Linux Command Line Course
From first principles to networking commands — how the shell resolves and runs a command, navigating and reading the filesystem, files and directories, finding things with find/grep/locate, permissions and ownership, processes and resource use, text processing and pipes, networking commands, package management, and shell scripting basics, plus a one-page cheat sheet.
I kept this short on purpose — seven pages, because the command line doesn't need two hundred pages, it needs the right forty commands explained properly. The section on how the shell resolves and runs a command is the one most "Linux basics" content skips entirely, and it's the thing that makes everything after it make sense. If you already script daily, skip to the networking commands section — that's the part most developers never fully learn.