The code below is taken entirely from a post by Brett Terpstra located at: http://brettterpstra.com/2011/03/07/watch-for-file-changes-and-refresh-your-browser-automatically/ This version below is modified to work better with Jekyll (see comment below)

watch.rb

#!/usr/bin/env ruby
# watch.rb by Brett Terpstra, 2011 <http://brettterpstra.com>
# with credit to Carlo Zottmann <https://github.com/carlo/haml-sass-file-watcher>

trap("SIGINT") { exit }

if ARGV.length < 2
  puts "Usage: #{$0} watch_folder keyword"
  puts "Example: #{$0} . mywebproject"
  exit
end

dev_extension = 'dev'
filetypes = ['css','html','htm','php','rb','erb','less','js']
watch_folder = ARGV[0]
keyword = ARGV[1]
puts "Watching #{watch_folder} and subfolders for changes in project files..."

while true do
  files = []

  if File.directory?(watch_folder)
    filetypes.each {|type|
      files += Dir.glob( File.join( watch_folder, "**", "*.#{type}" ) )
    }
  else
    # watch a single file only. useful in cases like Jekyll where every file is
    # touched each time we generate the site
    files += Dir.glob(File.absolute_path(watch_folder))
  end

  new_hash = files.collect {|f| [ f, File.stat(f).mtime.to_i ] }
  hash ||= new_hash
  diff_hash = new_hash - hash

  unless diff_hash.empty?
    hash = new_hash

    diff_hash.each do |df|
      puts "Detected change in #{df[0]}, refreshing"
      %x{osascript<<ENDGAME
        	tell application "Safari"
          	set windowList to every window
          	repeat with aWindow in windowList
          		set tabList to every tab of aWindow
          		repeat with atab in tabList
          			if (URL of atab contains "#{keyword}") then
          			  tell atab to do javascript "window.location.reload()"
          			end if
          		end repeat
          	end repeat
        	end tell
ENDGAME
}
    end
  end

  sleep 1
end

I modified this script from Brett's original to work better with Jekyll.

If only a single file is passed to the script then only watch that file for changes. This is useful in for static site generators like Jekyll where all the files are touched each time the site is updated.

To use in the single file mode for Jekyll do something like the following:

watch.rb _site/index.html localhost