udev配置文件理解

!/bin/sh -e

### BEGIN INIT INFO
# Provides:          udev
# Required-Start:    mountvirtfs
# Required-Stop:     
# Default-Start:     S
# Default-Stop:
# Short-Description: Start udevd, populate /dev and load drivers.
### END INIT INFO

export TZ=/etc/localtime  //自动设置时区

[ -d /sys/class ] || exit 1   //判断/sys/class 目录是否存在，如果不存在退出。给目录主要用来创建驱动设备
[ -r /proc/mounts ] || exit 1 //判断mount是否可读
[ -x /sbin/udevd ] || exit 1 //判断/sbin/udevd 是否可执行
[ -f /etc/default/udev ] && . /etc/default/udev //如果/etc/defult/udev文件存在，有则执行
[ -f /etc/udev/udev.conf ] && . /etc/udev/udev.conf //如果/etc/udev/udev.conf,有则执行

kill_udevd() {
        if [ -x /sbin/pidof ]; then
                pid=`/sbin/pidof -x udevd`
                [ -n "$pid" ] && kill $pid
        fi
}

export ACTION=add
# propagate /dev from /sys
echo -n "Starting udev"

# mount the tmpfs on /dev, if not already done
LANG=C awk "\$2 == \"/dev\" && \$4 == \"tmpfs\" { exit 1 }" /proc/mounts && {
        mount -n -o mode=0755 -t tmpfs none "/dev"
        mkdir -m 0755 /dev/pts
        mkdir -m 0755 /dev/shm
}

if [ "$DEVCACHE" != "" ]; then
	# Invalidate udev cache if the kernel or its bootargs/cmdline have changed
	[ -x /bin/uname ] && /bin/uname -mrspv > /tmp/uname || touch /tmp/uname
	[ -r /proc/cmdline ] && cat /proc/cmdline > /tmp/cmdline || touch /tmp/cmdline
	[ -r /proc/atags ] && cat /proc/atags > /tmp/atags || touch /tmp/atags
	if [ -e $DEVCACHE ] && \
	   cmp -s /tmp/uname /etc/udev/saved.uname && \
	   cmp -s /tmp/cmdline /etc/udev/saved.cmdline && \
	   cmp -s /tmp/atags /etc/udev/saved.atags; then
		(cd /; tar xf $DEVCACHE)
		not_first_boot=1
	fi
fi

# make_extra_nodes
kill_udevd > "/dev/null" 2>&1

        # trigger the sorted events
        echo -e '\000\000\000\000' > /proc/sys/kernel/hotplug
        /sbin/udevd -d

        /sbin/udevadm control env STARTUP=1
		if [ "$not_first_boot" != "" ];then
			/sbin/udevadm trigger --subsystem-nomatch=tty --subsystem-nomatch=mem --subsystem-nomatch=vc --subsystem-nomatch=vtconsole --subsystem-nomatch=misc --subsystem-nomatch=dcon --subsystem-nomatch=pci_bus  --subsystem-nomatch=graphics  --subsystem-nomatch=backlight --subsystem-nomatch=video4linux  --subsystem-nomatch=platform
			(/sbin/udevadm settle --timeout=3; /sbin/udevadm control env STARTUP=)&
			if [ "$DEVCACHE" != "" ]; then
				rm -f /tmp/uname
				rm -f /tmp/cmdline
				rm -f /tmp/atags
			fi
		else
			/sbin/udevadm trigger
			/sbin/udevadm settle
			if [ "$DEVCACHE" != "" ]; then
				echo -n " and populating dev cache"
				(cd /; tar cf $DEVCACHE dev)
				mv /tmp/uname /etc/udev/saved.uname
				mv /tmp/cmdline /etc/udev/saved.cmdline
				mv /tmp/atags /etc/udev/saved.atags
			fi
		fi

echo
exit 0