Linux 系统启动过程

Linux 系统启动过程

linux启动时我们会看到许多启动信息。

Linux系统的启动过程并不是大家想象中的那么复杂，其过程可以分为5个阶段：

• 内核的引导。
• 运行 init。
• 系统初始化。
• 建立终端 。
• 用户登录系统。

init程序的类型：

• SysV: init, CentOS 5之前, 配置文件： /etc/inittab。
• Upstart: init,CentOS 6, 配置文件： /etc/inittab, /etc/init/*.conf。
• Systemd： systemd, CentOS 7,配置文件： /usr/lib/systemd/system、 /etc/systemd/system。

内核引导

当计算机打开电源后，首先是BIOS开机自检，按照BIOS中设置的启动设备（通常是硬盘）来启动。

操作系统接管硬件以后，首先读入 /boot 目录下的内核文件。

运行init

init 进程是系统所有进程的起点，你可以把它比拟成系统所有进程的老祖宗，没有这个进程，系统中任何进程都不会启动。

init 程序首先是需要读取配置文件 /etc/inittab。

运行级别

许多程序需要开机启动。它们在Windows叫做"服务"（service），在Linux就叫做"守护进程"（daemon）。

init进程的一大任务，就是去运行这些开机启动的程序。

但是，不同的场合需要启动不同的程序，比如用作服务器时，需要启动Apache，用作桌面就不需要。

Linux允许为不同的场合，分配不同的开机启动程序，这就叫做"运行级别"（runlevel）。也就是说，启动时根据"运行级别"，确定要运行哪些程序。

Linux系统有7个运行级别(runlevel)：

• 运行级别0：系统停机状态，系统默认运行级别不能设为0，否则不能正常启动
• 运行级别1：单用户工作状态，root权限，用于系统维护，禁止远程登录
• 运行级别2：多用户状态(没有NFS)
• 运行级别3：完全的多用户状态(有NFS)，登录后进入控制台命令行模式
• 运行级别4：系统未使用，保留
• 运行级别5：X11控制台，登录后进入图形GUI模式
• 运行级别6：系统正常关闭并重启，默认运行级别不能设为6，否则不能正常启动

系统初始化

在init的配置文件中有这么一行： si::sysinit:/etc/rc.d/rc.sysinit　它调用执行了/etc/rc.d/rc.sysinit，而rc.sysinit是一个bash shell的脚本，它主要是完成一些系统初始化的工作，rc.sysinit是每一个运行级别都要首先运行的重要脚本。

它主要完成的工作有：激活交换分区，检查磁盘，加载硬件模块以及其它一些需要优先执行任务。

l5:5:wait:/etc/rc.d/rc 5

这一行表示以5为参数运行/etc/rc.d/rc，/etc/rc.d/rc是一个Shell脚本，它接受5作为参数，去执行/etc/rc.d/rc5.d/目录下的所有的rc启动脚本，/etc/rc.d/rc5.d/目录中的这些启动脚本实际上都是一些连接文件，而不是真正的rc启动脚本，真正的rc启动脚本实际上都是放在/etc/rc.d/init.d/目录下。

而这些rc启动脚本有着类似的用法，它们一般能接受start、stop、restart、status等参数。

/etc/rc.d/rc5.d/中的rc启动脚本通常是K或S开头的连接文件，对于以 S 开头的启动脚本，将以start参数来运行。

而如果发现存在相应的脚本也存在K打头的连接，而且已经处于运行态了(以/var/lock/subsys/下的文件作为标志)，则将首先以stop为参数停止这些已经启动了的守护进程，然后再重新运行。

这样做是为了保证是当init改变运行级别时，所有相关的守护进程都将重启。

至于在每个运行级中将运行哪些守护进程，用户可以通过chkconfig或setup中的"System Services"来自行设定。

建立终端

rc执行完毕后，返回init。这时基本系统环境已经设置好了，各种守护进程也已经启动了。

init接下来会打开6个终端，以便用户登录系统。在inittab中的以下6行就是定义了6个终端：

1:2345:respawn:/sbin/mingetty tty1
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

