sendmail, 4th Edition

Rewrite Through a Rule Set: $>set

Rules are organized in sets that can be thought of as subroutines. Occasionally, a series of rules can be common to two or more rule sets. To make the configuration file more compact and somewhat clearer, such common series of rules can be made into separate subroutines.

The RHS $>set operator tells sendmail to perform additional rewriting using a secondary set of rules. The set is the rule set name or number of that secondary set. If set is the name or number of a nonexistent rule set, the effect is the same as if the subroutine rules were never called (the workspace is unchanged).

If the set is numeric and is greater than the maximum number of allowable rule sets, sendmail prints the following error and skips that rule:

 bad ruleset bad_number (maximum max)

If the set is a name and the rule set name is undeclared, sendmail prints the following error and skips that rule:

 Unknown ruleset bad_name

Neither of these errors is caught when the configuration file is read. They are caught only when mail is sent because a rule set name can be a macro:

 $> $&{SET}

The $& prefix prevents the macro named {SET} from being expanded when the configuration file is read. Therefore, the name or number of the rule set cannot be known until mail is sent.

The process of calling another set of rules proceeds in five stages:

First

As usual, if the LHS matches the workspace, the RHS gets to rewrite the workspace.

Second

The RHS ignores the $>set part and rewrites the rest as usual. ...