test.rb

#!/usr/bin/env ruby

all_user_procs = %x[/bin/ps -e -o uid,pid | /bin/egrep "^\s+[0-9]{3,4}"]

pids_by_user = all_user_procs.lines.inject(Hash.new{|h,k| h[k] = []}) do |h,line|
  (uid,pid) = line.split.map(&:to_i)
  h[uid] << pid
  h
end