In the Ad hoc developer environments tutorial we looked at providing shell environments for when we need a quick’n’dirty way of getting hold of some tools.
In this tutorial we’ll take a look how to create reproducible shell environments given a declarative configuration file called a Nix expression.
This is the quickest approach to getting started with Nix:
use single command to invoke it via nix-shell
it works across different operating systems (Linux / MacOS)
you share the exact same environment with all developers
Developer environments allow you to:
provide CLI tools, such as psql, jq, tmux, etc
provide developer libraries, such as zlib, openssl, etc
set shell environment variables
execute bash during environment activation
At the top-level of your project create shell.nix with the following contents:
{ pkgs ? import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/3590f02e7d5760e52072c1a729ee2250b5560746.tar.gz") {} }:
pkgs.mkShell {
buildInputs = [
pkgs.which
pkgs.htop
pkgs.zlib
];
}
Note
To understand the first line, read through pinning nixpkgs tutorial.
We import nixpkgs and make a shell with which and htop available in $PATH.
zlib provides libraries and headers in case we’re compiling something against it.
To enter the environment:
$ nix-shell
these paths will be fetched (0.07 MiB download, 0.20 MiB unpacked):
/nix/store/072a6x7rwv5f8wr6f5s1rq8nnm767cfp-htop-2.2.0
copying path '/nix/store/072a6x7rwv5f8wr6f5s1rq8nnm767cfp-htop-2.2.0' from 'https://cache.nixos.org'...
[nix-shell:~]$
The command will start downloading the missing packages from the https://cache.nixos.org binary cache.
Once it’s done, you are dropped into a new
shell. This shell provides the packages specified in shell.nix.
Run htop to confirm that it is present. Quit the program by hitting
q.
Now, try which htop to check where the htop command is on disk.
You should see something similar to this:
[nix-shell:~]$ which htop
/nix/store/y3w2i8kfdbfj9rx287ad52rahjpgv423-htop-2.2.0/bin/htop
Given the following shell.nix:
{ pkgs ? import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/3590f02e7d5760e52072c1a729ee2250b5560746.tar.gz") {} }:
pkgs.mkShell {
buildInputs = [
pkgs.which
pkgs.htop
pkgs.zlib
];
shellHook = ''
echo hello
'';
MY_ENVIRONMENT_VARIABLE = "world";
}
Running nix-shell we observe:
$ nix-shell
hello
[nix-shell:~]$ echo $MY_ENVIRONMENT_VARIABLE
world
The shellHook section allows you to execute bash while entering the shell environment.
Any attributes passed to mkShell function are available once the shell environment is active.
direnv: Automatically activating the environment on directory changeBesides activating the environment for each project, every time you change
shell.nix you need to re-enter the shell.
You can use direnv to automate this process for you, with the downside that each developer needs
to install it globally.
direnvAt the top-level of your project run:
echo "use nix" > .envrc && direnv allow
The next time your launch your terminal and enter the top-level of your project direnv will check for changes.
$ cd myproject
direnv: loading myproject/.envrc
direnv: using nix
hello
Towards reproducibility: Pinning nixpkgs to see different ways to import nixpkgs
To quickly set up a Nix project read through Getting started Nix template.