This chapter provides a guided tour of Nix. Let's start from the perspective of an end user. Common operations at this level are to install and remove packages, ask what packages are installed or available for installation, and so on. These are operations on the user environment: the set of packages that a user sees. In a command line Unix environment, this means the set of programs that are available through the PATH environment variable. (In other environments it might mean the set of programs available on the desktop, through the start menu, and so on.) The terms installation and uninstallation are used in this context to denote the act of adding or removing packages from the user environment. In Nix, these operations are dissociated from the physical copying or deleting of files. Installation requires that the files constituting the package are present, but they may be present beforehand. Likewise, uninstallation does not actually delete any files; this is done automatically by running a garbage collector. User environments are manipulated through the nix-env command. The query operation can be used to see what packages are currently installed. $ nix-env -q MozillaFirebird-0.7 sylpheed-0.9.7 pan-0.14.2 (-q is actually short for --query --installed.) The package names are symbolic: they don't have any particular significance to Nix (as they shouldn't, since they are not unique—there can be many derivations with the same name). Note that these packages have many dependencies (e.g., Mozilla uses the gtk+ package) but these have not been installed in the user environment, though they are present on the system. Generally, there is no need to install such packages; only packages containing programs should be installed. To install packages, a Nix expression is required that tells Nix how to build that package. There is a set of standard of Nix expressions for many common packages. Assuming that you have downloaded and unpacked these, you can view the set of available packages: $ nix-env -qf pkgs/system/i686-suse-linux.nix gettext-0.12.1 sylpheed-0.9.7 aterm-2.0 gtk+-1.2.10 apache-httpd-2.0.48 pan-0.14.2 ... The Nix expression in the file i686-suse-linux.nix yields the set of packages for a SuSE Linux system running on x86 hardware. For other platforms, copy and modify this file for your platform as appropriate. [TODO: improve this] It is also possible to see the status of available packages, i.e., whether they are installed into the user environment and/or present in the system: $ nix-env -qf pkgs/system/i686-suse-linux.nix -P gettext-0.12.1 IP sylpheed-0.9.7 -- aterm-2.0 -P gtk+-1.2.10 This reveals that the sylpheed package is already installed, or more precisely, that exactly the same instantiation of sylpheed is installed. This guarantees that the available package is exactly the same as the installed package with regard to sources, dependencies, build flags, and so on. Similarly, we see that the gettext and gtk+ packages are present but not installed in the user environment, while the aterm package is not installed or present at all (so, if we were to install it, it would have to be built or downloaded first). The install operation is used install available packages from a Nix environment. To install the pan package (a newsreader), you would do: $ nix-env -i pkgs/system/i686-suse-linux.nix pan-0.14.2 Since installation may take a long time, depending on whether any packages need to be built or downloaded, it's a good idea to make nix-env run verbosely by using the -v (--verbose) option. This option may be repeated to increase the level of verbosity. A good value is 3 (-vvv). In fact, if you run this command verbosely you will observe that Nix starts to build many packages, including large and fundamental ones such as glibc and gcc. I.e., you are performing a source installation. This is generally undesirable, since installation from sources may require large amounts of disk and CPU resources. Therefore a binary installation is generally preferable. Rather than provide different mechanisms to create and perform the installation of binary packages, Nix supports binary deployment transparently through a generic mechanism of substitute expressions. If an request is made to build some Nix expression, Nix will first try to build any substitutes for that expression. These substitutes presumably perform an identical build operation with respect to the result, but require less resources. For instance, a substitute that downloads a pre-built package from the network requires less CPU and disk resources, and possibly less time. Nix's use of cryptographic hashes makes this entirely safe. It is not possible, for instance, to accidentally substitute a build of some package for a Solaris or Windows system for a build on a SuSE/x86 system. While the substitute mechanism is a generic mechanism, Nix provides two standard tools called nix-push and nix-push that maintain and use a shared cache of prebuilt derivations on some network site (reachable through HTTP). If you attempt to install some package that someone else has previously built and pushed into the cache, and you have done a pull to register substitutes that download these prebuilt packages, then the installation will automatically use these. For example, to pull from our cache of prebuilt packages (at the time of writing, for SuSE Linux/x86), use the following command: $ nix-pull http://losser.st-lab.cs.uu.nl/~eelco/nix-dist/ obtaining list of Nix archives at http://losser.st-lab.cs.uu.nl/~eelco/nix-dist... ... If nix-pull is run without any arguments, it will pull from the URLs specified in the file prefix/etc/nix/prebuilts.conf. Assuming that the pan installation produced no errors, it can be used immediately, that is, it now appears in a directory in the PATH environment variable. Specifically, PATH includes the entry prefix/var/nix/links/current/bin, where prefix/var/nix/links/current is just a symlink to the current user environment: $ ls -l /nix/var/nix/links/ ... lrwxrwxrwx 1 eelco ... 15 -> /nix/store/1871...12b0-user-environment lrwxrwxrwx 1 eelco ... 16 -> /nix/store/59ba...df6b-user-environment lrwxrwxrwx 1 eelco ... current -> /nix/var/nix/links/16 That is, current in this example is a link to 16, which is the current user environment. Before the installation, it pointed to 15. Note that this means that you can atomically roll-back to the previous user environment by pointing the symlink current at 15 again. This also shows that operations such as installation are atomic in the Nix system: any arbitrarily complex set of installation, uninstallation, or upgrade actions eventually boil down to the single operation of pointing a symlink somewhere else (which can be implemented atomically in Unix). What's in a user environment? It's just a set of symlinks to the files that constitute the installed packages. For instance: $ ls -l /nix/var/nix/links/16/bin lrwxrwxrwx 1 eelco ... MozillaFirebird -> /nix/store/35f8...4ae6-MozillaFirebird-0.7/bin/MozillaFirebird lrwxrwxrwx 1 eelco ... svn -> /nix/store/3829...fb5d-subversion-0.32.1/bin/svn ... Note that, e.g., svn = /nix/var/nix/links/current/bin/svn = /nix/var/nix/links/16/bin/svn = /nix/store/59ba...df6b-user-environment/bin/svn = /nix/store/3829...fb5d-subversion-0.32.1/bin/svn. Naturally, packages can also be uninstalled: $ nix-env -u pan-0.14.2