内容：
CentOS 的启动流程
服务管理(chkconfig)
手动制定系统开机服务脚本
一、centos6系统启动流程
综述过程：
POST-->BIOS(Boot Sequence)-->MBR(grub,446)-->Kernel-->initramfs（虚根文件）-->(ROOTFS)/sbin/init(/etc/inittab)
步骤详解：
1、POST：
POST：Power-On-Self-Test，加电自检，是BIOS功能的一个主要部分。负责完成对CPU、主板、内存、硬盘子系统、显示子系统、串并行接口、键盘、CD-ROM光驱等硬件情况的检测。
2、BIOS：
决定设备的启动顺序
3、grub(bootloader)
在centos5、6中，linux系统的MBR的bootloader是grub程序，windows的bootloader程序是ntloader：
windows: ntloader，仅是启动OS
Linux：功能丰富，提供菜单，允许用户选择要启动系统或不同的内核版本；把用户选定的内核装载到内存中的特定空间中，解压、展开，并把系统控制权移交给内核
由于linux的grub提供了丰富的功能，而MBR的bootloader仅仅446个字节，所以grub有分成了两个阶段，其中阶段二又有两部分（stage1_5,stage2）：
stage1：bootloader，446个字节，作用就是为了引导下一个阶段
stage1_5：此阶段的文件在初次安装系统时生成，只要是boot分区的文件系统驱动程序，作用是为了加载boot分区
[19:13 [email protected]/etc/init.d]# ll /boot/grub/*5 -rw-r--r--. 1 root root 13428 Jul 20 02:24 /boot/grub/e2fs_stage1_5 -rw-r--r--. 1 root root 12636 Jul 20 02:24 /boot/grub/fat_stage1_5 -rw-r--r--. 1 root root 11780 Jul 20 02:24 /boot/grub/ffs_stage1_5 -rw-r--r--. 1 root root 11772 Jul 20 02:24 /boot/grub/iso9660_stage1_5 -rw-r--r--. 1 root root 13284 Jul 20 02:24 /boot/grub/jfs_stage1_5 -rw-r--r--. 1 root root 11972 Jul 20 02:24 /boot/grub/minix_stage1_5 -rw-r--r--. 1 root root 14428 Jul 20 02:24 /boot/grub/reiserfs_stage1_5 -rw-r--r--. 1 root root 12040 Jul 20 02:24 /boot/grub/ufs2_stage1_5 -rw-r--r--. 1 root root 11380 Jul 20 02:24 /boot/grub/vstafs_stage1_5 -rw-r--r--. 1 root root 13980 Jul 20 02:24 /boot/grub/xfs_stage1_5
stage2：此阶段的作用是提供选择菜单（如果有多个内核系统），引导启动下一步的kernel内核启动和加载initramfs文件（作用是提供真正根分区的文件系统驱动程序，以便能加载根分区），其配置文件为/etc/grub.conf
[19:16 [email protected]/etc/init.d]# cat /etc/grub.conf # grub.conf generated by anaconda # # Note that you do not have to rerun grub after making changes to this file # NOTICE: You have a /boot partition. This means that # all kernel and initrd paths are relative to /boot/, eg. # root (hd0,0) # kernel /vmlinuz-version ro root=/dev/sda2 # initrd /initrd-[generic-]version.img #boot=/dev/sda default=0 timeout=5 splashimage=(hd0,0)/grub/splash.xpm.gz hiddenmenu title CentOS 6 (2.6.32-642.el6.x86_64) root (hd0,0) kernel /vmlinuz-2.6.32-642.el6.x86_64 ro root=UUID=81d6eec4-2af7-4166-bfb6-092432abb4c0 rd_NO_LUKS rd_NO_LVM LANG=en_US.UTF-8 rd_NO_MD SYSFONT=latarcyrheb-sun16 crashkernel=128M KEYBOARDTYPE=pc KEYTABLE=us rd_NO_DM rhgb quiet initrd /initramfs-2.6.32-642.el6.x86_64.img
说明： default：设定默认启动菜单项，默认为0开始 timeout：选择启动内核的停留时间 splashimage：启动的背景图片 hiddenmenu：隐藏选择菜单 title：启动时看到的选择内核的名称 root(hd0,0)：第一块硬盘，第一个分区，根系统的分区位置 kernel：内核文件的路径，此时会把boot文件当跟文件，所以不需要写成/boot/PATH initrd：根分区文件系统的驱动程序路径
由于在grub阶段只能进入了boot分区，所以kernel内核和initramfs文件都存在在boot分区
4、kernel
kernel内核阶段会把内核文件进行解压，初始化系统中各设备并做相关的配置工作，其中包括CPU、I/O、存储设备等。关于Linux的设备驱动程序的加载，有一部分驱动程序直接被编译进内核镜像中，另一部分驱动程序则是以模块的形式放在initrd(ramdisk)中。
initramfs的作用前面已经说明，下一步就是加载真正的/根分区
5、/sbin/init
