How can I have ruby logger log output to stdout as well as file?

You can write a pseudo IO class that will write to multiple IO objects. Something like:

class MultiIO
  def initialize(*targets)
     @targets = targets
  end

  def write(*args)
    @targets.each {|t| t.write(*args)}
  end

  def close
    @targets.each(&:close)
  end
end

Then set that as your log file:

log_file = File.open("log/debug.log", "a")
Logger.new MultiIO.new(STDOUT, log_file)

Every time Logger calls puts on your MultiIO object, it will write to both STDOUT and your log file.

Edit: I went ahead and figured out the rest of the interface. A log device must respond to write and close (not puts). As long as MultiIO responds to those and proxies them to the real IO objects, this should work.

@David's solution is very good. I've made a generic delegator class for multiple targets based on his code.

require 'logger'

class MultiDelegator
  def initialize(*targets)
    @targets = targets
  end

  def self.delegate(*methods)
    methods.each do |m|
      define_method(m) do |*args|
        @targets.map { |t| t.send(m, *args) }
      end
    end
    self
  end

  class <<self
    alias to new
  end
end

log_file = File.open("debug.log", "a")
log = Logger.new MultiDelegator.delegate(:write, :close).to(STDOUT, log_file)

If you're in Rails 3 or 4, as this blog post points out, Rails 4 has this functionality built in. So you can do:

# config/environment/production.rb
file_logger = Logger.new(Rails.root.join("log/alternative-output.log"))
config.logger.extend(ActiveSupport::Logger.broadcast(file_logger))

Or if you're on Rails 3, you can backport it:

# config/initializers/alternative_output_log.rb

# backported from rails4
module ActiveSupport
  class Logger < ::Logger
    # Broadcasts logs to multiple loggers. Returns a module to be
    # `extended`'ed into other logger instances.
    def self.broadcast(logger)
      Module.new do
        define_method(:add) do |*args, &block|
          logger.add(*args, &block)
          super(*args, &block)
        end

        define_method(:<<) do |x|
          logger << x
          super(x)
        end

        define_method(:close) do
          logger.close
          super()
        end

        define_method(:progname=) do |name|
          logger.progname = name
          super(name)
        end

        define_method(:formatter=) do |formatter|
          logger.formatter = formatter
          super(formatter)
        end

        define_method(:level=) do |level|
          logger.level = level
          super(level)
        end
      end
    end
  end
end

file_logger = Logger.new(Rails.root.join("log/alternative-output.log"))
Rails.logger.extend(ActiveSupport::Logger.broadcast(file_logger))