pry-rescue super-fast, painless, debugging for the (ruby) masses. (See Pry to the rescue!)

Usage

First gem install pry-rescue pry-stack_explorer. Then run your program with rescue instead of ruby:

rescue <script.rb> [arguments..]

If you're using Rails, you should add pry-rescue to the development section of your Gemspec and then run rails server using rescue:

rescue rails server

If you're using bundle exec the rescue should go after the exec:

bundle exec rescue rails server

If you're using Rack, you should use the middleware instead (though be careful to only include it in development!)

use PryRescue::Rack

If you want more fine-grained control over which parts of your code are rescued, you can also use the block form:

require 'pry-rescue'

def test
  raise "foo"
rescue => e
  raise "bar"
end

Pry.rescue do
  test
end

This will land you in a pry-session:

From: examples/example.rb @ line 4 Object#test:

    4: def test
    5:   raise "foo"
    6: rescue => e
 => 7:   raise "bar"
    8: end

RuntimeError: bar
from examples/example.rb:7:in `rescue in test'
[1] pry(main)>

Finally. If you're doing your own exception handling, you can ask pry to open on an exception that you've caught. For this to work you must be inside a Pry::rescue{ } block.

def test
  raise "foo"
rescue => e
  Pry::rescued(e)
end

Pry::rescue{ test }

cd-cause

If you need to find the reason that the exception happened, you can use the cd-cause command:

[1] pry(main)> cd-cause
From: examples/example.rb @ line 4 Object#test:

    4: def test
 => 5:   raise "foo"
    6: rescue => e
    7:   raise "bar"
    8: end

RuntimeError: foo
from examples/example.rb:5:in `test'
[1] pry(main)>

To get back from cd-cause you can either type <ctrl+d> or cd ...

try-again

Once you've used Pry's edit or edit-method commands to fix your code, you can issue a try-again command to re-run your code. (Either from the start in the case of using the rescue script, or from the block if you're using that API).

[1] pry(main)> edit-method
[2] pry(main)> whereami
From: examples/example.rb @ line 4 Object#test:

    4: def test
 => 5:   puts "foo"
    6: rescue => e
    7:   raise "bar"
    8: end
[3] pry(main)> try-again
foo

Peeking

Sometimes bugs in your program don't cause exceptions. Instead your program just gets stuck. Examples include infinite loops, slow network calls, or tests that take a suprisingly long time to run.

In this case it's useful to be able to open a pry console when you notice that your program is not going anywhere. To enable this feature you need to run:

rescue --peek <script.rb>

Then hit <ctrl+c> at any time to stop your program and have a peek at what it's actually doing. Hitting <ctrl-c> a second time will quit your program, if that's what you were trying to do.

Automatic peeking

Remembering to run your program with rescue --peek manually is a bit frustrating (as you don't know you need to peek, until you need to peek) and so pry-rescue also allows you to peek into any program at any time by sending it a SIGUSR2 signal.

The easiest way to do this is to hit <ctrl-z> to interrupt your program, then type kill -SIGUSR2 %1 && fg into bash. For example:

cirwin@localhost:/tmp/pry $ ruby examples/loop.rb
^Z
cirwin@localhost:/tmp/pry $ kill -SIGUSR2 %1 && fg
[1]  + continued  ./examples/loop.rb
Preparing to peek via pry!
Frame number: 0/4

From: ./examples/loop.rb @ line 10 Object#r
    10: def r
    11:   some_var = 13
    12:   loop do
 => 13:     x = File.readlines('lib/pry-rescue.rb')
    14:   end
    15: end
pry (main)>

If you find that that's a bit hard to remember (I know I do), then you can make it easier by adding the following alias to your ~/.bashrc or similar:

alias peek='kill -SIGUSR2 %1 && fg'

Then you can just type peek instead of kill -SIGUSR2 %1 && fg

Advanced peeking

You can configure which signal pry-rescue listens for by default by exporting the PRY_PEEK environment variable that suits your use-case best:

export PRY_PEEK=""    # don't autopeek at all
export PRY_PEEK=INT   # peek on SIGINT (<ctrl+c>)
export PRY_PEEK=USR1  # peek on SIGUSR1
export PRY_PEEK=USR2  # peek on SIGUSR2
export PRY_PEEK=EXIT  # peek on program exit

If it's only important for one program, then you can also set the environment variable in ruby before requiring pry-rescue

ENV['PRY_PEEK'] = '' # disable SIGUSR2 handler
require "pry-rescue"

Finally, you can enable peeking into programs that do not include pry-rescue by configuring ruby to always load one (or several) of these files:

export RUBYOPT=-rpry-rescue/peek/int   # peek on SIGINT (<ctrl-c>)
export RUBYOPT=-rpry-rescue/peek/usr1  # peek on SIGUSR1
export RUBYOPT=-rpry-rescue/peek/usr2  # peek on SIGUSR2
export RUBYOPT=-rpry-rescue/peek/exit  # peek on program exit

These last examples relies on having pry-rescue in the load path (i.e. at least in the gemset, or Gemfile of the program). If that is not true, you can use absolute paths. The hook files do not require the whole of pry-rescue, nor is any of pry itself loaded until you trigger the signal.

export RUBYOPT=-r/home/cirwin/src/pry-rescue/lib/pry-rescue/peek/usr2

pry-stack explorer

If you're running rubinius, or ruby-1.9, then you can use pry-rescue alongside pry-stack\_explorer. This gives you the ability to move up or down the stack so that you can get a better idea of why your function ended up in a bad state. Run example2.rb to get a feel for what this is like.

Known bugs

ruby 2.0, 1.9.3, 1.9.2 – no known bugs
ruby 1.9.1 — not supported
ruby 1.8.7 — occasional incorrect values for self
ree 1.8.7 — no known bugs
jruby 1.7 (1.8 mode and 1.9 mode) — no known bugs
jruby 1.6 (1.8 mode and 1.9 mode) — incorrect value for self in NoMethodErrors
rbx (1.8 mode and 1.9 mode) – does not catch some low-level errors (e.g. ZeroDivisionError)

Meta-fu

Released under the MIT license, see LICENSE.MIT for details. Contributions and bug-reports are welcome.