Authentication of users via a shell script

Clarification from my previous email. From http://freeradius.org/features/authentication.html If a password is not available locally for some reason, the server can pass the authentication to another system such as LDAP, PAM, Unix (/etc/passwd), Kerberos, Active Directory, or RADIUS server via RADIUS proxying. Local programs (e.g. CGI scripts) can also be used to authenticate users via shell scripts or any other method. Perl orPython scripts can be pre-loaded into the server, which significantly lowers the cost of running such programs. How do I do the authentication of users via a shell script? Thanks again!