Ngspice consists of the simulator and a front-end for data analysis and plotting. Input to the simulator is a netlist file, including commands for circuit analysis and output control. Interactive ngspice can plot data from a simulation on a PC or a workstation display.

Ngspice on Linux (and OSs like Cygwin, BCD, Solaris ...) uses the X Window System for plotting (see Chapt. 18.3) if the environment variable DISPLAY is available. Otherwise, a console mode (non-graphical) interface is used. If you are using X on a workstation, the DISPLAY variable should already be set; if you want to display graphics on a system different from the one you are running ngspice or ngutmeg on, DISPLAY should be of the form machine:0.0. See the appropriate documentation on the X Window System for more details.

The MS Windows versions of ngspice and ngnutmeg will have a native graphics interface (see Chapt. 18.1).

The front-end may be run as a separate `stand-alone' program under the name ngnutmeg. ngnutmeg is a subset of ngspice dedicated to data evaluation, still made available for historical reasons. Ngnutmeg will read in the `raw' data output file created by ngspice -r or by the write command during an interactive ngspice session.