Julia Evans

Some notes on using nix

Recently I started using a Mac for the first time. The biggest downside I’ve noticed so far is that the package management is much worse than on Linux. At some point I got frustrated with homebrew because I felt like it was spending too much time upgrading when I installed new packages, and so I thought – maybe I’ll try the nix package manager!

nix has a reputation for being confusing (it has its whole own programming language!), so I’ve been trying to figure out how to use nix in a way that’s as simple as possible and does not involve managing any configuration files or learning a new programming language. Here’s what I’ve figured out so far! We’ll talk about how to:

  1. install packages with nix
  2. build a custom nix package for a C++ program called paperjam
  3. install a 5-year-old version of hugo with nix

As usual I’ve probably gotten some stuff wrong in this post since I’m still pretty new to nix. I’m also still not sure how much I like nix – it’s very confusing! But it’s helped me compile some software that I was struggling to compile otherwise, and in general it seems to install things faster than homebrew.

what’s interesting about nix?

People often describe nix as “declarative package management”. I don’t care that much about declarative package management, so here are two things that I appreciate about nix:

  1. It provides binary packages (hosted at https://cache.nixos.org/) that you can quickly download and install
  2. For packages which don’t have binary packages, it makes it easier to compile them

I think that the reason nix is good at compiling software is that:

  • you can have multiple versions of the same library or program installed at a time (you could have 2 different versions of libc for instance). For example I have two versions of node on my computer right now, one at /nix/store/4ykq0lpvmskdlhrvz1j3kwslgc6c7pnv-nodejs-16.17.1 and one at /nix/store/5y4bd2r99zhdbir95w5pf51bwfg37bwa-nodejs-18.9.1.
  • when nix builds a package, it builds it in isolation, using only the specific versions of its dependencies that you explicitly declared. So there’s no risk that the package secretly depends on another package on your system that you don’t know about. No more fighting with LD_LIBRARY_PATH!
  • a lot of people have put a lot of work into writing down all of the dependencies of packages

I’ll give a couple of examples later in this post of two times nix made it easier for me to compile software.

how I got started with nix

here’s how I got started with nix:

  1. Install nix. I forget exactly how I did this, but it looks like there’s an official installer and an unofficial installer from zero-to-nix.com. The instructions for uninstalling nix on MacOS with the standard multi-user install are a bit complicated, so it might be worth choosing an installation method with simpler uninstall instructions.
  2. Put ~/.nix-profile/bin on my PATH
  3. Install packages with nix-env -iA nixpkgs.NAME
  4. That’s it.

Basically the idea is to treat nix-env -iA like brew install or apt-get install.

For example, if I want to install fish, I can do that like this:

nix-env -iA nixpkgs.fish

This seems to just download some binaries from https://cache.nixos.org – pretty simple.

Some people use nix to install their Node and Python and Ruby packages, but I haven’t been doing that – I just use npm install and pip install the same way I always have.

some nix features I’m not using

There are a bunch of nix features/tools that I’m not using, but that I’ll mention. I originally thought that you had to use these features to use nix, because most of the nix tutorials I’ve read talk about them. But you don’t have to use them.

I won’t go into these because I haven’t really used them and there are lots of explanations out there.

where are nix packages defined?

I think packages in the main nix package repository are defined in https://github.com/NixOS/nixpkgs/

It looks like you can search for packages at https://search.nixos.org/packages. The two official ways to search packages seem to be:

  • nix-env -qaP NAME, which is very extremely slow and which I haven’t been able to get to actually work
  • nix --extra-experimental-features 'nix-command flakes' search nixpkgs NAME, which does seem to work but is kind of a mouthful. Also all of the packages it prints out start with legacyPackages for some reason

I found a way to search nix packages from the command line that I liked better:

  1. Run nix-env -qa '*' > nix-packages.txt to get a list of every package in the Nix repository
  2. Write a short nix-search script that just greps packages.txt (cat ~/bin/nix-packages.txt | awk '{print $1}' | rg "$1")

One of nix’s major design choices is that there isn’t one single bin with all your packages, instead you use symlinks. There are a lot of layers of symlinks. A few examples of symlinks:

  • ~/.nix-profile on my machine is (indirectly) a symlink to /nix/var/nix/profiles/per-user/bork/profile-111-link/
  • ~/.nix-profile/bin/fish is a symlink to /nix/store/afkwn6k8p8g97jiqgx9nd26503s35mgi-fish-3.5.1/bin/fish

When I install something, it creates a new profile-112-link directory with new symlinks and updates my ~/.nix-profile to point to that directory.

