Ask HN: Linux foundations training material for junior colleagues?

I'm finding myself often teaching junior colleagues command line and other Linux basics, then sending them material (be official docs, a blog or a StackOverflow page or a man page command) to look at to review what I've explained. It's in the context of work so I have to rush through it, so I can finish my own projects.

All this to ask: do you have recommended fast-paced training of Linux command-line, bash scripting, then core systems stuff (how ssh works, strace, /proc and so forth)? I was looking at the Linux Professional Institute as I once took a kernel class there, but their partners don't have great reviews and the training seems video-based (from what I could tell).

Any suggested resources? I was thinking Red Hat but it's quite pricey and RH-centric.