Sendmail, 3rd Edition

Write a Delivery Agent Script

The program that is driven by the prog delivery agent can be a compiled executable binary, a shell script, or even a perl(1) script. The limitation on the kind of program that can be run is made by the sh(1) shell (if sh -c is used in the A=) or by execve(2) (if it is launched directly from the P=). You need to read the manuals on your system to determine your limitations. For example, not all versions of sh(1) allow constructs such as the following in scripts:

#!/usr/local/bin/perl

When this appears as the first line of a script, the #! tells sh(1) or execve(2) to run the program whose pathname follows, to execute the commands in the script.^[8]

In writing a program for mail delivery using the prog delivery agent, some unexpected problems can arise. We will illustrate, using fragments from a Bourne shell script.

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^[9] 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 ...