从上面可以看出在2、3、4、5的运行级别中都将以respawn方式运行mingetty程序，mingetty程序能打开终端、设置模式。

同时它会显示一个文本登录界面，这个界面就是我们经常看到的登录界面，在这个登录界面中会提示用户输入用户名，而用户输入的用户将作为参数传给login程序来验证用户的身份。

https://avg.163.com/topic/detail/9807901 https://avg.163.com/topic/detail/9807900 https://avg.163.com/topic/detail/9807899 https://avg.163.com/topic/detail/9807897 https://avg.163.com/topic/detail/9807898 https://avg.163.com/topic/detail/9807896 https://avg.163.com/topic/detail/9807895 https://avg.163.com/topic/detail/9807894 https://avg.163.com/topic/detail/9807892 https://avg.163.com/topic/detail/9807893 https://avg.163.com/topic/detail/9807890 https://avg.163.com/topic/detail/9807889 https://avg.163.com/topic/detail/9807888 https://avg.163.com/topic/detail/9807887 https://avg.163.com/topic/detail/9807886 https://avg.163.com/topic/detail/9807884 https://avg.163.com/topic/detail/9807885 https://avg.163.com/topic/detail/9807883 https://avg.163.com/topic/detail/9807865 https://avg.163.com/topic/detail/9807876 https://avg.163.com/topic/detail/9807711 https://avg.163.com/topic/detail/9807866 https://avg.163.com/topic/detail/9807862 https://avg.163.com/topic/detail/9807864 https://avg.163.com/topic/detail/9807863 https://avg.163.com/topic/detail/9807861 https://avg.163.com/topic/detail/9807860 https://avg.163.com/topic/detail/9807859 https://avg.163.com/topic/detail/9807858 https://avg.163.com/topic/detail/9807856 https://avg.163.com/topic/detail/9807857 https://avg.163.com/topic/detail/9807855 https://avg.163.com/topic/detail/9807854 https://avg.163.com/topic/detail/9807844 https://avg.163.com/topic/detail/9807837 https://avg.163.com/topic/detail/9807838 https://avg.163.com/topic/detail/9807836 https://avg.163.com/topic/detail/9807833 https://avg.163.com/topic/detail/9807834 https://avg.163.com/topic/detail/9807835 https://avg.163.com/topic/detail/9807832 https://avg.163.com/topic/detail/9807831 https://avg.163.com/topic/detail/9807830 https://avg.163.com/topic/detail/9807829 https://avg.163.com/topic/detail/9807828 https://avg.163.com/topic/detail/9807827 https://avg.163.com/topic/detail/9807826 https://avg.163.com/topic/detail/9807824 https://avg.163.com/topic/detail/9807823 https://avg.163.com/topic/detail/9807821 https://avg.163.com/topic/detail/9807818 https://avg.163.com/topic/detail/9807819 https://avg.163.com/topic/detail/9807806 https://avg.163.com/topic/detail/9807805 https://avg.163.com/topic/detail/9807804 https://avg.163.com/topic/detail/9807803 https://avg.163.com/topic/detail/9807802 https://avg.163.com/topic/detail/9807801 https://avg.163.com/topic/detail/9807800 https://avg.163.com/topic/detail/9807799 https://avg.163.com/topic/detail/9807795 https://avg.163.com/topic/detail/9807794 https://avg.163.com/topic/detail/9807791 https://avg.163.com/topic/detail/9807793 https://avg.163.com/topic/detail/9807787 https://avg.163.com/topic/detail/9807779 https://avg.163.com/topic/detail/9807777 https://avg.163.com/topic/detail/9807775 https://avg.163.com/topic/detail/9807774 https://avg.163.com/topic/detail/9807773 https://avg.163.com/topic/detail/9807772 https://avg.163.com/topic/detail/9807771 https://avg.163.com/topic/detail/9807770 https://avg.163.com/topic/detail/9807769 https://avg.163.com/topic/detail/9807768 https://avg.163.com/topic/detail/9807767 https://avg.163.com/topic/detail/9807766 https://avg.163.com/topic/detail/9807763 https://avg.163.com/topic/detail/9807765 https://avg.163.com/topic/detail/9807761 https://avg.163.com/topic/detail/9807762 https://avg.163.com/topic/detail/9807759 https://avg.163.com/topic/detail/9807760 https://avg.163.com/topic/detail/9807745 https://avg.163.com/topic/detail/9807744 https://avg.163.com/topic/detail/9807743 https://avg.163.com/topic/detail/9807742 https://avg.163.com/topic/detail/9807741 https://avg.163.com/topic/detail/9807740 https://avg.163.com/topic/detail/9807739 https://avg.163.com/topic/detail/9807738 https://avg.163.com/topic/detail/9807736 https://avg.163.com/topic/detail/9807735 https://avg.163.com/topic/detail/9807734 https://avg.163.com/topic/detail/9807733 https://avg.163.com/topic/detail/9807731 https://avg.163.com/topic/detail/9807732 https://avg.163.com/topic/detail/9807726 