Sometimes you will want to ensure that a file is sourced instead of executed. This ensures, among other things, that any environment variables that the script defines remain in your current shell after the script completes.
To do so, use the following to check whether the file was sourced or run in a sub-shell
(return 0 2>/dev/null) && sourced=1 || sourced=0 echo "sourced=$sourced"
Bash allows return statements only from functions and in a scripts top level scope IF it is sourced. If you try to use a return statement outside of a function in a non-sourced context it returns an error.