This chapter discusses how to do package management with Nix, i.e., how to obtain, install, upgrade, and erase components. This is the “user’s” perspective of the Nix system — people who want to create components should consult .

The main command for package management is nix-env. You can use it to install, upgrade, and erase components, and to query what components are installed or are available for installation. In Nix, different users can have different “views” on the set of installed applications. That is, there might be lots of applications present on the system (possibly in many different versions), but users can have a specific selection of those active — where “active” just means that it appears in a directory in the user’s PATH. Such a view on the set of installed applications is called a user environment, which is just a directory tree consisting of symlinks to the files of the active applications. Components are installed from a set of Nix expressions that tell Nix how to build those components, including, if necessary, their dependencies. There is a collection of Nix expressions called the Nix Package collection that contains components ranging from basic development stuff such as GCC and Glibc, to end-user applications like Mozilla Firefox. (Nix is however not tied to the Nix Package collection; you could write your own Nix expressions based on it, or completely new ones.) You can download the latest version from . Assuming that you have downloaded and unpacked a release of Nix Packages, you can view the set of available components in the release: $ nix-env -qaf nixpkgs-version ant-blackdown-1.4.2 aterm-2.2 bash-3.0 binutils-2.15 bison-1.875d blackdown-1.4.2 bzip2-1.0.2 ... where nixpkgs-version is where you’ve unpacked the release. It is also possible to see the status of available components, i.e., whether they are installed into the user environment and/or present in the system: $ nix-env -qasf nixpkgs-version ... -PS bash-3.0 --S binutils-2.15 IPS bison-1.875d ... The first character (I) indicates whether the component is installed in your current user environment. The second (P) indicates whether it is present on your system (in which case installing it into your user environment would be a very quick operation). The last one (S) indicates whether there is a so-called substitute for the component, which is Nix’s mechanism for doing binary deployment. It just means that Nix know that it can fetch a pre-built component from somewhere (typically a network server) instead of building it locally. So now that we have a set of Nix expressions we can build the components contained in them. This is done using nix-env -i. For instance, $ nix-env -f nixpkgs-version -i subversion will install the component called subversion (which is, of course, the Subversion version management system). When you do this for the first time, Nix will start building Subversion and all its dependencies. This will take quite a while — typically an hour or two on modern machines. Fortunately, there is a faster way (so do a Ctrl-C on that install operation!): you just need to tell Nix that pre-built binaries of all those components are available somewhere. This is done using the nix-pull command, which must be supplied with a URL containing a manifest describing what binaries are available. This URL should correspond to the Nix Packages release that you’re using. For instance, if you obtained a release from , then you should do: $ nix-pull http://nix.cs.uu.nl/dist/nix/nixpkgs-0.6pre1554/MANIFEST If you then issue the installation command, it should start downloading binaries from nix.cs.uu.nl, instead of building them from source. This might still take a while since all dependencies must be downloaded, but on a reasonably fast connection such as an DSL line it’s on the order of a few minutes. Naturally, packages can also be uninstalled: $ nix-env -e subversion Upgrading to a new version is just as easy. If you have a new release of Nix Packages, you can do: $ nix-env -f nixpkgs-version -u subversion This will only upgrade Subversion if there is a “newer” version in the new set of Nix expressions, as defined by some pretty arbitrary rules regarding ordering of version numbers (which generally do what you’d expect of them). To just unconditionally replace Subversion with whatever version is in the Nix expressions, use -i instead of -u; -i will remove whatever version is already installed. You can also upgrade all components for which there are newer versions: $ nix-env -f nixpkgs-version -u '*' Sometimes it’s useful to be able to ask what nix-env would do, without actually doing it. For instance, to find out what packages would be upgraded by nix-env -u '*', you can do $ nix-env ... -u '*' --dry-run (dry run; not doing anything) upgrading `libxslt-1.1.0' to `libxslt-1.1.10' upgrading `graphviz-1.10' to `graphviz-1.12' upgrading `coreutils-5.0' to `coreutils-5.2.1' If you grow bored of specifying the Nix expressions using -f all the time, you can set a default location: $ nix-env -I nixpkgs-version After this you can just say, for instance, nix-env -u '*'.Setting a default using -I currently clashes with using Nix channels, since nix-channel --update calls nix-env -I to set the default to the Nix expressions it downloaded from the channel, replacing whatever default you had set.

