sendmail, 4th Edition

Specify a Default with $:

The $: operator can be used as an alternative to the -a switch (or in conjunction with it). The $: operator, when it stands between the $( and $), specifies a default to use instead of the key, should a lookup fail:

R$- . uucp      $: $(uucp $1 $: $1.uucp $)

Here, the $- part of the LHS is looked up in the uucp database. If it is found, the $( to the $) in the RHS expression is replaced by the data from that database. If it is not found, the $: causes the expression to be replaced with the $- LHS part and a .uucp suffix ($1.uucp).

This version of our rule further simplifies the contents of the database file. With this rule, the database file would contain information such as the following:

lady    lady
sonya   sonya

The -a is still used as before to append a .localuucp to each successful match:

Kuucp hash -a.localuucp /etc/mail/uucp

In the RHS expression, the $: must follow the key or it loses its special meaning:

$(name key $:  default  $)

If the $:default wrongly precedes the key, it is used as the key, lookups fail, and replacements are not as expected. If the $: is present but the default is missing, a failed lookup returns an empty workspace.