wait4ifup

sharewall is very good

#!/bin/sh
#
#     Shorewall interface helper utility - V4.2
#
#     This program is under GPL [http://www.gnu.org/licenses/old-licenses/gpl-2.0.txt]
#
#     (c) 2007 - Tom Eastep ([e-mail])
#
#	This file is installed in /usr/share/shorewall/wait4ifup
#
#	Shorewall documentation is available at http://www.shorewall.net
#
#	This program is free software; you can redistribute it and/or modify
#	it under the terms of Version 2 of the GNU General Public License
#	as published by the Free Software Foundation.
#
#	This program is distributed in the hope that it will be useful,
#	but WITHOUT ANY WARRANTY; without even the implied warranty of
#	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
#	GNU General Public License for more details.
#
#	You should have received a copy of the GNU General Public License
#	along with this program; if not, write to the Free Software
#	Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
#
#	If an error occurs while starting or restarting the firewall, the
#	firewall is automatically stopped.
#
#	The firewall uses configuration files in /etc/shorewall/ - skeleton
#	files is included with the firewall.
#
#	wait4ifup <interface> [ <seconds to wait> ]
#

interface_is_up() {
    [ -n "$(ip link list dev $1 2> /dev/null | grep -e '[<,]UP[,>]')" ] 
}

case $# in
    1)
	timeout=60
	;;
    2)
	timeout=$2
	;;
    *)
	echo "usage: $(basename $0) <interface> [ <timeout> ]"
	exit 2
	;;
esac

while [ $timeout -gt 0 ]; do
    interface_is_up $1 && exit 0
    sleep 1
    timeout=$(( $timeout - 1 ))
done

exit 1