line originating from a gem:

BacktraceShortener.filters.unshift(Proc.new do |backtrace|
  backtrace.reject { |line| line.include?(Gem.dir) }
end)

Another example which truncates the backtrace to include only the first 10 lines:

BacktraceShortener.filters.unshift(Proc.new { |backtrace| backtrace[0, 10] }

If you need to access the full backtrace while debugging, you can call the_exception.full_backtrace.