Unable to programmatically run a Jekyll build - ruby

Question

I'm trying to run a Jekyll build programatically, but I'm having issues in that the build crashes when trying to load some gems used by the Jekyll site's plugins.
My code looks like this (line breaks added for readability):
%x(cd #{#config['input_directory']} &&
bundle install &&
bundle exec jekyll build --config #{File.join(#config['input_directory'], "_config.yml")} --source #{#config['input_directory']} --destination #{#config['output_directory']} --trace)
When this runs, I get a stack trace that looks like this:
/_plugins/jekyll_lunr_js_search.rb:80:in `require': cannot load such file -- nokogiri (LoadError)
from /_plugins/jekyll_lunr_js_search.rb:80:in `<top (required)>'
from /ruby-1.9.3-p448/gems/jekyll-1.3.0/lib/jekyll/site.rb:77:in `require'
from /ruby-1.9.3-p448/gems/jekyll-1.3.0/lib/jekyll/site.rb:77:in `block (2 levels) in setup'
from /ruby-1.9.3-p448/gems/jekyll-1.3.0/lib/jekyll/site.rb:76:in `each'
from /ruby-1.9.3-p448/gems/jekyll-1.3.0/lib/jekyll/site.rb:76:in `block in setup'
from /ruby-1.9.3-p448/gems/jekyll-1.3.0/lib/jekyll/site.rb:75:in `each'
from /ruby-1.9.3-p448/gems/jekyll-1.3.0/lib/jekyll/site.rb:75:in `setup'
from /ruby-1.9.3-p448/gems/jekyll-1.3.0/lib/jekyll/site.rb:29:in `initialize'
from /ruby-1.9.3-p448/gems/jekyll-1.3.0/lib/jekyll/commands/build.rb:5:in `new'
from /ruby-1.9.3-p448/gems/jekyll-1.3.0/lib/jekyll/commands/build.rb:5:in `process'
from /ruby-1.9.3-p448/gems/jekyll-1.3.0/bin/jekyll:77:in `block (2 levels) in <top (required)>'
from /ruby-1.9.3-p448/gems/commander-4.1.5/lib/commander/command.rb:180:in `call'
from /ruby-1.9.3-p448/gems/commander-4.1.5/lib/commander/command.rb:180:in `call'
from /ruby-1.9.3-p448/gems/commander-4.1.5/lib/commander/command.rb:155:in `run'
from /ruby-1.9.3-p448/gems/commander-4.1.5/lib/commander/runner.rb:402:in `run_active_command'
from /ruby-1.9.3-p448/gems/commander-4.1.5/lib/commander/runner.rb:78:in `run!'
from /ruby-1.9.3-p448/gems/commander-4.1.5/lib/commander/delegates.rb:11:in `run!'
from /ruby-1.9.3-p448/gems/commander-4.1.5/lib/commander/import.rb:10:in `block in <top (required)>'
The Gemfile of the input_directory looks like this, so I'm not sure why it's not able to load nokogiri.
source 'https://rubygems.org'
gem 'directory_watcher', '= 1.4.1' # http://stackoverflow.com/questions/15591000/jekylls-auto-doesnt-work
gem 'kramdown'
gem 'nokogiri'
gem 'yui-compressor'
gem "jekyll"
group :development do
gem 'capistrano'
gem 'rvm-capistrano'
end
Any pointers here are welcome. I get the feeling there are better ways of doing this Jekyll build programmatically too, this just seemed to be simplest to me.

Answer 1

Run bundle exec jekyll to ensure that it sets up the gem path correctly for Jekyll. If you're calling the program from an app that uses Bundler itself, you may need to also wrap the execution in a call to Bundler.with_clean_env.
The complete command should look like this:
Bundler.with_clean_env do
%x(cd #{#config['input_directory']} &&
bundle install &&
bundle exec jekyll build --config #{File.join(#config['input_directory'], "_config.yml")} --source #{#config['input_directory']} --destination #{#config['output_directory']} --trace)
end