Install on Your Running System

Welcome to the second Nix pill. In the first pill we briefly described Nix.

Now we'll install Nix on our running system and understand what changed in our system after the installation. If you're using NixOS, Nix is already installed; you can skip to the next pill.

For installation instructions, please refer to the Nix Reference Manual on Installing Nix.

Installation

These articles are not a tutorial on using Nix. Instead, we're going to walk through the Nix system to understand the fundamentals.

The first thing to note: derivations in the Nix store refer to other derivations which are themselves in the Nix store. They don't use libc from our system or anywhere else. It's a self-contained store of all the software we need to bootstrap up to any particular package.

Note: In a multi-user installation, such as the one used in NixOS, the store is owned by root and multiple users can install and build software through a Nix daemon. You can read more about multi-user installations here.

The beginnings of the Nix store

Start looking at the output of the install command:

copying Nix to /nix/store..........................

That's the /nix/store we were talking about in the first article. We're copying in the necessary software to bootstrap a Nix system. You can see bash, coreutils, the C compiler toolchain, perl libraries, sqlite and Nix itself with its own tools and libnix.

You may have noticed that /nix/store can contain not only directories, but also files, still always in the form «hash-name».

The Nix database

Right after copying the store, the installation process initializes a database:

initialising Nix database...

Yes, Nix also has a database. It's stored under /nix/var/nix/db. It is a sqlite database that keeps track of the dependencies between derivations.

The schema is very simple: there's a table of valid paths, mapping from an auto increment integer to a store path.

Then there's a dependency relation from path to paths upon which they depend.

You can inspect the database by installing sqlite (nix-env -iA sqlite -f '<nixpkgs>') and then running sqlite3 /nix/var/nix/db/db.sqlite.

Note: If this is the first time you're using Nix after the initial installation, remember you must close and open your terminals first, so that your shell environment will be updated.

Important: Never change /nix/store manually. If you do, then it will no longer be in sync with the sqlite db, unless you really know what you are doing.

The first profile

Next in the installation, we encounter the concept of the profile:

creating /home/nix/.nix-profile
installing 'nix-2.1.3'
building path(s) `/nix/store/a7p1w3z2h8pl00ywvw6icr3g5l9vm5r7-user-environment'
created 7 symlinks in user environment

A profile in Nix is a general and convenient concept for realizing rollbacks. Profiles are used to compose components that are spread among multiple paths under a new unified path. Not only that, but profiles are made up of multiple "generations": they are versioned. Whenever you change a profile, a new generation is created.

Generations can be switched and rolled back atomically, which makes them convenient for managing changes to your system.

Let's take a closer look at our profile:

$ ls -l ~/.nix-profile/
bin -> /nix/store/ig31y9gfpp8pf3szdd7d4sf29zr7igbr-nix-2.1.3/bin
[...]
manifest.nix -> /nix/store/q8b5238akq07lj9gfb3qb5ycq4dxxiwm-env-manifest.nix
[...]
share -> /nix/store/ig31y9gfpp8pf3szdd7d4sf29zr7igbr-nix-2.1.3/share

That nix-2.1.3 derivation in the Nix store is Nix itself, with binaries and libraries. The process of "installing" the derivation in the profile basically reproduces the hierarchy of the nix-2.1.3 store derivation in the profile by means of symbolic links.

The contents of this profile are special, because only one program has been installed in our profile, therefore e.g. the bin directory points to the only program which has been installed (Nix itself).

But that's only the contents of the latest generation of our profile. In fact, ~/.nix-profile itself is a symbolic link to /nix/var/nix/profiles/default.

In turn, that's a symlink to default-1-link in the same directory. Yes, that means it's the first generation of the default profile.

Finally, default-1-link is a symlink to the nix store "user-environment" derivation that you saw printed during the installation process.

We'll talk about manifest.nix more in the next article.

Nixpkgs expressions

More output from the installer:

downloading Nix expressions from `http://releases.nixos.org/nixpkgs/nixpkgs-14.10pre46060.a1a2851/nixexprs.tar.xz'...
unpacking channels...
created 2 symlinks in user environment
modifying /home/nix/.profile...

Nix expressions are written in the Nix language and used to describe packages and how to build them. Nixpkgs is the repository containing all of the expressions: https://github.com/NixOS/nixpkgs.

The installer downloaded the package descriptions from commit a1a2851.

The second profile we discover is the channels profile. ~/.nix-defexpr/channels points to /nix/var/nix/profiles/per-user/nix/channels which points to channels-1-link which points to a Nix store directory containing the downloaded Nix expressions.

Channels are a set of packages and expressions available for download. Similar to Debian stable and unstable, there's a stable and unstable channel. In this installation, we're tracking nixpkgs-unstable.

Don't worry about Nix expressions yet, we'll get to them later.

Finally, for your convenience, the installer modified ~/.profile to automatically enter the Nix environment. What ~/.nix-profile/etc/profile.d/nix.sh really does is simply to add ~/.nix-profile/bin to PATH and ~/.nix-defexpr/channels/nixpkgs to NIX_PATH. We'll discuss NIX_PATH later.

Read nix.sh, it's short.

FAQ: Can I change /nix to something else?

You can, but there's a good reason to keep using /nix instead of a different directory. All the derivations depend on other derivations by using absolute paths. We saw in the first article that bash referenced a glibc under a specific absolute path in /nix/store.

You can see for yourself, don't worry if you see multiple bash derivations:

$ ldd /nix/store/*bash*/bin/bash
[...]

Keeping the store in /nix means we can grab the binary cache from nixos.org (just like you grab packages from debian mirrors) otherwise:

  • glibc would be installed under /foo/store

  • Thus bash would need to point to glibc under /foo/store, instead of under /nix/store

  • So the binary cache can't help, because we need a different bash, and so we'd have to recompile everything ourselves.

After all /nix is a sensible place for the store.

Conclusion

We've installed Nix on our system, fully isolated and owned by the nix user as we're still coming to terms with this new system.

We learned some new concepts like profiles and channels. In particular, with profiles we're able to manage multiple generations of a composition of packages, while with channels we're able to download binaries from nixos.org.

The installation put everything under /nix, and some symlinks in the Nix user home. That's because every user is able to install and use software in her own environment.

I hope I left nothing uncovered so that you think there's some kind of magic going on behind the scenes. It's all about putting components in the store and symlinking these components together.

Next pill...

...we will enter the Nix environment and learn how to interact with the store.