home-manager/docs/manual/internals/activation.md

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Activation

Activating a Home Manager configuration ensures that the built configuration is introduced into the user's environment. The activation is performed by a suitably named script {command}activate. This script is generated as part of the configuration build and will be placed in the root of the build output.

The activation script is implemented in the Bash language and consists of initialization code followed by a number of activation script blocks. These blocks are specified using the home.activation option. The blocks may have dependencies among themselves and the generated activation script will contain the blocks serialized such that the dependencies are satisfied. A dependency cycle causes a failure when the configuration is built.

Historically, the activation script has been responsible for creating a new generation of the home-manager Nix profile. The more modern way, however, is to let the activation driver that is, the software calling the activation script manage the profile. Indeed, in some cases we may not have a home-manager profile at all! This is the case when Home Manager is used as a NixOS or nix-darwin module, in these cases the system profile will contain references to the corresponding Home Manager configurations.

Note, to maintain backwards compatibility, the old activation script behavior is still the default. To choose the new mode of operation you have to call the activation script with the command line option --driver-version 1. The old behavior is available using --driver-version 0, or simply omit it entirely.

Unfortunately, driver software need to support both modes of operation for the time being since a user may wish to activate an old generation that contains an activation script that does not support --driver-version. To determine whether support is available, check the {file}gen-version file in the configuration build output root. If the file is missing then the activation script does not support --driver-version. If the file exists and contains the integer 1 or higher, then --driver-version 1 is supported.