So… I’m not going to answer your question, feel free to ignore me.

It’s of course possible to do so and the most obvious way is to use dd since on Linux devices, including disks, are files. Consequently you can indeed “save” the whole system from the CLI.

That being said I would argue it’s a bit waste of time unless you have a very specific, and usually rare, use case e.g testing OSes themselves. Most likely I imagine (and again I’m not directly answering your question here so please do feel free to fix my assumptions or ignore this entirely) you “just” want to “quickly” go from a “broken” state to one where you can “work” again.

It might be because you are doing something “weird” e.g tinkering with the OS itself or lack of “trust” in your current setup.

Here my recommendation would be instead to have a “work” OS and then other partitions, or even virtual machines (not containers) dedicated to testing because it’s truly a great way to learn BUT it shouldn’t come at he risk of your data or your time.

Finally, one of the bounding resource is the speed of your disk and your time to focus. I find that installing a “fresh” OS from a modern USB stick is fast, like take 2 coffees fast. I installed Ubuntu just yesterday, twice, so rather confident about that comment.

What is indeed slow is to copy YOUR files because they are larges and numerous.

So… finally, the “trick” do NOT copy your files despite reinstalling the system! Instead, have a dedicated /home partition so that if you reinstall the OS, your files are untouched. Yes you might have to install a couple of software but if you keep track of them via e.g ~/.history (which BTW will be saved in that situation) you will be able to e.g grep apt install it and be back on track in minutes.

TL;DR: /home partition that is not deleted on OS reinstallation is often IMHO the most efficient way to go.