Throwaway development environments with Nix.
I use Nix a lot.
Nix is a bunch of different things. It’s a programming language, designed for expressing a build pipeline. It’s a package manager. It (well, NixOS) is an operating system based on that package manager.
Today I’d like to talk about the package manager. Specifically, a lovely gateway into the rest of the ecosystem, nix-shell.
Some people will tell you that the point of Nix is to set up your software so it can be built with Nix, which allows you to tightly control all dependencies and emit something that is as close to reproducible as possible. I am all for this, but if we can tightly control the dependencies without actually building inside a Nix environment, we’ve still improved the reproducibility a lot, and it’s not that hard.
To follow along, you’ll need to first install Nix. It works on Linux and macOS; if you use Windows, I recommend checking out the Windows Subsystem for Linux.
Running an arbitrary program with nix-shell
nix-shell does two things. It will read an environment specification from a file named shell.nix and load up a bash shell (which you can override) with that environment present. Or it will do the same thing with a list of packages supplied on the command line.
For example, if I want to run the cowsay program, I don’t have to install it: I can load a shell with that program.
$ nix-shell -p cowsay
this path will be fetched (0.01 MiB download, 0.05 MiB unpacked):
/nix/store/x87xaaad225x5x9gv15mn01mf204kycv-cowsay-3.04
copying path ‘/nix/store/x87xaaad225x5x9gv15mn01mf204kycv-cowsay-3.04’ from ‘https://cache.nixos.org’…

[nix-shell:~]$ cowsay hello

< hello >

    \   ^__^
     \  (oo)\_______
        (__)\       )\/\
            ||----w |
            ||     ||

Running nix-shell -p cowsay will download cowsay from the nixpkgs package repository, and then launch a bash shell with that program available, by adding it to the PATH.
We can verify that:
[nix-shell:~]$ echo $PATH
…:/nix/store/x87xaaad225x5x9gv15mn01mf204kycv-cowsay-3.04/bin:…
There it is: cowsay, downloaded to the Nix store.
(You’ll see more than just that in the PATH, and you may see nix-shell download a lot of extra packages the first time you use it. Just sit tight.)
You may be wondering what that long string of letters and numbers is in the package directory name. It’s a hash of all the inputs, which means that if one of the dependencies change, the hash will change too. The source of the program is also considered an input, so if we upgrade cowsay to a new version (I find it awesome that there are so many versions of this program), the hash will also change.
This means that we can have various different versions all available in our local store at once, and they won’t collide with each other. Perhaps not so important for cowsay, but when we start dealing with programming languages, this gets interesting.

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