Declarative Package Management in NixOS

# Declarative Package Management {#sec-declarative-package-mgmt} With declarative package management, you specify which packages you want on your system by setting the option [](#opt-environment.systemPackages). For instance, adding the following line to `configuration.nix` enables the Mozilla Thunderbird email application: ```nix { environment.systemPackages = [ pkgs.thunderbird ]; } ``` The effect of this specification is that the Thunderbird package from Nixpkgs will be built or downloaded as part of the system when you run `nixos-rebuild switch`. ::: {.note} Some packages require additional global configuration such as D-Bus or systemd service registration so adding them to [](#opt-environment.systemPackages) might not be sufficient. You are advised to check the [list of options](#ch-options) whether a NixOS module for the package does not exist. ::: You can get a list of the available packages as follows: ```ShellSession $ nix-env -qaP '*' --description nixos.firefox firefox-23.0 Mozilla Firefox - the browser, reloaded ... ``` The first column in the output is the *attribute name*, such as `nixos.thunderbird`. Note: the `nixos` prefix tells us that we want to get the package from the `nixos` channel and works only in CLI tools. In declarative configuration, use `pkgs` prefix (variable). To "uninstall" a package, remove it from [](#opt-environment.systemPackages) and run `nixos-rebuild switch`. ```{=include=} sections customizing-packages.section.md adding-custom-packages.section.md ```

This section explains declarative package management in NixOS, where users specify desired packages by setting the `environment.systemPackages` option in `configuration.nix`. For instance, adding `pkgs.thunderbird` will include the Thunderbird email client. Running `nixos-rebuild switch` then builds or downloads these packages. A crucial note advises checking the list of options for packages requiring additional global configuration (e.g., D-Bus, systemd services). Users can list available packages using `nix-env -qaP '*' --description`, noting that `nixos` is a CLI prefix while `pkgs` is used in declarative configuration. To uninstall a package, it must be removed from `environment.systemPackages`, followed by `nixos-rebuild switch`.