ip route show table main | grep -Ev "^default" | while read ROUTE; do
  ip route add table wan2 $ROUTE
done

ip route add table wan2 default via 1.2.3.4 dev ethX

ip rule add from 10.1.2.0/24 table wan2

#!/bin/sh
set -e

MARK=100
TABLE_NAME=wan2
DEV=eth2
GATEWAY=192.168.4.1

IPTABLES=/sbin/iptables
IP=/sbin/ip

$IP route flush table $TABLE_NAME

$IP route show table main | grep -Ev "^default" | while read ROUTE; do
    $IP route add table $TABLE_NAME $ROUTE
done

$IP route add table $TABLE_NAME default dev $DEV via $GATEWAY

while true; do
    ip rule del table $TABLE_NAME 2>/dev/null || break
done

$IP rule add from 10.1.2.0/24 table $TABLE_NAME
....
$IP route flush cache