This is a great question!

It’s hard to really wrap your head around it without doing a ton of low-level taking things apart and putting them together differently.

But to answer, it’s pretty impressive the extent to which a full Linux install of any distro tends to just be like a bunch of legos put together in one particular way.

Theoretically, there’s no reason why you couldn’t ship-of-Thesius one distro into another. You’d have to have a good idea of what the differences between the two are, but it can certainly be done.

There’s a thing called a “chroot.” It’s basically a whole OS installed in a subdirectory on another whole OS. And there’s a command (also called “chroot”) that can be used to tell the parent OS to “give me a shell in the chroot OS – as in run the /path/to/chroot/bin/bash (or whatever) executable in ‘The Matrix’ such that that process thinks that the chroot is the root OS.”) That lets you do some pretty cool stuff like building an OS to be installed on another box. But when you run in the chroot, it doesn’t load the guest OS’s kernel or (typically) init system or anything. The processes run on the host system’s kernel.

And it’s entirely possible to have the guest chroot system be a whole different distro than the host. (Though some distros will have tools that make it easier to chroot into a guest chroot of the same distro.) Which implies that you can just kindof substitute one distro’s kernel for another distro’s, right?

Turns out the answer to that question is “at least mostly yes.” Quick funny personal story. I started working somewhere recently where they allowed new hires a choice between Windows, Mac, or Linux on their work laptop. I chose Linux, but didn’t like the distro they pre-installed on it. (KDE Neon. I preferred Arch. Insert hate here.) But the laptop had secure boot enabled and the PC support department wasn’t willing to let me disable that. The laptop would only allow certain kernels to boot. Windows and some kernels from some unknown set of Linux distros.

Just as a quick aside, the way it knew how to deny a specific kernel from running or allow another to run was with signatures. Canonical which makes Ubuntu includes cryptographic signatures in the kernel file identifying that kernel image as made and certified by Canonical. (Microsoft does roughly the same thing for Windows kernels.) The secure boot system on the laptop has a list of trusted certificates. If the kernel that the bootloader (which is also signed, by the way) asks the secure boot system to boot is signed by one of those certificates, it boots. If not, secure boot denies the request. Theoretically more certificates can probably be configured/trusted, but that wasn’t an option in my case.

But I still wanted to run Arch! Now, KDE Neon uses the Ubuntu kernel, so I knew that was one I could boot without access to the secure boot config. So I grabbed the .deb for the Ubuntu kernel, wrote a script to convert the .deb for the Ubuntu kernel into an Arch package. (Arch doesn’t use .debs or .rpms. It uses “pacman packages”.) I installed that arch package, configured the bootloader to point to the arch install including that Ubuntu kernel, and booted it. Viola! Arch (mostly) without secure boot access!

What I was running was really kindof 95% Arch and 5% Ubuntu kernel. Kindof a Frankenstein’s monster of OS’s. But it worked perfectly.

And theoretically, just about any part of a distro can be replaced with the equivalent from another distro. (Or from the upstream/source version.) You could technically take a Fedora system and replace the package manager with apt (I’m guessing there isn’t an rpm package that would install apt on your Fedora, so you might have to make it yourself or just build it from source and install it manually) pointed at Ubuntu repositories and transform Fedora piece-by-piece into Ubuntu. It’d be a pretty wild and messy process. And it would probably be easier to just reformat and install Ubuntu. But it could be done.

Similarly, you could replace the init system. Artix is a fork of Arch that gives a choice of init systems whereas Arch only supports Systemd. And it’s kindof another Frankenstein’s monster of an OS because it still relies heavily on the Arch repos. But it works.

Would it be enough to be able to run .deb packages on fedora?

Unpacking a .deb on Fedora, or unpacking an .rpm on Ubuntu isn’t a big deal. The files inside are often actually identical.

But would not be useful because the files inside usually rely on shared libraries, which may or may not already be installed. Those shared libraries are installed in different places on each Linux distro. Figuring out which ones to ask for (and making sure the program can find them) is the real work that the .Deb or .RPM installers do.

A fun way to try this out is with Portable Apps. Anything called a “portable app” either doesn’t use additional libraries, or carries the libraries it needs with it.

If you find a portable app for Ubunutu, there’s a good chance the Fedora version is an identical file, and works fine on Ubuntu. There’s lots of reasons it might not work, but it can be fun to try.

For the most part, the only reason any Linux program is unavailable on a different version of Linux is that no one has bothered to build the necessary installer for that combination of program and OS.

.RPM was supposed to solve this by being universal, since any other OS can implement it to match .Deb was supposed to solve this by being universal, since any other OS can implement it to match (about 60% actually do). I think Flatpacks and Snaps might solve this by being universal, at some point…

Source: I’ve built installer packages for various operating systems.

