Remove duplicate passwd file lines by uid

remove-duplicate-uids.rb

#!/usr/bin/env ruby

=begin
How would I write code to delete lines from a passwd file that has duplicate uids?
Example entry from the passwd file looks like:
  `nobody:*:-2:-2:Unprivileged User:/var/empty:/usr/bin/false`

Bonus points if you can do it in a single pass through the file.
=end

require 'benchmark'

ruby_way = Benchmark.measure do

  File.open('/etc/passwd').each_line do |line|
    uid_hash = Hash.new(0)
    uid = line.split(':')
    uid_hash[uid[3]] = uid_hash[uid[3]] + 1
    puts line if uid_hash[uid[3]] != 1
  end
end

unix_way = Benchmark.measure do
  `awk -F: 'uid[$3]++ {print $0}' /etc/passwd`
end

puts "Ruby solution: #{ruby_way.to_s}"
puts "Unix solution: #{unix_way.to_s}"

#!/usr/bin/env bash

# Print unique UIDs and the first duplicate 
while read line; do
  i=$(echo $line|cut -d: -f3)
  [[ ${passwds[$i]+exists} ]] || passwds[$i]="${line}"
done < /etc/passwd

printf '%s\n' "${passwds[@]}"

#!/usr/bin/env bash

# Print unique UIDs and the last duplicate
while read line; do
  passwds[$(echo $line|cut -d: -f3)]="${line}"
done < /etc/passwd

printf '%s\n' "${passwds[@]}"