My main system (Xubuntu 20.04.) is in deep trouble. I see no hardware problems, but only very basic things are working. Web is down. My RAID is down. Several other things are flakey. I don't know if it's configuration or the actual programs.
I've made a clone of the main disk, so I can be a bit adventurous, but there are some basic tasks that I would like to do, but don't know how. The first is re-installing all the software I can. Not just the default stuff, but every package I ever installed. SO:
I've read about how to do "something else" to do a refresh of the initial set of programs So I can already do that.
There are various kinds of configuration files. I've been running Ubuntu for a couple of decades so there's no chance I remember them all; is there a way to find their locations? Can I ensure they are safe, or restorable?
How do I reinstall the packages I have installed manually. I've got lists of all packages from three sources: (1) dpkg --get-selections, (2) apt-mark and (3) apt list, but don't see how to use them directly to cause packages to be refreshed.
In response to the first answer, I tried this script
apt update
dpkg --set-selections </PACKAGE-LISTS/b5-getsel
apt-get dselect-upgrade
but it made no changes to the installed packages, in spite
of the file having >3000 lines, specifically including vim(1),
which I sorely miss.
How can I debug this?