Shell scripting question

[Paul Schmehl]

I'm thinking about writing an rc.subr script that sucks in variables from a 
conf file.  Since the rc.firewall script does just that, I thought I'd take 
a look at it.  But I can't understand what it's doing.

Here's the code:

# Suck in the configuration variables.
if [ -z "${source_rc_confs_defined}" ]; then
        if [ -r /etc/defaults/rc.conf ]; then
                . /etc/defaults/rc.conf
                source_rc_confs
        elif [ -r /etc/rc.conf ]; then
                . /etc/rc.conf
        fi
fi

Neither rc.conf nor source_rc_confs appears anywhere else in the script, so 
how does this suck in the variables?  And what does the syntax ". 
/etc/rc.conf" do?

Here's what I understand so far.  If the variable source_rc_confs_defined 
is a zero length string, then if /etc/defaults/rc.conf is readable, then do 
something with it.  I have no idea what the next line "source_rc_confs" 
does.  Else, if /etc/rc.conf is readable, then do something with that.

Can someone explain what all this does please?

Paul Schmehl (pauls at utdallas.edu)
Adjunct Information Security Officer
The University of Texas at Dallas
http://www.utdallas.edu/ir/security/