View source for Development environment with nix-shell

Nix can be used to provide some kind of '''virtual environment''' through the <code>nix-shell</code> command.

If you already have a nix package definition of your project it's easy: Just use <code>nix-shell</code> instead of <code>nix-build</code> and you will end up in a bash shell that reproduce the build-environment of your package. You can also override[https://nixos.org/nixpkgs/manual/#sec-pkg-override] your package in a <code>shell.nix</code> file to add test and coverage dependencies, that are not necessary for the actual build of the package, but that you want for your development environment.

But, if you don't (or you don't want to) have a package definition you can still use a nix-shell to provide a reproducible development environment. To do so, you have to create a <code>shell.nix</code> file at the root of your repository. For example, if you want to have Ruby 3.2 and not one provided by your system you can write:
<syntaxhighlight lang="nix">
{ pkgs ? import <nixpkgs> {} }:
pkgs.mkShell {
  # nativeBuildInputs is usually what you want -- tools you need to run
  nativeBuildInputs = with pkgs.buildPackages; [ ruby_3_2 ];
}
</syntaxhighlight>

Then just run:
{{Commands|$ nix-shell}}

Or, to be more explicit:
{{Commands|$ nix-shell shell.nix}}

Now you have ruby 3.2 available in your shell:
<syntaxHighlight lang=bash>
$ ruby --version
ruby 3.2.2 (2023-03-30 revision e51014f9c0) [x86_64-linux]
</syntaxHighlight>

To be sure that the tools installed on your system will not interfere with the dependencies that you've defined in the shell you can use the <code>--pure</code> option.

If you'd like to load a local nix expression into a shell you can do it by modifying the earlier example a little bit:
<syntaxHighlight lang=nix>
{ pkgs ? import <nixpkgs> {} }:
pkgs.mkShell {
  # buildInputs is for dependencies you'd need "at run time",
  # were you to to use nix-build not nix-shell and build whatever you were working on
  buildInputs = [
    (import ./my-expression.nix { inherit pkgs; })
  ];
}
</syntaxHighlight>

If you want to see how to manually run the various phases of a given derivation from a nix-shell (useful to debug), see [[Nixpkgs/Create_and_debug_packages#Using_nix-shell_for_package_development]]. 

== nix develop ==

For [[Flakes]]-based projects (<code>flake.nix</code> file in project root),
we replace <code>nix-shell</code> with <code>nix develop</code>

Example: Building Nix in a development shell, to get [[Incremental builds]] = faster recompiles

<pre>
git clone https://github.com/NixOS/nix --depth 1
cd nix
nix develop
</pre>

Now what? Let's read the manual:

<pre>
less README.md
less doc/manual/src/contributing/hacking.md
</pre>

The contributing guide for Nix says:

<pre>
To build all dependencies and start a shell in which all environment
variables are set up so that those dependencies can be found:

```console
$ nix-shell
```

To build Nix itself in this shell:

```console
[nix-shell]$ ./bootstrap.sh
[nix-shell]$ ./configure $configureFlags --prefix=$(pwd)/outputs/out
[nix-shell]$ make -j $NIX_BUILD_CORES
```
</pre>

So, in our <code>nix develop</code> shell, we run

<pre>
./bootstrap.sh
./configure $configureFlags --prefix=$(pwd)/outputs/out
make -j $NIX_BUILD_CORES
</pre>

This will compile Nix to <code>./outputs/out/bin/nix</code>

Let's make some changes to the source code, and run <code>make</code> again.<br>
Now the compilation should be much faster (see [[Incremental builds]])

== stdenv.mkDerivation ==

Let's assume you have a <code>default.nix</code> file

<syntaxHighlight lang=nix>
{ stdenv, python }:
stdenv.mkDerivation {
  buildInputs = [ python ];
  name = "some-package";
  version = "0.0.1";
  src = /home/yourname/path/to/project; # can be a local path, or fetchFromGitHub, fetchgit, ...
}
</syntaxHighlight>

Then you can start a development shell with

<syntaxHighlight lang=bash>
nix-shell -E 'with import <nixpkgs> { }; callPackage ./default.nix { }'
</syntaxHighlight>

In this shell, you can run the phases of stdenv.mkDerivation:

<syntaxHighlight lang=bash>
# clean build: copy sources from /nix/store
echo "src = $src" && cd $(mktemp -d) && unpackPhase && cd *

