Question

I’m editing a text-document styled on its own way of which Emacs doesn’t recognize the markup, and opens it in fundamental-mode. Tab indentation is the most problematic behaviour.

Can I fix tab width with a runtime command, only for the current session?

Particularly I’d avoid any permanent in-file configuration.

Answer 1

You can use M-x set-variable to set variables at runtime and only for that buffer and session, ideally you can set the offset variable (for instance in c-common-mode it’s c-basic-offset) and you can change this value to the desired tab width (although I recommend you use spaces instead of tabs for consistency as tab characters \t are usually 8 spaces wide.