# 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.

```bash
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`).