Tony Finn

Runnable Flakes (Nix From First Principles: Flake Edition #9)

#Nix

This is part 9 of the Nix from First Principles: Flake Edition series.

In the last two parts, I covered using flakes for two different types of output: packages and developer environments. This time I'll cover a third type of output - runnable commands. These are what power the nix run command that you've seen a few times throughout the series, and now you will find out how to get this functionality for your own flakes.

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Flakes and Developer Environments (Nix From First Principles: Flake Edition #8)

#Nix

This is part 8 of the Nix from First Principles: Flake Edition series.

While packages are the most common type of item you'll find contained in flakes, another type of item is a development shell. This is an environment that you can enter with the nix develop command. By default, each package will provide a development shell which is the build environment used to build it. By running just nix develop without specifying which shell is wanted, nix will put you in a bash shell with the environment from the default package.

However, sometimes you may wish to set up additional tools in a development environment, for example you may have scripts written in Python and want to have Python available as for a developer to use, yet you do not want to include Python as a dependency in your built package. Or you might want to set some environment variables so that logging is set to debug mode in your development environment.

You can customise the nix develop environment with the devShells attribute set in a flake.

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What about flakes then? (Nix From First Principles: Flake Edition #7)

#Nix

This is part 7 of the Nix from First Principles: Flake Edition series.

One of the big new concepts in Nix is the flake, which is a new standard format for nix projects to declare all their outputs. At the beginning of this series, I mentioned them as being one of the major components of modern Nix, and now this series has introduced enough of a foundation it's time to explain flakes themselves.

What is a flake?

A flake is a standard format for describing a collection of Nix resources. These resources can be packages, of the type described in previous posts, intended to be used to install some software to your system. There's also a number of other types of resources that can be exposed by the flake which we'll cover in later posts. Some examples of these are:

  • developer environment descriptions
  • modules for configuring NixOS systems
  • runnable commands

By standardising a format to list all these outputs (compared to the ad-hoc files this series has been using until now), Nix allows a number of higher level tools to operate on these resources, like the nix profile install and nix run commands that have been shown in earlier parts.

Finally, the development of the flake based tooling allowed the Nix team to resolve some pain points with the older generation of tooling, such as making the process of recording versions for reproducibility more ergonomic with lock files, or making better ways to identify newer versions of the same package to allow tools to manage updates.

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Nixpkgs - How Not to Reinvent the Wheel (Nix From First Principles: Flake Edition #6)

#Nix

This is part 6 of the Nix from First Principles: Flake Edition series.

In the last section I discussed creating your first derivation, which allows you to make a first nix package. As you might have noticed, the default execution environment is incredibly barebones, to the point that you needed to include such fundamental tools as chmod and cp. If that process had to be repeated by every Nix user, it would be very inconvenient. Luckily, there is nixpkgs, which provides a number of packages that the community has already built, along with the standard environment which includes a number of tools to use in building your own. You may remember installing some of these packages in part 3.

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Overview (Nix From First Principles: Flake Edition #1)

#Nix

This is part 1 of the Nix from First Principles: Flake Edition series.

One technology that I'd been hearing more and more over the last two years has been Nix, in its various forms. The big draws I've heard about include declarative, portable, cross lang dependency specification, and the ability to have a config file declare the entire state of a system without storing huge docker container artifacts for every permutation.

Actually learning Nix though felt much harder than it needed to be. The biggest problem is that the documentation you need is often fragmented among the different subprojects, and also the transition that Nix is in at the moment from the current standard methods of default.nix, configuration.nix and shell.nix to a new world of flakes.

As an outsider looking in, the flake world felt kind of in limbo at the moment where anyone I talked to indicated that flakes fix so many of the problems with current nix, but also they're still officially experimental so the official documentation declines to mention flakes.

After spending a fair amount of time understanding Nix for myself, I've decided to write my own guide which explains it from scratch in a world using only the new Nix features, with the nix CLI and flakes for the use cases I want from Nix. Credit to ianthehenry for his guide on Nix focused on the "old world" approach, and Xe Iaso for explaining flakes. The information included in this guide is largely from synthesising their blogs and the official documentation.

This first post will describe the major parts of the Nix ecosystem.

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Installation (Nix From First Principles: Flake Edition #2)

#Nix

This is part 2 of the Nix from First Principles: Flake Edition series.

Nix can be installed on Linux or Mac systems. If you're using the Nix based Linux distribution, NixOS, then you'll already have it installed. Otherwise the official nix website provides the most up to date installation steps, which at the time of writing are

Linux:

sh <(curl -L https://nixos.org/nix/install) --daemon

Mac:

sh <(curl -L https://nixos.org/nix/install)

Windows (run in WSL):

sh <(curl -L https://nixos.org/nix/install) --no-daemon

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Nix Package Manager CLI (Nix From First Principles: Flake Edition #3)

#Nix

This is part 3 of the Nix from First Principles: Flake Edition series.

The foundational part of Nix is the Nix package manager. This is a tool that allows you to install packages. The "first layer" of Nix usage is to use this tool to install packages. The Nix package manager, similar to projects like os-rpmtree (used in Fedora Silverblue) or snapper (used in snap based Linux distros), will allow you to easily revert package installs or updates if something goes wrong.

This post covers the basics of using the nix package manager at the command line - while most users use nix through more advanced workflows, the basics explained here will be useful in understanding those workflows, as well as for users who want to dip their toes into Nix by treating it like a more traditional package manager to start out.

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Just Enough Nixlang (Nix From First Principles: Flake Edition #4)

#Nix

This is part 4 of the Nix from First Principles: Flake Edition series.

This is a crash course in Nix's language, which I'll also sometimes call Nixlang from here on, to avoid confusing it with the Nix package manager. For more detailed instructions see the manual. The Nix language is used heavily within Nix, for defining packages, environments, runnable commands and library functions.

Nix's language is expression oriented. This means everything, including up to the level of an entire program, has an output value. It will be most familiar if you have experimented with functional langages such as Haskell, but it is less complicated than those languages, so you don't need to have mastered one to start.

To start experimenting with it, you can run the nix repl command which will let you type in Nix expressions and see the output.

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Derivations (Nix From First Principles: Flake Edition #5)

#Nix

This is part 5 of the Nix from First Principles: Flake Edition series.

So, time to explain the nix derivation. A Nix derivation is a special type of set created from the derivation function which describes how to obtain and build a package. The Nix package manger will then evaluate this derivation, and use the result to copy the built package into the Nix store, which is effectively a folder with all locally Nix built or installed packages in it. The subfolders are hashes derived from the inputs to the expression that built the stored packages.

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