# dirty build: keep cache files from last buildPhase, to compile faster
# this is useful to make many small changes to a large project
# after each change, just run `buildPhase`
#cd $HOME/path/to/project

configurePhase

buildPhase # most time is spent here

checkPhase && installPhase && fixupPhase
</syntaxHighlight>

== cross env ==

The comments in the code snippets on <code>nativeBuildInputs</code> and <code>buildInputs</code> above might seem pedantic --- who cares about build-time vs run-time when we're just making a dev environment, not a real package! However, the distinction becomes of practical importance if one wants a cross compilation development environment. In that case one would begin file with something like:
<syntaxHighlight lang=nix>
{ pkgs ? import <nixpkgs> { crossSystem.config = "exotic_arch-unknown-exotic_os"; } }:
</syntaxHighlight>
and <code>nativeBuildInputs</code> would be for the native platform, while <code>buildInputs</code> would be for the foreign platform. That's a much more practical distinction: any tool that's miscategorized one won't be able to run, and any library that's miscategorized one won't be able to link!

== direnv ==

One of the limitations of nix-shell is that you can't use a shell other than bash. Thankfully, there is Direnv [[https://direnv.net/]] with the support of Nix[[https://github.com/direnv/direnv/wiki/Nix]] to overcome this limitation. Also, Direnv provides some nice features like loading the environment automatically when you enter your project directory and show the loaded variables to you (explicit is always better;-)).

First, install Direnv:
<syntaxHighlight lang=bash>
nix-env -i direnv
</syntaxHighlight>

Now, you need to add one more file in the root of your repository named <code>.envrc</code> that contains only this:
<syntaxHighlight lang=bash>
use_nix
</syntaxHighlight>

Then depending on the shell you are using, you need to add a line in your configuration file. See the ''Setup'' section of the doc[https://direnv.net/]. For example, for Zsh put in your <code>~/.zshrc.local</code>:
<syntaxHighlight lang=bash>
eval "$(direnv hook zsh)"
</syntaxHighlight>

Then, still at the root of your repository, run:
<syntaxHighlight lang=bash>
$ direnv allow .
direnv: loading .envrc
direnv: using nix
[...]
 +SIZE +SOURCE_DATE_EPOCH +STRINGS +STRIP +TEMP +TEMPDIR +TMP +TMPDIR +_PATH +buildInputs +builder +checkPhase +cmakeFlags +configureFlags +doCheck +enableParallelBuilding +name +nativeBuildInputs +out +postCheck +preCheck +preConfigure +propagatedBuildInputs +propagatedNativeBuildInputs +shell +src +stdenv +system +testInputs +version ~PATH
</syntaxHighlight>

Bonus: you can see all the variables set by the nix-shell :)

Now you can leave your project and the environment will be unloaded:
<syntaxHighlight lang=bash>
$ cd ..
direnv: unloading
</syntaxHighlight>

No need to use <code>direnv allow</code> anymore, the next time you go to your project the environment will be loaded!

More explanation and configuration tweaks can be found in the Direnv wiki [https://github.com/direnv/direnv/wiki/Nix].

== Troubleshooting ==

When compiling software which links against local files (e.g. when compiling with rust's cargo), you may encounter the following problem:
<syntaxHighlight lang=bash>
= note: impure path `/[...]' used in link
</syntaxHighlight>
This happens due to a specialty in nix: <code>ld</code> is wrapped in a shell script which refuses to link against files not residing in the nix store, to ensure the purity of builds. Obviously this is not useful when building locally, for example in your home directory. To disable this behavior simply set
<syntaxHighlight lang=bash>
NIX_ENFORCE_PURITY=0
</syntaxHighlight>
in the nix-shell.

=== No GSettings schemas are installed on the system ===
When working with gtk, the <code>XDG_DATA_DIRS</code> must contain a path to the gtk schemas, if not an application may crash with the error above.

For packages we use <code>wrapGAppsHook</code> in <code>nativeBuildInputs</code>, however in nix-shell this is not working as expected.
To get your application to work in nix-shell you will need to add the following to your <code>mkShell</code> expression:
<syntaxhighlight lang="nix">
mkShell {
  ...
  buildInputs = [ gtk3 ];
  shellHook = ''
     export XDG_DATA_DIRS=$GSETTINGS_SCHEMAS_PATH
  '';
}
</syntaxhighlight>

This may also called: <code>$GSETTINGS_SCHEMA'''S'''_PATH</code>.

=== Icons not working ===
Similar to the Gsettings issue, icons can be added with XDG_DATA_DIRS:
<pre> XDG_DATA_DIRS=...:${hicolor-icon-theme}/share:${gnome3.adwaita-icon-theme}/share</pre>
[[Category:Development]]
[[Category:nix]]