Linux from scratch

From the list ... I would go with:

Arch Linux - if you want to learn how to partition you drive, setup bootloader, manage packages from terminal, setup EFI, and such, probably tasks that will help you for example in cases when windows would override bootlaoder/efi and you want to recover/fix your Linux OS without reinstall, or you broken it and you want to fix it.

Nix OS - if you want to learn how to serialize your OS to an reproducible state with later ways to use Nix packages on other distros if needed. If you break it, you will just revert to older revision or reinstall everything in a matter of minutes (if not seconds) from few files. You will learn way different set of skills than Arch as you will only 'define' what you want your partition and bootloader should look, and NixOS will take care of that almost magically, but you could use Nix packages on top of Arch or any other distro.

Gentoo - if you want to learn how all those packages are build and installed from sources, how to patch and modify them when needed, how much time you gain with distro with prebuild packages if you decide to build everything, but on the other hand how much performance you 'could' gain from optimized builds or how easy is to install anything you want in any way you want. (Didn't use Gentoo myself, as installing stuff from AUR on my desktop Arch is all I need to experience how much time it could take :) So Gentoo would probably not be the hardest but most time consuming, but I could be wrong)

Operating Systems are lame, I use butterflies like a real programmer

You're in a good position to "master" NixOS, but I don't think it'll help you to achieve your goal of getting better at Linux.

NixOS will help you with declarative systems ("mastering" NixOS will help you working with it's sibling Guix for example), but because so many things are specific to the declarative configuration, it won't help you in many other places (not with you current knowledge, apart from functional progamming I guess).

If you want to "master" linux, maybe check out linuxfromscratch.org

Edit 1 (spelling)

As both arch and gentoo has very good documentation, and is very standard Linux as it “always” has been, I don’t see anything hard about them. Nix is a different beast and you need to learn some new approaches.

Sorry, but I can't follow. Shouldn't an OS be easy to use so that it gets out of the way so that you have more time for the real work you have to do?

For example I tried out using plain Fedora Server for my NAS but went back to unRAID. I just didn't want to spend all that time figuring out why the samba server isn't visible or why the drives aren't spinning down, etc etc.

Lfs but you learn a ton.

You could always go down the Slackware route. It's only hard because it does some stuff differently and doesn't have a huge userbase.

My question is, which among these is considered to be hardest and thus by mastering it, one can master linux to atleast some part?

It does not matter which distribution you use. For example, I am using Arch for over 10 years. Have I gained any new knowledge since then? Sure. But not because I use Arch but because I had to do certain tasks. Or because I was interested in certain things. I even go so far as to say that I got a lot of my Linux knowledge under Mandrake / Mandriva (comparable to Ubuntu) because that was the distribution I used initially.

From my point of view, it is therefore only important that you want to learn something. Whether you use Ubuntu, OpenSuse, Arch or Gentoo for this is basically irrelevant.

Apart from that, I don't think Arch is a difficult distribution to install or use. For example, many of the commands in the official installation guide can be executed without any changes. Or if you want to make it even easier, just use archinstall which has been an official part of the iso file for some time.

As for maintaining Arch, I've been doing exactly two things for years.

Before an update, I check if anything has been released at https://archlinux.org/news/ that affects my installations (this can be automated with tools like informant). It is important to follow these instructions.

And from time to time I synchronize my configuration files with the Pacnew files. There are tools for this as well (https://wiki.archlinux.org/title/Pacman/Pacnew_and_Pacsave#Managing_.pac*_files).

In other words, just use the distribution that suits you and learn what you need. Learning things on spec that you may never need is useless in my opinion.

I think Gentoo and Arch are quite similar but Gentoo needs more work. Not harder, but more annoying and time-consuming.

I've not done Gentoo since about 2006/7 after running as a primary OS for about 2-3 years - It taught me lots about system maintenance. I then move to SUSE and *BSD desktops before coming back to Windows as my OS for daily use (have to for work).

I don't think it really matters what poison you choose, just dig down in to it's inner workings.

None of them are hard at all. They require some different skills though. Due to few defaults you have to spent more time with setting it up than for example Ubuntu which made many decisions for you. It's not too bad either way though. All the distros are supposed to be usable and not hard, that's the wrong mindset.

You won't learn much about Linux by using NixOS, as the configuration and skills are not really transferable (yet?). It fills a niche which is useful for development. Its goals are different and I would exclude it from the list.

Arch Linux is very simple and a bit difficult at the same time. As an experienced Linux User or administrator you can easily set it up and get it running like any other distro. The difficulty comes from you having to know what you need and want. This is the actual learning curve. Many packages have sane defaults which do not differ too much from other distributions so you spend most time with customizing it to your needs, but otherwise it is just a rolling release distro which works just fine and is easy to manage once it is all up and running. You have to make many decisions though.

The same is mostly true for Gentoo, but they use other tools and it invites its users to tinker with the software if they want to. You spend most of the time waiting for the compiler hence I personally don't really like working with it. It is a neat idea, but it's getting old really fast.

Personally I stick to Arch for my desktop system, but use RHEL or Debian based OS on any servers I manage. So my real answer is another option: Learn by operating and using a stable established distribution and once you are comfortable with the tools and everything you can branch out and dig your teeths into something else if you feel you need to. This would build more transferable skills especially in a professional setting.

After all if you want to take the difficult approach, rather stick to the learning "distro" Linux from Scratch (it's a guide) which can teach you quite a lot about Linux.