I accidently rebooted during pacman update and now my system dosen't show option to boot into endeavour os. Systemd just shows option to boot into firmware interface please help!

Oh my God. Flashbacks to the first time I fucked up my Arch installation like a decade ago. This is a solid run-through of a very character-building exercise 😂
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Pretty sure pacman runs mkinitcpio by itself, but I guess a second time for good measure couldn't hurt
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This is the best comment here.
I couldn't figure out how to mount /dev/sda1 and did pacman -Syu and then I mounted it once I figured it out now pacman says there is nothing to do.
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- Nevermind I ran a script that looped through all packages in the output of pacman -Qk and reinstalled them.

Not an endavour/arch user, but have been in similar situation.

What I did:

boot into live USB
mount the problematic rootfs
chroot to it
run pacman update

Is this work for every system? Like Fedora?
- Well , except the pacman part. The chroot part should certainly work.
- Yup. Mount your disk and chroot into it.
- You may need to adapt the last part to your needs.
  Example:
  for Fedora, you'd use dnf instead of pacman
  if your bootloader is broken, you'd want to run grub-install or grub-mkconfig
  if your initramfs doesn't recognize your new partition, you'd want to regenerate it with the current fstab or crypttab

Boot to a liveUSB of the distro of your choice, create a chroot to your install, and then run a Pacman update from there.

Googling “Arch rescue chroot” should point you in the right direction. Good luck!

Will this work from slax linux? I am sorry if I seem like I can't fix the issue myself seeing as you have given the resources for me to do so but what would be the exact steps to do that?
- I’ve never used Slax but it should, boot the liveUSB and enter terminal.
  The general process is:
  Boot to live Slax
  Mount your install
  Mount /proc, /sys, /dev
  Enter the chroot
  Check if networking is working
  Attempt to run commands in your chroot
  Exit the chroot
  Unmount everything
  Boot back to your install
  https://wiki.archlinux.org/title/Chroot
- It should work, afaik chroot always use the binaries of the system you chrooted, so you will be able to use pacman normally. I don´t remember if chroot will mount the efi partition by default, you can do this before go to chroot (again, I'm have some memory issues but I believe that /dev does not mount as well if you just use chroot, this is why arch have arch-chroot that mounts this kind of stuff but you can mount before so it should work).
  Assuming you are using systemd boot on efi partition (that is likelly if you have not changed the installer defaults), what I would do:
  On your live CD run sudo fdisk -l to get what is the efi partition, usually will be /dev/sdb1 since sda will be your usb, you should be able to see something like that.
  Then you will mount your endevour partition, in your situation should be sudo mount /dev/sdb2 /mnt/mydisk but check your fdisk command output.
  Now you will have to mount the efi partition sudo mount /dev/sdb1 /mnt/mydisk/efi
  Then you can use chroot /mnt/mydisk/ and proceed to do a pacman -Syu, this should trigger the post scripts that create the kernel images on the efi partition.
- You have to chroot into the mount point of your root partition
This one, I did it recently when my girlfriend uninstalled python that was necessary to run the process of creation the image of your kernel in the efi partition and happened the same thing, the update process removed the old images from efi partition but was not able to copy the new.

- i dunno why you gotta call me out like that...
- hey thanks man
for a simple solution: git + gnu stow. life changing.
something like:
~/dotfiles:

i3/ .config/ i3/ config kitty/ .config/ ...
sh

stow */
symlinks everything nicely :)
Why version them if you could simply have backups? I rarely want old config files...
- I don't know about you, but I tend to make lots of little changes to my setup all the time. Versioning makes it easy to roll back those individual changes, and to tell which change broke what. Sure, you could accomplish the same thing with backups, but versioning offers additional information with negligible cost.
  Why not stick them in a git repo?
- Versioning is one of those things that you don't realize you need until it's too late. Also, commits have messages that can be used to explain why something was done, which can be useful to store info without infodumping comments into your files.
- In my experience, it's shockingly useful. For example, maybe I want to try out a new set of Emacs packages. I can make a branch, mess around for a bit, and even keep that configuration for a week or two. Then if I find out it's not good, I can switch back
- I also rarely want old versions of my code, but I still use git. A very nice feature, besides the essential backup quality, is to synchronize dotfiles between machines and merge configs together if they diverge.

