zsh bash compatibility question

[William Scott <wgscott@xxxxxxxxxxxxxxxxxx>]

Hi Citizens:

I've got a file that I want users to be able to source from either zsh  
or bash


If, while in zsh, I type

source /whole/path/to/sourcefile

and have   dirname $0 embedded within the sourcefile

In zsh, it will report back the full directory path for the directory  
in which this file is located, the same way it does for shell scripts.

In bash, I just get the location of the bash executable itself.

Is there an equivalent command that will work in bash (or preferably  
both shells)?

Thanks.

Bill Scott