This is not the recommended way to use FluidSynth, because the Qsynth graphical interface is much easier to use. Qsynth automatically configures most of FluidSynth's settings by default, allowing you to avoid focus on how you want to use FluidSynth, rather than on how to use FluidSynth.

If you want to use FluidSynth in a terminal, you can use the fluidsynth command. The default sample-rate is 44.1 kHz, so if you want to use JACK at a different sample rate, you need to use the -r flag, like this: fluidsynth -r 48000

When you start FluidSynth from a terminal, it will normally start a shell of its own. How to use this shell is beyond the scope of the Musicians' Guide, but you can get basic help by running the "help" command from the FluidSynth command line.