BIOS->Bootloader-->Kernel-->init-->service-->login-->bash

1.Bootloader启动kernel通过/boot/grub/grub.conf
       root(hd0,0)
       kernel /vmlinuz-2.6.32-358.el6.x86_64 ro root=/dev/sda1 
       initrd /initramfs-2.6.32-358.el6.x86_64.img

[root@oracle ~]# more /boot/grub/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/mapper/vg_oracle-lv_root
#          initrd /initrd-[generic-]version.img
#boot=/dev/sda
default=0                          ---默认情况下，找的是第一个title
timeout=5                          ---设置5s的选择时间
splashimage=(hd0,0)/grub/splash.xpm.gz           --加载的时候红色按钮
hiddenmenu                                       --隐藏title选项
password --md5 xxxxxxxxxxxxxxxxxxxxxxxxxx        
title Red Hat Enterprise Linux (2.6.32-358.el6.x86_64)
        root (hd0,0)
        kernel /vmlinuz-2.6.32-358.el6.x86_64 ro root=/dev/mapper/vg_oracle-lv_root rd_NO_LUKS LANG=en_US.UTF-8 rd_LVM_LV=vg_oracle/lv_root rd_NO_MD rd_LVM_LV=vg_oracle/lv_swap SYSFONT=latarc
yrheb-sun16 crashkernel=128M  KEYBOARDTYPE=pc KEYTABLE=us rd_NO_DM rhgb quiet
        initrd /initramfs-2.6.32-358.el6.x86_64.img


title linux safe mode
        root (hd0,0)
        kernel /vmlinuz-2.6.32-358.el6.x86_64 ro root=LABEL=/ vga=0x314 selinux=off 4       --4表示safe mode
        initrd /initramfs-2.6.32-358.el6.x86_64.img