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1# Activation script {#sec-activation-script}
2
3The activation script is a bash script called to activate the new
4configuration which resides in a NixOS system in `$out/activate`. Since its
5contents depend on your system configuration, the contents may differ.
6This chapter explains how the script works in general and some common NixOS
7snippets. Please be aware that the script is executed on every boot and system
8switch, so tasks that can be performed in other places should be performed
9there (for example letting a directory of a service be created by systemd using
10mechanisms like `StateDirectory`, `CacheDirectory`, ... or if that's not
11possible using `preStart` of the service).
12
13Activation scripts are defined as snippets using
14[](#opt-system.activationScripts). They can either be a simple multiline string
15or an attribute set that can depend on other snippets. The builder for the
16activation script will take these dependencies into account and order the
17snippets accordingly. As a simple example:
18
19```nix
20system.activationScripts.my-activation-script = {
21 deps = [ "etc" ];
22 # supportsDryActivation = true;
23 text = ''
24 echo "Hallo i bims"
25 '';
26};
27```
28
29This example creates an activation script snippet that is run after the `etc`
30snippet. The special variable `supportsDryActivation` can be set so the snippet
31is also run when `nixos-rebuild dry-activate` is run. To differentiate between
32real and dry activation, the `$NIXOS_ACTION` environment variable can be
33read which is set to `dry-activate` when a dry activation is done.
34
35An activation script can write to special files instructing
36`switch-to-configuration` to restart/reload units. The script will take these
37requests into account and will incorporate the unit configuration as described
38above. This means that the activation script will "fake" a modified unit file
39and `switch-to-configuration` will act accordingly. By doing so, configuration
40like [systemd.services.\<name\>.restartIfChanged](#opt-systemd.services) is
41respected. Since the activation script is run **after** services are already
42stopped, [systemd.services.\<name\>.stopIfChanged](#opt-systemd.services)
43cannot be taken into account anymore and the unit is always restarted instead
44of being stopped and started afterwards.
45
46The files that can be written to are `/run/nixos/activation-restart-list` and
47`/run/nixos/activation-reload-list` with their respective counterparts for
48dry activation being `/run/nixos/dry-activation-restart-list` and
49`/run/nixos/dry-activation-reload-list`. Those files can contain
50newline-separated lists of unit names where duplicates are being ignored. These
51files are not create automatically and activation scripts must take the
52possibility into account that they have to create them first.
53
54## NixOS snippets {#sec-activation-script-nixos-snippets}
55
56There are some snippets NixOS enables by default because disabling them would
57most likely break your system. This section lists a few of them and what they
58do:
59
60- `binsh` creates `/bin/sh` which points to the runtime shell
61- `etc` sets up the contents of `/etc`, this includes systemd units and
62 excludes `/etc/passwd`, `/etc/group`, and `/etc/shadow` (which are managed by
63 the `users` snippet)
64- `hostname` sets the system's hostname in the kernel (not in `/etc`)
65- `modprobe` sets the path to the `modprobe` binary for module auto-loading
66- `nix` prepares the nix store and adds a default initial channel
67- `specialfs` is responsible for mounting filesystems like `/proc` and `sys`
68- `users` creates and removes users and groups by managing `/etc/passwd`,
69 `/etc/group` and `/etc/shadow`. This also creates home directories
70- `usrbinenv` creates `/usr/bin/env`
71- `var` creates some directories in `/var` that are not service-specific
72- `wrappers` creates setuid wrappers like `ping` and `sudo`