7.4 KiB
Introduction
Nix is a purely functional package manager. This means that it
treats packages like values in purely functional programming languages
such as Haskell — they are built by functions that don’t have
side-effects, and they never change after they have been built. Nix
stores packages in the Nix store, usually the directory
/nix/store, where each package has its own unique subdirectory such
as
/nix/store/b6gvzjyb2pg0kjfwrjmg1vfhh54ad73z-firefox-33.1/
where b6gvzjyb2pg0… is a unique identifier for the package that
captures all its dependencies (it’s a cryptographic hash of the
package’s build dependency graph). This enables many powerful
features.
Multiple versions
You can have multiple versions or variants of a package installed at the same time. This is especially important when different applications have dependencies on different versions of the same package — it prevents the “DLL hell”. Because of the hashing scheme, different versions of a package end up in different paths in the Nix store, so they don’t interfere with each other.
An important consequence is that operations like upgrading or uninstalling an application cannot break other applications, since these operations never “destructively” update or delete files that are used by other packages.
Complete dependencies
Nix helps you make sure that package dependency specifications are complete. In general, when you’re making a package for a package management system like RPM, you have to specify for each package what its dependencies are, but there are no guarantees that this specification is complete. If you forget a dependency, then the package will build and work correctly on your machine if you have the dependency installed, but not on the end user's machine if it's not there.
Since Nix on the other hand doesn’t install packages in “global”
locations like /usr/bin but in package-specific directories, the
risk of incomplete dependencies is greatly reduced. This is because
tools such as compilers don’t search in per-packages directories such
as /nix/store/5lbfaxb722zp…-openssl-0.9.8d/include, so if a package
builds correctly on your system, this is because you specified the
dependency explicitly. This takes care of the build-time dependencies.
Once a package is built, runtime dependencies are found by scanning
binaries for the hash parts of Nix store paths (such as r8vvq9kq…).
This sounds risky, but it works extremely well.
Multi-user support
Nix has multi-user support. This means that non-privileged users can
securely install software. Each user can have a different profile,
a set of packages in the Nix store that appear in the user’s PATH.
If a user installs a package that another user has already installed
previously, the package won’t be built or downloaded a second time.
At the same time, it is not possible for one user to inject a Trojan
horse into a package that might be used by another user.
Atomic upgrades and rollbacks
Since package management operations never overwrite packages in the Nix store but just add new versions in different paths, they are atomic. So during a package upgrade, there is no time window in which the package has some files from the old version and some files from the new version — which would be bad because a program might well crash if it’s started during that period.
And since packages aren’t overwritten, the old versions are still there after an upgrade. This means that you can roll back to the old version:
$ nix-env --upgrade some-packages
$ nix-env --rollback
Garbage collection
When you uninstall a package like this…
$ nix-env --uninstall firefox
the package isn’t deleted from the system right away (after all, you might want to do a rollback, or it might be in the profiles of other users). Instead, unused packages can be deleted safely by running the garbage collector:
$ nix-collect-garbage
This deletes all packages that aren’t in use by any user profile or by a currently running program.
Functional package language
Packages are built from Nix expressions, which is a simple functional language. A Nix expression describes everything that goes into a package build action (a “derivation”): other packages, sources, the build script, environment variables for the build script, etc. Nix tries very hard to ensure that Nix expressions are deterministic: building a Nix expression twice should yield the same result.
Because it’s a functional language, it’s easy to support building variants of a package: turn the Nix expression into a function and call it any number of times with the appropriate arguments. Due to the hashing scheme, variants don’t conflict with each other in the Nix store.
Transparent source/binary deployment
Nix expressions generally describe how to build a package from source, so an installation action like
$ nix-env --install firefox
could cause quite a bit of build activity, as not only Firefox but
also all its dependencies (all the way up to the C library and the
compiler) would have to built, at least if they are not already in the
Nix store. This is a source deployment model. For most users,
building from source is not very pleasant as it takes far too long.
However, Nix can automatically skip building from source and instead
use a binary cache, a web server that provides pre-built
binaries. For instance, when asked to build
/nix/store/b6gvzjyb2pg0…-firefox-33.1 from source, Nix would first
check if the file https://cache.nixos.org/b6gvzjyb2pg0….narinfo
exists, and if so, fetch the pre-built binary referenced from there;
otherwise, it would fall back to building from source.
Nix Packages collection
We provide a large set of Nix expressions containing hundreds of existing Unix packages, the Nix Packages collection (Nixpkgs).
Managing build environments
Nix is extremely useful for developers as it makes it easy to
automatically set up the build environment for a package. Given a Nix
expression that describes the dependencies of your package, the
command nix-shell will build or download those dependencies if
they’re not already in your Nix store, and then start a Bash shell in
which all necessary environment variables (such as compiler search
paths) are set.
For example, the following command gets all dependencies of the Pan newsreader, as described by its Nix expression:
$ nix-shell '<nixpkgs>' -A pan
You’re then dropped into a shell where you can edit, build and test the package:
[nix-shell]$ unpackPhase
[nix-shell]$ cd pan-*
[nix-shell]$ configurePhase
[nix-shell]$ buildPhase
[nix-shell]$ ./pan/gui/pan
Portability
Nix runs on Linux and macOS.
NixOS
NixOS is a Linux distribution based on Nix. It uses Nix not just for
package management but also to manage the system configuration (e.g.,
to build configuration files in /etc). This means, among other
things, that it is easy to roll back the entire configuration of the
system to an earlier state. Also, users can install software without
root privileges. For more information and downloads, see the NixOS
homepage.
License
Nix is released under the terms of the GNU LGPLv2.1 or (at your option) any later version.