Dunno if anyone would know but, basically, I want something similar to Ctrl-r, when you're using Bash.

eshell-isearch-backward kind of gets at it but it seems to fail at detecting commands that have definitely been used in the past, randomly (and finds them when invoked a second time…; the other issue is there doesn't seem to be an easy way to reinvoke it. Commands like eshell-isearch-repeat-backward don't seem to work like anticipated).

Figured I'd throw out a line, in case anyone knew.