The canonical example of employing existing tools might be McIlroy's shell script used as a critique of Knuth's solution to word counting [search for "knuth" here: https://www.princeton.edu/~hos/mike/transcripts/mcilroy.htm].

In fairness, I think a more intellectually honest critique would have been to write the script using literate programming. Sure it might have required McIlroy to write a tool for literate shell programming -- the will to do so being the magic trick separating Knuth from mortals. The world might have been better for it.