How can I get the list of all active network interfaces programmatically? My objective is to determine the IP address for Ethernet on a device. But assuming that there are connections like WiFi and USB Ethernet, is there a particular way just to get the Ethernet IP ?

On a shell I would do:
networksetup -listallhardwareports

which would give me a mapping of Hardware Port to Device. From that I would grep the device for Hardware Port: Ethernet and then use ifaddr struct to get my IP. 

I know the ifaddr part, please suggest how to do the first part programmatically.
Also, is there an alternative approach ?
There may be several different ways to accomplish what you're asking, however, I'll just throw this out there.

I have a MacBook Pro that doesn't have a built-in Ethernet Port so in my examples I'll use Hardware Port: Wi-Fi since I tested this in both examples below and it worked, however you can change it to Hardware Port: Ethernet if that is what the output of networksetup -listallhardwareports shows for you.

If you just want to output the target device's IP Address to stdout, use the following example:

$ ipconfig getifaddr $(networksetup -listallhardwareports | awk '/Hardware Port: Wi-Fi/{getline; print $2}')
$ 192.168.1.100


If you want to assign it to a variable in a script, use the following example:

ipAddress="$(ipconfig getifaddr $(networksetup -listallhardwareports | awk '/Hardware Port: Wi-Fi/{getline; print $2}'))"



Explanation: 

The relevant output of networksetup -listallhardwareports for my system is:

Hardware Port: Wi-Fi
Device: en0
Ethernet Address: 28:cf:e3:10:a4:cd


(Note: That is not my real MAC Address)

Using $(...) command substitution in order to have something to pass to ipconfig getifaddr &lt;args&gt; I determine the Hardware Port's Device Name with the output of networksetup -listallhardwareports and pipe | it through awk which is looking for Hardware Port: Wi-Fi and uses the get line function, which reads the next line after the match and is passed to print $2, which in essence prints the second part of the line following the match, which in the case is en0 and that gets passed to ipconfig getifaddr as its argument in the first example, e.g ipconfig getifaddr en0. The output of which is the IP Address.

In the second example a second $(...) command substitution is used around the complex command line used for the stdout example to assign it to the ipAddress variable when used in a script.

Note:  This is really meant to be an example as I've not coded it to account for the device being down.  In other words if the device doesn't have and IP Address there will be no output to stdout and nothing assigned to the ipAddress variable in the script.  Although in the script it can be coded to test whether or not the ipAddress variable is empty and act accordingly.