There are a number of situations where snapd needs to notify a snap that something has happened. For example, when a snap is upgraded, it may need to run some sort of migration on the previous version's data in order to make it consumable by the new version. Or when an interface is connected or disconnected, the snap might need to obtain attributes specific to that connection. These types of situations are handled by hooks.
A hook is defined as an executable contained within the meta/hooks/ directory
inside the snap. The file name of the executable is the name of the hook (e.g.
the upgrade hook executable would be meta/hooks/upgrade).
As long as the file name of the executable corresponds to a supported hook name,
that's all one needs to do in order to utilize a hook within their snap. Note
that hooks, like apps, are executed within a confined environment. By default
hooks will run with no plugs; if a hook needs more privileges one can use the
top-level attribute hooks in snap.yaml to request plugs, like so:
hooks: # Top-level YAML attribute, parallel to `apps`
upgrade: # Hook name, corresponds to executable name
plugs: [network] # Or any other plugs required by this hook
Note that hooks will be called with no parameters. If they need more information
from snapd (or need to provide information to snapd) they can utilize the
snapctl command (for more information on snapctl, see snapctl -h).
Note: The development of specific hooks is ongoing. None are currently supported.