Celluloid::Loggerのerrorがthorのerrorに負ける ref: http://qiita.com/hotakasaito/items/3af13db980addab8152f

file0.txt

require 'thor'
require 'celluloid'
include Celluloid::Logger
Celluloid.logger = ::Logger.new("sample.log")

class SampleThor < Thor
  desc "test", "sample"
  def test
    error "test"
  end
end

SampleThor.start(ARGV)

file1.txt

Thor::Shell::Basic.class_eval do
  def error(statement)
    Celluloid.logger.error(statement) if Celluloid.logger
  end
end

file2.txt

require 'thor'
require 'celluloid'
include Celluloid::Logger
Celluloid.logger = ::Logger.new("sample.log")

Thor::Shell::Basic.class_eval do
  def error(statement)
    Celluloid.logger.error(statement) if Celluloid.logger
  end
end

class SampleThor < Thor
  desc "test", "sample"
  def test
    error "test"
  end
end

SampleThor.start(ARGV)