sendmail, 4th Edition

When Is a Macro Expanded?

A sendmail macro can be expanded either immediately or at runtime, depending on where the expansion takes place in the configuration file.

Macros are expanded in rule sets as the configuration file is read and parsed by sendmail, and (beginning with V8.7) so are macros in rule set names (Macros in Rule Set Names on page 686) and in database maps declared with the K configuration command (The K Configuration Command on page 882). In other configuration lines, expansion is deferred until sendmail actually needs to use that value. In yet others, macros are neither recognized nor expanded.

To illustrate, macros used in header commands are not expanded until the headers of a mail message are processed:

H?x?Full-Name: $x

Here, $x ($x on page 851) can change as sendmail is running. It contains as its value the full name of the sender. Clearly, this macro should not be expanded until that full name is known.

On the other hand, macros in rules are always expanded when the configuration file is read. Therefore, macros such as $x should never be used in rules because the configuration file is read long before mail is processed:

R$x        ($x)

Rules such as this won’t work because $x lacks a value when the configuration file is read. This rule will be expanded to become meaningless:

R      (  )

Note that the $digit positional operator (Copy by Position: $digit on page 661) in the RHS cannot be used to reference defined macros in the LHS. Consider this example, in which {HOST} has the value ...