************************part 1: 系统启动*********************
1.系统的启动流程
-->power on
-->bios(basic input output system),加电自检
-->看看mbr
-->grub | lilo
-->内核(vmlinuz-2.6.32-71.el6.i686) | initrdxxxx.img(RHEL5) initramfs-2.6.32-71.el6.i686.img(RHEL6)
-->init -->启动一个超级进程
-->/etc/inittab -->确定runlevel
-->/etc/rc.sysinit(主机名,ip,检查磁盘,挂载文件系统) --/etc/rc.sysinit -> rc.d/rc.sysinit：/etc/rc.sysinit是/etc/rc.d/rc.sysinit的软链接
-->/etc/rc.d/rcX.d/ -->启动运行级别X的脚本
-->/etc/rc.local --/etc/rc.local -> rc.d/rc.local：/etc/rc.local是/etc/rc.d/rc.local的软链接
2.经典/etc/inittab
#
# inittab This file describes how the INIT process should set up
# the system in a certain run-level.
#
# Author: Miquel van Smoorenburg, <miquels@drinkel.nl.mugnet.org>
# Modified for RHS Linux by Marc Ewing and Donnie Barnes
#
# Default runlevel. The runlevels used by RHS are:
# 0 - halt (Do NOT set initdefault to this) --关机
# 1 - Single user mode --单用户
# 2 - Multiuser, without NFS (The same as 3, if you do not have netwo***ng) --无网络,无NFS
# 3 - Full multiuser mode --字符模式,完全多用户模式
# 4 - unused --保留
# 5 - X11 --图形
# 6 - reboot (Do NOT set initdefault to this) --重启
#
id:5:initdefault: <--确定runlevel
# System initialization.
si::sysinit:/etc/rc.d/rc.sysinit <--初始化操作系统的基本东西：主机名,ip,检查磁盘,挂载文件系统
l0:0:wait:/etc/rc.d/rc 0 <--对应不同运行级别
l1:1:wait:/etc/rc.d/rc 1
l2:2:wait:/etc/rc.d/rc 2
l3:3:wait:/etc/rc.d/rc 3
l4:4:wait:/etc/rc.d/rc 4
l5:5:wait:/etc/rc.d/rc 5
l6:6:wait:/etc/rc.d/rc 6
# Trap CTRL-ALT-DELETE
ca::ctrlaltdel:/sbin/shutdown -t3 -r now <--ctl alt del实现重启
# When our UPS tells us power has failed, assume we have a few minutes
# of power left. Schedule a shutdown for 2 minutes from now.
# This does, of course, assume you have powerd installed and your
# UPS connected and wo***ng correctly.
pf::powerfail:/sbin/shutdown -f -h 2 "Power Failure; System Shutting Down" <--UPS
# If power was restored before the shutdown kicked in, cancel it.
pr:12345:powerokwait:/sbin/shutdown -c "Power Restored; Shutdown Cancelled"
# Run gettys in standard runlevels
1:2345:respawn:/sbin/mingetty tty1 <--终端
2:2345:respawn:/sbin/mingetty tty2
3:2345:respawn:/sbin/mingetty tty3
4:2345:respawn:/sbin/mingetty tty4
5:2345:respawn:/sbin/mingetty tty5
6:2345:respawn:/sbin/mingetty tty6
# Run xdm in runlevel 5
x:5:respawn:/etc/X11/prefdm -nodaemon <--图形界面
3. id:runlevel:action:process
--id: -->两个字符的特殊符号
--runlevel -->运行级别,,0,6不能设置成initdefault,生产环境常用3
--action -->动作
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