If you're looking for something simple and unixy, Zim is a pretty good choice[1]. It's an offline GTK-based GUI application for creating personal wikis that saves all the wiki pages as Markdown files and can export your wiki as HTML using various templates. Zim has been available in many linux package repos for over a decade and is GPL-2.0 licensed.
[0] https://en.wikipedia.org/wiki/Personal_wiki?useskin=vector#S...
Usually you end with wiki or plain text files.
Some popular offers:
There's also a URL with a textbox that can save small snippets of text. No auth needed to save, but auth needed to retrieve the saved contents. This helps when I'm on the road and need to quickly save something.