jekyll serve --watch と同時にブラウザで開く。

Rakefile

require 'bundler/setup'
require 'thread'
require 'launchy'

desc 'preview'
task :preview do
  Thread.new do
    sleep 1
    Launchy.open 'http://localhost:9001/'
  end

  sh 'bundle exec jekyll serve --watch'
end