Nixos useful stuff

Managing system configurations

• List system configurations: nix-env --list-generations --profile /nix/var/nix/profiles/system
• Creating a new configuration:
  • Simulate building the configuration (as well as syntax checking): nixos-rebuild dry-build
  • Build and apply new configuration: nixos-rebuild switch
  • Update packages and build new configuration: nixos-rebuild switch --upgrade
• Rollback to previous configuration: nixos-rebuild switch --rollback
  • Currently rolling back to another configuration than the previous one is not supported, but multiple rollback commands can be executed one after the other.

Managing nix channels

Nix channels are where packages are published, they are equivalent of the "releases" in other distributions.

The current stable channel can generally be found by going on nixos.org and looking for the "announcements" page, or by going on the channels directory and looking for the highest version number.

It is generally safe to switch between channels, with the exception that after switching to a newer channel which changes the database schema, it might then be impossible to roll back to a previous channel (which should not be needed anyway).

• Show the current channel: nix-channel --list | grep nixos
• Switch to a different channel: nix-channel --add {channel url, e.g. https://channels.nixos.org/nixos-1.2} nixos
• Update packages from the current channel: nix-channel --update, note that the new packages will not be used until the next nixos-rebuild. As such, nixos-rebuild switch --upgrade might be more useful.

Freeing up disk space

Nixos does not delete old versions of packages that are used by previous system configurations: they are kept so that a rollback is always possible. This can make the disk usage go up quickly, so commands exist to delete old configurations and optimise disk usage if needed.

• Delete all system configurations except the currently in use: nix-collect-garbage -d
  • Only delete all system configuration older than a certain age: nix-collect-garbage --delete-older-than {time ago (e.g. 2d)}
  • Accepts a --dry-run flag that simulates the deletions but without actually deleting anything
• Delete packages not used by any system configuration, without deleting system configurations themselves: nix-collect-garbage
• Optimise disk usage by hard-linking together files that appear in different packages/version with exactly the same content: nix-store --optimise

Update packages without using too much disk space

Putting all the above together, updating packages keeping only the current configuration as backup:

• nix-collect-garbage -d to remove all system configurations except the one currently in use
• nixos-rebuild switch --upgrade to upgrade all packages into a new system configuration
• nix-store --optimise to reduce disk usage of new configuration

Testing packages

Any user can create a temporary shell with some additional packages installed by running nix-shell -p {packages}. The packages do not persist after the shell has been closed.

User management

See the Nixos manual.

In order to manage users in a declarative-only way:

• Set users.mutableUsers = false;: users will now only exist if declared in the configuration. Imperative commands for managing users and groups, such as useradd, are no longer available.
• Configuration example for adding a user name alice:

users.users.alice = {
    isNormalUser = true;
    description = "Alice Foobar";
    extraGroups = [
        "wheel"
        "networkmanager"
    ];
    openssh.authorizedKeys.keys = [
        "ssh-dss AAAAB3Nza... alice@foobar"
    ];
    hashedPassword = "$6$DICyZPtOZcWUIULB$hOaPtCKeTJUxHeFDyBUI9nGeX5LCLAqvZOEtAZQTFbLuum5upMt10.9LtGmY01Fc7w.Wi2OFwA5YHyJJC18MM/";
};

• The hashed password can be obtained by running mkpasswd -m sha-512 (included in the mkpasswd package)