# My first overlay, for emacs
**URL:** https://discourse.nixos.org/t/my-first-overlay-for-emacs/7570
**Category:** Uncategorized
**Created:** [June 8, 2020, 3:55pm UTC](https://discourse.nixos.org/t/my-first-overlay-for-emacs/7570 "2020-06-08T15:55:08Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@JeffreyBenjaminBrown](https://discourse.nixos.org/u/JeffreyBenjaminBrown)
#### Post date: [June 8, 2020, 3:55pm UTC](https://discourse.nixos.org/t/my-first-overlay-for-emacs/7570/1 "2020-06-08T15:55:08Z")
In the course of [trying to install org-roam for Emacs](([emacsPackages.org-roam: init at 0.1.2 by rasendubi · Pull Request #80280 · NixOS/nixpkgs · GitHub](https://github.com/NixOS/nixpkgs/pull/80280)), I’ve realized that I have to understand overlays.
# Questions about the [NixOS wiki page on overlays](https://nixos.wiki/wiki/Overlays)
The first section, “Applying overlays manually”, includes just the code snippet `import { overlays = [overlay1 overlay2]; }`. Where would one use that? In the `configuration.nix` file? From the nix repl?
The next section, “Applying overlays automatically”, includes “user level” and “system level” subsections. I’m the only user of my system, and the only way I ever build is via sudo, so I’m interested in the system level, right?
If so, in the “system level” subsection it says, “If you want your overlays to be accessible by nix tools and also in the system-wide configuration, add nixpkgs-overlays to your NIX\_PATH:”
```auto
NIX_PATH="$NIX_PATH:nixpkgs-overlays=/etc/nixos/overlays"
```
That looks like a bash command. Is there some bash file that gets loaded every time I start the OS, to which I should add that line? I don’t see `.bashrc` or `.bash_profile` in my home directory.
Then there’s a third subsection called “Using nixpkgs.overlays from configuration.nix as in your NIX\_PATH”. Is that an alternative to the “system level” section earlier? It’s the only one that mentions the `configuration.nix` file.
My `.nix` files do not mention `nix.nixPath`, so it seems like I should add this:
```auto
nix.nixPath =
# Prepend default nixPath values.
options.nix.nixPath.default ++
# Append our nixpkgs-overlays.
["nixpkgs-overlays=/etc/nixos/overlays-compat/"]
;
```
and then put this code into `/etc/nixos/overlays-compat/overlays.nix`:
```auto
self: super:
with super.lib;
let
# Load the system config and get the `nixpkgs.overlays` option
overlays = (import { }).config.nixpkgs.overlays;
in
# Apply all overlays to the input of the current "main" overlay
foldl' (flip extends) (_: super) overlays self
```
# Questions about using `emacs-overlay`
The [emacs-overlay](https://github.com/nix-community/emacs-overlay#latest-master-each-rebuild) README says that “probably the most convenient way to pull in this overlay is by just fetching the tarball of latest master on rebuild”, and provides the following code for doing so:
```auto
{
nixpkgs.overlays = [
(import (builtins.fetchTarball {
url = https://github.com/nix-community/emacs-overlay/archive/master.tar.gz;
}))
];
}
```
Where does that code go? Should I create an `emacs-overlay.nix` file in `/etc/nixos/overlays-compat/`, containing that code and nothing else? Will the `foldl' (flip ...` line in `overlays.nix` find and incorporate it?
---
### Author:  [@jtojnar](https://discourse.nixos.org/u/jtojnar)
#### Post date: [June 8, 2020, 4:52pm UTC](https://discourse.nixos.org/t/my-first-overlay-for-emacs/7570/2 "2020-06-08T16:52:57Z")
> [@JeffreyBenjaminBrown](#):
>
> Questions about the NixOS wiki page on overlays
Would be nice to [link](https://nixos.wiki/wiki/Overlays) it here.
> [@JeffreyBenjaminBrown](#):
>
> The first section, “Applying overlays manually”, includes just the code snippet `import { overlays = [overlay1 overlay2]; }` . Where would one use that? In the `configuration.nix` file? From the nix repl?
Anywhere you would use `import { }`. Most prominently in `shell.nix` but it is a Nix expression so it can be used anywhere in Nix.
> [@JeffreyBenjaminBrown](#):
>
> The next section, “Applying overlays automatically”, includes “user level” and “system level” subsections. I’m the only user of my system, and the only way I ever build is via sudo, so I’m interested in the system level, right?
