ics guardfile

.Guardfile

# A sample Guardfile
# More info at https://github.com/guard/guard#readme

require "~/work/scripts/ruby/calendars/cal_concatenator"
require 'logger'
require 'guard/guard'

module ::Guard
  class CalGuard < ::Guard::Guard
    
    def concatenate
      # How to get "paths"?
      this_dir = File.expand_path(File.dirname(modfile))
      logs_dir = "#{this_dir}/logs"
      results_dir = "#{this_dir}/results"
      [logs_dir,results_dir].each do |this_dir|
        Dir.mkdir(this_dir) unless File.directory?(this_dir)
      end
      log = Logger.new("#{logs_dir}/ics_mod.log", 'monthly')
      log.level = Logger::INFO

      apple_guid = CalConcatenator.apple_guid(modfile)
      unless apple_guid
        log.error "An apple-generateduid could not be found within '#{filename}'."
        return(exit)
      end
      common_name = CalConcatenator.get_common_name(apple_guid)
      unless common_name
        log.error "The common name could not be determined for apple-generateduid: '#{apple_guid}'."
        return(exit)
      end
      last_slash_pos = modfile.rindex('/')
      parent_dirname = modfile[0,last_slash_pos]
      unless File.directory? parent_dirname
        log.error "Folder: '#{parent_dirname}' was not found."
        return(exit)
      end

      catter = CalConcatenator.new
      outfilepath = File.join(results_dir, "#{common_name}.ics")  
      catter.concatenate_calendars(parent_dirname, outfilepath)
      if File.exists?(outfilepath)
        log.info("The #{common_name} calendar was modified.")
      else
        log.info("The #{common_name} calendar was deleted.")
      end      
    end
    
    def run_on_change(paths)
      concatenate
    end

    def run_on_deletion(paths)
      concatenate
    end
  end
end

guard 'cal_guard' do
  ignore_paths 'wi'
  watch(%r{^(.+\.ics)$})    { |m| "self.concatenate(m[1])"}
end

You should pass paths to concatenate L.50 and L.54 and change concatenate signature to def concatenate(paths) L.11.

Also, L.61 you can remove the block, modified paths will be passed to run_on_change and then passed to concatenate.