I’m wondering how you could get the same moving progress bar you see
when you run :TOhtml on a big file using VimScript.
Also, when you do :vimgrep, you can see the files displayed in
succession in the command window. How does one create this kind of
progressive command window update using VimScript?
Take a look at this question. It might lead you in the right direction. You’re likely interested in the echo command
:help echo