rails_root.rb

# sometimes you need to calculate the rails_root from a subdirectory of a rails' app, eg, lib/capistrano/tasks/db.rake
# this code does it properly, but is verbose.  golf it!
#

      rails_root = File.expand_path(Dir.pwd)
      until rails_root == '/'                                                                                                                                   
        if %w[ Gemfile app public ].all?{|entry| test(?e, "#{ rails_root }/#{ entry }")}
          break
        else
          rails_root = File.dirname(rails_root)
        end
        rails_root = File.dirname(rails_root)
      end
      raise 'no rails_root!' if rails_root == '/'

Pathname.pwd.ascend do |x|; (x + "Gemfile").exist? and break x; end or raise "none"

I'd feel confident enough if it had Gemfile and app dir.

require "pathname"

rails_root = Pathname.pwd.ascend.select do |pn|
  File.file?(pn.join("Gemfile")) && File.directory?(pn.join("app"))
end

raise "Not found" if rails_root.nil?

@joelparkerhenderson - oh! Pathname.ascend is teh sweet!

@bradpauly - yes on both 'Gemfile' and 'app' !

based on your awesome suggestions:

Pathname.pwd.ascend{|d| f = d.join('config/application.rb') and f.exist? && f.read.match('Rails') && (break(d))}

# or

Pathname.pwd.ascend{|d| (d.join('config/application.rb').read.match('Rails') && (break(d))) rescue nil}

# or

Pathname.pwd.ascend{|d| (d.join('Gemfile').read.match('rails') && (break(d))) rescue 42}

@ahoward Nice, I like reading config/application.rb.