[CentOS] Network Utilization

I'm attempting to create a script that will indicate the % utilization
of a network interface. I need this to export a one shot output that can
be called by nagios.

I've tried the following tools but I was unable to find a way to output
just the utilization, and just one time: bmon, iftop, ifstat, bwm,
dstat, rrdtool, iptraf.

Any ideas?

Thanks,
Scott