sendmail, 4th Edition

Duplicates Discarded

When sendmail gathers its list of recipients, it views a program to run as just another recipient. Before performing any delivery, it sorts the list of recipients and discards any duplicates. Ordinarily, this is just the behavior that is desired, but discarding duplicate programs from the aliases(5) file^[202] can cause some users to lose mail. To illustrate, consider a program that notifies the system administrator that mail has arrived for a retired user:

#!/bin/sh
/usr/ucb/mail -s gone postmaster

This script reads everything (the mail message) from its standard input and feeds what it reads to the /usr/ucb/mail program. The command-line arguments to mail are a subject line of gone and a recipient of postmaster. Now consider two aliases that use this program:

george: "|/usr/local/bin/gone"
ben:    "|/usr/local/bin/gone"

When mail is sent to both george and ben, sendmail aliases each to the program |/usr/local/bin/gone. But because both of the addresses are identical, sendmail discards one.

To avoid this problem, design all delivery programs to require at least one unique argument. For example, the previous program should be rewritten to require the user’s name as an argument:

#!/bin/sh
if [ ${#} -ne 2 ]; then
        echo $0 needs a username.
        exit
fi
/usr/ucb/mail -s "$1 gone" postmaster

By requiring a username as an argument, the once-faulty aliases are made unique:

george: "|/usr/local/bin/gone george"
ben:    "|/usr/local/bin/gone ben"

Although the program paths are still the ...