If you call

ngspice

ngspice will start, load spinit (16.5) and .spiceinit (16.6, if available), and then waits for your manual input. Any of the commands described in 17.5 may be chosen, but many of them are useful only after a circuit has been loaded by

ngspice 1 -> source adder-mos.cir

others require the simulation being done already (e.g. plot):

ngspice 2 ->run
ngspice 3 ->plot allv

If you call ngspice from the command line with a circuit file as parameter:

ngspice adder-mos.cir

ngspice will start, load the circuit file, parse the circuit (same circuit file as above, containing only dot commands (see Chapt. 15) for analysis and output control). ngspice then just waits for your input. You may start the simulation by issuing the run command. Following completion of the simulation you may analyze the data by any of the commands given in Chapt. 17.5.