Linux内核提供了一种机制，使得有热插拔事件（比如插入或拔出U盘）发生时可以执行一个程序，在本文中我称之为hotplug程序。内核在调用hotplug程序时会传递一个命令行参数，这个参数是发生热插拔事件的子系统名称，常见的有usb, module, drivers, net等。此外内核在调用hotplug程序是还会设置一些环境变量，如SUBSYSTEM, ACTION, PRODUCT, TYPE, INTERFACE, DEVPATH等。
下面是一系列实际的热插拔事件的例子，每一行表示hotplug程序被调用一次：
1 SUBSYSTEM=net, ACTION=linkup, PRODUCT=, TYPE=, INTERFACE=eth0, DEVPATH=/class/net/eth0
2 SUBSYSTEM=module, ACTION=add, PRODUCT=, TYPE=, INTERFACE=, DEVPATH=/module/ehci_hcd
3 SUBSYSTEM=platform, ACTION=add, PRODUCT=, TYPE=, INTERFACE=, DEVPATH=/devices/platform/ehci_hcd
4 SUBSYSTEM=drivers, ACTION=add, PRODUCT=, TYPE=, INTERFACE=, DEVPATH=/bus/platform/drivers/ehci_hcd
5 SUBSYSTEM=usb_host, ACTION=add, PRODUCT=, TYPE=, INTERFACE=, DEVPATH=/class/usb_host/usb1
6 SUBSYSTEM=usb, ACTION=add, PRODUCT=, TYPE=, INTERFACE=, DEVPATH=/devices/platform/ehci_hcd/usb1
7 SUBSYSTEM=module, ACTION=add, PRODUCT=, TYPE=, INTERFACE=, DEVPATH=/module/ohci_hcd
8 SUBSYSTEM=platform, ACTION=add, PRODUCT=, TYPE=, INTERFACE=, DEVPATH=/devices/platform/ohci_hcd
9 SUBSYSTEM=drivers, ACTION=add, PRODUCT=, TYPE=, INTERFACE=, DEVPATH=/bus/platform/drivers/ohci_hcd
10 SUBSYSTEM=usb_host, ACTION=add, PRODUCT=, TYPE=, INTERFACE=, DEVPATH=/class/usb_host/usb2
11 SUBSYSTEM=usb, ACTION=add, PRODUCT=0/0/206, TYPE=9/0/1, INTERFACE=9/0/0, DEVPATH=/devices/platform/ehci_hcd/usb1/1-0:1.0
12 SUBSYSTEM=usb, ACTION=add, PRODUCT=, TYPE=, INTERFACE=, DEVPATH=/devices/platform/ohci_hcd/usb2
13 SUBSYSTEM=scsi_host, ACTION=add, PRODUCT=, TYPE=, INTERFACE=, DEVPATH=/class/scsi_host/host0
14 SUBSYSTEM=usb, ACTION=add, PRODUCT=18a5/216/112, TYPE=0/0/0, INTERFACE=8/6/80, DEVPATH=/devices/platform/ehci_hcd/usb1/1-1/1-1:1.0
15 SUBSYSTEM=usb, ACTION=add, PRODUCT=0/0/206, TYPE=9/0/0, INTERFACE=9/0/0, DEVPATH=/devices/platform/ohci_hcd/usb2/2-0:1.0
16 SUBSYSTEM=usb, ACTION=add, PRODUCT=, TYPE=, INTERFACE=, DEVPATH=/devices/platform/ehci_hcd/usb1/1-1
17 SUBSYSTEM=scsi_generic, ACTION=add, PRODUCT=, TYPE=, INTERFACE=, DEVPATH=/class/scsi_generic/sg0
18 SUBSYSTEM=scsi_device, ACTION=add, PRODUCT=, TYPE=, INTERFACE=, DEVPATH=/class/scsi_device/0:0:0:0
19 SUBSYSTEM=scsi, ACTION=add, PRODUCT=, TYPE=, INTERFACE=, DEVPATH=/devices/platform/ehci_hcd/usb1/1-1/1-1:1.0/host0/target0:0:0/0:0:0:0
20 SUBSYSTEM=block, ACTION=add, PRODUCT=, TYPE=, INTERFACE=, DEVPATH=/block/sda
21 SUBSYSTEM=module, ACTION=add, PRODUCT=, TYPE=, INTERFACE=, DEVPATH=/module/ufsd
