A hook is an executable 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.

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.

Default shell environment: A hook script can only assume a POSIX-compliant shell environment for its execution. If your script needs a specific shell, such as Bash or Zsh, it needs to be explicitly declared within the script’s hashbang header (#!/bin/bash, for example). Your snap also needs to ensure your chosen shell is available.

Accessing resources

If a hook requires access to system resources outside of a snap’s confined environment, it will need to use slots and plugs via the interface mechanism to access any required resources.

When using Snapcraft to build the snap, the interface definition will go inside snapcraft.yaml, and the snapcraft command create a snap.yaml within the snap to hold the required metadata.

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 snapctl --help and snapctl <command> --help for details on how to use the tool.

The configure hook

The configure hook is called when the following actions happen:

  • initial snap installation
  • snap refresh
  • whenever the user runs snap set to 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

The 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 install hook

The 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.

The interface hooks

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.

The pre-refresh hook

The 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.

The post-refresh hook

The 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.

The remove hook

The 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.

The prepare-device hook

This hook is only supported in gadget snaps.

See The gadget snap documentation for more details.