Sometimes you want to execute a particular command, but only when you're at home. Examples would be running fetchmail (or fetchnews) through cron, but you don't want this to run when you're in the train, connected to the Internet through GPRS...

My idea here would be to create a script (say "athome.sh") which returns 0 if you're at home, and 1 otherwise. The key of the script is that the MAC address of your (default) gateway is unique.

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#!/bin/sh

GW=$(/sbin/ip route | awk '/default/ {print $3}');
MGW=$(/sbin/arp -e | grep ${GW} | awk '{print $3}');

if [ "${MGW}" = "00:11:22:33:44:55" ]
then
  exit 0;
else
  exit 1;
fi

With this script, you can then run athome.sh && fetchmail. If you aren't home, athome.sh will return 1 and the fetchmail command will never be executed. When you are, the command returns 0 and fetchmail is launched.