https://avg.163.com/topic/detail/9807725 https://avg.163.com/topic/detail/9807723 https://avg.163.com/topic/detail/9807724 https://avg.163.com/topic/detail/9807713 https://avg.163.com/topic/detail/9807712 https://avg.163.com/topic/detail/9807711 https://avg.163.com/topic/detail/9807710 https://avg.163.com/topic/detail/9807709 https://avg.163.com/topic/detail/9807708 https://avg.163.com/topic/detail/9807707 https://avg.163.com/topic/detail/9807706 https://avg.163.com/topic/detail/9807704 https://avg.163.com/topic/detail/9807702 https://avg.163.com/topic/detail/9807701 https://avg.163.com/topic/detail/9807695 https://avg.163.com/topic/detail/9807686 https://avg.163.com/topic/detail/9807692 https://avg.163.com/topic/detail/9807688 https://avg.163.com/topic/detail/9807489 https://avg.163.com/topic/detail/9807683 https://avg.163.com/topic/detail/9807682 https://avg.163.com/topic/detail/9807491 https://avg.163.com/topic/detail/9807681 https://avg.163.com/topic/detail/9807679 https://avg.163.com/topic/detail/9807678 https://avg.163.com/topic/detail/9807677 https://avg.163.com/topic/detail/9807676 https://avg.163.com/topic/detail/9807673 https://avg.163.com/topic/detail/9807675 https://avg.163.com/topic/detail/9807674 https://avg.163.com/topic/detail/9807672 https://avg.163.com/topic/detail/9807670 https://avg.163.com/topic/detail/9807665 https://avg.163.com/topic/detail/9807661 https://avg.163.com/topic/detail/9807656 https://avg.163.com/topic/detail/9807653 https://avg.163.com/topic/detail/9807652 https://avg.163.com/topic/detail/9807647 https://avg.163.com/topic/detail/9807650 https://avg.163.com/topic/detail/9807648 https://avg.163.com/topic/detail/9807649 https://avg.163.com/topic/detail/9807645 https://avg.163.com/topic/detail/9807646 https://avg.163.com/topic/detail/9807642 https://avg.163.com/topic/detail/9807640 https://avg.163.com/topic/detail/9807638 https://avg.163.com/topic/detail/9807637 https://avg.163.com/topic/detail/9807636 https://avg.163.com/topic/detail/9807632 https://avg.163.com/topic/detail/9807630 https://avg.163.com/topic/detail/9807623 https://avg.163.com/topic/detail/9807622 https://avg.163.com/topic/detail/9807621 https://avg.163.com/topic/detail/9807619 https://avg.163.com/topic/detail/9807620 https://avg.163.com/topic/detail/9807618 https://avg.163.com/topic/detail/9807617 https://avg.163.com/topic/detail/9807616 https://avg.163.com/topic/detail/9807615 https://avg.163.com/topic/detail/9807399 https://avg.163.com/topic/detail/9807614 https://avg.163.com/topic/detail/9807613 https://avg.163.com/topic/detail/9807612 https://avg.163.com/topic/detail/9807611 https://avg.163.com/topic/detail/9807610 https://avg.163.com/topic/detail/9807609 https://avg.163.com/topic/detail/9807608 https://avg.163.com/topic/detail/9807600 https://avg.163.com/topic/detail/9807598 https://avg.163.com/topic/detail/9807591 https://avg.163.com/topic/detail/9807588 https://avg.163.com/topic/detail/9807589 https://avg.163.com/topic/detail/9807587 https://avg.163.com/topic/detail/9807586 https://avg.163.com/topic/detail/9807585 https://avg.163.com/topic/detail/9807584 https://avg.163.com/topic/detail/9807562 https://avg.163.com/topic/detail/9807583 https://avg.163.com/topic/detail/9807582 https://avg.163.com/topic/detail/9807576 https://avg.163.com/topic/detail/9807573 https://avg.163.com/topic/detail/9807572 https://avg.163.com/topic/detail/9807547 https://avg.163.com/topic/detail/9807561 https://avg.163.com/topic/detail/9807560 https://avg.163.com/topic/detail/9807559 https://avg.163.com/topic/detail/9807557 https://avg.163.com/topic/detail/9807558 https://avg.163.com/topic/detail/9807554 https://avg.163.com/topic/detail/9807553 https://avg.163.com/topic/detail/9807551 https://avg.163.com/topic/detail/9807552 https://avg.163.com/topic/detail/9807550 https://avg.163.com/topic/detail/9807549 https://avg.163.com/topic/detail/9807540 https://avg.163.com/topic/detail/9807546 https://avg.163.com/topic/detail/9807547 https://avg.163.com/topic/detail/9807534 https://avg.163.com/topic/detail/9807543 https://avg.163.com/topic/detail/9807529 https://avg.163.com/topic/detail/9807528 