Bash, Zsh, sh, tcsh, and the TUI tools built on top of them — prompts, scripting, multiplexers, and terminal troubleshooting.
After accidentally catting a binary file, the terminal shows strange characters or wrong colors — the shell is fine, the terminal's display state is what broke.
Commands seem to vanish between sessions, or history from multiple terminals overwrites instead of combining — how history writing works and the fix for each.
Opening a new terminal tab shouldn't take a visibly annoying second or two — how to find which specific config line is responsible, rather than guessing.
Wrong or missing colors specifically inside Windows Terminal connected to WSL, even though the same shell config looks fine over plain SSH.
Every shell in daily use today descends from a 1971 command interpreter, branching through PWB, Bourne, and C shell into the families still visible now.
The two essential shell debugging tools: one traces exactly what a script actually executes, the other catches whole categories of bugs before it ever runs.
Setting up fzf, a general-purpose fuzzy finder that plugs into history search, file finding, and practically any list-based shell workflow.
Writing shell scripts that run correctly under any POSIX-compliant shell, not just whichever one happens to be installed on your own machine.
GNU screen is older and less feature-rich than tmux, but still genuinely useful, and often already pre-installed on systems where tmux isn't.
Installing Starship, a fast, shell-agnostic prompt that works identically across Bash, Zsh, and fish, and configuring which information it shows.