  We report on photometry and high resolution spectroscopy of the chemically
peculiar Jupiter-family Comet (hereafter JFC) 21P/Giacobini-Zinner. Comet 21P
is a well known member of the carbon-chain depleted family but displays also a
depletion of amines. We monitored continuously the comet over more than seven
months with the two TRAPPIST telescopes (TN and TS), covering a large
heliocentric distance range from 1.60 au inbound to 2.10 au outbound with a
perihelion at 1.01 au on September 10, 2018. We computed and followed the
evolution of the dust (represented by Af$\rho$) and gas production rates of the
daughter species OH, NH, CN, C$_3$, and C$_2$ and their relative abundances to
OH and to CN over the comet orbit. We compared them to those measured in the
previous apparitions. The activity of the comet and its water production rate
reached a maximum of (3.72$\pm$0.07)$\times$10$^{28}$ molec/s on August 17,
2018 (r$_h$=1.07 au), 24 days before perihelion. The peak value of A(0)f$\rho$
was reached on the same date (1646$\pm$13) cm in the red filter. The abundance
ratios of the various species are remarkably constant over a large range of
heliocentric distances, before and after perihelion, showing a high level of
homogeneity of the ices in the surface of the nucleus. The behaviour and level
of the activity of the comet is also remarkably similar over the last five
orbits. About the coma dust colour, 21P shows reflectively gradients similar to
JFCs. We obtained a high resolution spectrum of 21P with UVES at ESO VLT one
week after perihelion. Using the CN B-X (0,0) violet band, we measured
$^{12}$C/$^{13}$C and $^{14}$N/$^{15}$N isotopic ratios of 100$\pm$10 and
145$\pm$10, respectively, both in very good agreement with what is usually
found in comets.
