CentOS RedHat启动过程

打开PC的电源，BIOS开机自检，按BIOS中设置的启动设备(通常是硬盘)启动，接着启动设备上安装的引导程序lilo或grub开始引导Linux，Linux首先进行内核的引导，接下来执行init程序，init程序调用了rc.sysinit和rc等程序，rc.sysinit和rc当完成系统初始化和运行服务的任务后，返回init；init启动了mingetty后，打开了终端供用户登录系统，用户登录成功后进入了Shell，这样就完成了从开机到登录的整个启动过程。





init进程起来后，系统启动的控制权移交给init进程。

/sbin/init进程是所有进程的父进程，当init起来之后，它首先会读取配置文件/etc/inittab，进行以下工作：

1）执行系统初始化脚本(/etc/rc.d/rc.sysinit)，对系统进行基本的配置，以读写方式挂载根文件系统及其它文件系统，到此系统基本算运行起来了，后面需要进行运行级别的确定及相应服务的启动；

2）确定启动后进入的运行级别；

3)执行/etc/rc.d/rc，该文件定义了服务启动的顺序是先K后S，而具体的每个运行级别的服务状态是放在/etc/rc.d/rcn.d（n=0~6）目录下，所有的文件均链接至/etc/init.d下的相应文件。

4）/etc/rc.d/rc$RUNLEVEL  # $RUNLEVEL为缺省的运行模式

5) /etc/rc.d/rc.local

6）启动虚拟终端/sbin/mingetty

7）在运行级别5上运行X

这时呈现给用户的就是最终的登录界面。

至此，系统启动过程完毕。

说明：

系统启动运行级别的概念以及服务的定制方法；

当initrd 可以正常检测和装载之后，最后的工作就基本上由操作系统来进行了。当系统的init进程起来之后系统启动的控制权移交给init进程。

/sbin/init 进程是所有进程的父进程，当init起来之后，它首先会读取配置文件/etc/inittab，进行以下工作：

1）执行系统初始化脚本(/etc/rc.d/rc.sysinit)，对系统进行基本的配置，以读写方式挂载根文件系统及其它文件系统，后面需要进行运行级别的确定及相应服务的启动，（从这个角度可以看出如果要定义系统的init动作，需要修改/etc/rc.d/rc.sysinit脚本）。它的主要工作有：配置selinux，系统时钟，内核参数（/etc/sysctl.conf），hostname，启用wap分区，根文件系统的检查和二次挂载（读写），激活RAID和LVM设备，启用磁盘quota检查并挂载其它文件系统。

2）通过对/etc/inittab文件

 

[root@sky etc]# cat inittab
# inittab is only used by upstart for the default runlevel.
#
# ADDING OTHER CONFIGURATION HERE WILL HAVE NO EFFECT ON YOUR SYSTEM.
#
# System initialization is started by /etc/init/rcS.conf
#
# Individual runlevels are started by /etc/init/rc.conf
#
# Ctrl-Alt-Delete is handled by /etc/init/control-alt-delete.conf
#
# Terminal gettys are handled by /etc/init/tty.conf and /etc/init/serial.conf,
# with configuration in /etc/sysconfig/init.
#
# For information on how to write upstart event handlers, or how
# upstart works, see init(5), init(8), and initctl(8).
#
# Default runlevel. The runlevels used are:
#   0 - halt (Do NOT set initdefault to this)
#   1 - Single user mode
#   2 - Multiuser, without NFS (The same as 3, if you do not have networking)
#   3 - Full multiuser mode
#   4 - unused
#   5 - X11
#   6 - reboot (Do NOT set initdefault to this)
#
id:5:initdefault:

[root@sky init]# cat rc.conf
# rc - System V runlevel compatibility
#
# This task runs the old sysv-rc runlevel scripts.  It
# is usually started by the telinit compatibility wrapper.
#
# Do not edit this file directly. If you want to change the behaviour,
# please create a file rc.override and put your changes there.

start on runlevel [0123456]

stop on runlevel [!$RUNLEVEL]

task

export RUNLEVEL
console output
exec /etc/rc.d/rc $RUNLEVEL
[root@sky init]#

[root@sky ~]# cat /etc/rc.d/rc
#! /bin/bash
#
# rc            This file is responsible for starting/stopping
#               services when the runlevel changes.
#
# Original Author:
#               Miquel van Smoorenburg, <miquels@drinkel.nl.mugnet.org>
#

set -m

# check a file to be a correct runlevel script
check_runlevel ()
{
# Check if the file exists at all.
[ -x "$1" ] || return 1
is_ignored_file "$1" && return 1
return 0
}

