command line – launchctl: Run script when community interface is added/comes up?

I am seeking to run a script when ‘bridge100’ interface is added with an IP. How is that this completed?

I see this publish so as to add the next, nevertheless I am undecided how one can perceive that ‘bridge100’ was the interface added and likewise has an IP simply by these information change timestamps:

<?xml model="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Pc//DTD PLIST 1.0//EN" 
 "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist model="1.0">
<dict>
  <key>Label</key>
  <string>ifup.ddns</string>

  <key>LowPriorityIO</key>
  <true/>

  <key>ProgramArguments</key>
  <array>
    <string>/Customers/Shared/bin/ddns-update.sh</string>
  </array>

  <key>WatchPaths</key>
  <array>
    <string>/and so forth/resolv.conf</string>
    <string>/Library/Preferences/SystemConfiguration/NetworkInterfaces.plist</string>
    <string>/Library/Preferences/SystemConfiguration/com.apple.airport.preferences.plist</string>
  </array>

  <key>RunAtLoad</key>
  <true/>
</dict>
</plist>

or a watch on:

/Library/Preferences/SystemConfiguration/com.apple.airport.preferences.plist