https://avg.163.com/topic/detail/9807527 https://avg.163.com/topic/detail/9807526 https://avg.163.com/topic/detail/9807524 https://avg.163.com/topic/detail/9807523 https://avg.163.com/topic/detail/9807522 https://avg.163.com/topic/detail/9807521 https://avg.163.com/topic/detail/9807517 https://avg.163.com/topic/detail/9807519 https://avg.163.com/topic/detail/9807514 https://avg.163.com/topic/detail/9807518 https://avg.163.com/topic/detail/9807512 https://avg.163.com/topic/detail/9807507 https://avg.163.com/topic/detail/9807497 https://avg.163.com/topic/detail/9807496 https://avg.163.com/topic/detail/9807498 https://avg.163.com/topic/detail/9807495 https://avg.163.com/topic/detail/9807494 https://avg.163.com/topic/detail/9807493 https://avg.163.com/topic/detail/9807492 https://avg.163.com/topic/detail/9807491 https://avg.163.com/topic/detail/9807490 https://avg.163.com/topic/detail/9807487 https://avg.163.com/topic/detail/9807489 https://avg.163.com/topic/detail/9807485 https://avg.163.com/topic/detail/9807488 https://avg.163.com/topic/detail/9807484 https://avg.163.com/topic/detail/9807473 https://avg.163.com/topic/detail/9807483 https://avg.163.com/topic/detail/9807482 https://avg.163.com/topic/detail/9807480 https://avg.163.com/topic/detail/9807475 https://avg.163.com/topic/detail/9807479 https://avg.163.com/topic/detail/9807478 https://avg.163.com/topic/detail/9807476 https://avg.163.com/topic/detail/9807463 https://avg.163.com/topic/detail/9807461 https://avg.163.com/topic/detail/9807353 https://avg.163.com/topic/detail/9807459 https://avg.163.com/topic/detail/9807458 https://avg.163.com/topic/detail/9807457 https://avg.163.com/topic/detail/9807456 https://avg.163.com/topic/detail/9807455 https://avg.163.com/topic/detail/9807452 https://avg.163.com/topic/detail/9807451 https://avg.163.com/topic/detail/9807332 https://avg.163.com/topic/detail/9807445 https://avg.163.com/topic/detail/9807435 https://avg.163.com/topic/detail/9807434 https://avg.163.com/topic/detail/9807433 https://avg.163.com/topic/detail/9807432 https://avg.163.com/topic/detail/9807431 https://avg.163.com/topic/detail/9807430 https://avg.163.com/topic/detail/9807429 https://avg.163.com/topic/detail/9807427 https://avg.163.com/topic/detail/9807426 https://avg.163.com/topic/detail/9807425 https://avg.163.com/topic/detail/9807424 https://avg.163.com/topic/detail/9807423 https://avg.163.com/topic/detail/9807422 https://avg.163.com/topic/detail/9807421 https://avg.163.com/topic/detail/9807417 https://avg.163.com/topic/detail/9807396 https://avg.163.com/topic/detail/9807415 https://avg.163.com/topic/detail/9807414 https://avg.163.com/topic/detail/9807412 https://avg.163.com/topic/detail/9807405 https://avg.163.com/topic/detail/9807399 https://avg.163.com/topic/detail/9807398 https://avg.163.com/topic/detail/9807397 https://avg.163.com/topic/detail/9807395 https://avg.163.com/topic/detail/9807386 https://avg.163.com/topic/detail/9807393 https://avg.163.com/topic/detail/9807392 https://avg.163.com/topic/detail/9807391 https://avg.163.com/topic/detail/9807390 https://avg.163.com/topic/detail/9807388 https://avg.163.com/topic/detail/9807384 https://avg.163.com/topic/detail/9807377 https://avg.163.com/topic/detail/9807367 https://avg.163.com/topic/detail/9807369 https://avg.163.com/topic/detail/9807366 https://avg.163.com/topic/detail/9807365 https://avg.163.com/topic/detail/9807363 https://avg.163.com/topic/detail/9807362 https://avg.163.com/topic/detail/9807361 https://avg.163.com/topic/detail/9807359 https://avg.163.com/topic/detail/9807358 https://avg.163.com/topic/detail/9807355 https://avg.163.com/topic/detail/9807357 https://avg.163.com/topic/detail/9807356 https://avg.163.com/topic/detail/9807354 https://avg.163.com/topic/detail/9807353 https://avg.163.com/topic/detail/9807352 https://avg.163.com/topic/detail/9807349 https://avg.163.com/topic/detail/9807347 https://avg.163.com/topic/detail/9807343 https://avg.163.com/topic/detail/9807342 https://avg.163.com/topic/detail/9807334 https://avg.163.com/topic/detail/9807332 https://avg.163.com/topic/detail/9807331 https://avg.163.com/topic/detail/9807302

