General-purpose operating system (Linux, BSD, Plan 9, etc)
Special-purpose operating system (RTOS)
Vendor-purpose operating system (iOS, Android)
I had a very good experience with UEFI on multiple machines, without any bootloaders, by with installing the kernel into the esp/EFI/BOOT/BOOTX64.EFI (the special removable drive path), and bundling a custom made initramfs into the kernel itself.
See: the CONFIG_INITRAMFS_SOURCE kernel config, and https://wiki.gentoo.org/wiki/Custom_Initramfs for making your own initramfs.
I had static installs of busybox, lvm, and cryptsetup in the initramfs, and a tiny init script.
The init script:
mount -t proc none /proc
mount -t sysfs none /sys
echo "Welcome to Red-Moon..."
cryptsetup open /dev/sda3 sda3_crypt || busybox sh
lvm lvchange -a ly red-moon || busybox sh
mount -o ro /dev/mapper/red--moon-root /mnt/root || busybox sh
exec switch_root /mnt/root /sbin/init || busybox sh
@qrsbrwn how do you let them break things without causing lasting breakage?
plan9 is used in commercial network storage solutions, lighting systems, cell phone towers, and more, and used to be used in large scale compute clusters
even haiku, which is still in beta and is arguably even less known, has found commercial use as an embedded os for high end audio mastering equipment
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