Hook commands

See also: Hook, Ops

In Juju, a hook tool (or ‘hook command’) is a Bash script located in /var/lib/juju/tools/unit-<app name>-<unit ID> that a charm uses to communicate with its Juju unit agent in response to a hook.

In the Juju ecosystem, in [Ops])(https://ops.readthedocs.io/en/latest/), hook tools are accessed through Ops constructs, specifically, those constructs designed to be used in the definition of the event handlers associated with the Ops events that translate Juju hooks. For example, when your charm calls ops.Unit.is_leader, in the background this calls ~/hooks/unit-name/leader-get; its output is wrapped and returned as a Python True/False value.

In Juju, you can use hook commands for troubleshooting.

Example: Use relation-get to change relation data

# Get the relation ID

$ juju show-unit synapse/0

...
  - relation-id: 7
    endpoint: synapse-peers
    related-endpoint: synapse-peers
   application-data:
      secret-id: secret://1234
    local-unit:
      in-scope: true


# Check the output:
$ juju exec --unit synapse/0 "relation-get -r 7 --app secret-id synapse/0"
secret://1234

# Change the data:
juju exec --unit synapse/0 "relation-set -r 7 --app secret-id=something-else"

# Check the output again to verify the change.