Authentication of users via a shell script

[Craig Smith]

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!