libalpine: add available_ifaces

a sorted list of available network interfaces thats not a part of
any bridge or bond

libalpine.sh.in

@@ -81,6 +81,27 @@ is_xen_dom0() {
 	is_xen && \
 	grep -q "control_d" /proc/xen/capabilities
 }
+
+# list of acailable network interfaces thats not part of any bridge or bond
+available_ifaces() {
+        local iflist= ifpath= iface= i=
+        sorted_ifindexes=$(
+                for i in /sys/class/net/*/ifindex; do
+                        [ -e "$i" ] || continue
+                        echo -e "$(cat $i)\t$i";
+                done | sort -n | awk '{print $2}')
+        for i in $sorted_ifindexes; do
+                ifpath=${i%/*}
+                iface=${ifpath##*/}
+                # skip interfaces that are part of a bond or bridge
+                if [ -d "$ifpath"/master/bonding ] || [ -d "$ifpath"/brport ]; then
+                        continue
+                fi
+                iflist="${iflist}${iflist:+ }$iface"
+        done
+        echo $iflist
+}
+
 # from OpenBSD installer
 
 # Ask for a password, saving the input in $resp.