sendmail.rb

#!/usr/bin/ruby

require "base64"
require "optparse"

params = {
  from: nil,
  to: [],
  subject: "",
  message: nil,
  attachments: [],
}

# Parse command-line arguments into params
opts = OptionParser.new do |opts|
  opts.on("-f FROM") { |v| params[:from] = v }
  opts.on("-t TO") { |v| params[:to].push(v) }
  opts.on("-s SUBJECT") { |v| params[:subject] = v }
  opts.on("-a ATTACHMENT") { |v| params[:attachments].push(v) }
  opts.on("-m MESSAGE") { |v| params[:message] = v }
end
begin
  opts.parse!
  raise OptionParser::MissingArgument.new("-t") if params[:to].empty?
rescue OptionParser::InvalidOption, OptionParser::MissingArgument
  $stderr.puts $!.to_s
  $stderr.puts opts
  exit 1
end
params[:from] ||= "hoge@piyo.com" # FIXME
params[:message] ||= $stdin.read

IO.popen("/usr/sbin/sendmail -i -t", "w") do |sm|
  # From: Alice <alice@example.com>
  sm.puts "From: #{params[:from]}"

  # To: Bob <bob@example.com>,Charley <charley@example.com>
  sm.puts "To: #{params[:to].join(",")}"

  # Subject: Hello, Bob
  sm.puts "Subject: #{params[:subject]}"

  # <empty line>
  sm.puts

  # <message>
  sm.puts params[:message]
end