I'm job hunting again. When I find a tempting position, one that looks great in every way except it requires some skill "ABC" I'm not familiar with, I get a book or find online tutorials, and study it. A few days later another tempting position comes to my attention, and it requires know-how with "DEF". I'm rusty on it, so I switch to studying that. This continues, and I soon find myself with a long list of "Things to study: ABC, DEF, GHI, ..."
None of these are simple topics taking an hour of reading or a simple how-to, but whole technologies, areas of science, deep abstract techniques which take practice, exploration, making toy projects, involvement on forums, etc. (Simple things that do take only a small effort, fine, I just go ahead and do.) Maybe a full week or two of intense effort could get me to the point of hire-ready competence, more likely a month or two since I still have to job hunt and deal with life.
If I had a talented fortune teller to tell me which job I'll actually be hired for, suppose it turns out to be the DEF-needing job, I could study only DEF and ignore ABC, GHI, and the rest.
How to decide which topics to study with minimal wasted effort? Especially how to know when to stick with something I'm already studying vs. switching when a new interesting position comes to my attention?
Additional: To be clear, I'm not talking about "study" as in college, but activities of skill-building and it's more active, building demos, working with others on pet projects etc than it is book learning or practice quizzes. There is time, money, equipment invested in some cases, so it's important to decide wisely.