как в bash-скрипте проверить, поднят интерфейс или опущен?

  GNU nano 2.3.1                                                              File: /etc/init.d/routing                                                                                                                        Modified  

#!/sbin/runscript
# Copyright 1999-2012 Gentoo Foundation
# Distributed under the terms of the GNU General Public License v2
# $Header: $

depend() {
    use net.eth1 net.eth2
    before net
}

function test_if {
   ip link show $1 | grep -q 'state $2'
}

start() {
  while ip route show | grep -q "default"
  do
    route del default
  done

  if test_if eth1 DOWN && test_if eth2 DOWN ; then
    echo "No internet connection"
  fi;
  if test_if eth1 UP && test_if eth2 DOWN ; then
    ebegin "Default route eth1"
      ip route add default scope global \
        nexthop via 192.168.1.1 dev eth1 weight 1
    eend $?
  fi;
  if test_if eth1 DOWN && test_if eth2 UP ; then
    ebegin "Default route eth2"
      ip route add default scope global \
        nexthop via 192.168.2.1 dev eth2 weight 1
    eend $?
  fi;
  if test_if eth1 UP && test_if eth2 UP ; then
    ebegin "Default route: eth1 and eth2"
      ip route add default scope global \
        nexthop via 192.168.1.1 dev eth1 weight 1 \
        nexthop via 192.168.2.1 dev eth2 weight 1
    eend $?
  fi;
}

is_interface_up() {
	/sbin/ifconfig|grep "^$1"
}

  if is_interface_up eth1 && ! is_interface_up eth2 ; then
    ebegin "Default route eth1"
      ip route add default scope global \
        nexthop via 192.168.1.1 dev eth1 weight 1
    eend $?
  fi;

fn if_st {
  ip link | grep -q $1:' <.*,UP,.*>'
}

You want to look at the nodes in

/sys/class/net/

I experimented with mine:

Wire Plugged in:

eth0/carrier:1
eth0/operstate:unknown

Wire Removed:

eth0/carrier:0
eth0/operstate:down

Wire Plugged in Again:

eth0/operstate:up
eth0/carrier:1