Commands, as described in detail in Chapt. 17.5, may be used interactively, but also as a command script enclosed in .control ... .endc lines. The scripts may contain expressions (see Chapt. 17.2). The expressions may work upon simulation output vectors (of node voltages, branch currents), as well as upon predefined or user defined vectors and variables, and are invoked after the simulation. Parameters from 2.8.1 defined by the .param statement are not allowed in these expressions. However you may define such parameters with .csparam (2.10). Again the expression syntax (see Chapt. 17.2) will deviate from the one for parameters or B sources listed in 2.8.1 and 5.1.

If you want to use parameters from 2.8.1 inside your control script, you may use .csparam (2.10) or apply a trick by defining a voltage source with the parameter as its value, and then have it available as a vector (e.g. after a transient simulation) with a then constant output (the parameter). A feedback from here back into parameters (2.13.1) is never possible. Also you cannot access non-linear sources of the preceding simulation. However you may start a first simulation inside your control script, then evaluate its output using expressions, change some of the element or model parameters with the alter and altermod statements (see Chapt. 17.5.3) and then automatically start a new simulation.

Expressions and scripting are powerful tools within ngspice, and we will enhance the examples given in Chapt. 21 continuously to describe these features.