Yeah, but I find dig faster since all the whois servers decided they needed to also return 10 pages of legal bullshit, and easier to check a bunch of possible domains in a loop. Whois is probably more up to date as zone updates are batched so it's possible something returns NXDOMAIN but has actually been registered.

How are you constructing your dig? Is a null dig result always an indication of an unregistered domain?

Usually something like this:

cat /tmp/wants.txt | while read d; do echo -n $d:; dig $d in ns | grep '>>HEADER<<' | awk '{print $6}'; done

You get NXDOMAIN if it doesn't exist. That doesn't necessarily mean you can register it though. Maybe it's expired but in the grace period, or some other kind of exits-but-disabled state. If it does exist it might be for sale. Anyway, for quickly checking what's probably unregistered from the shell this one liner works great. A whois query usually takes about a second. Dig takes about a tenth of a second, and DNS queries aren't rate limited like WHOIS.