Linux provides a flexible multi-operating system boot loader named grub (GRand Unified Bootloader) that can be used to boot different operating systems. Like Windows bootloaders, it can also sit on the Master Boot Record (MBR) of the boot device and transfer control to specific operating system kernels depending on either a user type-in or a default. Grub's behavior is controlled by /boot/grub/grub.conf.
Previously, there was lilo, which is still seen occasionally (especially when booting from cd/dvd), and even older, the loadlin program which used DOS to boot Linux.
Here's a grub configuration file from a Debian Squeeze machine with two boot-up choices:
#
# DO NOT EDIT THIS FILE
#
# It is automatically generated by grub-mkconfig using templates
# from /etc/grub.d and settings from /etc/default/grub
#
### BEGIN /etc/grub.d/00_header ###
if [ -s $prefix/grubenv ]; then
load_env
fi
set default="0"
if [ "${prev_saved_entry}" ]; then
set saved_entry="${prev_saved_entry}"
save_env saved_entry
set prev_saved_entry=
save_env prev_saved_entry
set boot_once=true
fi
function savedefault {
if [ -z "${boot_once}" ]; then
saved_entry="${chosen}"
save_env saved_entry
fi
}
function load_video {
insmod vbe
insmod vga
insmod video_bochs
insmod video_cirrus
}
insmod part_msdos
insmod ext2
set root='(hd0,msdos1)'
search --no-floppy --fs-uuid --set bb37e57f-0535-4a67-8f1d-f11cdd6098f3
if loadfont /usr/share/grub/unicode.pf2 ; then
set gfxmode=640x480
load_video
insmod gfxterm
fi
terminal_output gfxterm
insmod part_msdos
insmod ext2
set root='(hd0,msdos1)'
search --no-floppy --fs-uuid --set bb37e57f-0535-4a67-8f1d-f11cdd6098f3
set locale_dir=($root)/boot/grub/locale
set lang=en
insmod gettext
set timeout=5
### END /etc/grub.d/00_header ###
### BEGIN /etc/grub.d/05_debian_theme ###
set menu_color_normal=cyan/blue
set menu_color_highlight=white/blue
### END /etc/grub.d/05_debian_theme ###
### BEGIN /etc/grub.d/10_linux ###
menuentry 'Debian GNU/Linux, with Linux 2.6.32-5-amd64' --class debian --class gnu-linux --class gnu --class os {
insmod part_msdos
insmod ext2
set root='(hd0,msdos1)'
search --no-floppy --fs-uuid --set bb37e57f-0535-4a67-8f1d-f11cdd6098f3
echo 'Loading Linux 2.6.32-5-amd64 ...'
linux /boot/vmlinuz-2.6.32-5-amd64 root=UUID=bb37e57f-0535-4a67-8f1d-f11cdd6098f3 ro quiet
echo 'Loading initial ramdisk ...'
initrd /boot/initrd.img-2.6.32-5-amd64
}
menuentry 'Debian GNU/Linux, with Linux 2.6.32-5-amd64 (recovery mode)' --class debian --class gnu-linux --class gnu --class os {
insmod part_msdos
insmod ext2
set root='(hd0,msdos1)'
search --no-floppy --fs-uuid --set bb37e57f-0535-4a67-8f1d-f11cdd6098f3
echo 'Loading Linux 2.6.32-5-amd64 ...'
linux /boot/vmlinuz-2.6.32-5-amd64 root=UUID=bb37e57f-0535-4a67-8f1d-f11cdd6098f3 ro single
echo 'Loading initial ramdisk ...'
initrd /boot/initrd.img-2.6.32-5-amd64
}
### END /etc/grub.d/10_linux ###
### BEGIN /etc/grub.d/20_linux_xen ###
### END /etc/grub.d/20_linux_xen ###
### BEGIN /etc/grub.d/30_os-prober ###
### END /etc/grub.d/30_os-prober ###
### BEGIN /etc/grub.d/40_custom ###
# This file provides an easy way to add custom menu entries. Simply type the
# menu entries you want to add after this comment. Be careful not to change
# the 'exec tail' line above.
### END /etc/grub.d/40_custom ###
### BEGIN /etc/grub.d/41_custom ###
if [ -f $prefix/custom.cfg ]; then
source $prefix/custom.cfg;
fi
### END /etc/grub.d/41_custom ###
# inittab is no longer used when using systemd. # # ADDING CONFIGURATION HERE WILL HAVE NO EFFECT ON YOUR SYSTEM. # # Ctrl-Alt-Delete is handled by /etc/systemd/system/ctrl-alt-del.target # # systemd uses 'targets' instead of runlevels. By default, there are two main targets: # # multi-user.target: analogous to runlevel 3 # graphical.target: analogous to runlevel 5 # # To set a default target, run: # # ln -s /lib/systemd/system/.target /etc/systemd/system/default.target #
The idea for init is that soft-links that start with a capital ``S'' are executed at startup; soft-links with a leading capital ``K'' are executed at shutdown (you can see this behavior in /sbin/rc2 on Solaris and /etc/rc.d/rc on RedHat).
The Linux init package usually includes a utility named runlevel that you can run to determine the current machine's run level.
Notice the location of the actual init shell scripts can vary (/etc, /etc/rc.d, etc.), even between different versions of UNIX. For instance, Solaris: /etc/rc?.d (/etc/rc2.d is typical), CentOS Linux: /etc/rc.d
On some systems, you can use chkconfig to control the maze of links.
# chkconfig --list # show what is on and what is off for runlevels anacron 0:off 1:off 2:on 3:on 4:on 5:on 6:off atd 0:off 1:off 2:off 3:on 4:on 5:on 6:off atieventsd 0:off 1:off 2:off 3:off 4:off 5:on 6:off auditd 0:off 1:off 2:on 3:on 4:on 5:on 6:off autofs 0:off 1:off 2:off 3:on 4:on 5:on 6:off # chkconfig --level 2345 sendmail on # have sendmail start 2345 # chkconfig --del sendmail # remove sendmail from chkconfig management # chkconfig --add sendmail # add sendmail to chkconfig management
Beware that many versions of UNIX/Linux choose to use symbolic links between a common script directory and the particular runlevel directory. Treat any modifications to the startup shell scripts as you would any other program -- edit, test (reboot), document, debug until it works.
In a major site, you may find that weekly reboots at an off time (such as early on Sunday morning) are automatically done to discover any bad interactions among boot modifications that might have been made recently.
In Windows 2008, Microsoft eliminated the boot.ini file. Now only ``magic'' commands will let you modify boot parameters. Microsoft's bcdedit.exe or EasyBCD will let you do things such as set up multi-boot.
Look at for a short summary of the boot sequence for W2k8, Vista, and Windows 7.
What is the BCD store?
The Boot Configuration Data (BCD) store contains boot configuration parameters and controls how the operating system is started in Microsoft® Windows Vista® and Microsoft® Windows Server® 2008 operating systems. These parameters were previously in the Boot.ini file (in BIOS-based operating systems) or in the nonvolatile RAM (NVRAM) entries (in Extensible Firmware Interface–based operating systems). You can use the Bcdedit.exe command-line tool to affect the Windows® code which runs in the pre-operating system environment by adding, deleting, editing, and appending entries in the BCD store. Bcdedit.exe is located in the WindowsSystem32 directory of the Windows Vista partition.
[From http://technet.microsoft.com/en-us/library/cc721886.aspx]