Moving from CachyOS to NixOS

The plan keeps your disk as it is. NixOS goes into new btrfs subvolumes next to CachyOS's, and /home stays the same @home subvolume. Your files, game libraries, app data, ~/.ssh, Firefox profile and Steam settings never move, and CachyOS stays bootable until you remove it.

1. Before you start

• Push this repo (git push) and the latest Eiziv/philly. The install fetches philly from GitHub.
• Back up what you can't get back to another disk or machine, in case something goes wrong with the partition: ~/.ssh, ~/.config/keys (the imsh upload key), ~/.config/Signal (including config.json, which holds the key to Signal's database), ~/.config/mozilla, game saves that aren't in a cloud.
• Optional: a read-only snapshot of /home to roll back to: sudo btrfs subvolume snapshot -r /home /home/.snapshot-pre-nixos
• Get the NixOS installer (26.05, graphical or minimal) on a USB stick.

2. Install

Boot the installer. Wired network works out of the box.

sudo -i

# The btrfs partition's top level, to make NixOS's subvolumes.
mkdir -p /mnt/top
mount -o subvolid=5 /dev/nvme0n1p2 /mnt/top
btrfs subvolume create /mnt/top/@nixos
btrfs subvolume create /mnt/top/@nix
btrfs subvolume create /mnt/top/@nixos-log
umount /mnt/top

# Mount the new system the way hosts/desktop/hardware-configuration.nix does.
o=noatime,compress=zstd
mount -o subvol=@nixos,$o /dev/nvme0n1p2 /mnt
mkdir -p /mnt/{nix,var/log,home,boot}
mount -o subvol=@nix,$o /dev/nvme0n1p2 /mnt/nix
mount -o subvol=@nixos-log,$o /dev/nvme0n1p2 /mnt/var/log
mount -o subvol=@home,$o /dev/nvme0n1p2 /mnt/home
mount /dev/nvme0n1p1 /mnt/boot

# Compare the generated hardware config with the one in the repo; if the
# kernel modules or anything else differ, keep the generated lines.
nixos-generate-config --root /mnt --show-hardware-config > /tmp/hw.nix
diff /tmp/hw.nix /mnt/home/vizie/git/dot/hosts/desktop/hardware-configuration.nix

# philly is a private repo: sign in to GitHub for this root shell.
nix --extra-experimental-features 'nix-command flakes' shell nixpkgs#gh -c gh auth login
git config --global credential.helper '!gh auth git-credential'

# Install. The extra cache is where the CachyOS kernel comes from.
nixos-install --flake /mnt/home/vizie/git/dot#desktop \
  --option extra-substituters https://attic.xuyh0120.win/lantian \
  --option extra-trusted-public-keys lantian:EeAUQ+W+6r7EtwnmYjeVwx5kOGEBpjlBfPlzGlTNvHc=

# It asks for a root password at the end. Then set yours:
nixos-enter --root /mnt -c 'passwd vizie'
reboot

The NixOS boot menu (systemd-boot) becomes the default. CachyOS's Limine stays in the firmware boot menu (F8 at the ASUS logo), and also on the ESP, next to it.

3. First boot

• The login screen is the same gtkgreet over the blurred wallpaper.
• Home Manager replaces the old symlinks in ~/.config (pointing into ~/git/dotfiles) with its own. The old ones are kept beside them with a .pre-nixos suffix, so nothing is overwritten.
• Bolt installs from Flathub in the background on the first boot with network (systemctl status flatpak-bolt).
• "Command not found" suggestions need nix-index's database once: nix-index (takes a few minutes).
• Check the background programs: systemctl --user list-units --type=service.

4. What changed

What is different from CachyOS, all on purpose:

• Packages are generic x86-64 builds, apart from the kernel (Zen 4) and proton-cachyos (x86-64-v3). CachyOS rebuilds everything for v3/v4.
• Snapper root snapshots are replaced by NixOS generations: each rebuild is a boot menu entry. /home was never snapshotted on CachyOS either.
• GTK theme: set in modules/home/theming.nix. nwg-look can't save over those files any more.
• Signal theme is built into the package: edit, rebuild, config/signal/restart.
• Bitwarden autostart: the same XDG autostart entry, with NixOS's program name.
• fish: only CachyOS's default fish config was there, and your shell is zsh, so fish isn't set up.
• vkBasalt's ReShade shaders in ~/.config/vkBasalt/reshade-shaders stay in your home folder; they aren't in this repo (vkBasalt.conf doesn't use them).
• vkSumi: its config is kept, but the layer itself wasn't installed on CachyOS (there is no nixpkgs package for it either).

Unchanged on purpose, but worth knowing:

• kitty's font: kitty.conf asks for JetBrains Mono, which was never installed, so kitty has been showing Noto Sans Mono. That's kept. Add nerd-fonts.jetbrains-mono to fonts.packages in modules/nixos/desktop.nix to get the font the config names.
• The login screen opens a kitty terminal behind gtkgreet (hl.exec_cmd("kitty") in config/greetd/hyprland.lua), running as the greeter user. If gtkgreet ever crashed, that shell would be on screen before anyone logs in. If it's a leftover from debugging, delete that line.
• ~/.ssh/authorized_keys has a key labelled chris@echoz.io, which lets that key log in as you over SSH (port 22 is open, as before). It stays in your home folder.
• mDNS (port 5353) stays closed in the firewall, as ufw had it.
• sched-ext schedulers stay off, as before. See the comment in modules/nixos/cachyos.nix to try one.

5. Going back to CachyOS

It still boots (firmware boot menu). Its ~/.config links now point into NixOS's store, which CachyOS doesn't have, so rerun the old repo's script there: ~/git/dotfiles/symlink.sh.

6. Once you're happy

Remove CachyOS: boot NixOS, mount the top level, delete its subvolumes (@, @root, @srv, @cache, @tmp, @log), then its kernels and Limine files from the ESP (look first: sudo ls -R /boot; NixOS's own files are /boot/EFI/nixos, /boot/EFI/systemd, /boot/EFI/BOOT and /boot/loader), and its firmware boot entry (efibootmgr).