10.1.1 Getting Acquainted with Filters

Suppose we wish to implement an authentication method based on the user name and the user's calling station ID. We have a database of user names with valid IDs, and the new method should authenticate a user only if the combination {user_name, id} is found in this database.

We write a filter program that reads its standard input line by line. Each input line must consist of exactly two words: the user name and the calling station ID. For each input line, the program prints 0 if the {user_name, id} is found in the database and 1 otherwise. Let's suppose for the sake of example that the database is a plaintext file and the filter is written in a shell programming language. Then it will look like

#! /bin/sh

DB=/var/db/userlist

while read NAME CLID
do
    if grep "$1:$2" $DB; then
        echo "0"
    else
        echo "1"
    fi
done

This document was generated by Sergey Poznyakoff on December, 6 2008 using texi2html 1.78.