22 SUBSYSTEM=block, ACTION=add, PRODUCT=, TYPE=, INTERFACE=, DEVPATH=/block/sda/sda1
系统默认的热插拔处理程序是/sbin/hotplug。在MP800H上/sbin/hotplug用的是diethotplug-0.4，在
华硕公开的源码中有它的源代码。这是一个瘦身版的hotplug程序，它的主要工作是在usb存储设备插入时进行自动挂载，挂载目录在/tmp/usbmounts下面，如/tmp/usbmounts/sda1。
我们可以通过修改/proc/sys/kernel/hotplug来重新指定hotplug程序，通过定制hotplug程序，我们可以按我们希望的方式来处理热插拔事件，比如插入无线网卡时自动连接无线网。下面我将给出一个实现此功能的程序实例。
程序由两个shell脚本和一个配置文件组成：hotplug脚本是我们定制的hotplug程序，wifi_monitor脚本是无线网络监控程序，它们都放在/usr/local/sbin下。wifi.conf是无线网络配置文件，放在/usr/local/etc下。
首先用我们的hotplug程序替换系统默认的/sbin/hotplug
1 # echo /usr/local/sbin/hotplug > /proc/sys/kernel/hotplug
程序是这样工作的：热插拔事件发生时，内核执行我们的hotplug程序，即/usr/local/sbin/hotplug。hotplug程序先调用/sbin/hotplug，让它完成它该做的工作。然后判断该事件是否为usb无线网卡插入事件，如果是则执行wifi_monitor，如果是无线网络连接事件则通过DHCP获取IP地址。wifi_monitor根据wifi.conf配置文件尝试连接无线网络。它先扫描可用无线网络，如果找到指定的网络则进行连接，否则等5秒钟再重新扫描。如果扫描3次都没有找到，则等待10秒钟后重新读入配置文件再次开始扫描循环。wifi_monitor还可以处理无线网络断线的情况，即发现无线网络断开时重新开始扫描循环。
/usr/local/sbin/hotplug
#!/bin/sh
# let /sbin/hotplug do the dirty work first
/sbin/hotplug $1
#define log file. set to /dev/null if you want to suppress logging
LOG=/var/log/hotplug.log
# log the event
echo "[`date`] hotplug SUBSYSTEM=$SUBSYSTEM, ACTION=$ACTION, PRODUCT=$PRODUCT, TYPE=$TYPE, INTERFACE=$INTERFACE, DEVPATH=$DEVPATH" >> $LOG
EVENT="$SUBSYSTEM.$INTERFACE.$ACTION";
if [ "$EVENT" = "net.wlan0.add" ] ; then
# start wifi monitor
/usr/local/sbin/wifi_monitor $INTERFACE &
elif [ "$EVENT" = "net.wlan0.linkup" ] ; then
/sbin/udhcpc -i $INTERFACE -t 15 -b -q -s /etc/udhcpc.script >> $LOG 2>&1
fi
/usr/local/sbin/wifi_monitor
#!/bin/sh
PATH=/sbin:/usr/sbin:/bin:/usr/bin:/usr/local/sbin:/usr/local/bin
WPA_CTRL_INTERFACE=/var/lock/wpa_supplicant
WPA_SUPPLICANT_PID=/var/lock/wpa_supplicant.pid
WPA_SUPPLICANT_CONF=/tmp/wpa_supplicant.conf
WIFI_CONF=/usr/local/etc/wifi.conf
if [ -z "$1" ] ; then
echo "Usage: wifi_monitor <interface>"
exit 1
fi
IFACE=$1
PIDFILE=/var/lock/wifi_monitor.$IFACE.pid
SYSFS="/sys/class/net/$IFACE"
if [ -e $PIDFILE ] && grep -q wifi_monitor /proc/`cat $PIDFILE`/cmdline ; then
echo "wifi_monitor: another instance is running, quiting..."
exit
fi
read_conf() {
for v in ESSID SECURITY WEP_AUTH WEP_KEYINDEX WEP_KEY WPA_PASSPHRASE ; do unset CONF_$v ; done
for line in `grep -E '^ *[A-Z_]+ *= *[^ #]+' $WIFI_CONF|sed -e 's/#.*//' -e 's/ *//g'` ; do eval CONF_$line ; done
}
reload() {
echo "wifi_monitor: reloading..."
