Is voltage and current sources linear or nonlinear?
Only elements that have both inputs and outputs can be considered to be linear. "Linear" is a term that applies to a system with inputs and outputs, since it describes a particular kind of relationship between them. This means that simple voltage and current sources (when seen as ideal circuit elements) are neither linear nor non-linear.

In essence, a system is linear if it follows the superposition principle. Note that in reality, all physical circuit elements are non-linear, but in practice they behave as linear systems for some input range.

See also this answer for the difference between linear and affine, which is important for some circuit elements.