Hey! Great questions.

It seems like what you’re asking about are more what I’d think of as components of an a Linux “system” or “install.”

First off, it’s definitely worth saying that there aren’t a lot of rules that would apply to “all” Linux systems. Linux is huge in embedded systems, for instance, and it’s not terribly uncommon to find embedded Linux systems with no shells, no DE/WM, and no package manager. (I’m not 100% sure a filesystem is technically necessary. If it is, you can probably get away with something that’s… kinda sorta a filesystem. But I’ll get to that.)

Also, it’s very common to find “headless” systems without any graphical system whatsoever. Just text-mode. These are usually either servers that are intended to be interacted with over a network or embedded systems without screens. But there are a lot of them in the wild.

There’s also Linux From Scratch. You can decide for yourself whether it qualifies as a “distribution”, but it’s a way of running Linux on (typically) a PC (including things like DE’s) without a package manager.

All that I’d say are truly necessary for all Linux systems are 1) a bootloader, 2) a Linux kernel, 3) A PID 1 process which may or may not be an init system. (The “PID 1 process” is just the first process that is run by the Linux kernel after the kernel starts.)

The “bunch of default applications and daemons” feels like three or four different items to me:

• Systemd is an example of an “init system.” There are several available. OpenRC, Runit, etc. It’s main job is to manage/supervise the daemons. Ensure they’re running when they’re supposed to be. (I’ll mention quickly here that Systemd has a lot more functionality built in than just for managing daemons and gets a bad rap for it. Network configuration, cron, dbus for communication between processes, etc. But it still probably qualifies as “an init system.” Just not just an init system.)
• Daemons are programs that kindof run in the background and handle various things.
• Coreutils are probably something I’d list separately from user applications. Coreutils are mostly for interacting with low-ish level things. Formatting filesystems. Basic shell commands. Things like that.
• User applications are the programs that you run on demand and interact with. Terminal emulators, browsers compilers, things like that. (I’ll admit the line between coreutils and user applications might be a little fuzzy.)

As for your question about graphical systems, X11 and Wayland work a little differently. X11 is a graphical system that technically can be run without a desktop environment or window manager, but it’s pretty limited without one. The DE/WM runs as one or more separate processes communicating with X11 to add functionality like a taskbar, window decorations, the ability to have two or more separate windows and move them around and switch between them, etc. A Wayland “compositor” is generally the same process handling everything X11 would handle plus everything the DE/WM would handle. (Except for the Weston compositor that uses different “shells” for DE/WM kind of functionality.)

As far as things that might be missing from your list, I’ll mention the initrd/initramfs. Typically, the way things are done, when the Linux kernel is first loaded by the bootloader, it an “initial ramdisk” is also loaded. Basically, it creates a filesystem that lives only in ram and populates it from an archive file called an “initramfs”. (“initrd” is the older way to do the same thing.) Sometimes the initramfs is bundled into the same file as the kernel itself. But, that initial ramdisk provides an initial set of tools necessary to load the “main” root filesystem. The initramfs can also do some cool things like handling full disk encryption.

So, the whole list of typical components for a PC-installed Linux system to be interacted with directly as I’d personally construct it would be something like:

• Bootloader
• Linux Kernel
• Initramfs
• Filesystem(s)
• Shell(s)
• Init system
• Daemons
• Coreutils
• Graphical system (X11 or Wayland potentially with a DE/WM.)
• User applications
• Package manager

But techinically, you could have a functional, working “Linux system” with just:

• Bootloader
• Linux Kernel
• Either a nonvolatile filesystem or initrd/initramfs (and I’m not 100% sure this one is even strictly necessary)
• A PID 1 process

Hopefully this all helps and answers your questions! Never stop learning. :D

I would say, that from most important to least important components are:

1. kernel
2. init system (systemd, openrc, runit…)
3. C library (glibc, musl)
4. filesystem
5. coreutils
6. shell
7. bootloader
8. package manager
9. x11/Wayland (if any)
10. sound system (if any)
11. WM (if any)
12. DE (if any)

I think you would also need an initial run process such as systemd or the sysV runlevels.

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Also:

• init system, without which you’d be left with only one program running at a time
• some programs are written in interpreted language (e.g python, shell, perl), so the interpreter would also be required
• C library, without which none of the above would function (yes, even if all the programs are statically compiled, it still has that library included with each executable)
• this one is not necessary for the runtime, but is needed for creating a working system: toolchain – preprocessor, compiler, linker, assembler – all the stuff for transforming the source code into executables

Another comment mentioned Linux From Scratch, I’d totally recommend that, but it would take so much of your time manually building stuff (which is why it is so educational). If you don’t have the time, you may want to opt with Gentoo instead.

You pretty much got it, except for the fifth point.

A desktop environment (“DE”) is separate from the compositor (X11 or Wayland), but can’t exist without it.

