sendmail, 4th Edition

Rewrite-and-Return Prefix: $@

The flow of rules is such that each and every rule in a series of rules (a rule set) is given a chance to match the workspace:

 R xxx yyy
 R yyy zzz

The first rule matches xxx in the workspace and rewrites the workspace to contain yyy. The first rule then tries to match the workspace again but, of course, fails. The second rule then tries to match the workspace. Because the workspace contains yyy, a match is found, and the RHS rewrites the workspace to be zzz.

There will often be times when one rule in a series performs the appropriate rewrite and no subsequent rules need to be called. In the earlier example, suppose xxx should only become yyy and that the second rule should not be called. To solve problems such as this, sendmail offers the $@ prefix for use in the RHS.

The $@ prefix tells sendmail that the current rule is the last one that should be used in the current rule set. If the LHS of the current rule matches, any rules that follow (in the current rule set) are ignored:

 R xxx $@ yyy
 R yyy zzz

If the workspace contains anything other than xxx, the first rule does not match, and the second rule is called. But if the workspace contains xxx, the first rule matches and rewrites the workspace. The $@ prefix for the RHS of that rule prevents the second rule (and any subsequent rules in that rule set) from being called.

Note that the $@ also prevents looping. The $@ tells sendmail to skip further rules and to rewrite only once. The difference between $@