First , is the name of the script you are calling .getdir or just getdir? Your example command says ./getdir somedir, but you refer to it as .getdir in your variable and comment.
If that's just a typo, it looks to me that there may be a discrepancy on what getdir does and/or takes as input and what you are asking it to do. In your example, getdir takes a directory as input, but in your script, you pass a parameter. Is this parameter a directory? Maybe getdir expecting a path and your parameter is just a name (or vise versa)?

This part has the proper syntax to work:

NETDIR=$(${GETDIR} ${PARAM})
echo ${NETDIR}