When sendmail first starts, the -q command-line switch (Section 11.8.1) tells it how often to process its queues. The form of that command-line switch looks like this:

-qinterval

The interval is an expression composed of numbers and letters that sets the time interval between queue processing runs. The following, for example, sets the interval to be once every 2 hours, 13 minutes, 7 seconds:

-q2h13m7s

In typical installations, the interval is usually expressed only in minutes:

-q15m

When sendmail first starts, it finds the -q command-line switch, then places the interval value into the ${queue_interval} macro. That value is a text expression containing three positions:

hours:minutes:seconds  

If the interval is longer than a day, that number of days (and possibly weeks or months) is expressed in hours in the hours position. If any of the three positions is zero, it is expressed as 00. If any of the three positions has a value less than 10, it is zero padded on the left. For example, a -q0h9m12s would yield this value in the ${queue_interval} macro:

00:09:12

One possible use for this macro might be to cause rules to function differently depending on whether the -q command-line switch contains an interval. Consider, for example, the following mc configuration file lines:

LOCAL_RULESETS
Squeuegroup
R $*                        $: $&{queue_interval} $| $1
R $+ : $+ : $+ $| $*        $@
... select queue groups here

Here, under LOCAL_RULESETS, we declare the queuegroup rule set (Section 11.4.5), which is used to select queue groups for particular addresses. The first rule prefixes the workspace with the value of the ${queue_interval} macro, and a $| operator. The second rule checks the workspace to the left of the $| to see if it looks like a time expression. If it does (if sendmail was run with a -q interval), we skip (the RHS $@) queue group selection.

This scheme allows the same configuration file to be used for two daemons. One will be the initial delivery daemon and will be run without a queue interval. The other will be the queue processing daemon and will run with a queue interval.

${queue_interval} is transient. If it is defined in the configuration file or in the command line, that definition can be ignored by sendmail. Note that ${queue_interval} is defined after the configuration file is read. Therefore, although it won't change thereafter, a $& prefix is still necessary when you reference it in rules (that is, use $&{queue_interval}, not ${queue_interval}).