(1)进入根分区后，会运行第一个进程init，PID为1，/sbin/init进程是系统其他所有进程的父进程，当它接管了系统的控制权先之后，它首先会去读取/etc/inittab文件来执行相应的脚本进行系统初始化
[19:18 [email protected]/etc/init.d]# cat /etc/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:3:initdefault:
运行级别：为系统运行或维护等目的而设定；0-6：7个级别
0：关机
1：单用户模式(root自动登录), single, 维护模式，该模式就是所谓的破解root密码
2: 多用户模式，启动网络功能，但不会启动NFS；维护模式
3：多用户模式，正常模式；文本界面
4：预留级别；可同3级别
5：多用户模式，正常模式；图形界面
6：重启
默认级别：3, 5
切换级别：init#
查看级别：runlevel; who -r
[20:20 [email protected]/etc/init.d]# runlevel N 5 #N表示上次运行的级别，5表示切换的本次的运行级别
(2)运行/etc/rc.d/rc.sysinit脚本，此脚本的大概功能如下
(1) 设置主机名 (2) 设置欢迎信息 (3) 激活udev和selinux (4) 挂载/etc/fstab文件中定义的文件系统 (5) 检测根文件系统，并以读写方式重新挂载根文件系统 (6) 设置系统时钟 (7) 激活swap设备 (8) 根据/etc/sysctl.conf文件设置内核参数 (9) 激活lvm及software raid设备 (10) 加载额外设备的驱动程序 (11) 清理操作
(3)sysinit脚本执行完成，系统就可以顺利工作了，只是还需要启动系统所需要的各种服务，这样主机才可以提供相关的网络和主机功能，因此还会根据相应的运行级别，运行相应的系统服务。
执行/etc/rc.d/rc脚本。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/rc*.d（*=0~6）目录下，所有的文件均是指向/etc/init.d下相应文件的符号链接。rc.sysinit通过分析/etc/inittab文件来确定系统的启动级别，然后才去执行/etc/rc.d/rcN.d下的文件。
以rc3.d为例：
/etc/rc.d/rc3.d目录，该目录下的内容全部都是以 S 或 K 开头的链接文件，都链接到"/etc/rc.d/init.d"目录下的各种shell脚本。S表示的是启动时需要start的服务内容，K表示关机时需要关闭的服务内容，后面的数字代表优先次序。
[19:45 [email protected]/etc/init.d]# ll /etc/rc.d/rc3.d/ total 0 lrwxrwxrwx. 1 root root 16 Jul 20 02:20 K01smartd -> ../init.d/smartd lrwxrwxrwx. 1 root root 17 Jul 20 02:18 K02oddjobd -> ../init.d/oddjobd lrwxrwxrwx. 1 root root 17 Jul 20 02:22 K05wdaemon -> ../init.d/wdaemon lrwxrwxrwx. 1 root root 16 Jul 20 02:21 K10psacct -> ../init.d/psacct lrwxrwxrwx. 1 root root 19 Jul 20 02:18 K10saslauthd -> ../init.d/saslauthd lrwxrwxrwx. 1 root root 22 Jul 20 02:18 K15htcacheclean -> ../init.d/htcacheclean lrwxrwxrwx. 1 root root 15 Jul 20 02:18 K15httpd -> ../init.d/httpd lrwxrwxrwx. 1 root root 18 Aug 26 19:04 K15svnserve -> ../init.d/svnserve lrwxrwxrwx. 1 root root 24 Jul 20 02:20 K30spice-vdagentd -> ../init.d/spice-vdagentd lrwxrwxrwx. 1 root root 16 Jul 20 02:18 K36mysqld -> ../init.d/mysqld lrwxrwxrwx. 1 root root 17 Jul 20 02:17 K50dnsmasq -> ../init.d/dnsmasq lrwxrwxrwx. 1 root root 13 Jul 20 02:16 K60nfs -> ../init.d/nfs lrwxrwxrwx. 1 root root 18 Jul 20 02:20 K61nfs-rdma -> ../init.d/nfs-rdma lrwxrwxrwx. 1 root root 20 Jul 20 02:16 K69rpcsvcgssd -> ../init.d/rpcsvcgssd lrwxrwxrwx. 1 root root 17 Jul 20 02:16 K73winbind -> ../init.d/winbind lrwxrwxrwx. 1 root root 14 Jul 20 03:15 K74ntpd -> ../init.d/ntpd lrwxrwxrwx. 1 root root 17 Jul 20 02:17 K75ntpdate -> ../init.d/ntpdate lrwxrwxrwx. 1 root root 19 Jul 20 02:20 K75quota_nld -> ../init.d/quota_nld lrwxrwxrwx. 1 root root 16 Jul 20 02:17 K76ypbind -> ../init.d/ypbind lrwxrwxrwx. 1 root root 24 Sep 1 17:09 K84NetworkManager -> ../init.d/NetworkManager lrwxrwxrwx. 1 root root 24 Jul 20 02:17 K84wpa_supplicant -> ../init.d/wpa_supplicant lrwxrwxrwx. 1 root root 21 Jul 20 02:15 K87restorecond -> ../init.d/restorecond lrwxrwxrwx. 