I think this means that if I install a new version of fish and I don’t like it, I can easily go back just by running nix-env --rollback – it’ll move me to my previous profile directory.

uninstalling packages doesn’t delete them

If I uninstall a nix package like this, it doesn’t actually free any hard drive space, it just removes the symlinks.

$ nix-env --uninstall oil

I’m still not sure how to actually delete the package – I ran a garbage collection like this, which seemed to delete some things:

$ nix-collect-garbage
85 store paths deleted, 74.90 MiB freed

But I still have oil on my system at /nix/store/8pjnk6jr54z77jiq5g2dbx8887dnxbda-oil-0.14.0.

There’s a more aggressive version of nix-collect-garbage that also deletes old versions of your profiles (so that you can’t rollback)

$ nix-collect-garbage -d --delete-old

That doesn’t delete /nix/store/8pjnk6jr54z77jiq5g2dbx8887dnxbda-oil-0.14.0 either though and I’m not sure why.


It looks like you can upgrade nix packages like this:

nix-channel --update
nix-env --upgrade

(similar to apt-get update && apt-get upgrade)

I haven’t really upgraded anything yet. I think that if something goes wrong with an upgrade, you can roll back (because everything is immutable in nix!) with

nix-env --rollback

Someone linked me to this post from Ian Henry that talks about some confusing problems with nix-env --upgrade – maybe it doesn’t work the way you’d expect? I guess I’ll be wary around upgrades.

next goal: make a custom package of paperjam

After a few months of installing existing packages, I wanted to make a custom package with nix for a program called paperjam that wasn’t already packaged.

I was actually struggling to compile paperjam at all even without nix because the version I had of libiconv I has on my system was wrong. I thought it might be easier to compile it with nix even though I didn’t know how to make nix packages yet. And it actually was!

But figuring out how to get there was VERY confusing, so here are some notes about how I did it.

how to build an example package

Before I started working on my paperjam package, I wanted to build an example existing package just to make sure I understood the process for building a package. I was really struggling to figure out how to do this, but I asked in Discord and someone explained to me how I could get a working package from https://github.com/NixOS/nixpkgs/ and build it. So here are those instructions:

step 1: Download some arbitrary package from nixpkgs on github, for example the dash package:

wget https://raw.githubusercontent.com/NixOS/nixpkgs/47993510dcb7713a29591517cb6ce682cc40f0ca/pkgs/shells/dash/default.nix -O dash.nix

step 2: Replace the first statement ({ lib , stdenv , buildPackages , autoreconfHook , pkg-config , fetchurl , fetchpatch , libedit , runCommand , dash }: with with import <nixpkgs> {}; I don’t know why you have to do this, but it works.

step 3: Run nix-build dash.nix

This compiles the package

step 4: Run nix-env -i -f dash.nix

This installs the package into my ~/.nix-profile

That’s all! Once I’d done that, I felt like I could modify the dash package and make my own package.

how I made my own package

paperjam has one dependency (libpaper) that also isn’t packaged yet, so I needed to build libpaper first.

Here’s libpaper.nix. I basically just wrote this by copying and pasting from other packages in the nixpkgs repository. My guess is what’s happening here is that nix has some default rules for compiling C packages (like “run make install”), so the make install happens default and I don’t need to configure it explicitly.

with import <nixpkgs> {};

stdenv.mkDerivation rec {
  pname = "libpaper";
  version = "0.1";

  src = fetchFromGitHub {
    owner = "naota";
    repo = "libpaper";
    rev = "51ca11ec543f2828672d15e4e77b92619b497ccd";
    hash = "sha256-S1pzVQ/ceNsx0vGmzdDWw2TjPVLiRgzR4edFblWsekY=";

  buildInputs = [ ];

  meta = with lib; {
    homepage = "https://github.com/naota/libpaper";
    description = "libpaper";
    platforms = platforms.unix;
    license = with licenses; [ bsd3 gpl2 ];

Basically this just tells nix how to download the source from GitHub.

I built this by running nix-build libpaper.nix

Next, I needed to compile paperjam. Here’s a link to the nix package I wrote. The main things I needed to do other than telling it where to download the source were:

  1. add some extra build dependencies (like asciidoc)
  2. set some environment variables for the install (installFlags = [ "PREFIX=$(out)" ];) so that it installed in the correct directory instead of /usr/local/bin.

