In Bash, the correct way to get the full path to a shell script is ${BASH_SOURCE[0]}, since $0 returns bash if the script is being sourced. For example, try running this in your terminal:

source <(echo 'echo $0')

It will print bash.

Does anyone know how to do this for Bourne shell scripts? For example, what would be the contents of a script foo such that if I typed this:

sh -c '. foo'

it would output the full path to the script?