Read the lines of a csv file and extract a certain field and list the unique values in that field and sort that list based on how many times a value is repeated

gistfile1.rb

cdrs = File.readlines ARGV[0] 
cdrs.shift 1 # Delete the header row 
cdrs.map! { |x| x.split(',')[3] } # Keep only the callid puts 
duplicate_callids = cdrs.group_by(&:freeze). # Group by occurence 
                    sort_by { |k, v| v.length}.reverse. # Sort by number of occurences desc 
                    map { |x| "#{x[0]} - #{x[1].length}" if x[1].length > 1 }. # Format for printing compact