Determine if a script was sourced or executed

Bash only allows return in a function or at the top level of a sourced script. We can call it in a subshell and use the exit code to determine if the script is being sourced or not.

(return 0 2>/dev/null) && sourced=1 || sourced=0

If all we want to do is to halt execution at some point if we're sourcing the script, we can use the simpler return 0 2>/dev/null as a short-circuit.

#!/usr/bin/env bash

_a_function() {
  echo "useful stuff"
}

return 0 2>/dev/null

echo "I'll only print when script is executed"
_a_function

Example usage:

source ./script.sh
# _a_function now available for use
_a_function
# useful stuff

. ./script
# I'll only print when script is executed
# useful stuff

Source: stackoverflow