brctl doesn't give an exit code indicating success or failure. # echo "alias bridge off" >> /etc/modules.conf ; brctl addbr br0 ; echo $? br_add_bridge: Package not installed 0 Ideally, there would at least be a 'test if bridging support is available' command; 'brctl show > /dev/null' would suffice, if it was going to return failure from that.