NixOS On Raspberry Pi: Configuration Changes Not Saving?

by Alex Johnson 57 views

Are you wrestling with NixOS on your Raspberry Pi and finding that your configuration changes aren't sticking after a reboot? You're not alone! This is a common hiccup, and we'll dive into the likely causes and how to fix them. Based on the provided scenario, we'll troubleshoot why the changes made in your flake.nix file, specifically the installation of the cowsay package, seem to vanish after a reboot. We'll explore the critical aspects of NixOS configuration and how they relate to the Raspberry Pi to ensure your changes are saved and applied correctly. Let's get started!

Understanding the Problem: Why Configurations Disappear

The core issue lies in how NixOS manages its system configurations. Unlike traditional Linux distributions that store configurations in files directly on the root filesystem, NixOS uses a declarative, immutable approach. This means that the system's state is defined in a series of Nix expressions, primarily in your flake.nix file. These expressions are then used to build a complete system configuration, which is then made active through a process known as a switch. When you make changes to your flake.nix and run nixos-rebuild switch, you're essentially telling NixOS to build a new system configuration based on the updated definitions and activate it.

One of the most common pitfalls is that the configuration isn't fully applied or it's not being properly written to the boot partition. This could happen if the system isn't correctly mounting the root filesystem, if the configurations aren't being built with the correct options, or if there's an issue during the switch operation. When a reboot happens, the system may revert to a previous configuration or fail to load the latest changes. This can be frustrating, especially when you're trying to customize your Raspberry Pi setup. The crucial element here is understanding how NixOS handles these configurations, how it stores them, and how it applies them during a system boot.

The Role of flake.nix and nixos-rebuild switch

Your flake.nix file is your source of truth for your system's configuration. It's where you define everything, from installed packages to network settings and user accounts. The nixos-rebuild switch command is the key to applying these configurations. When you run this command, NixOS does several things:

  1. Evaluates the flake.nix: NixOS reads your flake.nix file and evaluates the Nix expressions within it. This process determines the desired system state.
  2. Builds the System Configuration: Based on the evaluated expressions, NixOS builds a complete system configuration, including all the necessary packages and settings.
  3. Activates the Configuration: The switch part of the command makes this new configuration active. This typically involves updating the system's bootloader, creating new generations of the system, and ensuring that the new configuration takes effect on the next boot.

If any of these steps fail or are incomplete, your configuration changes may not persist. This is why it's crucial to ensure that the flake.nix file is correct, the build process completes without errors, and the switch command runs successfully. Let's delve deeper into how to troubleshoot this process.

Troubleshooting and Solutions: Ensuring Your Configuration Persists

Given the provided steps, the issue lies in the configuration not being properly activated or retained across reboots. Here's a breakdown of the likely causes and how to address them:

1. Verify the Boot Partition and Root Filesystem

The first thing to check is that the root filesystem is correctly mounted and that the boot partition is accessible. The root filesystem contains all the system files, including the Nix store, and must be correctly mounted for your configurations to persist. The boot partition contains the bootloader and the system's kernel, and it must be set up properly for the system to boot and load the correct configuration. Ensure your fileSystems."/" is correctly pointing to the root partition's UUID, especially on the Raspberry Pi where these settings can be sensitive. Double-check this by inspecting your flake.nix and confirming the UUID matches the actual partition on your SD card.

2. Confirm the nixos-rebuild switch Process

Pay close attention to the output of the nixos-rebuild switch command. Any errors during the build or activation process can lead to configuration issues. Make sure the command completes successfully and without errors. Check for any warnings related to the bootloader configuration or filesystem mounts. If any errors occur, carefully review the error messages and address the issues accordingly.

3. Check the Systemd Service for NixOS

NixOS utilizes systemd to manage its services. You can investigate the status of the NixOS systemd service to see if any errors are occurring during the boot process or during the activation of new configurations. To check the service status, you can use the command sudo systemctl status nixos-systemd-rebuild.service. This command provides detailed logs, and any errors present in the logs will pinpoint the exact issue. Reviewing this log can give you clues about why your configuration isn't being applied correctly. Examine the log for any failures during the activation phase or any issues related to mounting the root filesystem or activating the new configuration.

4. Examine the boot.loader.grub.device Setting

While not directly applicable to a Raspberry Pi with an extlinux bootloader, the general concept applies to how the bootloader is configured. On a Raspberry Pi, ensure the boot partition is correctly identified. Make sure that the bootloader (typically extlinux) is correctly configured to boot from the right partition and that it knows where the root filesystem is located. Incorrect bootloader configurations can lead to the system booting from an older configuration, thus negating your changes.

5. Review Flake Inputs and Outputs

Ensure that you correctly specify the inputs and outputs within your flake.nix. Incorrect inputs may lead to the system using an older version of your configuration or even failing to build your system configuration correctly. The outputs section should correctly define your NixOS configuration.

Step-by-Step Fix and Explanation

Let's refine the troubleshooting steps based on the provided scenario. Here’s a breakdown of the steps to remedy the issue:

  1. Verify the flake.nix: Double-check your flake.nix file to ensure everything is correctly configured, especially the root filesystem UUID and package definitions. Also, make sure there are no typos or syntax errors.
  2. nixos-rebuild switch with Careful Observation: Run sudo nixos-rebuild switch --flake /etc/nixos#thermidor. Monitor the output closely for any error messages. Errors during the build or switch process are a primary cause of configuration failures. If there are any errors, address them before proceeding.
  3. Reboot and Verify: After a successful nixos-rebuild switch, reboot your system using sudo reboot. Once the system reboots, log in and verify the configuration changes. Run cowsay hi to confirm that the cowsay package is installed and working.
  4. Check System Logs: If the changes still aren't persisting, examine the system logs for more clues. Use commands like journalctl -b to view the boot logs. Look for any errors related to the boot process, filesystem mounts, or NixOS configuration activation. Also, verify the status of the NixOS systemd service using sudo systemctl status nixos-systemd-rebuild.service. Any errors in the logs will give you a clear direction to fix the issue.

Addressing the cowsay Issue

The reason the cowsay command is missing after the reboot is likely due to the configuration not being applied correctly. Ensure that the package is correctly defined in your flake.nix file, and that the nixos-rebuild switch process is completing without errors. After a successful nixos-rebuild switch and reboot, cowsay hi should work.

Advanced Troubleshooting: Digging Deeper

If the above steps don't resolve the issue, you may need to delve deeper into advanced troubleshooting techniques:

Inspecting the Nix Store

The Nix store is where all your packages are installed. You can inspect it to see if cowsay is actually present in the store and if the symlinks are correctly set up. You can use the nix-store command-line tool to list the packages. For example, you can try `nix-store -q --requisites /nix/store/$(nix-instantiate --eval -E 'with import {}; writeText