Chapter 3. Enter the Environment
Welcome to the third Nix pill. In the second pill we installed Nix on our running system. Now we can finally play with it a little, these things also apply to NixOS users.
3.1. Enter the environment
If you're using NixOS, you can skip to the next step.
In the previous article we created a Nix user, so let's start by switching
to it with su - nix. If your
~/.profile got evaluated, then you should now be able
to run commands like
If that's not the case:
$ source ~/.nix-profile/etc/profile.d/nix.sh
To remind you,
~/.nix-profile/etc points to the
derivation. At this point, we are in our Nix user profile.
3.2. Install something
Finally something practical! Installation into the Nix environment is an
interesting process. Let's install
hello, a simple CLI
tool which prints
Hello world and is mainly used to test compilers
and package installations.
Back to the installation:
$ nix-env -i hello installing 'hello-2.10' [...] building '/nix/store/0vqw0ssmh6y5zj48yg34gc6macr883xk-user-environment.drv'... created 36 symlinks in user environment
Now you can run
hello. Things to notice:
We installed software as a user, and only for the Nix user.
It created a new user environment. That's a new generation of our Nix user profile.
The nix-env tool manages environments, profiles and their generations.
helloby derivation name minus the version. I repeat: we specified the derivation name (minus the version) to install it.
We can list generations without walking through the
$ nix-env --list-generations 1 2014-07-24 09:23:30 2 2014-07-25 08:45:01 (current)
Listing installed derivations:
$ nix-env -q nix-2.1.3 hello-2.10
So, where did
hello really get installed?
which hello is
~/.nix-profile/bin/hello which points to the store.
We can also list the derivation paths with nix-env -q --out-path. So
that's what those derivation paths are called: the
output of a build.
3.3. Path merging
At this point you probably want to run
man to get some documentation.
Even if you
already have man system-wide outside of the Nix environment, you can
install and use it within Nix with nix-env -i man-db. As
usual, a new generation will be created, and
~/.nix-profile will point to
Let's inspect the profile a bit:
$ ls -l ~/.nix-profile/ dr-xr-xr-x 2 nix nix 4096 Jan 1 1970 bin lrwxrwxrwx 1 nix nix 55 Jan 1 1970 etc -> /nix/store/ig31y9gfpp8pf3szdd7d4sf29zr7igbr-nix-2.1.3/etc [...]
Now that's interesting. When only
nix-2.1.3 was installed,
bin was a
nix-2.1.3. Now that we've actually installed some things
hello), it's a real directory, not a symlink.
$ ls -l ~/.nix-profile/bin/ [...] man -> /nix/store/83cn9ing5sc6644h50dqzzfxcs07r2jn-man-1.6g/bin/man [...] nix-env -> /nix/store/ig31y9gfpp8pf3szdd7d4sf29zr7igbr-nix-2.1.3/bin/nix-env [...] hello -> /nix/store/58r35bqb4f3lxbnbabq718svq9i2pda3-hello-2.10/bin/hello [...]
Okay, that's clearer now.
nix-env merged the paths from the installed derivations.
which man points to the Nix profile, rather than the
~/.nix-profile/bin is at the head
3.4. Rolling back and switching generation
The last command installed
man. We should be at generation 3, unless
you changed something in the middle. Let's say we want to rollback to the
$ nix-env --rollback switching from generation 3 to 2
Now nix-env -q does not list
ls -l `which man` should now be your system copy.
Enough with the rollback, let's go back to the most recent generation:
$ nix-env -G 3 switching from generation 2 to 3
I invite you to read the manpage of
nix-env requires an operation
to perform, then there are common options for all operations, as well as
options specific to each operation.
3.5. Querying the store
So far we learned how to query and manipulate the environment. But all of the environment components point to the store.
To query and manipulate the store, there's the
nix-store command. We can do some interesting things, but we'll
only see some queries for now.
To show the direct runtime dependencies of
$ nix-store -q --references `which hello` /nix/store/fg4yq8i8wd08xg3fy58l6q73cjy8hjr2-glibc-2.27 /nix/store/58r35bqb4f3lxbnbabq718svq9i2pda3-hello-2.10
The argument to
nix-store can be anything as long as it points to the
Nix store. It will follow symlinks.