ifconfig $IFACE down
iwconfig $IFACE essid off
}
quit() { exit; }
cleanup() {
[ -e $WPA_SUPPLICANT_PID ] && pkill -x wpa_supplicant
rm -f $WPA_SUPPLICANT_PID
rm -rf $WPA_CTRL_INTERFACE
rm -f $PIDFILE
}
setup_wifi() {
[ -n "$CONF_ESSID" ] || { echo "Error: ESSID not defined!" ; return; }
# scan for AP
ifconfig $IFACE up
local count=0
until [ $count -ge 3 ] || iwlist $IFACE scanning|grep -q $CONF_ESSID ;
do
echo "AP $CONF_ESSID not found, sleeping...";
sleep 5;
let 'count=count+1';
done
if [ $count -lt 3 ] ; then
[ "x$CONF_SECURITY" != "xWPA" ] && [ -e $WPA_SUPPLICANT_PID ] && kill `cat $WPA_SUPPLICANT_PID`
case "$CONF_SECURITY" in
NONE)
iwconfig $IFACE key off
iwconfig $IFACE essid "$CONF_ESSID"
;;
WEP)
[ -n "$CONF_WEP_KEY" ] || { echo "Error: WEP_KEY not defined!" ; return; }
if [ -n "$CONF_WEP_KEYINDEX" ] ; then
iwconfig $IFACE key $CONF_WEP_AUTH [$CONF_WEP_KEYINDEX] "$CONF_WEP_KEY"
else
iwconfig $IFACE key $CONF_WEP_AUTH "$CONF_WEP_KEY"
fi
iwconfig $IFACE essid "$CONF_ESSID"
;;
WPA)
[ -n "$CONF_WPA_PASSPHRASE" ] || { echo "Error: WPA_PASSPHRASE not defined!" ; return; }
if [ ! -e $WPA_SUPPLICANT_PID ] || ! pgrep -x wpa_supplicant > /dev/null ; then
# generate the wpa_supplicant.conf
if [ "x$LAST_WPA_PASSPHRASE" != "x$CONF_ESSID.$CONF_WPA_PASSPHRASE" ] ; then
WPA_PSK=`wpa_passphrase "$CONF_ESSID" "$CONF_WPA_PASSPHRASE"|grep -o 'psk=[0-9a-z]\{64\}'`
LAST_WPA_PASSPHRASE="$CONF_ESSID.$CONF_WPA_PASSPHRASE"
fi
cat >$WPA_SUPPLICANT_CONF<<-EOF
ctrl_interface=$WPA_CTRL_INTERFACE
network={
ssid="$CONF_ESSID"
scan_ssid=1
proto=WPA RSN
key_mgmt=WPA-PSK
pairwise=CCMP TKIP
group=CCMP TKIP
$WPA_PSK
}
EOF
wpa_supplicant -B -Dwext -i$IFACE -c$WPA_SUPPLICANT_CONF -P$WPA_SUPPLICANT_PID
fi
;;
esac
fi
}
trap 'reload' HUP
trap 'quit' INT QUIT TERM
trap 'cleanup' EXIT
echo $$ > $PIDFILE
# wait until the kernel structure is ready. FIXME: Is this really necessary?
until [ -d $SYSFS ] ; do sleep 1; done
while [ -d $SYSFS ] ; do
carrier=`cat $SYSFS/carrier 2>/dev/null`
if [ "x$carrier" != "x1" ] ; then
# reload configuration
read_conf
setup_wifi
fi
sleep 10
done
/usr/local/etc/wifi.conf - WEP实例
ESSID=MYESSID
# wireless security: NONE, WEP or WPA
SECURITY=WEP
# WEP security mode: open or restricted
WEP_AUTH=restricted
WEP_KEYINDEX=1
WEP_KEY="06af3154902d96506b3cbe6bae"
/usr/local/etc/wifi.conf - WPA实例
ESSID=MYESSID
# wireless security: NONE, WEP or WPA
SECURITY=WPA
WPA_PASSPHRASE="testtest"