Namely, de-facto, or one of, in Linux. Mature. No GUI. Open-source and free.
What is it? GPG or anything else?
For a separate file(s), or directory(ies), and not for the entire disk or partition.
Sorry I’m not sure I understand what it is you think I’m missing. It’s FOSS, works on Linux, has a CLI, works for both files and directories… please enlighten me what I got wrong?