sendmail, 4th Edition

When looking up information or performing actions with the $( and $) operators, it is sometimes necessary to provide positional substitution arguments. To illustrate , consider an entry such as this in a hypothetical database source file:

hostA %0!%1@%2

With such an entry in place, and having built the database, the following rule could be used to perform a lookup:

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

Here, if the workspace contains the address joe@hostA.uucp , the LHS matches, causing it to be rewritten as hostA!joe@mailhost .

See Section 23.4.2 for a full description of how $@ is used in this way.