[aclug-L] opposite of basename?

["Bates, Rod" <Rod.Bates@xxxxxxxxxxxxxxxxxx>]

Is there a command which does the opposite of basename,
i.e strips off the simple file name at the right end and
returns the path leading to it?  

I need to write a script which refers to some files relative
to where the script is located.  Applying such a function
to $0 would do it.  Suggestions on other approaches are also
welcome 

Rodney Bates