On the surface, the biggest difference between distros will be the package manager and the update cadence. Most package managers are generally comparable so I won’t get into that. The cadence has to do with release type - rolling or fixed - and the speed with which updates are released. Do you want the newest packages, LTS or somewhere in the middle? This is probably the first big decision to make when choosing a distro. The only real must-have here is you want a distro that provides timely security updates. Even a highly stable LTS should be pushing out security updates asap.

Then you have default package choices, which are often superficial like DE or default apps. This can all be changed so it’s not much of a concern. But there could also be more impactful choices like whether a distro uses systemd or glibc vs musl. The mainstream distros tend to use systemd and glibc, which is generally good, but know that you have other options if your specific use case requires it. There’s also package availability, meaning the number of packages available in the repository, although this is less important than it used to be because you have options like Flatpak or Nix for getting packages that aren’t in your distro’s repository.

There are also some distros created with a specific use case in mind, such as Alpine for containers or Kali for testing network security.

Finally, you have structure and governance. Some distros have corporate backing, others are community supported and still others aren’t much more than a hobby. The ones with corporate backing typically have options for paid support. In general, you want something with stable and competent governance where it will continue to thrive even as team members change. You can find examples of this in corporate-backed distros as well as community distros.

So your biggest choices are going to be cadence, structure/governance, and whether you may need paid support now or in the future.

As for what distro developers actually do… First, they build the tooling and infrastructure to make their distro work - package manager, packaging tools, repository, etc. Then, they are responsible for packaging everything available in the distro. They are pulling in source code for all these apps, compiling it and putting binaries in the repository. They rebuild packages as required when there are updates to the source code. Some distros like Arch will build vanilla packages, meaning they don’t make changes to upstream code. Others may apply their own patches for various reasons. Some like Red Hat will provide patches to upstream apps requested by customers as part of their paid support services. So let’s say something isn’t working the way you need it for some random FOSS app included with the distro. You can put in a request and they will change it for you.

As for your specific question about simulating Ubuntu on Fedora, that is not possible. They each use their own distinct package manager and repository. They generally have similar packages, but they are not interchangeable. However, there are tools like distrobox and distros like VanillaOS that have mechanisms for using another distro’s packages. These use containers under the hood so it’s not quite the same as just installing .deb on Fedora or .rpm on Ubuntu.

what else?

fedora is usually more updated(newer packages and newer kernel) and it uses zram, ubuntu use swap from default, and ubuntu push snap, fedora, like others, come with flatpak pre-installed

Just wondering if there could be a way to “simulate”, lets say ubuntu on fedora.

distrobox

The differences between distros are the things you mentioned. They all use the Linux kernel, so the differences are in the DE, installer, theme, default packages, and package manager. These changes come about from design choices: rolling vs versioned releases, stability goals, FOSS vs proprietary packages/repositories, things like systemd vs alternatives, and overall goals/use cases (lightweight, server, etc).

A distro can be as little as a theme change. The famous Hannah Montana Linux is KUbuntu with a custom theme, icon pack, and Hannah Montana as the background.

https://linuxreviews.org/Hannah_Montana_Linux

Distros

• are putting together a set of packages in repos.
• the repos are either close to upstream, or they backport security fixes. Everything else is not secure
• make working, secure, sometimes branded bundles including Desktop, some apps, some specific software
• the bundles get updated and if it is a point release, upgraded to a new set of packages. That is called a “Distro version”
• This ensures new features and security fixes
• the Distros care about bug reports, work with upstream, getting new contributors, packaging (bundling the packages, presets, libraries into a set with a name, handling dependencies etc.)
• Distros also often package and build their own Kernel or multiple ones. These kernels are general purpose most often, even though there is the kernel-hardened or Oracles “unbreakable kernel” (whatever that is). Also there is a lts Kernel that has backported security fixes, as well as other releases of the kernel like git (latest of everything)
• Distros take care of the versioning, so not every package is always the latest but tested to work with other packages.
• Distros also implement security systems like SELinux and Apparmor with matching configurations

So you see that is highly complex. So stay as close to upstream as possible to get the best experience. I think of the main distros as

• Debian + Ubuntu
• Fedora + the RHEL stuff or clones (Oracle, Alma, Rocky etc)
• Opensuse, SEL
• Arch
• Gentoo
• Alpine (busybox and musl, not real Gnu+Linux)
• NixOS
• GUIX
• ClearLinux
• Coreboot (yes that is a Linux distro)
• Slackware and other probably outdated projects
• small ones with different focus

All the others are either downstream modifications of these, or less known. Some Line ublue, EndeavorOS etc. also just take an upstream distro and change very little.

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How much snap you have/need. Or the stability I guess, but in my experience that hasn’t been a problem yet.

Fedora ships Btrfs and Zram by default.

Really nothing. Want to distribute hop without the hassle change your theme.