Been here before, but didn't bother asking for help. Just used a liveusb to grab what I needed and reinstalled. I need to learn how to chroot ...

Same, though I did try to chroot. Totally failed though. Luckily I had backups, which I learned never backed up properly.
The lessons I learned were to never trust a GUI and always make sure your backups are viable. I still have a copy of a duplicity backup form ~2020 that I hope one day to recover, if anythings in it.

To add to the other responses, after you recovered your stuff you could probably like moving to an immutable OS if you risk having power issues often, the transactions won't be applied until everything is done so if anything happens during a transaction you'll just remain at your last usable state

I had the same thought, but didn't want to sound insensitive.
Saying "Your fault, using Arch for something important is a bad idea, you should have made a backup before", while he fears all his important data is gone, would have been rude and very unhelpful.
But immutable distros solve these issues, yes. Since I switched to Silverblue I've never been more relaxed than ever. If something goes bad, I just select the old state and everything works, and updates never get applied incompletely like here.
- I'm sorry if I sounded insensitive, it wasn't my intention, just thought that since many others had already given a solution to the data and even OS recovery I could chip in to add something that they might find useful, if they don't mind switching away from Arch.
  I hope mine would be a reassuring suggestion more than anything
- It happens, that's less of a worry then
Note that this isn't about immutability but atomicity. Current immutable usually have that feature aswell but you don't need immutability to achieve it.
- Yeah you're right, however searching "linux distro with atomic updates" doesn't seem to turn up much, as you say, in most cases the two features happen to come together and the distros that have them are mostly known for the former

If nothing else, your files are all fine. You can mount your drive on a different system (like a live USB) and copy all your files.

There is nothing worth of freaking out in your situation. Your files shouldn't have been impacted at all.

Boot from LiveUSB and reinstall the packages you were updating, maybe reinstall grub too.

There are tons of guides for this in the Internet, like this one: https://www.jeremymorgan.com/tutorials/linux/how-to-reinstall-boot-loader-arch-linux/

Edit: since you probably use systemd-boot, as I can see from your post, obviously the grub part of my comment shouldn't be done. Replace those parts with systemd-boot reinstallation. Even better if pacman will update it, because there's probably some hook already to do things manually and you won't have to touch systemd-boot at all

Thank you all for offering advice. I did eventually get it working and repaired all the packages.

Other people will probably give you better answers, but I think the solution is quite easy: chroot and relauch the update.

Pretty much all the other answers are coming down to this, yeah.

Does Timeshift work on Arch? If so I would look into it, saved my ass a few times.

Timeshift definitely works on Arch (I use it before every update) but it isn't going to help OP if he hasnt taken an image already
- Yeah I know, that was a somewhat pretentious “backups” hint.

Boot a live Linux, chroot into your system, run pacman again and fix your systemd boot to include a fallback option for the next time this happens.

Assuming you have access to a secondary computer to make a LiveUSB

boot a live disk/USB on your PC and copy the data you want off. Then reinstall the OS.
If you haven’t got a drive you can move data to, from the live OS, partition your disk and move the data to the new partition CAUTION ON PARTITIONING

Can't help but I just did this myself. Was a fairly fresh install so I didn't lose anything other than have to reconfigure some stuff and install some things.

Buuuuut

What happened dto me was something crashed during the update and my computer went to a black screen. So I just left it for a bit to hopefully finish even without the display. Turned the computer off and my nvme was just gone. Ended up having to get a new one.

Did systemd or grub not even show up?
- Nope. I think the drive just died at a bad time honestly. I've had issues with it in the passed and the computer itself came from an e-scrap pile because the water pump for the CPU cooler was dead. Has worked great since swapping that out until the nvme died. Even after installing the new nvme and reinstalling EOS I couldnt see the old nvme.