nixos-and-flakes-book/docs/development/intro.md

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# Development Environments on NixOS
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NixOS's reproducibility makes it ideal for building development environments. However, if you're used to other distros, you may encounter problems because NixOS has its own logic. We'll briefly explain this below.
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On NixOS, it's recommended to only install common tools in the global environment, such as `git`, `vim`, `emacs`, `tmux`, `zsh`, etc. The development environment of each language should be an independent environment for each project.
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You should NOT install the development environment of each language in the global environment. The project environment should be completely isolated from each other and will not affect each other.
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In the following sections, we'll introduce how the development environment works in NixOS.
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## Creating a Development Environment
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We can create a development environment using `pkgs.mkShell { ... }` and open an interactive Bash shell of this development environment using `nix develop`.
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To see how `pkgs.mkShell` works, let's take a look at [its source code](https://github.com/NixOS/nixpkgs/blob/master/pkgs/build-support/mkshell/default.nix).
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```nix
{ lib, stdenv, buildEnv }:
# A special kind of derivation that is only meant to be consumed by the
# nix-shell.
{ name ? "nix-shell"
, # a list of packages to add to the shell environment
packages ? [ ]
, # propagate all the inputs from the given derivations
inputsFrom ? [ ]
, buildInputs ? [ ]
, nativeBuildInputs ? [ ]
, propagatedBuildInputs ? [ ]
, propagatedNativeBuildInputs ? [ ]
, ...
}@attrs:
let
mergeInputs = name:
(attrs.${name} or [ ]) ++
(lib.subtractLists inputsFrom (lib.flatten (lib.catAttrs name inputsFrom)));
rest = builtins.removeAttrs attrs [
"name"
"packages"
"inputsFrom"
"buildInputs"
"nativeBuildInputs"
"propagatedBuildInputs"
"propagatedNativeBuildInputs"
"shellHook"
];
in
stdenv.mkDerivation ({
inherit name;
buildInputs = mergeInputs "buildInputs";
nativeBuildInputs = packages ++ (mergeInputs "nativeBuildInputs");
propagatedBuildInputs = mergeInputs "propagatedBuildInputs";
propagatedNativeBuildInputs = mergeInputs "propagatedNativeBuildInputs";
shellHook = lib.concatStringsSep "\n" (lib.catAttrs "shellHook"
(lib.reverseList inputsFrom ++ [ attrs ]));
phases = [ "buildPhase" ];
# ......
# when distributed building is enabled, prefer to build locally
preferLocalBuild = true;
} // rest)
```
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`pkgs.mkShell { ... }` is a special derivation (Nix package). Its `name`, `buildInputs`, and other parameters are customizable, and `shellHook` is a special parameter that will be executed when `nix develop` enters the environment.
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Here is a `flake.nix` that defines a development environment with Node.js 18 installed:
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```nix
{
description = "A Nix-flake-based Node.js development environment";
inputs = {
nixpkgs.url = "github:nixos/nixpkgs/nixos-23.05";
};
outputs = { self , nixpkgs ,... }: let
# system should match the system you are running on
# system = "x86_64-linux";
system = "x86_64-darwin";
in {
devShells."${system}".default = let
pkgs = import nixpkgs {
inherit system;
overlays = [
(self: super: rec {
nodejs = super.nodejs-18_x;
pnpm = super.nodePackages.pnpm;
yarn = (super.yarn.override { inherit nodejs; });
})
];
};
in pkgs.mkShell {
# create an environment with nodejs-18_x, pnpm, and yarn
packages = with pkgs; [
node2nix
nodejs
pnpm
yarn
];
shellHook = ''
echo "node `${pkgs.nodejs}/bin/node --version`"
'';
};
};
}
```
Create an empty folder, save the above configuration as `flake.nix`, and then execute `nix develop` (or more precisely, you can use `nix develop .#default`), you will find that you have entered a nodejs 18 development environment, you can use `node` `npm` `pnpm` `yarn` and other commands. And when you just entered, `shellHook` was also executed, outputting the current version of nodejs.
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## Enter the build environment of any Nix package
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Now let's take a look at `nix develop`, first read the help document output by `nix develop --help`:
```
Name
nix develop - run a bash shell that provides the build environment of a derivation
Synopsis
nix develop [option...] installable
# ......
```
It tells us that `nix develop` accepts a parameter `installable`, which means that we can enter the development environment of any installable Nix package through it, not just the environment created by `pkgs.mkShell`.
