[qsort]

I feel like "do one thing and do it well" is an oversimplification:

(i) Make each program do one thing well. To do a new job, build afresh rather than complicate old programs by adding new "features".

(ii) Expect the output of every program to become the input to another, as yet unknown, program. Don't clutter output with extraneous information. Avoid stringently columnar or binary input formats. Don't insist on interactive input.

(iii) Design and build software, even operating systems, to be tried early, ideally within weeks. Don't hesitate to throw away the clumsy parts and rebuild them.

(iv) Use tools in preference to unskilled help to lighten a programming task, even if you have to detour to build the tools and expect to throw some of them out after you've finished using them

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[1] https://archive.org/details/bstj57-6-1899/page/n3/mode/2up

[PetitPrince]

I agree with that. And like most rule of thumbs, it's very useful to go beyond the prescriptive part and ask yourself "why". And as qsort described, doing "one thing well" enables you to have desirable traits (easy testability, low cost refactoring, etc.).