Unable to read MAC address of router at login

Question

I'm using the following bash function to get the MAC address of the current router when logging in. The script works when I run it after logging in but doesn't work from my login hook (running as root). The script fails even after sleeping for 30 seconds prior to running. I know the interface is up because my mail client is finished checking mail by the time the script pings the router runs the arp command. I could run it as a cron job, but I'd like to understand why it fails during login.
<pre style="border: 1px solid #ddd; padding-left: .75ex; padding-top: .25em; padding-bottom: .25em; margin-top: .5em; margin-bottom: .5em; margin-left: 1ex; max-width: 80ex; overflow: auto; font-size: 10px; font-family: Monaco, 'Courier New', Courier, monospace; color: #444; background: #eee; line-height: normal">get_route_mac() {
ip=$(netstat -nrfinet | awk '/^default/{print $2; exit}')
[[ "$ip" = *.*.*.* ]] || return 1
[ ${1:-0} -gt 0 ] && ping -nc$1 $ip 2>&1 >/dev/null
arp -n $ip | sed -n 's/.* at \(.*:.*:.*:.*:.*:.*\) on .*/\1/p'
</pre>
The idea is to do certain things when joining different networks. Especially when connected to an unknown network (stolen laptop recovery). I would like to support 10.3.9, so I'm avoiding launchd for now.

Answer 1

PWD=/
SHLVL=1
SECURITYSESSIONID=ceb19270
_=/usr/bin/printenv
uid=0(root) gid=0(wheel) groups=0(wheel)
The script works much better when it can find the required programs. :0)
which netstat awk ping arp sed
/usr/sbin/netstat
/usr/local/bin/awk
/sbin/ping
/usr/sbin/arp
/usr/bin/sed
<pre style="border: 1px solid #ddd; padding-left: .75ex; padding-top: .25em; padding-bottom: .25em; margin-top: .5em; margin-bottom: .5em; margin-left: 1ex; max-width: 80ex; overflow: auto; font-size: 10px; font-family: Monaco, 'Courier New', Courier, monospace; color: #444; background: #eee; line-height: normal">get_route_mac() {
ip=$(/usr/sbin/netstat -nrfinet | /usr/local/bin/awk '/^default/{print $2; exit}')
[[ "$ip" = *.*.*.* ]] || return 1
[ ${1:-0} -gt 0 ] && /sbin/ping -nc$1 $ip 2>&1 >/dev/null
/usr/sbin/arp -n $ip | /usr/bin/sed -n 's/.* at (.*:.*:.*:.*:.*:.*) on .*/ /p'
}</pre>
Thanks for your help, Bob!