Date By Fabien

This post aims to teach how to leverage nix via the nix-shell feature in order to ease the distribution of reproducible environment.

What is Nix ?

Nix is a purely functional package manager. It manages packages independently from your system by maintaining a package store in /nix/store. This makes Nix convenient because various softwares and libraries can be installed without the fear of breaking the base system provided by your Linux distribution, nor having to handle potential conflicts in the versions of dependencies. Furthermore, as the Nix store is a graph of cryptographic hashes of package’s build dependencies, then it brings the guarantee reproducible environments.

How to install nix

This page describes installation instructions.

We'll use the single user installation process (the user needs to be able to sudo -i):

sh <(curl -L --no-daemon
. ~/.nix-profile/etc/profile.d/

Now let's verify our nix installation is working as expected:

nix --version
nix (Nix) 2.12.0

Using the nix-shell to setup an environment

A Nix shell environment gives access to specified packages.

For instance, this command enhances the current shell environment to make cowsay and fortune available in the PATH:

$ nix-shell -p cowsay fortune
these 3 paths will be fetched (1.76 MiB download, 6.34 MiB unpacked):
copying path '/nix/store/fkrh0bzwymq0220fscz7grd3yrh5hzsd-cowsay-3.04' from ''...
copying path '/nix/store/4agvv4d3jl9lcwxd46qjlkzcibsbryvz-recode-3.7.9' from ''...
copying path '/nix/store/k5dfq7qj0vp10jyb2pn780f323f4vdzm-fortune-mod-3.6.1' from ''...

$ type cowsay fortune
cowsay is hashed (/nix/store/fkrh0bzwymq0220fscz7grd3yrh5hzsd-cowsay-3.04/bin/cowsay)
fortune is hashed (/nix/store/k5dfq7qj0vp10jyb2pn780f323f4vdzm-fortune-mod-3.6.1/bin/fortune)

The Nix project maintains a binary cache then packages are usually just downloaded from the cache.

However this command does not guarantee the same versions of packages will be installed when the same command runs on another machine. Indeed package definitions are maintained in the nixpkgs project, and to ensure reproducibility the version of nixpkgs must be pinned.

By default, running nix-shell, uses the default nixpkgs channel, which might be set to a different version across nix installations.

$ nix-instantiate --eval -E '(import <nixpkgs> {}).lib.version'

The nix-shell command can be run with a pinned version of nixpkgs, by doing so we get the guarantee run a reproducible shell environment:

$ nix-shell -I nixpkgs= -p cowsay

Now let use our new knowledge to get a Python 3.9 shell with various Python libraries and ipython:

$ nix-shell -I nixpkgs= -p \
python39 python39Packages.tox python39Packages.flake8 python39Packages.requests \

$ type python tox flake8 ipython
python is /nix/store/h4h5rxs0hzpzvz37yrwv1k2na1acgzww-python3-3.9.15/bin/python
tox is hashed (/nix/store/0iifww8anqsg84apj0dklrpiqjwn1nzy-python3.9-tox-3.27.1/bin/tox)
flake8 is hashed (/nix/store/mri6xdgqa5b4hj7by88mlidksi1h7kd2-python3.9-flake8-5.0.4/bin/flake8)
ipython is /nix/store/1kgkssy7lkgsxpjii618ddjq2v03473x-python3.9-ipython-8.4.0/bin/ipython

$ python --version && flake8 --version && tox --version && ipython --version
Python 3.9.15
5.0.4 (mccabe: 0.7.0, pycodestyle: 2.9.1, pyflakes: 2.5.0) CPython 3.9.15 on Linux
3.27.1 imported from /nix/store/0iifww8anqsg84apj0dklrpiqjwn1nzy-python3.9-tox-3.27.1/lib/python3.9/site-packages/tox/

$ exit

# Note that running again the nix-shell command will enter the shell instantanously as all
# binaries have been fetched into /nix/store already.

If you try the same commands as above on your machine you should see the extact same output.

Currently, nixpkgs owns definitions for around 80,000 packages. You can search for available packages on

A simple shell.nix definition

The nix-shell command looks for a shell.nix file in the current directory and if it exists the shell environment is loaded. This is handy in order to share with co-workers a common and reproducible work environment for a given project. Since it is a pure text file, it can also be easily versioned with git.

As the most simple example of shell.nix to deploy the previous Python environment:

{ pkgs ? import (fetchTarball "") {} }:

let fooScript = pkgs.writeScriptBin "" ''
  echo $FOO

in pkgs.mkShell {
  name = "My-project build environment";
  buildInputs = [
  shellHook = ''
    echo "Welcome in $name"
    export FOO="BAR"

This shell.nix sample describes a shell with:

  • Some Python packages available
  • A script available in the PATH
  • Some commands (via shellHook to run a shell startup)

Enter the shell by typing: nix-shell.

To go further

In this post we learned the basic steps to bootstrap a simple shell environment with Nix. However more complex and reproducible environment setups can be built via a Nix shell, like the setup of services (MariaDB, Zookeeper, ...), installation of additional scripts, compilation/installation of softwares and libraries not available in nixpkgs, but this goes beyond that simple introdution.

Here are some interesting resources to continue your learning.