...Which is why I said it was ugly. The grep probably won't break, the cut is more likely to. This is in part because the 'ip' command isn't very well designed. A better solution I found is

  ip addr show wlan0 | grep -Po 'inet \K[\d.]+'

Which is less likely to break. If your system supports it,

  hostname -I

Works great. However, this seems to be an ubuntuism, and it doesn't work on my system.

  ip addr show eth0 | awk '/inet/ { print $2 }'

Will be better ;) 'inet[::space::] ' not working with ipv6 addresses.

I though there was a specific desire for ipv4 addrs. That's why the space was there.