shell 获取网关地址

获取网关的办法有很多，有通过UDP+ioctl的，有通过netlink的，也有通过shell读取的。

这里介绍的方法是通过route解析其命令输出结果，并取出有效的gateway。

BusyBox v1.19.4 (2016-04-11 16:47:28 CST) built-in shell (ash)
Enter 'help' for a list of built-in commands.

  _______                     ________        __
 |       |.-----.-----.-----.|  |  |  |.----.|  |_
 |   -   ||  _  |  -__|     ||  |  |  ||   _||   _|
 |_______||   __|_____|__|__||________||__|  |____|
          |__| W I R E L E S S   F R E E D O M
 -----------------------------------------------------
 ATTITUDE ADJUSTMENT (Attitude Adjustment, r42647)
 -----------------------------------------------------
  * 1/4 oz Vodka      Pour all ingredients into mixing
  * 1/4 oz Gin        tin with ice, strain into glass.
  * 1/4 oz Amaretto
  * 1/4 oz Triple sec
  * 1/4 oz Peach schnapps
  * 1/4 oz Sour mix
  * 1 splash Cranberry juice
 -----------------------------------------------------
root@OpenWrt:~# 
root@OpenWrt:~# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.1.1     0.0.0.0         UG    0      0        0 br-lan
192.168.1.0     0.0.0.0         255.255.255.0   U     0      0        0 br-lan
root@OpenWrt:~# route -n | grep UG | awk '{print $2}'
192.168.1.1
root@OpenWrt:~#