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The Linux Boot Process
What happens before the login prompt
Linux Users of Victoria
on 7 November 2001.
This talk is based on
an earlier talk presented to LUV.
Note: many links on this page point to files on your local file
system. If you are not running Linux then they probably will not work. There
are also links to man pages. These work in konqueror, and may or may not
work with other browsers. If the link does not work just use the
man command from a Linux command line.
Boot sequence summary
Load boot sector from one of:
The boot order can be changed from within the BIOS.
BIOS setup can be entered by pressing a key during bootup. The exact key
depends varies, but is often one of Del, F1, F2, or F10.
DOS in the context includes MS-DOS, Win95, and Win98.
- BIOS loads and execute the first 512 bytes off the disk (/dev/hda)
- Standard DOS MBR will:
- look for a primary partition (/dev/hda1-4) marked bootable
- load and execute first 512 bytes of this partition
- can be restored with fdisk /mbr from DOS
- does not understand filesystems
- code and kernel image to be loaded is stored as raw disk offsets
- uses the BIOS routines to load
- load menu code, typically /boot/boot.b
- prompt for (or timeout to default) partition or kernel
- for "image=" (ie Linux) option load kernel image
- for "other=" (ie DOS) option load first 512 bytes of the partition
One minute guide to installing a new kernel
- copy kernel image (bzImage) and modules to /boot and /lib/modules
- edit /etc/lilo.conf
- run /sbin/lilo
- reboot to test
- Understands file systems
- config lives in /boot/grub/menu.lst or /boot/boot/menu.lst
- initialise devices
- (optionally loads initrd, see below)
- mounts root filesystem
- specified by lilo or loadin with root= parameter
- kernel prints: VFS: Mounted root (ext2 filesystem) readonly.
- runs /sbin/init which is process number 1 (PID=1)
- init prints: INIT: version 2.76 booting
- can be changed with boot= parameter to lilo, eg
boot=/bin/sh can be useful to rescue a system which is having
Allows setup to be performed before root FS is mounted
- lilo or loadlin loads ram disk image
- kernel runs /linuxrc
- load modules
- initialise devices
- /linuxrc exits
- "real" root is mounted
- kernel runs /sbin/init
Details in /usr/src/linux/Documentation/initrd.txt
(part of the kernel source).
- reads /etc/inittab (see
man inittab which specifies the scripts
- Run boot scripts:
- debian: run /etc/init.d/rcS which runs:
- redhat: /etc/rc.d/rc.sysinit
script which: loads modules, check root FS and mount RW, mount local FS,
setup network, and mount remote FS
- switches to default runlevel eg 3.
- 0 halt
- 1 single user
- 2-4 user defined
- 5 X11 only (0 or 1 text console)
- 6 Reboot
- Default is defined in /etc/inittab, eg:
- The current runlevel can be changed by running
where # is the new runlevel, eg typing telinit 6 will reboot.
Run Level programs
- Scripts in /etc/rc*.d/* are symlinks to /etc/init.d
- Scripts prefixed with S will be started when the runlevel is entered, eg
- Scripts prefixed with K will be killed when the runlevel is entered, eg
- X11 login screen is typically started by one of S99xdm,
S99kdm, or S99gdm.
- Run programs for specified run level
- /etc/inittab lines:
- 1:2345:respawn:/sbin/getty 9600 tty1
- Always running in runlevels 2, 3, 4, or 5
- Displays login on console (tty1)
- 2:234:respawn:/sbin/getty 9600 tty2
- Always running in runlevels 2, 3, or 4
- Displays login on console (tty2)
- l3:3:wait:/etc/init.d/rc 3
- Run once when switching to runlevel 3.
- Uses scripts stored in /etc/rc3.d/
- ca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -r now
- Run when control-alt-delete is pressed
- debian runs
- /etc/rcS.d/S* scripts
- /etc/rc3.d/S* scripts
- redhat runs
- /etc/rc.d/rc.sysinit script
- /etc/rc.d/rc3.d/S* scripts