Question

The objective is to have two simple ways to source some code, say func.R, containing a function. Calling R CMD BATCH func.R initializes the function and evaluates is. Within a session, issuing source("func.R") simply initializes the function.
Any idea?

Answer 1

I think that the interactive() function might work.

This function returns TRUE when R is being used interactively and FALSE otherwise. So just use if (interactive())

i.e. the equivalent is

if (!interactive()) {
  main()
}