Profiles and user environments are Nix’s mechanism for implementing the ability to allow different users to have different configurations, and to do atomic upgrades and rollbacks. To understand how they work, it’s useful to know a bit about how Nix works. In Nix, components are stored in unique locations in the Nix store (typically, /nix/store). For instance, a particular version of the Subversion component might be stored in a directory /nix/store/dpmvp969yhdqs7lm2r1a3gng7pyq6vy4-subversion-1.1.3/, while another version might be stored in /nix/store/5mq2jcn36ldlmh93yj1n8s9c95pj7c5s-subversion-1.1.2. The long strings prefixed to the directory names are cryptographic hashes160-bit truncations of SHA-256 hashes encoded in a base-32 notation, to be precise. of all inputs involved in building the component — sources, dependencies, compiler flags, and so on. So if two components differ in any way, they end up in different locations in the file system, so they don’t interfere with each other. shows a part of a typical Nix store.

Of course, you wouldn’t want to type $ /nix/store/dpmvp969yhdq...-subversion-1.1.3/bin/svn every time you want to run Subversion. Of course we could set up the PATH environment variable to include the bin directory of every component we want to use, but this is not very convenient since changing PATH doesn’t take effect for already existing processes. The solution Nix uses is to create directory trees of symlinks to activated components. These are called user environments and they are components themselves (though automatically generated by nix-env), so they too reside in the Nix store. For instance, in the user environment /nix/store/5mq2jcn36ldl...-user-env contains a symlink to just Subversion 1.1.2 (arrows in the figure indicate symlinks). This would be what we would obtain if we had done $ nix-env -i subversion on a set of Nix expressions that contained Subversion 1.1.2. This doesn’t in itself solve the problem, of course; you wouldn’t want to type /nix/store/0c1p5z4kda11...-user-env/bin/svn either. That’s why there are symlinks outside of the store that point to the user environments in the store; for instance, the symlinks default-42-link and default-43-link in the example. These are called generations since every time you perform a nix-env operation, a new user environment is generated based on the current one. For instance, generation 43 was created from generation 42 when we did $ nix-env -i subversion mozilla on a set of Nix expressions that contained Mozilla and a new version of Subversion. Generations are grouped together into profiles so that different users don’t interfere with each other if they don’t want to. For example: $ ls -l /nix/var/nix/profiles/ ... lrwxrwxrwx 1 eelco ... default-42-link -> /nix/store/0c1p5z4kda11...-user-env lrwxrwxrwx 1 eelco ... default-43-link -> /nix/store/3aw2pdyx2jfc...-user-env lrwxrwxrwx 1 eelco ... default -> default-43-link This shows a profile called default. The file default itself is actually a symlink that points to the current generation. When we do a nix-env operation, a new user environment and generation link are created based on the current one, and finally the default symlink is made to point at the new generation. This last step is atomic on Unix, which explains how we can do atomic upgrades. (Note that the building/installing of new components doesn’t interfere in any way with old components, since they are stored in different locations in the Nix store.) If you find that you want to undo a nix-env operation, you can just do $ nix-env --rollback which will just make the current generation link point at the previous link. E.g., default would be made to point at default-42-link. You can also switch to a specific generation: $ nix-env --switch-generation 43 which in this example would roll forward to generation 43 again. You can also see all available generations: $ nix-env --list-generations Actually, there is another level of indirection not shown in the figure above. You generally wouldn’t have /nix/var/nix/profiles/some-profile/bin in your PATH. Rather, there is a symlink ~/.nix-profile that points to your current profile. This means that you should put ~/.nix-profile/bin in your PATH (and indeed, that’s what the initialisation script /nix/etc/profile.d/nix.sh does). This makes it easier to switch to a different profile. You can do that using the command nix-env --switch-profile: $ nix-env --switch-profile /nix/var/nix/profiles/my-profile $ nix-env --switch-profile /nix/var/nix/profiles/default These commands switch to the my-profile and default profile, respectively. If the profile doesn’t exist, it will be created automatically. You should be careful about storing a profile in another location than the profiles directory, since otherwise it might not be used as a root of the garbage collector (see section ). All nix-env operations work on the profile pointed to by ~/.nix-profile, but you can override this using the --profile option (abbreviation -p): $ nix-env -p /nix/var/nix/profiles/other-profile -i subversion This will not change the ~/.nix-profile symlink.

