a simple command line alarm clock, I use it to play some music when the alarm goes off.

alarm.rb

#!/usr/bin/ruby
#
# Usage: ruby alarm.rb <time>
#
# <time> is either a number of seconds (e.g. "50"), minutes and seconds
# (e.g. "5:30"), or hours minutes and seconds (e.g. "7:30:00"). After the
# specified amount of time has passed, ring_alarm will be called and the
# program will exit.
#

def ring_alarm
  puts "BBRRRRRRRRIIINNNGGGG!!!!"
  `mplayer /Users/jeremy/Music/Maps/Maps-Vicissitude/04\\ I\\ Heard\\ Them\\ Say.mp3`
end

def parse_time(string)
  parts = string.split(":", -1).map(&:to_f)

  raise ArgumentError, "too many parts" if parts.length > 3
  raise ArgumentError, "can't parse negative numbers" if parts.any? { |x| x < 0 }

  parts.unshift(0) until parts.length == 3
  hours, minutes, seconds = parts
  seconds = hours * 3600 + minutes * 60 + seconds
  seconds.to_i
end

if ARGV.empty?
  puts "Usage: #$0 <time>"
  exit
end

alarm_at = Time.now + parse_time(ARGV.first)
puts "Setting alarm for #{alarm_at}."

loop do
  sleep 1
  if Time.now >= alarm_at
    ring_alarm
    exit
  end
  puts "#{alarm_at.to_i - Time.now.to_i}..."
end