家里路由器不在自己卧室，用树莓派考虑用wifi，之前用Raspberry官方系统，按照教程写的wpa.conf可以连接wifi，后来重新刷ubuntu mate 16.04就不好用了。
各种找原因，后来原因找到了，wpa.conf的配置文件中的引号要用双引号的。
所以下面这样是不行的：
network={
ssid='TP-LINK_4A24E6'
key_mgmt=WPA-PSK
psk="12345678"
}
要像下面这样：
network={
ssid="TP-LINK_4A24E6"
key_mgmt=WPA-PSK
psk="12345678"
}
pi@xm:~$ sudo ifup wlan0
Internet Systems Consortium DHCP Client 4.3.3
Copyright 2004-2015 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/
Listening on LPF/wlan0/b8:27:eb:9b:2f:e9
Sending on LPF/wlan0/b8:27:eb:9b:2f:e9
Sending on Socket/fallback
DHCPDISCOVER on wlan0 to 255.255.255.255 port 67 interval 3 (xid=0x5a78f148)
DHCPDISCOVER on wlan0 to 255.255.255.255 port 67 interval 4 (xid=0x5a78f148)
DHCPREQUEST of 192.168.1.107 on wlan0 to 255.255.255.255 port 67 (xid=0x48f1785a)
DHCPOFFER of 192.168.1.107 from 192.168.1.1
DHCPACK of 192.168.1.107 from 192.168.1.1
bound to 192.168.1.107 -- renewal in 2914 seconds.
pi@xm:~$ ifconfig
enxb827ebce7abc Link encap:Ethernet HWaddr b8:27:eb:ce:7a:bc
inet addr:192.168.1.106 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::2b38:5262:3cef:f35b/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:787 errors:0 dropped:0 overruns:0 frame:0
TX packets:443 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:59985 (59.9 KB) TX bytes:55314 (55.3 KB)
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:65536 Metric:1
RX packets:91 errors:0 dropped:0 overruns:0 frame:0
TX packets:91 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1
RX bytes:7761 (7.7 KB) TX bytes:7761 (7.7 KB)
wlan0 Link encap:Ethernet HWaddr b8:27:eb:9b:2f:e9
inet addr:192.168.1.107 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::ba27:ebff:fe9b:2fe9/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:58 errors:0 dropped:6 overruns:0 frame:0
TX packets:55 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:8485 (8.4 KB) TX bytes:8798 (8.7 KB)