[[ch-faq]]
== Frequently Asked Questions (FAQ)

=== Why is there a collision error when switching generation?

Home Manager currently installs packages into the user environment, precisely as if the packages were installed through `nix-env --install`. This means that you will get a collision error if your Home Manager configuration attempts to install a package that you already have installed manually, that is, packages that shows up when you run `nix-env --query`.

For example, imagine you have the `hello` package installed in your environment

[source,console]
----
$ nix-env --query
hello-2.10
----

and your Home Manager configuration contains

[source,nix]
----
home.packages = [ pkgs.hello ];
----

Then attempting to switch to this configuration will result in an error similar to

[source,console]
----
$ home-manager switch
these derivations will be built:
  /nix/store/xg69wsnd1rp8xgs9qfsjal017nf0ldhm-home-manager-path.drv
[…]
Activating installPackages
replacing old ‘home-manager-path’
installing ‘home-manager-path’
building path(s) ‘/nix/store/b5c0asjz9f06l52l9812w6k39ifr49jj-user-environment’
Wide character in die at /nix/store/64jc9gd2rkbgdb4yjx3nrgc91bpjj5ky-buildenv.pl line 79.
collision between ‘/nix/store/fmwa4axzghz11cnln5absh31nbhs9lq1-home-manager-path/bin/hello’ and ‘/nix/store/c2wyl8b9p4afivpcz8jplc9kis8rj36d-hello-2.10/bin/hello’; use ‘nix-env --set-flag priority NUMBER PKGNAME’ to change the priority of one of the conflicting packages
builder for ‘/nix/store/b37x3s7pzxbasfqhaca5dqbf3pjjw0ip-user-environment.drv’ failed with exit code 2
error: build of ‘/nix/store/b37x3s7pzxbasfqhaca5dqbf3pjjw0ip-user-environment.drv’ failed
----

The solution is typically to uninstall the package from the environment using `nix-env --uninstall` and reattempt the Home Manager generation switch.

You could also opt to unistall _all_ of the packages from your profile with `nix-env --uninstall '*'`.

=== Why are the session variables not set?
:foreign-env: https://github.com/oh-my-fish/plugin-foreign-env

Home Manager is only able to set session variables automatically if it manages your Bash, Z shell, or fish shell configuration. To enable such management you use <<opt-programs.bash.enable>>, <<opt-programs.zsh.enable>>, or <<opt-programs.fish.enable>>.

If you don't want to let Home Manager manage your shell then you will have to manually source the `~/.nix-profile/etc/profile.d/hm-session-vars.sh` file in an appropriate way. In Bash and Z shell this can be done by adding

[source,bash]
----
. "$HOME/.nix-profile/etc/profile.d/hm-session-vars.sh"
----

to your `.profile` and `.zshrc` files, respectively. The `hm-session-vars.sh` file should work in most Bourne-like shells. For fish shell, it is possible to source it using {foreign-env}[the foreign-env plugin]

[source,bash]
----
fenv source "$HOME/.nix-profile/etc/profile.d/hm-session-vars.sh" > /dev/null
----

=== How to set up a configuration for multiple users/machines?
:post-your-homenix: https://www.reddit.com/r/NixOS/comments/9bb9h9/post_your_homemanager_homenix_file/

A typical way to prepare a repository of configurations for multiple logins and machines is to prepare one "top-level" file for each unique combination.

For example, if you have two machines, called "kronos" and "rhea" on which you want to configure your user "jane" then you could create the files

- `kronos-jane.nix`,
- `rhea-jane.nix`, and
- `common.nix`

in your repository.
On the kronos and rhea machines you can then make
`~jane/.config/home-manager/home.nix`
be a symbolic link to the corresponding file in your configuration repository.

The `kronos-jane.nix` and `rhea-jane.nix` files follow the format

[source,nix]
----
{ ... }:

{
  imports = [ ./common.nix ];

  # Various options that are specific for this machine/user.
}
----

while the `common.nix` file contains configuration shared across the two logins. Of course, instead of just a single `common.nix` file you can have multiple ones, even one per program or service.

You can get some inspiration from the {post-your-homenix}[Post your home-manager home.nix file!] Reddit thread.

=== Why do I get an error message about `ca.desrt.dconf` or `dconf.service`?

You are most likely trying to configure something that uses dconf
but the DBus session is not aware of the dconf service.
The full error you might get is

----
error: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name ca.desrt.dconf was not provided by any .service files
----

or

----
error: GDBus.Error:org.freedesktop.systemd1.NoSuchUnit: Unit dconf.service not found.
----

The solution on NixOS is to add

[source,nix]
programs.dconf.enable = true;

to your system configuration.

=== How do I install packages from Nixpkgs unstable?

If you are using a stable version of Nixpkgs but would like to install some particular packages from Nixpkgs unstable – or some other channel – then you can import the unstable Nixpkgs and refer to its packages within your configuration. Something like

[source,nix]
----
{ pkgs, config, ... }:

let

  pkgsUnstable = import <nixpkgs-unstable> {};

in

{
  home.packages = [
    pkgsUnstable.foo
  ];

  # …
}
----

should work provided you have a Nix channel called `nixpkgs-unstable`.

You can add the `nixpkgs-unstable` channel by running

[source,console]
----
$ nix-channel --add https://nixos.org/channels/nixpkgs-unstable nixpkgs-unstable
$ nix-channel --update
----

Note, the package will not be affected by any package overrides, overlays, etc.

=== How do I override the package used by a module?
:nixpkgs-overlays: https://nixos.org/nixpkgs/manual/#chap-overlays

By default Home Manager will install the package provided by your chosen `nixpkgs` channel but occasionally you might end up needing to change this package. This can typically be done in two ways.

1. If the module provides a `package` option, such as `programs.beets.package`, then this is the recommended way to perform the override. For example,
+
[source,nix]
programs.beets.package = pkgs.beets.override { enableCheck = true; };

2. If no `package` option is available then you can typically override the relevant package using an {nixpkgs-overlays}[overlay].
+
For example, if you want to use the `programs.skim` module but use the `skim` package from Nixpkgs unstable, then a configuration like
+
[source,nix]
----
{ pkgs, config, ... }:

let

  pkgsUnstable = import <nixpkgs-unstable> {};

in

{
  programs.skim.enable = true;

  nixpkgs.overlays = [
    (self: super: {
      skim = pkgsUnstable.skim;
    })
  ];

  # …
}
----
+
should work OK.