It may not make sense to you right now, but let's print reverse
$ nix-store -q --referrers `which hello` /nix/store/58r35bqb4f3lxbnbabq718svq9i2pda3-hello-2.10 /nix/store/fhvy2550cpmjgcjcx5rzz328i0kfv3z3-env-manifest.nix /nix/store/yzdk0xvr0b8dcwhi2nns6d75k2ha5208-env-manifest.nix /nix/store/mp987abm20c70pl8p31ljw1r5by4xwfw-user-environment /nix/store/ppr3qbq7fk2m2pa49i2z3i32cvfhsv7p-user-environment
Was it what you expected? It turns out that our environments depend upon
Yes, that means that the environments are in the store, and since they contain symlinks to
therefore the environment depends upon
Two environments were listed, generation 2 and generation 3, since these are the ones that had
hello installed in them.
manifest.nix file contains metadata about the environment, such as
which derivations are installed. So that
nix-env can list, upgrade
or remove them. And yet again, the current
manifest.nix can be found at
The closures of a derivation is a list of all its dependencies, recursively, including absolutely everything necessary to use that derivation.
$ nix-store -qR `which man` [...]
Copying all those derivations to the Nix store of another machine makes
you able to run
man out of the box on that other machine. That's the
base of deployment using Nix, and you can already foresee the potential when
deploying software in the cloud (hint:
A nicer view of the closure:
$ nix-store -q --tree `which man` [...]
With the above command, you can find out exactly why a runtime dependency, be it direct or indirect, exists for a given derivation.
The same applies to environments. As an exercise, run
nix-store -q --tree ~/.nix-profile, and see that the
first children are direct dependencies of the user environment:
the installed derivations, and the
3.7. Dependency resolution
There isn't anything like
apt which solves a SAT problem in order to
satisfy dependencies with lower and upper bounds on versions. There's no need
for this because all the dependencies are static: if a derivation X depends on a derivation Y,
then it always depends on it. A version of X which depended on Z would be a different derivation.
3.8. Recovering the hard way
$ nix-env -e '*' uninstalling 'hello-2.10' uninstalling 'nix-2.1.3' [...]
Oops, that uninstalled all derivations from the environment, including
Nix. That means we can't even run
nix-env, what now?
Previously we got
nix-env from the environment. Environments
are a convenience for the user, but Nix is still there in the store!
First, pick one
ls /nix/store/*nix-2.1.3, say
The first option is to rollback:
$ /nix/store/ig31y9gfpp8pf3szdd7d4sf29zr7igbr-nix-2.1.3/bin/nix-env --rollback
The second option is to install Nix, thus creating a new generation:
$ /nix/store/ig31y9gfpp8pf3szdd7d4sf29zr7igbr-nix-2.1.3/bin/nix-env -i /nix/store/ig31y9gfpp8pf3szdd7d4sf29zr7igbr-nix-2.1.3/bin/nix-env
So where are we getting packages from? We said something about this already in the second article. There's a list of channels from which we get packages, although usually we use a single channel. The tool to manage channels is nix-channel.
$ nix-channel --list nixpkgs http://nixos.org/channels/nixpkgs-unstable
If you're using NixOS, you may not see any output from the above command (if you're using the default), or you may see a channel whose name begins with "nixos-" instead of "nixpkgs".
That's essentially the contents of
To update the channel run nix-channel --update.
That will download the new Nix expressions (descriptions of the packages),
create a new generation of the channels profile and unpack it under
This is quite similar to apt-get update. (See this table for a rough mapping between Ubuntu and NixOS package management.)
We learned how to query the user environment and to manipulate it by installing and uninstalling software. Upgrading software is also straightforward, as you can read in the manual (nix-env -u will upgrade all packages in the environment).
Everytime we change the environment, a new generation is created. Switching between generations is easy and immediate.
Then we learned how to query the store. We inspected the dependencies and reverse dependencies of store paths.
We saw how symlinks are used to compose paths from the Nix store, a useful trick.
A quick analogy with programming languages: you have the heap with all the objects, that corresponds to the Nix store. You have objects that point to other objects, those correspond to derivations. This is a suggestive metaphor, but will it be the right path?
3.11. Next pill
...we will learn the basics of the Nix language. The Nix language is used to describe how to build derivations, and it's the basis for everything else, including NixOS. Therefore it's very important to understand both the syntax and the semantics of the language.