History

One line: The shell keeps the commands you type in memory and writes them to a file, but only at certain moments, and bash, zsh and fish each choose a different file, a different format, and different moments.

Lesson	Level	What it settles
The history is a list in memory	101	HISTSIZE against HISTFILESIZE, the write at exit, history -r, and bash 3.2's history -a, history -c and HISTSIZE=-1; zsh's SAVEHIST, zsh -f and macOS's /etc/zshrc; fish writing each command as it runs
Two terminals, one history file	201	when the last window to close wins, histappend, PROMPT_COMMAND='history -a' and history -n, Terminal.app's per-window history; zsh's APPEND_HISTORY, INC_APPEND_HISTORY and SHARE_HISTORY; fish
What gets remembered	101	HISTCONTROL, HISTIGNORE, and Ubuntu's /etc/skel/.bashrc; zsh's options and zshaddhistory; fish's leading space and fish_should_add_to_history
History expansion	101	!!, !$, !-3:2, ^old^new, :p and history -p, and the ! inside double quotes in bash 3.2 and 5.2; zsh; fish's !! abbreviation
Three history file formats	201	bash's #<seconds> lines, zsh's EXTENDED_HISTORY and metafied bytes, fish's - cmd: records

Planned

Rough order, not a promise:

• Searching back: Ctrl-R in bash and zsh, and fish's history pager. fzf's replacement for all three is in fzf