1 root root 14 Jul 20 02:16 K88sssd -> ../init.d/sssd lrwxrwxrwx. 1 root root 20 Jul 20 02:15 K89netconsole -> ../init.d/netconsole lrwxrwxrwx. 1 root root 15 Jul 20 02:15 K89rdisc -> ../init.d/rdisc lrwxrwxrwx. 1 root root 22 Jul 20 02:17 K92pppoe-server -> ../init.d/pppoe-server lrwxrwxrwx. 1 root root 19 Jul 20 03:15 K95firstboot -> ../init.d/firstboot lrwxrwxrwx. 1 root root 14 Jul 20 02:20 K95rdma -> ../init.d/rdma lrwxrwxrwx. 1 root root 14 Jul 20 02:20 K99rngd -> ../init.d/rngd lrwxrwxrwx. 1 root root 17 Jul 20 02:19 S01sysstat -> ../init.d/sysstat lrwxrwxrwx. 1 root root 22 Jul 20 02:20 S02lvm2-monitor -> ../init.d/lvm2-monitor lrwxrwxrwx. 1 root root 19 Jul 20 02:18 S08ip6tables -> ../init.d/ip6tables lrwxrwxrwx. 1 root root 18 Jul 20 02:15 S08iptables -> ../init.d/iptables lrwxrwxrwx. 1 root root 17 Jul 20 23:42 S10network -> ../init.d/network lrwxrwxrwx. 1 root root 16 Jul 20 02:21 S11auditd -> ../init.d/auditd lrwxrwxrwx. 1 root root 21 Jul 20 02:12 S11portreserve -> ../init.d/portreserve lrwxrwxrwx. 1 root root 17 Jul 20 02:17 S12rsyslog -> ../init.d/rsyslog lrwxrwxrwx. 1 root root 18 Jul 20 02:20 S13cpuspeed -> ../init.d/cpuspeed lrwxrwxrwx. 1 root root 20 Jul 20 02:18 S13irqbalance -> ../init.d/irqbalance lrwxrwxrwx. 1 root root 17 Jul 20 02:13 S13rpcbind -> ../init.d/rpcbind lrwxrwxrwx. 1 root root 19 Jul 20 02:16 S15mdmonitor -> ../init.d/mdmonitor lrwxrwxrwx. 1 root root 20 Jul 20 02:13 S22messagebus -> ../init.d/messagebus lrwxrwxrwx. 1 root root 17 Jul 20 02:18 S24nfslock -> ../init.d/nfslock lrwxrwxrwx. 1 root root 16 Jul 20 02:17 S24openct -> ../init.d/openct lrwxrwxrwx. 1 root root 17 Jul 20 02:18 S24rpcgssd -> ../init.d/rpcgssd lrwxrwxrwx. 1 root root 26 Jul 20 02:20 S25blk-availability -> ../init.d/blk-availability lrwxrwxrwx. 1 root root 14 Jul 20 02:15 S25cups -> ../init.d/cups lrwxrwxrwx. 1 root root 15 Jul 20 02:15 S25netfs -> ../init.d/netfs lrwxrwxrwx. 1 root root 15 Jul 20 02:20 S26acpid -> ../init.d/acpid lrwxrwxrwx. 1 root root 19 Jul 20 02:17 S26haldaemon -> ../init.d/haldaemon lrwxrwxrwx. 1 root root 19 Jul 20 02:15 S26udev-post -> ../init.d/udev-post lrwxrwxrwx. 1 root root 15 Jul 20 02:17 S27pcscd -> ../init.d/pcscd lrwxrwxrwx. 1 root root 16 Jul 20 02:18 S28autofs -> ../init.d/autofs lrwxrwxrwx. 1 root root 19 Jul 20 02:18 S50bluetooth -> ../init.d/bluetooth lrwxrwxrwx. 1 root root 15 Jul 20 03:15 S50kdump -> ../init.d/kdump lrwxrwxrwx. 1 root root 14 Jul 20 02:20 S55sshd -> ../init.d/sshd lrwxrwxrwx. 1 root root 13 Aug 31 21:43 S80cmd -> ../init.d/cmd lrwxrwxrwx. 1 root root 17 Jul 20 02:18 S80postfix -> ../init.d/postfix lrwxrwxrwx. 1 root root 19 Jul 20 02:13 S82abrt-ccpp -> ../init.d/abrt-ccpp lrwxrwxrwx. 1 root root 15 Jul 20 02:13 S82abrtd -> ../init.d/abrtd lrwxrwxrwx. 1 root root 16 Sep 7 16:52 S88testrv -> ../init.d/testrv lrwxrwxrwx. 1 root root 15 Jul 20 02:18 S90crond -> ../init.d/crond lrwxrwxrwx. 1 root root 13 Jul 20 02:17 S95atd -> ../init.d/atd lrwxrwxrwx. 1 root root 20 Jul 20 02:18 S99certmonger -> ../init.d/certmonger lrwxrwxrwx. 1 root root 11 Jul 20 02:15 S99local -> ../rc.local
