find_system_command

# File lib/shell/command-processor.rb, line 356
    def find_system_command(command)
      return command if /^\// =~ command
      case path = @system_commands[command]
      when String
        if exists?(path)
          return path
        else
          Shell.Fail Error::CommandNotFound, command
        end
      when false
        Shell.Fail Error::CommandNotFound, command
      end

      for p in @shell.system_path
        path = join(p, command)
        begin
          st = File.stat(path)
        rescue SystemCallError
          next
        else
          next unless st.executable? and !st.directory?
          @system_commands[command] = path
          return path
        end
      end
      @system_commands[command] = false
      Shell.Fail Error::CommandNotFound, command
    end