Call a D-Bus method and inspect available D-Bus services/objects from a shell script using busctl or gdbus

Steps

1. List available services on a bus (system or session) to find the target service name, using busctl's list command or gdbus's call/introspection commands.
2. Introspect the target service's object path with `busctl introspect <service> <object path>` (or `gdbus introspect`) to see its available interfaces, methods, properties, and signals.
3. Invoke a method non-interactively with `busctl call <service> <object path> <interface> <method> <signature> <args...>` (or the equivalent gdbus call form), matching the exact type signature the introspection output showed.
4. Read a property's current value with busctl's get-property command rather than calling a getter method, when the interface exposes it as a property.
5. Choose busctl when scripting in a systemd-centric shell environment (it ships with systemd) and gdbus when the environment is GLib/GNOME-centric, since both talk to the same bus but come from different projects with slightly different invocation syntax.