注意：正常级别下，最后启动一个服务S99local没有链接至/etc/rc.d/init.d一个服务脚本，而是指向了/etc/rc.d/rc.local脚本
不便或不需写为服务脚本放置于/etc/rc.d/init.d/目录，且又想开机时自动运行的命令，可直接放置于/etc/rc.d/rc.local文件中
6、等待用户登录
完成了系统服务的启动后，linux会启动终端或X-Window来等待用户登录，会执行"/sbin/mingetty"，而且执行了6个，所以linux会有6个纯文本终端(tty1,tty2...)，mingetty就是启动终端的命令。至此，系统就启动完毕了！
[19:51 [email protected]/etc/init.d]# cat /etc/init/tty.conf # tty - getty # # This service maintains a getty on the specified device. # # Do not edit this file directly. If you want to change the behaviour, # please create a file tty.override and put your changes there. stop on runlevel [S016] respawn instance $TTY exec /sbin/mingetty $TTY usage ‘tty TTY=/dev/ttyX - where X is console id‘
综述：POST --> Boot Sequence(BIOS) --> Boot Loader --> Kernel(ramdisk) --> rootfs--> switchroot--> /sbin/init-->(/etc/inittab, /etc/init/*.conf) --> 设定默认运行级别--> 系统初始化脚本rc.sysinit--> 关闭或启动对应级别的服务--> 启动终端
可参考下图(图片来自网络)：
二、系统服务管理
根据上面说明，我们可以知道，系统在启动后会根据想要的运行级别，运行相应的服务脚本，而这些系统服务的管理，可以用chkconfig来进行管理使用，这些服务脚本放置于/etc/rc.d/init.d中
chkconfig：跟新和确认系统服务信息
语法：chkconfig [--list] [--type type][name]：查看所有的服务列表详细信息
chkconfig --add name ：添加服务到系统服务
chkconfig --del name：删除系统服务
chkconfig --override name
chkconfig [--level levels] [--type type] name <on|off|reset|resetpriorities>
chkconfig [--level levels] [--type type] name
[19:58 [email protected]/etc/init.d]# chkconfig --list NetworkManager 0:off1:off2:off3:off4:off5:off6:off abrt-ccpp 0:off1:off2:off3:on4:off5:on6:off abrtd 0:off1:off2:off3:on4:off5:on6:off acpid 0:off1:off2:on3:on4:on5:on6:off atd 0:off1:off2:off3:on4:on5:on6:off auditd 0:off1:off2:on3:on4:on5:on6:off autofs 0:off1:off2:off3:on4:on5:on6:off blk-availability0:off1:on2:on3:on4:on5:on6:off bluetooth 0:off1:off2:off3:on4:on5:on6:off certmonger 0:off1:off2:off3:on4:on5:on6:off cmd 0:off1:off2:on3:on4:on5:on6:off cpuspeed 0:off1:on2:on3:on4:on5:on6:off crond 0:off1:off2:on3:on4:on5:on6:off cups 0:off1:off2:on3:on4:on5:on6:off dnsmasq 0:off1:off2:off3:off4:off5:off6:off firstboot 0:off1:off2:off3:off4:off5:off6:off haldaemon 0:off1:off2:off3:on4:on5:on6:off htcacheclean 0:off1:off2:off3:off4:off5:off6:off httpd 0:off1:off2:off3:off4:off5:off6:off ip6tables 0:off1:off2:on3:on4:on5:on6:off iptables 0:off1:off2:on3:on4:on5:on6:off irqbalance 0:off1:off2:off3:on4:on5:on6:off kdump 0:off1:off2:on3:on4:on5:on6:off lvm2-monitor 0:off1:on2:on3:on4:on5:on6:off mdmonitor 0:off1:off2:on3:on4:on5:on6:off messagebus 0:off1:off2:on3:on4:on5:on6:off mysqld 0:off1:off2:off3:off4:off5:off6:off netconsole 0:off1:off2:off3:off4:off5:off6:off netfs 0:off1:off2:off3:on4:on5:on6:off network 0:off1:off2:on3:on4:on5:on6:off
