is there a good book/course on "linux fundamentals"?
i am capable enough with linux to use it but there are a lot of concepts I don't actually understand, for example: permissions (users, groups, file permissions), networking, and key management (ssh, gpg).
i know to do all the things, but only by memorization, scripts and habits, not because i actually understand them. is there some central place to learn about it all? like a de facto "introduction to linux"?