Library/package development - message when loading

Question

is there any way to display a message when a user loads library(myCustomLibrary)? Upon loading, I want to display a message that tells the user how to run all the test functions.

Answer 1

Quick points (and updated edit in 2021):

• while if your package has a NAMESPACE, then .onLoad() is where you used to do this: but .onLoad() has been required to be 'silent'

• if your package has a NAMESPACE, then .onAttach() is where can call, preferably via packageStartupMessage() (which can be suppressed where cat() or message() cannot

• if your package does not have NAMESPACE, then you must add one now (and .First.lib() was where you used to do this) -- NAMESPACES have been mandatory for a few years now

• either way, use packageStartupMessage() instead of cat() so that users have a choice of suppressing this.