import shutil


def check_installed(command):
    """Check if the given command is installed.

    :param str command: name of the command

    :return: (indicator of the availability of the command, message saying
              command is not available)
    :rtype: tuple(bool, str)

    """
    if shutil.which(command):
        return True, ""
    else:
        return False, "Command '{}' is not found.".format(command)