File.open("/tmp/indexer_lock.tmp",'w') do |f|
  exit unless f.flock(File::LOCK_EX)
end

require 'rubygems'
require 'daemons'

Daemons.run('indexer.rb')

loop do
   # code executing your indexing

   sleep INDEXING_INTERVAL
end

File.new("/tmp/foo.lock").flock( File::LOCK_NB | File::LOCK_EX )

#!/usr/local/bin/bash
#

if ps -C $PROGRAM_NAME &> /dev/null ; then
   : #Program is already running.. appropriate action can be performed here (kill it?)
else
   #Program is not running.. launch it.
   $PROGRAM_NAME
fi

tmp_file = File.expand_path(File.dirname(__FILE__)) +  "/indexer.lock"
if File.exists?(tmp_file)
  puts "quitting"
  exit
else
  `touch #{tmp_file}`
end

.. do stuff ..

File.delete(tmp_file)

exit unless DATA.flock(File::LOCK_NB | File::LOCK_EX)

# your script here

__END__
DO NOT REMOTE: required for the DATA object above.

def the_method_my_cron_job_calls
  # something really expensive
end
lock_method :the_method_my_cron_job_calls

LockMethod.config.storage = Redis.new([...]) # a remote RedisToGo instance, perhaps?

def the_method_my_cron_job_calls
  # something really expensive
end
lock_method :the_method_my_cron_job_calls, (60*60) # automatically expire lock after an hour