A hook is an executable file that runs within a snap’s confined environment when a certain action occurs.
Common examples of actions requiring hooks include:
notifying a snap that something has happened
Example: If a snap has been upgraded, the snap may need to trigger a scripted migration process to port an old data format to the new one.
notifying a snap that a specific operation is in progress
Example: A snap may need to know when a specific interface connects or disconnects.
A hook is defined as an executable within a snap’s
meta/hooks/ directory, and consequently, also within
snap/hooks/ when building with snapcraft. See Snapcraft hook support for more information.
The filename of the executable is based on the name of the hook. If this file exists, snapd will execute the file when required by that hook’s action.
For example, the following excerpt registers an install hook making use of a network plug:
apps: ... hooks: install: plugs: [network]
Hooks are called with no parameters. When a hook needs to request or modify information in within snapd, they can do so via the snapctl tool, which is always available within a snap’s environment. See Using the snapctl tool for further details.
A hook is executed as a single transaction, where a transaction object holds all the configuration changes for that hook. These changes are invisible to the running system until the hook completely finishes. This allows for changes to be rolled back or unset if errors occur during the execution of a hook.
configure hook is called every time one the following actions happen:
snap set|unsetto change a configuration option
The hook should use
snapctl get to retrieve configuration values from snapd. If the hook exits with a non-zero status code, the configuration will not be applied.
For example, given the following command:
$ snap set mysnap username=foo password=bar
configure hook located within the mysnap snap at
meta/hooks/configure would be called to apply the configuration changes, if necessary.
The hook might look similar to:
#!/bin/sh -e username="$(snapctl get username)" password="$(snapctl get password)" if [ -z "$username" -o -z "$password" ]; then echo "Username and password are required." exit 1 fi mkdir -m 0600 $SNAP_DATA/options echo "username: $username" > $SNAP_DATA/options/credentials echo "password: $password" >> $SNAP_DATA/options/credentials
The same hook can also modify the configuration of a snap within the context of the current transaction. This is accomplished using
snapctl set and
snapctl unset. For more information see Adding Snap configuration and Using the snapctl tool.
install hook is called upon initial install only, i.e. it’s not called on subsequent refreshes.
The hook is executed before starting snap services (if it has any) and before the
configure hook. The install hook is the place for one-time actions, such as an early initialisation of a resource when installed for the first time.
This hook is only supported in Ubuntu Core 20 gadget snaps.
See Installation process in the Ubuntu Core documentation for more details.
Interface hooks are executed when an interface is either connected or disconnected via the interface’s plugs and slots mechanism.
They can be used to read or write attributes from a connection and, for example, acquire new resources, update internal options or update databases.
For further details, see Interface hooks.
pre-refresh hook is called whenever the snap gets refreshed.
This hook is executed for the already installed revision of the snap with its services still running (if the snap has any services) and before switching to the newly installed revision.
This hook is a good place for any maintenance or cleanup actions that prepare the snap for switching to the new revision. It’s also a good place to test whether a refresh will succeed, because if the test causes the hook to fail, the refresh will not proceed.
post-refresh hook is similar to
pre-refresh (above) in that it is called whenever the snap gets refreshed.
This hook is executed for the newly installed snap, before starting new services (if applicable). This hook is a good place for any extra actions that need to be performed for the new revision of the snap. It’s also a good place to test whether a refresh has succeeded, because if the test causes the hook to fail, the refresh will be rolled-back and the original state of the snap restored.
remove hook is called when the last revision of the snap gets removed from the system.
This hook is executed after stopping the services of the snap (if the snap has any services), therefore it’s useful for any custom cleanup logic.
This hook is only supported in gadget snaps.
See The gadget snap documentation for more details.
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