This chapter describes how you can modify and extend NixOS.

By default, NixOS’s nixos-rebuild command uses the NixOS and Nixpkgs sources provided by the nixos-unstable channel (kept in /nix/var/nix/profiles/per-user/root/channels/nixos). To modify NixOS, however, you should check out the latest sources from Git. This is done using the following command: $ nixos-checkout /my/sources or $ mkdir -p /my/sources $ cd /my/sources $ nix-env -i git $ git clone git://github.com/NixOS/nixpkgs.git This will check out the latest NixOS sources to /my/sources/nixpkgs/nixos and the Nixpkgs sources to /my/sources/nixpkgs. (The NixOS source tree lives in a subdirectory of the Nixpkgs repository.) If you want to rebuild your system using your (modified) sources, you need to tell nixos-rebuild about them using the -I flag: $ nixos-rebuild switch -I nixpkgs=/my/sources/nixpkgs If you want nix-env to use the expressions in /my/sources, use nix-env -f /my/sources/nixpkgs, or change the default by adding a symlink in ~/.nix-defexpr: $ ln -s /my/sources/nixpkgs ~/.nix-defexpr/nixpkgs You may want to delete the symlink ~/.nix-defexpr/channels_root to prevent root’s NixOS channel from clashing with your own tree.

NixOS has a modular system for declarative configuration. This system combines multiple modules to produce the full system configuration. One of the modules that constitute the configuration is /etc/nixos/configuration.nix. Most of the others live in the nixos/modules subdirectory of the Nixpkgs tree. Each NixOS module is a file that handles one logical aspect of the configuration, such as a specific kind of hardware, a service, or network settings. A module configuration does not have to handle everything from scratch; it can use the functionality provided by other modules for its implementation. Thus a module can declare options that can be used by other modules, and conversely can define options provided by other modules in its own implementation. For example, the module pam.nix declares the option security.pam.services that allows other modules (e.g. sshd.nix) to define PAM services; and it defines the option environment.etc (declared by etc.nix) to cause files to be created in /etc/pam.d. In , we saw the following structure of NixOS modules: { config, pkgs, ... }: { option definitions } This is actually an abbreviated form of module that only defines options, but does not declare any. The structure of full NixOS modules is shown in . { config, pkgs, ... }: { imports = [ paths of other modules ]; options = { option declarations }; config = { option definitions }; } The meaning of each part is as follows. This line makes the current Nix expression a function. The variable pkgs contains Nixpkgs, while config contains the full system configuration. This line can be omitted if there is no reference to pkgs and config inside the module. This list enumerates the paths to other NixOS modules that should be included in the evaluation of the system configuration. A default set of modules is defined in the file modules/module-list.nix. These don't need to be added in the import list. The attribute options is a nested set of option declarations (described below). The attribute config is a nested set of option definitions (also described below). shows a module that handles the regular update of the “locate” database, an index of all files in the file system. This module declares two options that can be defined by other modules (typically the user’s configuration.nix): services.locate.enable (whether the database should be updated) and services.locate.period (when the update should be done). It implements its functionality by defining two options declared by other modules: systemd.services (the set of all systemd services) and services.cron.systemCronJobs (the list of commands to be executed periodically by cron). { config, pkgs, ... }: with pkgs.lib; let locatedb = "/var/cache/locatedb"; in { options = { services.locate = { enable = mkOption { type = types.bool; default = false; description = '' If enabled, NixOS will periodically update the database of files used by the locate command. ''; }; period = mkOption { type = types.str; default = "15 02 * * *"; description = '' This option defines (in the format used by cron) when the locate database is updated. The default is to update at 02:15 at night every day. ''; }; }; }; config = { systemd.services.update-locatedb = { description = "Update Locate Database"; path = [ pkgs.su ]; script = '' mkdir -m 0755 -p $(dirname ${locatedb}) exec updatedb --localuser=nobody --output=${locatedb} --prunepaths='/tmp /var/tmp /media /run' ''; }; services.cron.systemCronJobs = optional config.services.locate.enable "${config.services.locate.period} root ${config.systemd.package}/bin/systemctl start update-locatedb.service"; }; }

An option declaration specifies the name, type and description of a NixOS configuration option. It is illegal to define an option that hasn’t been declared in any module. A option declaration generally looks like this: options = { name = mkOption { type = type specification; default = default value; example = example value; description = "Description for use in the NixOS manual."; }; }; The function mkOption accepts the following arguments. type The type of the option (see below). It may be omitted, but that’s not advisable since it may lead to errors that are hard to diagnose. default The default value used if no value is defined by any module. A default is not required; in that case, if the option value is ever used, an error will be thrown. example An example value that will be shown in the NixOS manual. description A textual description of the option, in DocBook format, that will be included in the NixOS manual. Here is a non-exhaustive list of option types: types.bool A Boolean. types.int An integer. types.str A string. types.lines A string. If there are multiple definitions, they are concatenated, with newline characters in between. types.path A path, defined as anything that, when coerced to a string, starts with a slash. This includes derivations. types.listOf t A list of elements of type t (e.g., types.listOf types.str is a list of strings). Multiple definitions are concatenated together. types.attrsOf t A set of elements of type t (e.g., types.attrsOf types.int is a set of name/value pairs, the values being integers). types.nullOr t Either the value null or something of type t. You can also create new types using the function mkOptionType. See lib/types.nix in Nixpkgs for details.