NixOS configurations built by `sudo nixos-rebuild` will only respect overlays set by [`nixpkgs.overlays`](https://nixos.org/nixos/options.html#nixpkgs.overlays), globally through `NIX_PATH` or locally for the root user (see [nixpkgs/pkgs/top-level/impure.nix at d903f1166b967c6a10de5fcc8cec1798be5b55ff · NixOS/nixpkgs · GitHub](https://github.com/NixOS/nixpkgs/blob/d903f1166b967c6a10de5fcc8cec1798be5b55ff/pkgs/top-level/impure.nix#L44-L57)).
> [@JeffreyBenjaminBrown](#):
>
> If so, in the “system level” subsection it says, “If you want your overlays to be accessible by nix tools and also in the system-wide configuration, add nixpkgs-overlays to your NIX\_PATH:”
>
> ```auto
> NIX_PATH="$NIX_PATH:nixpkgs-overlays=/etc/nixos/overlays"
>
> ```
>
> That looks like a bash command. Is there some bash file that gets loaded every time I start the OS, to which I should add that line? I don’t see `.bashrc` or `.bash_profile` in my home directory.
Yes, that is shell expression. You could set it in [`environment.extraInit`](https://nixos.org/nixos/options.html#environment.extraInit) NixOS option or [`environment.sessionVariables`](https://nixos.org/nixos/options.html#environment.sessionVariables). Or you can just use [`nix.nixPath`](https://nixos.org/nixos/options.html#nix.nixPath) option which also sets the environment variable globally. (Note that the latter two set the environment variable rather to append to it so you might need to add some items from the default `$NIX_PATH` like `nixos-config=/etc/nixos/configuration.nix`.)
> [@JeffreyBenjaminBrown](#):
>
> Then there’s a third subsection called “Using nixpkgs.overlays from configuration.nix as in your NIX\_PATH”. Is that an alternative to the “system level” section earlier? It’s the only one that mentions the `configuration.nix` file.
There are actually three ways to set up overlays and different tools support different methods.
There is [`nixpkgs.overlays`](https://nixos.org/nixos/options.html#nixpkgs.overlays) option, which sets up the overlays for the `pkgs` attribute passed to NixOS modules like `configuration.nix`. But tools like `nix-build` do not see it.
Then there is the local variant (path under your home directory) that `nix-{build,env,shell}` run as your user see.
And then there is the global variant (path pointed by `nixpkgs-overlays` in [`NIX_PATH`](https://nixos.org/nix/manual/#env-NIX_PATH) environment variable) that `nix-{build,env,shell}` see.
The “Using nixpkgs.overlays from configuration.nix as in your NIX\_PATH” section shows how to make the overlays you set through [`nixpkgs.overlays`](https://nixos.org/nixos/options.html#nixpkgs.overlays) visible in the global variant.
> [@JeffreyBenjaminBrown](#):
>
> The [emacs-overlay](https://github.com/nix-community/emacs-overlay#latest-master-each-rebuild) README says that “probably the most convenient way to pull in this overlay is by just fetching the tarball of latest master on rebuild”, and provides the following code for doing so:
>
> ```auto
> {
> nixpkgs.overlays = [
> (import (builtins.fetchTarball {
> url = https://github.com/nix-community/emacs-overlay/archive/master.tar.gz;
> }))
> ];
> }
>
> ```
That uses [`nixpkgs.overlays`](https://nixos.org/nixos/options.html#nixpkgs.overlays) option so it should go to a NixOS module like your `configuration.nix`.
---
### Author:  [@573](https://discourse.nixos.org/u/573)
#### Post date: [October 20, 2020, 5:24pm UTC](https://discourse.nixos.org/t/my-first-overlay-for-emacs/7570/3 "2020-10-20T17:24:38Z")
I also found out [over here at nixos-emacs](https://logs.nix.samueldr.com/nixos-emacs/2020-02-07), that I can choose the separate file per overlay approach (i. e. `~/.config/nixpkgs/overlays/emacs.nix`) as in:
```auto
# ~/.config/nixpkgs/overlays/emacsGit.nix
self: super:
{
inherit ((import (builtins.fetchTarball {
url = https://github.com/nix-community/emacs-overlay/archive/7678979b3ca51bf557b6bbe2a5b8aa8c4082ff26.tar.gz;
})) self super) emacsGit;
}
```