nix-env operations such as upgrades (-u) and uninstall (-e) never actually delete components from the system. All they do (as shown above) is to create a new user environment that no longer contains symlinks to the “deleted” components. Of course, since disk space is not infinite, unused components should be removed at some point. You can do this by running the Nix garbage collector. It will remove from the Nix store any component not used (directly or indirectly) by any generation of any profile. Note however that as long as old generations reference a component, it will not be deleted. After all, we wouldn’t be able to do a rollback otherwise. So in order for garbage collection to be effective, you should also delete (some) old generations. Of course, this should only be done if you are certain that you will not need to roll back. To delete all old (non-current) generations of your current profile: $ nix-env --delete-generations old Instead of old you can also specify a list of generations, e.g., $ nix-env --delete-generations 10 11 14 After removing appropriate old generations you can run the garbage collector as follows: $ nix-store --gc If you are feeling uncertain, you can also first view what files would be deleted: $ nix-store --gc --print-dead Likewise, the option --print-live will show the paths that won’t be deleted.

The roots of the garbage collector are all store paths to which there are symlinks in the directory prefix/nix/var/nix/gcroots. For instance, the following command makes the path /nix/store/d718ef...-foo a root of the collector: $ ln -s /nix/store/d718ef...-foo /nix/var/nix/gcroots/bar That is, after this command, the garbage collector will not remove /nix/store/d718ef...-foo or any of its dependencies. Subdirectories of prefix/nix/var/nix/gcroots are also searched for symlinks. Symlinks to non-store paths are followed and searched for roots, but symlinks to non-store paths inside the paths reached in that way are not followed to prevent infinite recursion.

If you want to stay up to date with a set of packages, it’s not very convenient to manually download the latest set of Nix expressions for those packages, use nix-pull to register pre-built binaries (if available), and upgrade using nix-env. Fortunately, there’s a better way: Nix channels. A Nix channel is just a URL that points to a place that contains a set of Nix expressions and a manifest. Using the command nix-channel you can automatically stay up to date with whatever is available at that URL. You can “subscribe” to a channel using nix-channel --add, e.g., $ nix-channel --add http://nix.cs.uu.nl/dist/nix/channels-v3/nixpkgs-unstable subscribes you to a channel that always contains that latest version of the Nix Packages collection. (Instead of nixpkgs-unstable you could also subscribe to nixpkgs-stable, which should have a higher level of stability, but right now is just outdated.) Subscribing really just means that the URL is added to the file ~/.nix-channels. Right now there is no command to “unsubscribe”; you should just edit that file manually and delete the offending URL. To obtain the latest Nix expressions available in a channel, do $ nix-channel --update This downloads the Nix expressions in every channel (downloaded from url/nixexprs.tar.bz2) and registers any available pre-built binaries in every channel (by nix-pulling url/MANIFEST). It also makes the union of each channel’s Nix expressions the default for nix-env operations. Consequently, you can then say $ nix-env -u '*' to upgrade all components in your profile to the latest versions available in the subscribed channels.