Linux

Unless you want to increase productivity in specific field (say some kinds of software development) you won't even need to remember anything more than cat, ls etc. In those specific usecases you'll become habituated to the command you use frequently, nobody becomes a grep ninja on day 0.

Whenever I need something mildly complex with ffmpeg or imagemagick, the right command is just a web search away, I rarely remember syntax of these anyway. I find commands less obvious and harder to find for windows shell (technically powershell is cross platform btw), but maybe that's because I'm not much familiar with windows-ism'

Tip: whenever you encounter a useful command syntax/one liner, save it with brief description. I find konsole's built in quick commands quite handy, some other terminals probably have such features too. Otherwise a simple markdown list is enough.