Linux系统的启动时通过读取不同的配置文件，执行相应的Shell脚本完成的。当然本文只是简单的从文件的角度分析，更深层次的本文没涉及。
主要读取了以下文件：
/boot/grub/grub.conf
/etc/inittab
/etc/rc5.d(rc.d) 0-99 Seq
/etc/passwd (Login,input username and password)
/etc/shadow
/etc/profile:init the env var of user
/etc/profile.d/*.sh
~/.bash_profile
~/.bash_history
~/.bashrc
/etc/bashrc
我们首先来看grub.conf文件
[root@ grub]# ll menu.lst
lrwxrwxrwx. 1 root root 11 7o?=o?= 19 10:52 menu.lst -> ./grub.conf
[root@ grub]# pwd
/boot/grub
grub.conf文件内容：
default=0
timeout=5
splashimage=(hd0,0)/boot/grub/splash.xpm.gz
hiddenmenu
title Red Hat Enterprise Linux (2.6.32-220.el6.i686)
root (hd0,0)
kernel /boot/vmlinuz-2.6.32-220.el6.i686 ro
root=UUID=ed98469d-857b-4ae5-91e4-118e0167ead7 rd_NO_LUKS rd_NO_LVM
LANG=en_US.UTF-8 rd_NO_MD quiet SYSFONT=lat arcyrheb-sun16 rhgb
crashkernel=auto KEYBOARDTYPE=pc KEYTABLE=us rd_NO_DM
initrd /boot/initramfs-2.6.32-220.el6.i686.img
执行runlevel命令可以看到启动级别是5
[root@ grub]# runlevel
N 5
我们进入etc目录，可以看到很多rcX.d目录
[root@ grub]# cd /etc/init.d
[root@ init.d]# cd /etc
[root@ etc]# cd rc
rc rc0.d/ rc1.d/ rc2.d/ rc3.d/ rc4.d/ rc5.d/
rc6.d/ rc.d/ rc.local rc.sysinit
我们进入/etc/profile.d目录，可以看到很多可执行脚本
[root@localhost test]# ll /etc/profile.d/*.sh
-rw-r--r--. 1 root root 1143 Apr 28 2010 /etc/profile.d/colorls.sh
-rw-r--r--. 1 root root 78 Apr 21 2010 /etc/profile.d/cvs.sh
-rw-r--r--. 1 root root 192 Dec 12 2005 /etc/profile.d/glib2.sh
-rw-r--r--. 1 root root 70 Aug 12 2010 /etc/profile.d/gnome-ssh-askpass.sh
-rwxr-xr-x. 1 root root 288 Sep 24 2009 /etc/profile.d/kde.sh
-rw-r--r--. 1 root root 2706 Sep 2 2010 /etc/profile.d/lang.sh
-rw-r--r--. 1 root root 108 Feb 7 2007 /etc/profile.d/less.sh
-rw-r--r--. 1 root root 933 Jun 30 2010 /etc/profile.d/qt.sh
-rw-r--r--. 1 root root 2142 Sep 8 2010 /etc/profile.d/udisks-bash-completion.sh
-rw-r--r--. 1 root root 269 May 19 2010 /etc/profile.d/vim.sh
-rw-r--r--. 1 root root 169 May 20 2009 /etc/profile.d/which2.sh
因为启动级别是5，所以我们进入rc5.d，可以看到很多文件，S表示当系统启动时执行，K表示当系统关闭时执行。
[root@localhost grub]# ll /etc/rc5.d/
K01certmonger K50netconsole K75ntpdate K89rdisc S11auditd S24avahi-daemon S26pcscd S82abrtd
K01smartd K50snmpd K76ipsec K95cgconfig S11portreserve S24nfslock S26udev-post S85qpidd
K02oddjobd K50snmptrapd K80kdump K95firstboot S12rsyslog S24openct S28autofs S90crond
K10psacct K50vsftpd K80sblim-sfcb S00microcode_ctl S13cpuspeed S24rpcgssd S30vboxadd S95atd
K10saslauthd K60nfs K80sssd S01sysstat S13irqbalance S24rpcidmapd S30vboxadd-x11 S97rhnsd
K15httpd K69rpcsvcgssd K84wpa_supplicant S02lvm2-monitor S13rpcbind S25cups S35vboxadd-service S98tog-pegasus
K20tomcat6 K73ypbind K86cgred S08ip6tables S15mdmonitor S25netfs S50bluetooth S99local
K36mysqld K74nscd K87restorecond S08iptables S22messagebus S26acpid S55sshd
K50dnsmasq K74ntpd K88nslcd S10network S23NetworkManager S26haldaemon S80postfix
我们再来看看用户主目录下的.bashrc文件内容
[root@localhost grub]# cat ~/.bashrc
# .bashrc
# User specific aliases and functions
alias rm='rm -i'
alias cp='cp -i'
alias mv='mv -i'
# Source global definitions
if [ -f /etc/bashrc ]; then
. /etc/bashrc
fi
我们再来看看用户主目录下的.bash_profile文件内容
[root@localhost grub]# cat ~/.bash_profile
# .bash_profile
# Get the aliases and functions
if [ -f ~/.bashrc ]; then
. ~/.bashrc
fi
# User specific environment and startup programs
PATH=$PATH:$HOME/bin
export PATH
完整的流程图如下图：