Option definitions are generally straight-forward bindings of values to option names, like config = { services.httpd.enable = true; }; However, sometimes you need to wrap an option definition or set of option definitions in a property to achieve certain effects: If a set of option definitions is conditional on the value of another option, you may need to use mkIf. Consider, for instance: config = if config.services.httpd.enable then { environment.systemPackages = [ ... ]; ... } else {}; This definition will cause Nix to fail with an “infinite recursion” error. Why? Because the value of config.services.httpd.enable depends on the value being constructed here. After all, you could also write the clearly circular and contradictory: config = if config.services.httpd.enable then { services.httpd.enable = false; } else { services.httpd.enable = true; }; The solution is to write: config = mkIf config.services.httpd.enable { environment.systemPackages = [ ... ]; ... }; The special function mkIf causes the evaluation of the conditional to be “pushed down” into the individual definitions, as if you had written: config = { environment.systemPackages = if config.services.httpd.enable then [ ... ] else []; ... }; A module can override the definitions of an option in other modules by setting a priority. All option definitions that do not have the lowest priority value are discarded. By default, option definitions have priority 1000. You can specify an explicit priority by using mkOverride, e.g. services.openssh.enable = mkOverride 10 false; This definition causes all other definitions with priorities above 10 to be discarded. The function mkForce is equal to mkOverride 50. In conjunction with mkIf, it is sometimes useful for a module to return multiple sets of option definitions, to be merged together as if they were declared in separate modules. This can be done using mkMerge: config = mkMerge [ # Unconditional stuff. { environment.systemPackages = [ ... ]; } # Conditional stuff. (mkIf config.services.bla.enable { environment.systemPackages = [ ... ]; }) ];

NixOS has many options, but some are of particular importance to module writers. environment.etc This set defines files in /etc. A typical use is: environment.etc."os-release".text = '' NAME=NixOS ... ''; which causes a file named /etc/os-release to be created with the given contents. system.activationScripts A set of shell script fragments that must be executed whenever the configuration is activated (i.e., at boot time, or after running nixos-rebuild switch). For instance, system.activationScripts.media = '' mkdir -m 0755 -p /media ''; causes the directory /media to be created. Activation scripts must be idempotent. They should not start background processes such as daemons; use systemd.services for that. systemd.services This is the set of systemd services. Example: systemd.services.dhcpcd = { description = "DHCP Client"; wantedBy = [ "multi-user.target" ]; after = [ "systemd-udev-settle.service" ]; path = [ dhcpcd pkgs.nettools pkgs.openresolv ]; serviceConfig = { Type = "forking"; PIDFile = "/run/dhcpcd.pid"; ExecStart = "${dhcpcd}/sbin/dhcpcd --config ${dhcpcdConf}"; Restart = "always"; }; }; which creates the systemd unit dhcpcd.service. The option wantedBy determined which other units pull this one in; multi-user.target is the default target of the system, so dhcpcd.service will always be started. The option serviceConfig.ExecStart provides the main command for the service; it’s also possible to provide pre-start actions, stop scripts, and so on. users.extraUsers users.extraGroups If your service requires special UIDs or GIDs, you can define them with these options. See for details.

With the command nix-build, you can build specific parts of your NixOS configuration. This is done as follows: $ cd /path/to/nixpkgs/nixos $ nix-build -A config.option where option is a NixOS option with type “derivation” (i.e. something that can be built). Attributes of interest include: system.build.toplevel The top-level option that builds the entire NixOS system. Everything else in your configuration is indirectly pulled in by this option. This is what nixos-rebuild builds and what /run/current-system points to afterwards. A shortcut to build this is: $ nix-build -A system system.build.manual.manual The NixOS manual. system.build.etc A tree of symlinks that form the static parts of /etc. system.build.initialRamdisk system.build.kernel The initial ramdisk and kernel of the system. This allows a quick way to test whether the kernel and the initial ramdisk boot correctly, by using QEMU’s -kernel and -initrd options: $ nix-build -A config.system.build.initialRamdisk -o initrd $ nix-build -A config.system.build.kernel -o kernel $ qemu-system-x86_64 -kernel ./kernel/bzImage -initrd ./initrd/initrd -hda /dev/null system.build.nixos-rebuild system.build.nixos-install system.build.nixos-generate-config These build the corresponding NixOS commands. systemd.units.unit-name.unit This builds the unit with the specified name. Note that since unit names contain dots (e.g. httpd.service), you need to put them between quotes, like this: $ nix-build -A 'config.systemd.units."httpd.service".unit' You can also test individual units, without rebuilding the whole system, by putting them in /run/systemd/system: $ cp $(nix-build -A 'config.systemd.units."httpd.service".unit')/httpd.service \ /run/systemd/system/tmp-httpd.service $ systemctl daemon-reload $ systemctl start tmp-httpd.service Note that the unit must not have the same name as any unit in /etc/systemd/system since those take precedence over /run/systemd/system. That’s why the unit is installed as tmp-httpd.service here.