I set the hashes by first leaving the hash empty, then running nix-build to get an error message complaining about a mismatched hash. Then I copied the correct hash out of the error message.

I figured out how to set installFlags just by running rg PREFIX in the nixpkgs repository – I figured that needing to set a PREFIX was pretty common and someone had probably done it before, and I was right. So I just copied and pasted that line from another package.

Then I ran:

nix-build paperjam.nix
nix-env -i -f paperjam.nix

and then everything worked and I had paperjam installed! Hooray!

next goal: install a 5-year-old version of hugo

Right now I build this blog using Hugo 0.40, from 2018. I don’t need any new features so I haven’t felt a need to upgrade. On Linux this is easy: Hugo’s releases are a static binary, so I can just download the 5-year-old binary from the releases page and run it. Easy!

But on this Mac I ran into some complications. Mac hardware has changed in the last 5 years, so the Mac Hugo binary I downloaded crashed. And when I tried to build it from source with go build, that didn’t work either because Go build norms have changed in the last 5 years as well.

I was working around this by running Hugo in a Linux docker container, but I didn’t love that: it was kind of slow and it felt silly. It shouldn’t be that hard to compile one Go program!

Nix to the rescue! Here’s what I did to install the old version of Hugo with nix.

installing Hugo 0.40 with nix

I wanted to install Hugo 0.40 and put it in my PATH as hugo-0.40. Here’s how I did it. I did this in a kind of weird way, but it worked (Searching and installing old versions of Nix packages describes a probably more normal method).

step 1: Search through the nixpkgs repo to find Hugo 0.40

I found the .nix file here https://github.com/NixOS/nixpkgs/blob/17b2ef2/pkgs/applications/misc/hugo/default.nix

step 2: Download that file and build it

I downloaded that file (and another file called deps.nix in the same directory), replaced the first line with with import <nixpkgs> {};, and built it with nix-build hugo.nix.

That almost worked without any changes, but I had to make two changes:

  • replace with stdenv.lib to with lib for some reason.
  • rename the package to hugo040 so that it wouldn’t conflict with the other version of hugo that I had installed

step 3: Rename hugo to hugo-0.40

I write a little post install script to rename the Hugo binary.

  postInstall = ''
    mv $out/bin/hugo $out/bin/hugo-0.40

I figured out how to run this by running rg 'mv ' in the nixpkgs repository and just copying and modifying something that seemed related.

step 4: Install it

I installed into my ~/.nix-profile/bin by running nix-env -i -f hugo.nix.

And it all works! I put the final .nix file into my own personal nixpkgs repo so that I can use it again later if I want.

reproducible builds aren’t magic, they’re really hard

I think it’s worth noting here that this hugo.nix file isn’t magic – the reason I can easily compile Hugo 0.40 today is that many people worked for a long time to make it possible to package that version of Hugo in a reproducible way.

that’s all!

Installing paperjam and this 5-year-old version of Hugo were both surprisingly painless and actually much easier than compiling it without nix, because nix made it much easier for me to compile the paperjam package with the right version of libiconv, and because someone 5 years ago had already gone to the trouble of listing out the exact dependencies for Hugo.

I don’t have any plans to get much more complicated with nix (and it’s still very possible I’ll get frustrated with it and go back to homebrew!), but we’ll see what happens! I’ve found it much easier to start in a simple way and then start using more features if I feel the need instead of adopting a whole bunch of complicated stuff all at once.

I probably won’t use nix on Linux – I’ve always been happy enough with apt (on Debian-based distros) and pacman (on Arch-based distros), and they’re much less confusing. But on a Mac it seems like it might be worth it. We’ll see! It’s very possible in 3 months I’ll get frustrated with nix and just go back to homebrew.

5-month update: rebuilding my nix profile

Update from 5 months in: nix is still going well, and I’ve only run into 1 problem, which is that every nix-env -iA package installation started failing with the error “bad meta.outputsToInstall”.

This script from Ross Light fixes that problem though. It lists every derivation installed in my current profile and creates a new profile with the exact same derivations. This feels like a nix bug (surely creating a new profile with the exact same derivations should be a no-op?) but I haven’t looked into it more yet.

Writing Javascript without a build system How do Nix builds work?