I frequently use a variety of ASCII command characters like ctrl-u to delete
the entire line or ctrl-a to jump to the front of a long line so I can make
some edits toward that side of the command or ctrl-e to jump to the end of
the command for the same reason. I sometimes even use ctrl-k to delete
everything after the cursor to the end of the line.
What I didn't realize until now is that any of those commands the modify the current line of the termianl prompt plus regular typing and hitting the backspace are all undoable.
So, if I just wiped out half the line (with ctrl-k) and I immediately regret
it, I can restore it with ctrl-_. The system keeps of history of the actions
you've taken, so you can keep hitting ctrl-_ to undo even further.
The ctrl-/ command does the same, per GNU's Undo Changes in the Emacs
docs.