How to get started using ngspice?
You can put interpreter command between .control and .endc commands in the circuit file. This way they should run together with the simulation.

PS. For more involved plotting and post-processing you can start with tabular data from SPICE and feed it to gnuplot or Python+numpy+matplotlib.

PS. Keep in mind that ngspice is based on Berkeley Spice 3 (rewritten in C) while most commercial SPICE engines were based on the 2nd version (in Fortran). Differences are mostly with ngspice supporting textual node names and arbitrary mathematical functions in dependent current/voltage sources. This last thing frequently causes headaches when trying to use manufacturer provided IC models which use the old (and unsupported in ngspice) POLY declaration.