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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
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