用户登录系统

一般来说，用户的登录方式有三种：

• （1）命令行登录
• （2）ssh登录
• （3）图形界面登录

对于运行级别为5的图形方式用户来说，他们的登录是通过一个图形化的登录界面。登录成功后可以直接进入 KDE、Gnome 等窗口管理器。

而本文主要讲的还是文本方式登录的情况：当我们看到mingetty的登录界面时，我们就可以输入用户名和密码来登录系统了。

Linux 的账号验证程序是 login，login 会接收 mingetty 传来的用户名作为用户名参数。

然后 login 会对用户名进行分析：如果用户名不是 root，且存在 /etc/nologin 文件，login 将输出 nologin 文件的内容，然后退出。

这通常用来系统维护时防止非root用户登录。只有/etc/securetty中登记了的终端才允许 root 用户登录，如果不存在这个文件，则 root 用户可以在任何终端上登录。

/etc/usertty文件用于对用户作出附加访问限制，如果不存在这个文件，则没有其他限制。

图形模式与文字模式的切换方式

Linux预设提供了六个命令窗口终端机让我们来登录。

默认我们登录的就是第一个窗口，也就是tty1，这个六个窗口分别为tty1,tty2 … tty6，你可以按下Ctrl + Alt + F1 ~ F6 来切换它们。

如果你安装了图形界面，默认情况下是进入图形界面的，此时你就可以按Ctrl + Alt + F1 ~ F6来进入其中一个命令窗口界面。

当你进入命令窗口界面后再返回图形界面只要按下Ctrl + Alt + F7 就回来了。

如果你用的vmware 虚拟机，命令窗口切换的快捷键为 Alt + Space + F1~F6. 如果你在图形界面下请按Alt + Shift + Ctrl + F1~F6 切换至命令窗口。

Linux 关机

在linux领域内大多用在服务器上，很少遇到关机的操作。毕竟服务器上跑一个服务是永无止境的，除非特殊情况下，不得已才会关机。

正确的关机流程为：sync > shutdown > reboot > halt

关机指令为：shutdown ，你可以man shutdown 来看一下帮助文档。

例如你可以运行如下命令关机：

sync 将数据由内存同步到硬盘中。

shutdown 关机指令，你可以man shutdown 来看一下帮助文档。例如你可以运行如下命令关机：

shutdown –h 10 ‘This server will shutdown after 10 mins’ 这个命令告诉大家，计算机将在10分钟后关机，并且会显示在登陆用户的当前屏幕中。

shutdown –h now 立马关机

shutdown –h 20:25 系统会在今天20:25关机

shutdown –h +10 十分钟后关机

shutdown –r now 系统立马重启

shutdown –r +10 系统十分钟后重启

reboot 就是重启，等同于 shutdown –r now

halt 关闭系统，等同于shutdown –h now 和 poweroff

最后总结一下，不管是重启系统还是关闭系统，首先要运行 sync 命令，把内存中的数据写到磁盘中。

关机的命令有 shutdown –h now halt poweroff 和 init 0 , 重启系统的命令有 shutdown –r now reboot init 6。