# Now find out what the current and what the previous runlevel are.
argv1="$1"
set $(/sbin/runlevel)
runlevel=$2
previous=$1
export runlevel previous

. /etc/init.d/functions

export CONSOLETYPE
do_confirm="no"
if [ -f /var/run/confirm ]; then
do_confirm="yes"
fi
UPSTART=
[ -x /sbin/initctl ] && UPSTART=yes
# See if we want to be in user confirmation mode
if [ "$previous" = "N" ]; then
if [ "$do_confirm" = "yes" ]; then
echo $"Entering interactive startup"
else
echo $"Entering non-interactive startup"
fi
fi

# Get first argument. Set new runlevel to this argument.
[ -n "$argv1" ] && runlevel="$argv1"

# Is there an rc directory for this new runlevel?
[ -d /etc/rc$runlevel.d ] || exit 0

# Set language, vc settings once to avoid doing it for every init script
# through functions
if [ -f /etc/sysconfig/i18n -a -z "${NOLOCALE:-}" ] ; then
. /etc/profile.d/lang.sh 2>/dev/null
export LANGSH_SOURCED=1
fi

# First, run the KILL scripts.
for i in /etc/rc$runlevel.d/K* ; do

# Check if the subsystem is already up.
subsys=${i#/etc/rc$runlevel.d/K??}
[ -f /var/lock/subsys/$subsys -o -f /var/lock/subsys/$subsys.init ] || continue
check_runlevel "$i" || continue

# Bring the subsystem down.
[ -n "$UPSTART" ] && initctl emit --quiet stopping JOB=$subsys
$i stop
[ -n "$UPSTART" ] && initctl emit --quiet stopped JOB=$subsys
done

# Now run the START scripts.
for i in /etc/rc$runlevel.d/S* ; do

# Check if the subsystem is already up.
subsys=${i#/etc/rc$runlevel.d/S??}
[ -f /var/lock/subsys/$subsys ] && continue
[ -f /var/lock/subsys/$subsys.init ] && continue
check_runlevel "$i" || continue

# If we're in confirmation mode, get user confirmation
if [ "$do_confirm" = "yes" ]; then
confirm $subsys
rc=$?
if [ "$rc" = "1" ]; then
continue
elif [ "$rc" = "2" ]; then
do_confirm="no"
fi
fi

update_boot_stage "$subsys"
# Bring the subsystem up.
[ -n "$UPSTART" ] && initctl emit --quiet starting JOB=$subsys
if [ "$subsys" = "halt" -o "$subsys" = "reboot" ]; then
export LC_ALL=C
exec $i start
fi
$i start
[ -n "$UPSTART" ] && initctl emit --quiet started JOB=$subsys
done
[ "$do_confirm" = "yes" ] && rm -f /var/run/confirm
exit 0
[root@sky ~]#

[root@sky rc5.d]# pwd
/etc/rc5.d
[root@sky rc5.d]# ls
K01smartd        K61nfs-rdma        K89netconsole    S10network         S24nfslock           S55sshd
K02oddjobd       K69rpcsvcgssd      K89rdisc         S11auditd          S24rpcgssd           S70spice-vdagentd
K05wdaemon       K73winbind         K92pppoe-server  S11portreserve     S25blk-availability  S80postfix
K10psacct        K74ntpd            K95firstboot     S12rsyslog         S25cups              S82abrt-ccpp
K10saslauthd     K75ntpdate         K95rdma          S13cpuspeed        S25netfs             S82abrtd
K15htcacheclean  K75quota_nld       K99rngd          S13irqbalance      S26acpid             S90crond
K15httpd         K76ypbind          S01sysstat       S13rpcbind         S26haldaemon         S95atd
K50dnsmasq       K84wpa_supplicant  S02lvm2-monitor  S15mdmonitor       S26udev-post         S99certmonger
K50kdump         K87restorecond     S08ip6tables     S22messagebus      S28autofs            S99local
K60nfs           K88sssd            S08iptables      S23NetworkManager  S50bluetooth
[root@sky rc5.d]#