Building a NixOS CD is as easy as configuring your own computer. The idea is to use another module which will replace your configuration.nix to configure the system that would be installed on the CD. Default CD/DVD configurations are available inside nixos/modules/installer/cd-dvd. To build them you have to set NIXOS_CONFIG before running nix-build to build the ISO. $ nix-build -A config.system.build.isoImage -I nixos-config=modules/installer/cd-dvd/installation-cd-minimal.nix Before burning your CD/DVD, you can check the content of the image by mounting anywhere like suggested by the following command: $ mount -o loop -t iso9660 ./result/iso/cd.iso /mnt/iso

Building, burning, and booting from an installation CD is rather tedious, so here is a quick way to see if the installer works properly: $ nix-build -A config.system.build.nixos-install $ dd if=/dev/zero of=diskimage seek=2G count=0 bs=1 $ yes | mke2fs -j diskimage $ mount -o loop diskimage /mnt $ ./result/bin/nixos-install

Complete NixOS GNU/Linux systems can be tested in virtual machines (VMs). This makes it possible to test a system upgrade or configuration change before rebooting into it, using the nixos-rebuild build-vm or nixos-rebuild build-vm-with-bootloader command. The tests/ directory in the NixOS source tree contains several whole-system unit tests. These tests can be runNixOS tests can be run both from NixOS and from a non-NixOS GNU/Linux distribution, provided the Nix package manager is installed. from the NixOS source tree as follows: $ nix-build tests/ -A nfs.test This performs an automated test of the NFS client and server functionality in the Linux kernel, including file locking semantics (e.g., whether locks are maintained across server crashes). It will first build or download all the dependencies of the test (e.g., all packages needed to run a NixOS VM). The test is defined in tests/nfs.nix. If the test succeeds, nix-build will place a symlink ./result in the current directory pointing at the location in the Nix store of the test results (e.g., screenshots, test reports, and so on). In particular, a pretty-printed log of the test is written to log.html, which can be viewed using a web browser like this: $ firefox result/log.html It is also possible to run the test environment interactively, allowing you to experiment with the VMs. For example: $ nix-build tests/ -A nfs.driver $ ./result/bin/nixos-run-vms The script nixos-run-vms starts the three virtual machines defined in the NFS test using QEMU/KVM. The root file system of the VMs is created on the fly and kept across VM restarts in ./hostname.qcow2. Finally, the test itself can be run interactively. This is particularly useful when developing or debugging a test: $ nix-build tests/ -A nfs.driver $ ./result/bin/nixos-test-driver starting VDE switch for network 1 > Perl statements can now be typed in to start or manipulate the VMs: > startAll; (the VMs start booting) > $server->waitForJob("nfs-kernel-nfsd"); > $client1->succeed("flock -x /data/lock -c 'sleep 100000' &"); > $client2->fail("flock -n -s /data/lock true"); > $client1->shutdown; (this releases client1's lock) > $client2->succeed("flock -n -s /data/lock true"); The function testScript executes the entire test script and drops you back into the test driver command line upon its completion. This allows you to inspect the state of the VMs after the test (e.g. to debug the test script). This and other tests are continuously run on the Hydra instance at nixos.org, which allows developers to be notified of any regressions introduced by a NixOS or Nixpkgs change. The actual Nix programming interface to VM testing is in NixOS, under lib/testing.nix. This file defines a function which takes an attribute set containing a nixpkgs attribute (the path to a Nixpkgs checkout), and a system attribute (the system type). It returns an attribute set containing several utility functions, among which the main entry point is makeTest. The makeTest function takes a function similar to that found in tests/nfs.nix (discussed above). It returns an attribute set containing (among others): test A derivation containing the test log as an HTML file, as seen above, suitable for presentation in the Hydra continuous build system. report A derivation containing a code coverage report, with meta-data suitable for Hydra. driver A derivation containing scripts to run the VM test or interact with the VM network interactively, as seen above.