What I found is that written evidence for knocking on wood (and for the history of gesture as a whole) is remarkably sparse. In the Penguin Guide to Superstitions, folklorist Steve Roud offers a skeptical take on the provenance of knocking on wood due to the near total absence of written records about it:
热气腾腾的煎饼摊、干净敞亮的菜市场、随处可见的口袋公园、社区小广场上欢快起舞的居民……“看得出来，他们中的大多数人都生活惬意、积极向上，这样的中国很温暖。”骑着共享单车，途经天津的大街小巷，所见的一幕幕，让格雷格如是感慨。
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The result is in the output directory that I mapped as a volume.