Just to mention it here, a linux program to generate signal in the audio (headphones) output http://www.comp.leeds.ac.uk/jj/linux/siggen.html (http://www.comp.leeds.ac.uk/jj/linux/siggen.html) . Install it, then run it in the terminal with padsp siggen , padsp is to provide the oss sound device /dev/dsp under the pulseaudio. Generates sine, triangle, square, sawtooth and pulse signals (with different duty cycles), of course it is limited by the sample rate of your sound device. It has an ncurses character screen interface, it is primitive but small and simple, and may well be good enough for some purpose.