The simulation flow in ngspice (input, simulation, output) may be controlled by dot commands (see Chapt. 15 and 16.4.1) in batch mode. There is, however, a much more powerful control scheme available in ngspice, traditionally coined `Interactive Interpreter', but being much more than just that. In fact there are several ways to use this feature, truly interactively by typing commands to the input, but also running command sequences as scripts or as part of your input deck in a quasi batch mode.

You may type in expressions, functions (17.2) or commands (17.5) into the input console to elaborate on data already achieved from the interactive simulation session.

Sequences of commands, functions and control structures (17.6) may be assembled as a script (17.8) into a file, and then activated by just typing the file name into the console input of an interactive ngspice session.

Finally, and most useful, is it to add a script to the input file, in addition the the netlist and dot commands. This is achieved by enclosing the script into .control ... .endc (see 16.4.3, and 17.8.7 for an example). This feature enables a wealth of control options. You may set internal (17.7) and other variables, start a simulation, evaluate the simulation output, start a new simulation based on these data, and finally make use of many options for outputting the data (graphically or into output files).

Historical note: The final releases of Berkeley Spice introduced a command shell and scripting possibilities. The former releases were not interactive. The choice for the scripting language was an early version of `csh', the C-shell, which was en vogue back then as an improvement over the ubiquitous Bourne Shell. Berkeley Spice incorporated a modified csh source code that, instead of invoking the unix `exec' system call, executed internal SPICE C subroutines. Apart from bug fixes, this is still how ngspice works.

The csh-like scripting language is active in .control sections. It works on `strings', and does string substitution of `environment' variables. You see the csh at work in ngspice with set foo = "bar"; set baz = "bar$foo", and in if, repeat, for, ... constructs. However, ngspice processes mainly numerical data, and support for this was not available in the c-sh implementation. Therefore, Berkeley implemented an additional type of variables, with different syntax, to access double and complex double vectors (possibly of length 1). This new variable type is modified with let, and can be used without special syntax in places where a numerical expression is expected: let bar = 4 * 5; let zoo = bar * 4 works. Unfortunately, occasionally one has to cross the boundary between the numeric and the string domain. For this purpose the $& construct is available – it queries a variable in the numerical let domain, and expands it to a c-sh string denoting the value. This lets you do do something like set another = "this is $&bar". It is important to remember that set can only operate on (c-sh) strings, and that let operates only on numeric data. Convert from numeric to string with $&, and from string to numeric with $.