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](03-enter-environment.md) pill.**
For installation instructions, please refer to the Nix Reference Manual on [ Installing Nix](https://nixos.org/manual/nix/stable/installation/installing-binary.html).
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: <https://nixos.org/manual/nix/stable/installation/installing-binary.html#multi-user-installation>.
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.
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.
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.
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`.
Nix expressions are written in the [Nix language](https://nix.dev/tutorials/nix-language) and used to describe packages and how to build them. [Nixpkgs](https://nixos.org/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`.
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.
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`.
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.
