Linux

How to read built-in help on Linux (`man`, `info`, `whatis`, `--help`, `apropos`, `tldr`) and how shells expand glob patterns — wildcards, character classes, brace expansion — plus how globs differ from regular expressions.

Day-to-day commands for moving around the filesystem (`cd`, `pwd`, `ls`, `tree`, `mkdir`) and inspecting space (`df` for mounted filesystems, `du` for file/directory size). Includes the common `df`-vs-`du` confusion and useful flags for both.

How Unix file permissions and ownership work — the `rwx` mode triad, octal notation, `chmod` / `chown` / `chgrp` usage, special bits (setuid, setgid, sticky), `umask` defaults, and a brief pointer to POSIX ACLs.

Reading files (`cat`, `less`), comparing them (`cmp`, `diff`), searching the filesystem (`find`), and managing symbolic vs hard links (`ln`). Plus the `find ... -exec` vs `xargs` distinction, which trips up most newcomers.

Grab-bag of useful Linux odds and ends — what wall/CPU/user/system time actually mean and how `time` reports them, brief notes on `jq` and `ffmpeg`, a pointer to `perf` for profiling, and GNOME / Ubuntu desktop tweaks (`gsettings` keys, Nautilus `Ctrl+L`, multi-monitor workspaces).