75 lines
3.7 KiB
Markdown
75 lines
3.7 KiB
Markdown
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# Activation script {#sec-activation-script}
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The activation script is a bash script called to activate the new
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configuration which resides in a NixOS system in `$out/activate`. Since its
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contents depend on your system configuration, the contents may differ.
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This chapter explains how the script works in general and some common NixOS
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snippets. Please be aware that the script is executed on every boot and system
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switch, so tasks that can be performed in other places should be performed
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there (for example letting a directory of a service be created by systemd using
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mechanisms like `StateDirectory`, `CacheDirectory`, ... or if that's not
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possible using `preStart` of the service).
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Activation scripts are defined as snippets using
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[](#opt-system.activationScripts). They can either be a simple multiline string
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or an attribute set that can depend on other snippets. The builder for the
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activation script will take these dependencies into account and order the
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snippets accordingly. As a simple example:
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```nix
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{
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system.activationScripts.my-activation-script = {
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deps = [ "etc" ];
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# supportsDryActivation = true;
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text = ''
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echo "Hallo i bims"
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'';
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};
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}
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```
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This example creates an activation script snippet that is run after the `etc`
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snippet. The special variable `supportsDryActivation` can be set so the snippet
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is also run when `nixos-rebuild dry-activate` is run. To differentiate between
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real and dry activation, the `$NIXOS_ACTION` environment variable can be
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read which is set to `dry-activate` when a dry activation is done.
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An activation script can write to special files instructing
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`switch-to-configuration` to restart/reload units. The script will take these
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requests into account and will incorporate the unit configuration as described
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above. This means that the activation script will "fake" a modified unit file
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and `switch-to-configuration` will act accordingly. By doing so, configuration
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like [systemd.services.\<name\>.restartIfChanged](#opt-systemd.services) is
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respected. Since the activation script is run **after** services are already
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stopped, [systemd.services.\<name\>.stopIfChanged](#opt-systemd.services)
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cannot be taken into account anymore and the unit is always restarted instead
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of being stopped and started afterwards.
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The files that can be written to are `/run/nixos/activation-restart-list` and
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`/run/nixos/activation-reload-list` with their respective counterparts for
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dry activation being `/run/nixos/dry-activation-restart-list` and
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`/run/nixos/dry-activation-reload-list`. Those files can contain
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newline-separated lists of unit names where duplicates are being ignored. These
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files are not create automatically and activation scripts must take the
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possibility into account that they have to create them first.
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## NixOS snippets {#sec-activation-script-nixos-snippets}
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There are some snippets NixOS enables by default because disabling them would
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most likely break your system. This section lists a few of them and what they
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do:
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- `binsh` creates `/bin/sh` which points to the runtime shell
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- `etc` sets up the contents of `/etc`, this includes systemd units and
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excludes `/etc/passwd`, `/etc/group`, and `/etc/shadow` (which are managed by
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the `users` snippet)
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- `hostname` sets the system's hostname in the kernel (not in `/etc`)
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- `modprobe` sets the path to the `modprobe` binary for module auto-loading
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- `nix` prepares the nix store and adds a default initial channel
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- `specialfs` is responsible for mounting filesystems like `/proc` and `sys`
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- `users` creates and removes users and groups by managing `/etc/passwd`,
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`/etc/group` and `/etc/shadow`. This also creates home directories
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- `usrbinenv` creates `/usr/bin/env`
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- `var` creates some directories in `/var` that are not service-specific
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- `wrappers` creates setuid wrappers like `sudo`
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