Hi everyone, I’m new to Linux and have been using it regularly for about two months now… I really like it because of all the possibilities it offers, and I’ve come to appreciate it a lot, anyway… I recently installed Fedora to learn more about Red Hat-based systems, and when I installed it, the boot manager I use didn’t appear because, as is often the case, the new system (Fedora) takes over the boot process… I’ve reconfigured the GRUB boot menu I use from CachyOS, but it turns out that of the three boot options that appear in the CachyOS GRUB menu, only the last one lets me access Fedora (the one labeled “Direct Boot”). The others—Fedora nvme0n1p3 (which is likely the ext4-formatted boot partition that Fedora created during installation) and Fedora nvme0n1p7 (which is the main BTFRS system)—don’t let me access Fedora… I get some error messages, and when I press a key on the keyboard, it takes me back to GRUB… Could someone help me set this up correctly, or maybe recommend that I just leave it as is, hahaha… I’m using a triple boot. (The error messages appearing on my screen basically say that it can’t find boot/vmlinuz-6.17.1-300.fc43.x86_64 y and the second error says that the kernel must be loaded)
It sounds like your GRUB-based CachyOS bootloader just isn’t designed to work with Fedora Linux. That’s not surprising.
You could (probably) script Fedora Linux to make the necessary changes to the CachyOS bootloader, but that is the sort of thing an advanced Linux user would do. There is nothing that would stop a newbie from doing it, but it might take you a bit longer to figure out, if you feel up to the challenge. 
To get you started, you can find the code that updates the boot loader snippets under /usr/lib/kernel/install.d. The files are ASCII text that you can edit with any plain text editor, but don’t edit the files there. If you do, your changes will be overwritten the next time the bootloader packages are updated. Instead, you should place your custom versions under /etc/kernel/install.d. The scripts must end in .install and they must be marked executable. They will run in the same order that the ls command shows them by default. If there are files by the same name under both /usr/lib/kernel/install.d and /etc/kernel/install.d, the latter will mask the former (i.e., only the one under /etc/kernel/install.d will run). I think what you want to do would require that you add a <whatever>.install script under /etc/kernel/install.d that would write new /<cachyos>/loader/entries/<whatever>.conf files whenever new kernels are installed in Fedora Linux. You would also need to add a mount entry in /etc/fstab so that the CachyOS booloader configuration snippets are mounted somewhere that your custom script can write.
Hope that helps
Oh, okay. Actually, I think I can boot into Fedora using the last or third entry in the GRUB list because I used Gemini to set everything up, and it recommended adding a menu entry to the /etc/grub.d/40_custom file in CachyOS
The code that Gemini generated, which I copied and pasted… Although I didn’t want to do it because it might not have worked… But I think thanks to that change, I can boot into Fedora using the third entry at the end of the list. Anyway, I’ll try what you recommend. Thanks so much for the suggestion.
Yes, that would be the boot entry that is currently working. Ideally, you would mount that partition somewhere under the Fedora Linux installation and write a script that would update that file with new menu entries whenever new kernels are added and remove old ones whenever old kernels are removed. If you don’t do that, then your Fedora Linux installation will be stuck with just the kernel version you added initially to that 40_custom file.