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GRUB

GRUB stands for Grand Unified Bootloader. GRUB is primarily used for booting Linux distributions. However, GRUB can work with other boot loaders. A common use-case scenario is for dual booting with a Microsoft operating system: it does this by doing a hand-off to the Windows bootloader for Microsoft operating systems.

GRUB uses the /boot/grub/grub.conf file. Sometimes you will see /boot/grub/menu.lst, but this file is simply a symbolic link to /boot/grub/grub.conf. Using the CentOS 6.5 distro, run the following command:

[root@localhost ~]# ls -l /boot/grub
 total 274
 -rw-r--r--. 1 root root 63 Jun 20 01:47    device.map
 -rw-r--r--. 1 root root 13380 Jun 20 01:47 e2fs_stage1_5
 -rw-r--r--. 1 root root 12620 Jun 20 01:47 fat_stage1_5
 -rw-r--r--. 1 root root 11748 Jun 20 01:47 ffs_stage1_5
 -rw-------. 1 root root 769 Jun 20 01:48   grub.conf
 -rw-r--r--. 1 root root 11756 Jun 20 01:47 iso9660_stage1_5
 -rw-r--r--. 1 root root 13268 Jun 20 01:47 jfs_stage1_5
 lrwxrwxrwx. 1 root root 11 Jun 20 01:47    menu.lst -> ./grub.conf
 -rw-r--r--. 1 root root 11956 Jun 20 01:47 minix_stage1_5
 -rw-r--r--. 1 root root 14412 Jun 20 01:47 reiserfs_stage1_5
 -rw-r--r--. 1 root root 1341 Nov 14 2010   splash.xpm.gz
 -rw-r--r--. 1 root root 512 Jun 20 01:47    stage1
 -rw-r--r--. 1 root root 126100 Jun 20 01:47 stage2
 -rw-r--r--. 1 root root 12024 Jun 20 01:47  ufs2_stage1_5
 -rw-r--r--. 1 root root 11364 Jun 20 01:47  vstafs_stage1_5
 -rw-r--r--. 1 root root 13964 Jun 20 01:47  xfs_stage1_5
 [root@localhost ~]#

From the preceding output, we can see /boot/grub/grub.conf and also the symbolic link /boot/grub/menu.lst.

We can view the actual /boot/grub/grub.conf file as follows:

[root@localhost ~]# cat /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/sda2
 # initrd /initrd-[generic-]version.img
 #boot=/dev/sda
 default=0
 timeout=5
 splashimage=(hd0,0)/grub/splash.xpm.gz
 hiddenmenu
 title CentOS (2.6.32-431.el6.x86_64)
 root (hd0,0)
 kernel /vmlinuz-2.6.32-431.el6.x86_64 ro root=UUID=05527d71-25b6-4931-a3bb-8fe505f3fa64 rd_NO_LUKS rd_NO_LVM LANG=en_US.UTF-8 rd_NO_MD SYSFONT=latarcyrheb-sun16 crashkernel=auto KEYBOARDTYPE=pc KEYTABLE=us rd_NO_DM rhgb quiet
 initrd /initramfs-2.6.32-431.el6.x86_64.img
 [root@localhost ~]#

From the preceding output, the common options would be as follows:

default=0: This means it is the first entry to boot from the menu.
timeout=5: This gives the amount of seconds (5, in this case) that the menu will be displayed for before the Linux kernel is booted or the Windows boot loader gets a hand-off from GRUB.
splashimage=(hd0,0)/grub/splash.xpm.gz: This is the background image of the boot menu.
root (hd0,0): This refers to the first hard disk and the first partition on the first hard disk.