#!/usr/bin/env ruby
raise "Requires your OS to support forking" unless Process.respond_to? :fork
require 'chronic'
require 'open3'

include Open3

# Usage
if ARGV.empty? or ARGV[0] == '-h' or ARGV[0] == '--help'
  puts <<-USAGE
Usage:  every [TIME INTERVAL]: [COMMAND]
Where:  [TIME INTERVAL] is a natural language expression of time
        [COMMAND] is the command you wish to run

Examples:
  # Delete a log file every five minutes
  every five minutes: rm /var/log/my_app.log

  # Restart nginx every morning
  every day at 1AM: /etc/init.d/nginx restart
  USAGE
  exit 0
end

# Validate and parse input
index = ARGV.index { |arg| arg[-1] == ':' }
raise "Parse error" if index.nil?

time_str = ARGV[0..index].join(' ').chomp(':').strip
command_str = ARGV[index+1..-1].join(' ').strip

raise "Parse error" if command_str.nil? or command_str.empty?

run_command = Proc.new do
  popen3 command_str do |stdin, stdout, stderr, wait_thread|
    status = wait_thread.value

    unless status.to_i == 0
      exit status.to_i
    end
  end
end

transform_time_str = Proc.new do
  if time_str.include? 'day'
    time_str.sub('day', 'tomorrow')
  else
    time_str + " from now"
  end
end


# Fork and process
if pid = fork
  puts "Forked with pid #{pid}"
  Process.detach pid
  exit 0
else
  $0 = "[every #{time_str}] #{command_str}"

  run_command.call

  loop do
    next_run = Chronic.parse(transform_time_str.call)
    if next_run.nil?
      exit 1
    end

    sleep 1 until Time.now >= next_run

    run_command.call
  end
end