depot/third_party/nixpkgs/nixos/doc/manual/configuration/declarative-packages.section.md

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# 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, simply remove it from
[](#opt-environment.systemPackages) and run `nixos-rebuild switch`.
```{=docbook}
<xi:include href="customizing-packages.section.xml" />
<xi:include href="adding-custom-packages.section.xml" />
```