qgis - Export a configured Atlas with a Python script / command line?

I just want to export a configured Atlas (composer with Atlas option and cover layer enabled)

I tried this :

composerView = qgis.utils.iface.activeComposers()[0]
composition = composerView.composition()
atlas = composition.atlasComposition()
composition.setAtlasMode(QgsComposition.ExportAtlas)

atlas.beginRender()
for i in range(0,atlas.numFeatures()):

    atlas.prepareForFeature(i)
    composition.exportAsPDF('C:\xxxx\test.pdf' + str(format(i)))
atlas.endRender()

But nothing happens.

Have anybody a clue?