Linux
7 curated resources, easiest first.
Shows how to make your user own a folder and everything inside it with chown -R, and optionally the group too. It's useful when files created with sudo have become unwritable.
du -hs gives a single human-readable total for a folder. The answer explains the -h (human-readable) and -s (summary) flags and how to sort folder sizes.
On Ubuntu, members of the sudo group can already use sudo, so you just add the user to that group instead of editing /etc/sudoers. The change takes effect at the user's next login.
Explains how dpkg, apt and the graphical installers relate, and how to install a downloaded .deb package from the terminal, including fixing missing dependencies afterwards.
A clear reference to shell redirection: > overwrites a file, >> appends, 2> captures errors, and tee shows output on screen while also writing it to a file.
sed -i 's/old/new/g' file edits a file in place, replacing every match. The answer breaks down each part of the command. Note that -i overwrites the file, so test without it first.
Uses find with -delete. The answer stresses running the same find command without -delete first to see what would go, and keeping -delete as the last argument, because in the wrong position it deletes everything.