def update(gems, ruby=None, runas=None, gem_bin=None):
    '''
    Update one or several gems.

    :param gems: string
        The gems to update.
    :param gem_bin: string : None
        Full path to ``gem`` binary to use.
    :param ruby: string : None
        If RVM or rbenv are installed, the ruby version and gemset to use.
        Ignored if ``gem_bin`` is specified.
    :param runas: string : None
        The user to run gem as.

    CLI Example:

    .. code-block:: bash

        salt '*' gem.update vagrant
    '''
    try:
        gems = gems.split()
    except AttributeError:
        pass

    return _gem(['update'] + gems,
                ruby,
                gem_bin=gem_bin,
                runas=runas)