By default, `nix develop` will try to use the following attributes in the flake outputs:
- `devShells.<system>.default`
- `packages.<system>.default`
If we use `nix develop /path/to/flake#<name>` to specify the flake package address and flake output name, then `nix develop` will try the following attributes in the flake outputs:
- `devShells.<system>.<name>`
- `packages.<system>.<name>`
- `legacyPackages.<system>.<name>`
Now let's try it out. First, test it to confirm that We don't have `c++` `g++` and other compilation-related commands in the current environment:
```shell
ryan in 🌐 aquamarine in ~
c++
c++: command not found
ryan in 🌐 aquamarine in ~
g++
g++: command not found
```
Then use `nix develop` to enter the build environment of the `hello` package in `nixpkgs`:
```shell
# login to the build environment of the package `hello`
ryan in 🌐 aquamarine in ~
nix develop nixpkgs#hello
ryan in 🌐 aquamarine in ~ via ❄️ impure (hello-2.12.1-env)
env | grep CXX
CXX=g++
ryan in 🌐 aquamarine in ~ via ❄️ impure (hello-2.12.1-env)
c++ --version
g++ (GCC) 12.3.0
Copyright (C) 2022 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
ryan in 🌐 aquamarine in ~ via ❄️ impure (hello-2.12.1-env)
g++ --version
g++ (GCC) 12.3.0
Copyright (C) 2022 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
```
We can see that the `CXX` environment variable have been set, and the `c++` `g++` and other commands can be used normally now.
In addition, we can also call every build phase of the `hello` package normally:
> The default execution order of all build phases of a Nix package is: `$prePhases unpackPhase patchPhase $preConfigurePhases configurePhase $preBuildPhases buildPhase checkPhase $preInstallPhases installPhase fixupPhase installCheckPhase $preDistPhases distPhase $postPhases`
```shell
# unpack source code
ryan in 🌐 aquamarine in /tmp/xxx via ❄️ impure (hello-2.12.1-env)
unpackPhase
unpacking source archive /nix/store/pa10z4ngm0g83kx9mssrqzz30s84vq7k-hello-2.12.1.tar.gz
source root is hello-2.12.1
setting SOURCE_DATE_EPOCH to timestamp 1653865426 of file hello-2.12.1/ChangeLog
ryan in 🌐 aquamarine in /tmp/xxx via ❄️ impure (hello-2.12.1-env)
ls
hello-2.12.1
ryan in 🌐 aquamarine in /tmp/xxx via ❄️ impure (hello-2.12.1-env)
cd hello-2.12.1/
# generate Makefile
ryan in 🌐 aquamarine in /tmp/xxx/hello-2.12.1 via ❄️ impure (hello-2.12.1-env)
configurePhase
configure flags: --prefix=/tmp/xxx/outputs/out --prefix=/tmp/xxx/outputs/out
checking for a BSD-compatible install... /nix/store/02dr9ymdqpkb75vf0v1z2l91z2q3izy9-coreutils-9.3/bin/install -c
checking whether build environment is sane... yes
checking for a thread-safe mkdir -p... /nix/store/02dr9ymdqpkb75vf0v1z2l91z2q3izy9-coreutils-9.3/bin/mkdir -p
checking for gawk... gawk
checking whether make sets $(MAKE)... yes
checking whether make supports nested variables... yes
checking for gcc... gcc
# ......
checking that generated files are newer than configure... done
configure: creating ./config.status
config.status: creating Makefile
config.status: creating po/Makefile.in
config.status: creating config.h
config.status: config.h is unchanged
config.status: executing depfiles commands
config.status: executing po-directories commands
config.status: creating po/POTFILES
config.status: creating po/Makefile
# build the package
ryan in 🌐 aquamarine in /tmp/xxx/hello-2.12.1 via C v12.3.0-gcc via ❄️ impure (hello-2.12.1-env) took 2s
buildPhase
build flags: SHELL=/run/current-system/sw/bin/bash
make all-recursive
make[1]: Entering directory '/tmp/xxx/hello-2.12.1'
# ......
ranlib lib/libhello.a
gcc -g -O2 -o hello src/hello.o ./lib/libhello.a
make[2]: Leaving directory '/tmp/xxx/hello-2.12.1'
make[1]: Leaving directory '/tmp/xxx/hello-2.12.1'
# run the built program
ryan in 🌐 aquamarine in /tmp/xxx/hello-2.12.1 via C v12.3.0-gcc via ❄️ impure (hello-2.12.1-env)
./hello
Hello, world!
```
This usage is mainly used to debug the build process of a Nix package, or to execute some commands in the build environment of a Nix package.
## `nix build`
The `nix build` command is used to build a software package and creates a symbolic link named `result` in the current directory, which points to the build result.
Here's an example:
```bash
# Build the package 'ponysay' from the 'nixpkgs' flake
nix build "nixpkgs#ponysay"
# Use the built 'ponysay' command
./result/bin/ponysay 'hey buddy!'
____________
< hey buddy! >
------------
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\
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```
## Other Commands
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There are other commands like `nix flake init`, which you can explore in [New Nix Commands][New Nix Commands]. For more detailed information, please refer to the documentation.
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## References
- [pkgs.mkShell - nixpkgs manual](https://nixos.org/manual/nixpkgs/stable/#sec-pkgs-mkShell)
- [A minimal nix-shell](https://fzakaria.com/2021/08/02/a-minimal-nix-shell.html)
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- [One too many shell, Clearing up with nix' shells nix shell and nix-shell - Yannik Sander](https://blog.ysndr.de/posts/guides/2021-12-01-nix-shells/)
[New Nix Commands]: https://nixos.org/manual/nix/stable/command-ref/new-cli/nix.html