Sendmail, 3rd Edition

The RHS

The purpose of the RHS in a rule is to rewrite the workspace. To make this rewriting more versatile, sendmail offers several special RHS operators. The complete list is shown in Table 18-2.

Table 18-2. RHS operators

RHS	§	Description or use
$digit	Section 18.7.1	Copy by position
$:	Section 18.7.2	Rewrite once (when used as a prefix), or specify the user in a delivery-agent “triple,” or specify the default value to return on a failed database-map lookup
$@	Section 18.7.3	Rewrite and return (when used as a prefix), or specify the host in a delivery-agent “triple”, or specify an argument to pass in a database-map lookup or action
$>set	Section 18.7.4	Rewrite through another rule set (such as a subroutine call that returns to the current position)
$#	Section 18.7.5	Specify a delivery agent or choose an action, such as to reject or discard a recipient, sender, connection, or message.
$[ $]	Section 18.7.6	Canonicalize hostname
$( $)	Section 23.4	Perform a lookup in an external database, file, or network service, or perform a change (such as dequoting), or store a value into a macro.
$&	Section 21.5.3	Delay conversion of a macro until runtime

Copy by Position: $digit

The $digit operator in the RHS is used to copy tokens from the LHS into the workspace. The digit refers to positions of LHS wildcard operators in the LHS:

R $+ @ $*    $2!$1
  ↑    ↑$1           $2 

Here, the $1 in the RHS indicates tokens matched by the first wildcard operator in the LHS (in this case, ...