Ubuntu Linux系统的启动过程

　　BIOS 自检

　　启动 GRUB/LILO

　　运行 Linux 的系统内核

　　读取系统引导配置文件 /etc/inittab 中的信息运行系统的第一个进程 init

　　执行系统初始化脚本 /etc/init.d/rcS

　　根据运行级别（X）配置服务执行 /etc/init.d/rcX.d/[KS]*

　　终止以“K”开头的服务

　　启动以“S”开头的服务

　　运行 getty 等待用户登录

　　执行系统初始化脚本 /etc/init.d/rcS 时会执行 /etc/init.d/networking 启动网络配置

　　在执行 /etc/init.d/rcX.d/ 时，若运行级别为 2~5

　　若安装了Gnome/KDE，还会运行 gdm/kdm

　　运行脚本 /etc/rc.local 执行本地的其他启动配置

　　INIT 及其配置文件

　　INIT 进程是由 Linux 内核引导的，是系统中的第一个进程，是系统其他进程的祖先进程，其进程号（PID）永远为 1。

　　INIT 进程在运行时将读取系统引导配置文件 /etc/inittab 中的信息。这些信息包括默认的运行级别和由 INIT 启动的进程。

cat /etc/inittab
# /etc/inittab: init(8) configuration.
# $Id: inittab,v 1.91 2002/01/25 13:35:21 miquels Exp $
# The default runlevel.
id:2:initdefault:
# Boot-time system configuration/initialization script.
# This is run first except when booting in emergency (-b) mode.
si::sysinit:/etc/init.d/rcS
# What to do in single-user mode.
~~:S:wait:/sbin/sulogin
# /etc/init.d executes the S and K scripts upon change
# of runlevel.
#
# Runlevel 0 is halt.
# Runlevel 1 is single-user.
# Runlevels 2-5 are multi-user.
# Runlevel 6 is reboot.
l0:0:wait:/etc/init.d/rc 0
l1:1:wait:/etc/init.d/rc 1
l2:2:wait:/etc/init.d/rc 2
l3:3:wait:/etc/init.d/rc 3
l4:4:wait:/etc/init.d/rc 4
l5:5:wait:/etc/init.d/rc 5
l6:6:wait:/etc/init.d/rc 6
# Normally not reached, but fallthrough in case of emergency.
z6:6:respawn:/sbin/sulogin
# What to do when CTRL-ALT-DEL is pressed.
ca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -r now
# Action on special keypress (ALT-UpArrow).
#kb::kbrequest:/bin/echo "Keyboard Request--edit /etc/inittab to let this work."
# What to do when the power fails/returns.
pf::powerwait:/etc/init.d/powerfail start
pn::powerfailnow:/etc/init.d/powerfail now
po::powerokwait:/etc/init.d/powerfail stop
# /sbin/getty invocations for the runlevels.
#
# The "id" field MUST be the same as the last
# characters of the device (after "tty").
#
# Format:
# :::
#
# Note that on most Debian systems tty7 is used by the X Window System,
# so if you want to add more getty's go ahead but skip tty7 if you run X.
#
1:2345:respawn:/sbin/getty 38400 tty1
2:23:respawn:/sbin/getty 38400 tty2
3:23:respawn:/sbin/getty 38400 tty3
4:23:respawn:/sbin/getty 38400 tty4
5:23:respawn:/sbin/getty 38400 tty5
6:23:respawn:/sbin/getty 38400 tty6
# Example how to put a getty on a serial line (for a terminal)
#
#T0:23:respawn:/sbin/getty -L ttyS0 9600 vt100
#T1:23:respawn:/sbin/getty -L ttyS1 9600 vt100
# Example how to put a getty on a modem line.
#
#T3:23:respawn:/sbin/mgetty -x0 -s 57600 ttyS3