At the end of the day, a DE is really just a “window manager” with a bunch of bundled applications, like taskbars/panels, a file manager, an app menu, etc. It’s as minimal or as feature rich as you want it to be.

The window manager dictates what to draw on the screen and where, but the compositor is what actually does the work. One is kind of useless without the other.

Hopefully that makes sense, I’m not a rocket surgeon.

The major parts of any distro are just bootloader, kernel, init, shell, and package system. The filesystem isn’t “part” of the distro, it’s just an abstraction layer to work with data on the drive, and should be considered independent of the packaged distribution itself.

With the above, you can run the basics of Linux on a device. The DE is not needed, and included packages and libraries are at the discretion of the maintainers. The included choices of all the above is the only thing that differentiates each distro.

If it helps in your understanding at all, back in the day, in order to install something like Slackware, you had to build each layer of these things manually like so: format and partition disk from disk in DOS, copy bootloader to newly partitioned HDD, boot to single user mode, compile kernel, add entries to bootloader, reboot from disk to Linux kernel, open TTY, set user and shell, reboot again, compile DE, set init level and basic services, reboot to DE, and then you had a Desktop.

Systemd has gone way and beyond what was supposed to be a replacement for init.rc.

Most important thing… not ALL Linux distros include systemd as the default init system. That’s the beauty of Linux (and POSIX in general), you can choose.

A biggie you miss is the toolchain: the compiler/binutils/linux-headers/libc/libstdc++ combination. The libc and usually libstdc++ are key components of any install. The other parts usually don’t make it to non-dev-desktops, but the distro couldn’t be made without them, so they’re virtually always available as packages.

Only exception is if the entire distro is cross-compiled or it’s made exclusively for containers, but those kinds of special distros break every rule imaginable anyway. Some might not even ship a bootloader or a Linux kernel by themselves.

The list is generally correct but these days, systemd has made quite an impact also. If a distribution uses systemd, it has one software to handle everything from booting (instead of grub), handling start and status of all system services etc. Its probably the largest change to the Linux ecosystem in a long time.

X11 and Wayland are desktop protocols, so things like desktop environments and window managers depends on one or them to be installed. Without them, you don’t get any graphics except for the console. It’s all built on top of one of those.

It’s easier to think about Linux on the context of what an individual application needs to run. Pretty much everything you do will have these components.

• configuration
• an executable
• a communication mechanism (dbus, networking, web server, etc)
• something that decides if the application runs or not (systemd, monit, docker/docker compose, kubernetes scheduler, or you as the user)
• a way of accepting input (keyboard and mouse, web requests, database queries, etc)
• a way of delivering an output (logging to unique log files, through syslog, or to stdout/stderr, showing something on a screen, playing a sound, returning a message to the client, etc)
• storage (optional)
• some cpu and memory capacity

That’s really it. If something isn’t working, it’s pretty much exclusively going to fall into one of those categories. What that means is going to vary significantly from app to app but understanding this is how literally everything works makes the troubleshooting process a lot easier.

I don’t know how helpful it is to split stuff out like that. Especially grouping so many things under “default applications and daemons”, which is most of what a desktop distro is. Also depends largely on a PC vs server setup.

should list an init system as its own bullet, which others have mentioned.

“one or more shells” doesn’t mean that much. Yes, every distro includes one but the only difference between a terminal and any other application is that a terminal needs to be able to escalate to root privileges. You can think of it as just another default (but special) application. A lot of stuff that people think about when they think of Linux distros is just various clever mechanisms for supporting the terminal shell. Like the PATH environment variable. If you are using actual desktop applications other than a terminal, there isn’t any interaction with the terminal shell application.

There’s also fwupd, for updating firmware (your hardware is gonna be running out of date/buggy/insecure code if you don’t have this).

The dbus daemon falls under the “daemons” bullet but it’s pretty important, like wayland/x11 it’s another IPC mechanism you need for programs to work correctly.

There’s also the sound system. PipeWire is the modern one that implements the interfaces of various other sound systems so existing applications work with it. https://pipewire.org/ (PipeWire also has its own IPC protocol like dbus/wayland/x11).

flatpak, snap, distrobox, toolbox, docker, podman, etc. for running sandboxed PC/server applications. I assume there are some programs that are flatpak-only these days.

gsettings/dconf for Windows-registry like config that many programs use.

There’s also plugging in an implementation of the glibc Name Service Switch, which allows libc to use a mechanism other than /etc/passwd and related files for user accounts, internet service names, DNS resolution, etc. . systemd can provide NSS implementations using its own user account mechanism.

Systemd.

You need the “Userland” programs. Basic things like ls, cp, cat, etc. Usually it’s GNU core tools, but there’s also BusyBox or BSD equivalents.

Package management is optional according to Slackware and Linux From Scratch.

A key part you left out is the init scripts. Without those you don’t have the fundamental under the hood flavor of a distribution.