Fixing installations
Super Grub2 Disk - Put it on a flash drive and if your grub gets borked, it can help you1 Rescatux - rescue utilities, also keep ‘em on a flash1
Command Line
CommandLineFu.com - people can post command line gems
ShellCheck - Check your shell scripts - it will find your errors!
cheat.sh - curl this website and get a cheatsheet
Filesystem stuff
Filesystem Hierarchy Standard - has the standards for waht files go where
Misc
Over The Wire - “Wargames” that will teach you Unix and Cybersecurity awesome lists - a crazy amount of lists of resources and other information