sendmail, 4th Edition

Specify Numbered Substitution with $@

For more complex substitutions, V8 sendmail offers use of the $@ operator in the RHS in conjunction with the $( and $) expressions in database maps.^[341] There can be multiple $@-prefixed texts between the key and the $: (if present) or the $), where each of the texts may itself be multiple rule-set expressions:

$(name key $@ text1  $@ text2 and text3 $: default  $)

Each $@text expression is numbered by position (from left to right):

$(name key $@ text1  $@ text2  $: default  $)
               ↑          ↑
1                                2

In this numbering scheme the key is always number 0, even if no $@s are listed.

These numbers correspond to literal % digit expressions in the data portion of a database map. For example:

lady   %0!%1@%2

When a lookup of the key in the RHS of the rule is successful, the returned value is examined for %digit expressions. Each such expression is replaced by its corresponding $@text from the rule. In the case of the preceding database map, %0 would be replaced with lady (the key), %1 with text1, and %2 with text2.

To illustrate, consider the earlier database entry and the following rule:

R$- @ $-.uucp   $: $(uucp $2 $@ $1 $@ mailhost $: $1@$2.uucp $)

If the workspace contains the address [e-mail], the LHS matches. The RHS rewrites only once because it is prefixed with the $: operator. The expression between the $( and $) causes the second $- from the LHS (the $2, the key) to be looked up in the database whose symbolic name is uucp. Because $2 references lady from the workspace, ...