Strain tuning of optical emission energy and polarization in monolayer and bilayer MoS<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow/><mml:mn>2</mml:mn></mml:msub></mml:math>

We use micro-Raman and photoluminescence (PL) spectroscopy at 300 K to investigate the influence of uniaxial tensile strain on the vibrational and optoelectronic properties of monolayer and bilayer MoS${}_{2}$ on a flexible substrate. The initially degenerate ${E}^{\ensuremath{'}}$ monolayer Raman mode is split into a doublet as a direct consequence of the strain applied to MoS${}_{2}$ through Van der Waals coupling at the sample-substrate interface. We observe a strong shift of the direct band gap of 48 meV/(% of strain) for the monolayer and 46 meV/% for the bilayer, whose indirect gap shifts by 86 meV/%. We find a strong decrease of the PL polarization linked to optical valley initialization for both monolayer and bilayer samples, indicating that scattering to the spin-degenerate $\ensuremath{\Gamma}$ valley plays a key role.