Stellar bars are known to gradually funnel gas to the central parts of disk galaxies. But it remains a matter of debate why the distribution of ionized gas along bars and in the circumnuclear regions varies among galaxies.We investigate the spatial distribution of star formation (SF) within bars of nearby low-inclination disk galaxies (i < 65 deg) from the S4G survey. We aim to link the loci of SF to global properties of the hosts (morphological type, stellar mass, gas fraction, and bar-induced gravitational torques), providing constraints for the conditions that regulate SF in bars. Using archival GALEX far- and near-UV imaging and continuum-subtracted H alpha images for barred galaxies and matched control samples of non-barred galaxies, we analyse (i) bar (2D) and disk (1D) stacks built from co-added UV images and (ii) the visual classification of the morphology of ionized regions (traced from H alpha and UV data) in individual galaxies.We find that massive, gas-poor, lenticular galaxies typically exhibit circumnuclear SF; this is probably related to bar-induced quenching of SF in the disk. SF at the bar ends, but not along the bar, is mostly found in early- and intermediate-type spirals; this most likely results from the interplay of gas flow, shocks, and enhanced shear in massive centrally concentrated galaxies with large bar amplitudes. Late-type gas-rich galaxies with high gravitational torques mainly show SF along the bar; we argue that this is a consequence of low shear among the faintest galaxies. Among early-type spirals the central UV emission is around 0.5 mag brighter in strongly barred galaxies, relative to their weakly barred counterparts; this is probably related to the efficiency of strong bars sweeping the disk gas and triggering central starbursts. On the contrary, in later types the UV emission is stronger at all radii in strongly barred galaxies than in weakly barred and non-barred ones.We conclude that star-forming bars are most common among late-type gas-rich galaxies, and that bars are important agents in the regulation of SF in disks.