How is ctrl + / working as undo in Zsh 5.7.1 on Debian 10.0? I have a Debian 10.0 system. It has zsh 5.7.1 (x86_64-debian-linux-gnu). In ZSH prompt, I type foo bar baz Then I type `alt` \+ `backspace` two times. The words `baz` and `bar` get deleted and only `foo` remains. Then I type `ctrl` \+ `/` two times to undo the deletions. The words `baz` and `bar` reappear. How is `ctrl` \+ `/` working as the undo command? I don't see it defined anywhere? $ echo / | grep -E "/|undo" # just testing my regex / $ bindkey -l .safe command emacs isearch main vicmd viins viopp visual $ bindkey -M .safe | grep -E "/|undo" $ bindkey -M command | grep -E "/|undo" $ bindkey -M emacs | grep -E "/|undo" "^X^U" undo "^Xu" undo "^_" undo I also have a macOS Catalina 10.15.7 with zsh 5.7.1 (x86_64-apple-darwin19.0) and `ctrl` \+ `/` does not do anything there.

There's no control character for `Control`+`/`, so there's no natural choice of character or character sequence that the terminal can send. Many terminals send `^_`, i.e. the same character as `Control`+`_`. This includes xterm, rxvt, konsole and all terminals based on the vte library (Gnome Terminal, Mate Terminal, lxterminal, Terminator, …), but not macOS's built-in Terminal application.

You can see what your terminal sends by typing `Ctrl`+`V` then `Ctrl`+`/`. `Ctrl`+`V` means “insert the next character literally” (even if it would be otherwise interpreted as a command).

(For the role of the terminal, see How do keyboard input and text output work?)

`^_` is the shortcut for undo in Emacs and applications with Emacs-inspired keyboard shortcuts.