Check out the redirectTo state definition property.
From the ui-router documentation:
Synchronously or asynchronously redirects Transitions to a different state/params
If this property is defined, a Transition directly to this state will be redirected based on the property's value.
https://ui-router.github.io/ng1/docs/latest/interfaces/state.statedeclaration.html#redirectto
Example:
state.state('root', {
url: "/",
redirectTo: "shop"
})
Full working example: here.
If you want any unknown route to redirect to /shop, you could use the following:
$urlRouterProvider.otherwise('/shop');