Provides an irb session to a running ruby process.
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bin 0.2.0.rc1 - Initial support for 1.9.2 Nov 25, 2010
examples Use ruby-debug on your target process, see README for details Aug 22, 2009
test Minor tweaks Oct 17, 2012
.gitignore Stop DRb when the client exits or if it's killed. Repeated hijacking … Sep 15, 2009
Rakefile Stop DRb when the client exits or if it's killed. Repeated hijacking … Sep 15, 2009
TODO 0.1.9 Sep 21, 2009


Hijack: Provides an irb session to an existing ruby process.


Hijack allows you to connect to any ruby process and execute code as if it were a normal irb session. Hijack does not require your target process to require any hijacking code, hijack is able to connect to any ruby process. It achieves this by using gdb to inject a payload into the process which starts up a DRb server, hijack then detaches gdb and reconnects via DRb. Please note that gdb will halt your target process while it is attached, though the injection process is very quick and your process should only be halted for a few milliseconds.

Hijack uses DRb over a unix socket file, so you need to be on the same machine as the process you want to hijack. This is by design for security reasons. You also need to run the hijack client as the same user as the remote process.

Using Hijack

$ hijack 16451
=> Hijacked 16451 (my_script.rb) (ruby 1.8.7 [i686-darwin9])

Using ruby-debug

Hijack can be used to start ruby-debug in your target process, for example:

$ hijack 61378
=> Hijacked 61378 (/opt/local/bin/thin) (ruby 1.8.7 [i686-darwin9])
>> hijack_debug_mode
=> true

We've enabled debug mode, but we still need to insert a breakpoint:

>> ActionController::Dispatcher.class_eval do
>>   class << self
>>     def dispatch_with_debugger(cgi, session_options, output)
>>       debugger
>>       dispatch_without_debugger(cgi, session_options, output)
>>     end
>>     alias_method :dispatch_without_debugger, :dispatch
>>     alias_method :dispatch, :dispatch_with_debugger
>>   end
>> end

Now tell hijack that we can start debugging:

>> hijack_debug_start

Point your browser at to trigger the breakpoint and ruby-debug's console will appear:

>> hijack_debug_start
(rdb:4) step
/Users/ian/Projects/zioko/vendor/rails/actionpack/lib/action_controller/dispatcher.rb:28 new(output).dispatch_cgi(cgi, session_options)
(rdb:4) backtrace
--> #0 /Users/ian/Projects/zioko/vendor/rails/actionpack/lib/action_controller/dispatcher.rb:28 in 'dispatch_without_debugger'
    #1 (eval):5 in 'dispatch'
    #2 /opt/local/lib/ruby/gems/1.8/gems/thin-1.2.2/lib/rack/adapter/rails.rb:81 in 'call'
    #3 /opt/local/lib/ruby/gems/1.8/gems/thin-1.2.2/lib/rack/adapter/rails.rb:69 in 'call'
    #4 /opt/local/lib/ruby/gems/1.8/gems/thin-1.2.2/lib/thin/connection.rb:76 in 'pre_process'
    #5 /opt/local/lib/ruby/gems/1.8/gems/thin-1.2.2/lib/thin/connection.rb:74 in 'pre_process'
    #6 /opt/local/lib/ruby/gems/1.8/gems/thin-1.2.2/lib/thin/connection.rb:57 in 'process'
    #7 /opt/local/lib/ruby/gems/1.8/gems/thin-1.2.2/lib/thin/connection.rb:42 in 'receive_data'

One caveat is that ruby-debug's 'irb' command will not work because the debug connection is remote, you also can't use hijack's irb console whilst debugging. A future version of hijack will hopefully allow you to switch between the two.

Monkey Patching

Just as in a normal irb session, you can redefine the code running in your target process.

This example redefines ActionController's dispatcher to print basic request activity (the example can be found in examples/rails_dispatcher.rb).

Start up a Thin web server:

$ thin start
>> Using rails adapter
>> Thin web server (v1.2.2 codename I Find Your Lack of Sauce Disturbing)
>> Maximum connections set to 1024
>> Listening on, CTRL+C to stop

In another console hijack the Thin process:

$ ps | grep thin
61160 ttys001    0:01.43 /opt/local/bin/ruby /opt/local/bin/thin start

$ hijack 61160
=> Hijacked 61160 (/opt/local/bin/thin) (ruby 1.8.7 [i686-darwin9])
>> ActionController::Dispatcher.class_eval do
?>   class << self
>>     def dispatch_with_spying(cgi, session_options, output)
>>       env = cgi.__send__(:env_table)
>>       puts "#{'%Y/%m/%d %H:%M:%S')} - #{env['REMOTE_ADDR']} - #{env['REQUEST_URI']}"
>>       dispatch_without_spying(cgi, session_options, output)
>>     end
>>     alias_method :dispatch_without_spying, :dispatch
>>     alias_method :dispatch, :dispatch_with_spying
>>   end
>> end

Point your browser to

Back in hijack you'll see your browsing activity:

2009/08/22 14:24:48 - - /
2009/08/22 14:24:53 - - /login
2009/08/22 14:24:54 - - /signup

Instead of pasting your code into hijack, you can pass hijack the -e option execute a local file on the target process.

Process Output

By default hijack will forward your process output to the hijack client. This can get a little messy if your trying to write code at the same time as the target process writes to STDOUT/STDERR. You can mute and unmute the process with:

>> hijack_mute
=> true
>> hijack_unmute
=> true

For ease of use, hijack helper methods are discoverable with tab completion. hi<tab><tab> will give you a list of available helpers.

Process Mirroring

DRb cannot dump objects to the hijack client for types that are not loaded in the client process. E.g if the remote process had required ActiveRecord and you tried to dump ActiveRecord::Base back to the client, DRb would instead return a DRb::Unknown object as ActiveRecord isn't loaded in the hijack client.

To work around this, when hijack connects to a remote process it will inspect all the files required by the process and also attempt to require them itself. This may not work for all object types however so you may still get a warning when an object cannot be dumped.