A command line script, available from the ngspice distribution at examples/control_structs/s-param.cir, together with the command wrs2p (see Chapt. 17.5.94) allows to calculate, print and plot the scattering parameters S11, S21, S12, and S22 of any two port circuit at varying frequencies.

The printed output using wrs2p is a Touchstone® version 1 format file. The file follows the format according to The Touchstone File Format Specification, Version 2.0, available from here. An example is given as number 13 on page 15 of that specification.