8 Giving more flexibility to an rc.d script

When invoked during startup or shutdown, an rc.d script is supposed to act on the entire subsystem it is responsible for. E.g., /etc/rc.d/netif should start or stop all network interfaces described by rc.conf(5). Either task can be uniquely indicated by a single command argument such as start or stop. Between startup and shutdown, rc.d scripts help the admin to control the running system, and it is when the need for more flexibility and precision arises. For instance, the admin may want to add the settings of a new network interface to rc.conf(5) and then to start it without interfering with the operation of the existing interfaces. Next time the admin may need to shut down a single network interface. In the spirit of the command line, the respective rc.d script calls for an extra argument, the interface name.

Fortunately, rc.subr(8) allows for passing any number of arguments to script's methods (within the system limits). Due to that, the changes in the script itself can be minimal.

How can rc.subr(8) gain access to the extra command-line arguments. Should it just grab them directly? Not by any means. Firstly, an sh(1) function has no access to the positional parameters of its caller, but rc.subr(8) is just a sack of such functions. Secondly, the good manner of rc.d dictates that it is for the main script to decide which arguments are to be passed to its methods.

So the approach adopted by rc.subr(8) is as follows: run_rc_command passes on all its arguments but the first one to the respective method verbatim. The first, omitted, argument is the name of the method itself: start, stop, etc. It will be shifted out by run_rc_command, so what is $2 in the original command line will be presented as $1 to the method, and so on.

To illustrate this opportunity, let us modify the primitive dummy script so that its messages depend on the additional arguments supplied. Here we go:

#!/bin/sh

. /etc/rc.subr

name="dummy"
start_cmd="${name}_start"
stop_cmd=":"
kiss_cmd="${name}_kiss"
extra_commands="kiss"

dummy_start()
{
        if [ $# -gt 0 ]; then
                echo "Greeting message: $*"
        else
                echo "Nothing started."
        fi
}

dummy_kiss()
{
        echo -n "A ghost gives you a kiss"
        if [ $# -gt 0 ]; then
                echo -n " and whispers: $*"
        fi
        case "$*" in
        *[.!?])
                echo
                ;;
        *)
                echo .
                ;;
        esac
}

load_rc_config $name
run_rc_command "$@"

What essential changes can we notice in the script?