  Blazars, one of the most extreme class of active galaxies, constitute so far
the largest known population of $\gamma$-ray sources and their number is
continuously growing in the Fermi catalogs. However in the latest release of
the Fermi catalog there is still a large fraction of sources that are
classified as blazar candidate of uncertain type (BCUs) for which optical
spectroscopic observations are necessary to confirm their nature and their
associations. In addition about 1/3 of the $\gamma$-ray point sources listed in
the Third Fermi-LAT Source Catalog (3FGL) are still unassociated and lacking an
assigned lower energy counterpart. Since 2012 we have been carrying out an
optical spectroscopic campaign to observe blazar candidates to confirm their
nature. In this paper, the sixth of the series, we present optical
spectroscopic observations for 30 $\gamma$-ray blazar candidates from different
observing programs we carried out with the TNG, WHT, OAN, SOAR and Magellan
telescopes. We found that 21 out of 30 sources investigated are BL Lac objects
while the remaining targets are classified as flat spectrum radio quasars
showing the typical broad emission lines of normal quasi stellar objects. We
conclude that our selection of $\gamma$-ray blazars candidates based on their
multifrequency properties continues to be a successful way to discover
potential low-energy counterparts of the FermiUnidentified Gamma-ray Sources
(UGSs) and to confirm the nature of BCUs.

High Energy Astrophysical Phenomena