Media featuring both optical gain and disorder, such as random lasers, represent formidable challenges as subjects of research due to the high complexity of the light propagation within them; however, dramatic advances in this nascent field have been furnished by the paradigm of applying wavefront shaping techniques to the beam pumping the system. We present here a theoretical and experimental study employing this approach in a gain medium where the disorder arises not from random scattering but rather from the multimodality of a waveguide: an amplifying multimode fiber. The shaping of the core-guided pump prior to its injection affects the complex, speckle-like patterns of excitation within the fiber volume. Thus we are offered the intriguing prospect of manipulating the spatially heterogeneous gain in a system where the disorder, albeit highly complex, may be fully understood in terms of the discrete eigenmodes of a well-known waveguide. We study our medium in two different configurations: as an optical amplifier, and as a fiber laser. In the first configuration we show that dependence upon the pump configuration, of the amplifier transmission function, surprisingly survives several physical mechanisms which at first sight would appear to severely limit it. This insight is then carried on to the lasing cavity configuration, where we find striking parallels between the behavior of the system and that of random lasers; more importantly, the pump shaping shows a strong ability to control the complex emission, in particular - to selectively favor individual spectral lines and stabilize single-mode operation.