三、手动配置系统启动服务
1、系统服务脚本特点：基本都可以提供start|stop|restart|status这几个状态值的接收
2、服务脚本存放位置
由上面的介绍可知，所有的系统服务脚本都放在/etc/rc.d/init.d文件夹中
[20:04 [email protected]/etc/init.d]# ls abrt-ccpp bluetooth functions kdump netfs oddjobd rdisc rsyslog svnserve abrtd certmonger haldaemon killall network openct rdma sandbox sysstat abrt-oops cmd halt lvm2-lvmetad NetworkManager pcscd restorecond saslauthd udev-post acpid cpuspeed htcacheclean lvm2-monitor nfs portreserve rngd single wdaemon atd crond httpd mdmonitor nfslock postfix rpcbind smartd winbind auditd cups ip6tables messagebus nfs-rdma pppoe-server rpcgssd spice-vdagentd wpa_supplicant autofs dnsmasq iptables mysqld ntpd psacct rpcidmapd sshd ypbind blk-availability firstboot irqbalance netconsole ntpdate quota_nld rpcsvcgssd sssd
3、服务脚本的编写格式
打开其中一个查看服务脚本的开头格式
[20:04 [email protected]/etc/init.d]# cat atd #!/bin/sh # # atd Starts/stop the "at" daemon # # chkconfig: 345 95 5 # description: Runs commands scheduled by the "at" command at the time # specified when "at" was run, and runs batch commands when the load # average is low enough. ### BEGIN INIT INFO # Provides: atd at batch # Required-Start: $local_fs # Required-Stop: $local_fs # Default-Start: 345 # Default-Stop: 95 # Short-Description: Starts/stop the "at" daemon # Description: Runs commands scheduled by the "at" command at the time # specified when "at" was run, and runs batch commands when the load # average is low enough. ### END INIT INFO
其中，有三行格式是必须要写上的#!/bin/bash，#/chkconfig，#description
# chkconfig: runlevels SS KK【SS：表示开始运行的次序，KK：表示关闭程序的次序，-表示都不启动】
因此可以手动编写系统服务：
(1)把服务脚本放置/etc/init.d目录下
(2)使用chkconfig --add 添加到系统服务
(3)重启系统让服务生效
[20:17 [email protected]/etc/init.d]# cat !$
cat testsrv
#!/bin/bash
#chkconfig 35 88 22
#description: test the system service
testsrv () {
case $1 in
start)
touch /var/lock/subsys/`basename $0` &> /dev/null
echo "start OK" ;;
stop)
rm -rf /var/lock/subsys/`basename $0` &> /dev/null
echo "stop OK" ;;
restart)
rm -rf /var/lock/subsys/`basename $0` &> /dev/null
touch /var/lock/subsys/`basename $0` &> /dev/null
echo "restart OK" ;;
status)
[ -e /var/lock/subsys/`basename $0` ] && echo "is running" || echo "is stopping" ;;
esac
}
CHOICE=$1
! [[ "$CHOICE" =~ start|stop|restart|status ]] && echo "useage:start|stop|restart|status" && exit 100
testsrv $CHOICE
[20:20 [email protected]/etc/init.d]# chkconfig --add testsrv
[20:20 [email protected]/etc/init.d]# chkconfig --list testsrv
testsrv 0:off1:off2:off3:on4:off5:on6:off
[20:20 [email protected]/etc/init.d]# service testsrv status
is stopping
[20:20 [email protected]/etc/init.d]# service testsrv start
start OK
[20:20 [email protected]/etc/init.d]# service testsrv status
is running