Both the following work here (CentOS 5).
ip addr show eth0 | awk '$1 == "inet" {gsub(/\/.*$/, "", $2); print $2}'
ifconfig eth0 | awk '/inet addr/ {gsub("addr:", "", $2); print $2}'
For OS X (v10.11 (El Capitan) at least):
ifconfig en0 | awk '$1 == "inet" {print $2}'