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Booting and Shuting Down

Booting and Shuting Down. WeeSan Lee <weesan@cs.ucr.edu>. Roadmap. Bootstrapping Boot Loaders Startup/Init Scripts Reboot & Shutdown Q&A. Bootstrapping (simplified version). BIOS. Boot Loader. Kernel Initialization. init. Runs scripts from /etc/rc[0-6].d/. Spawns getty processes.

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Booting and Shuting Down

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  1. Booting and Shuting Down WeeSan Lee <weesan@cs.ucr.edu>

  2. Roadmap • Bootstrapping • Boot Loaders • Startup/Init Scripts • Reboot & Shutdown • Q&A

  3. Bootstrapping (simplified version) BIOS Boot Loader Kernel Initialization init Runs scripts from /etc/rc[0-6].d/ Spawns getty processes Spawns Xdm/gdm processes login login

  4. BIOS • Basic Input/Output System • Contains information about the machine’s configuration. Eg. IDE controller, NIC • PC knows which device to boot from via BIOS • PC tries to run code from the MBR, ie. 1st 512 bytes, of the disk • MBR tells the PC to load the boot loader from certain disk partition • The boot loader loads the kernel

  5. Boot Loaders - LILO • Traditional and stable • /etc/lilo.conf • boot=/dev/hda • root=/dev/hda1 • timeout=5 • image=/boot/vmlinuz-2.6.20 • label=Linux • read-only • other=/dev/hdb1 • label=Windows • table=/dev/hdb • To install it • $ lilo • lilo must be run after every reconfiguration

  6. Boot Loaders – LILO (cont) • At LILO prompt • LILO: linux init=/sbin/init • LILO: linux init=/bin/bash • LILO: linux root=/dev/hda5 • LILO: linux single

  7. Boot Loaders – GRUB • GRand Unified Boot loader • Default on Red Hat and SuSe • Read configuration file at boot time • Understand filesystems and kernel executable formats • ie. Only need to know the device, disk partition and kernel filename • GRUB device (hd0,0) → /dev/hda1 or /dev/sda1 • To install GRUB (for the very first time) • $ grub-install ‘(hd0,0)’ • Edit /boot/grub/grub.conf

  8. Boot Loaders – GRUB (cont) • /boot/grub/grub.conf • default=0 • timeout=5 • splashimage=(hd0,0)/boot/grub/splash.xpm.gz • title CentOS (2.6.18-8.el5) • root (hd0,0) • kernel /boot/vmlinuz-2.6.18-8.el5 ro root=LABEL=/ • initrd /boot/initrd-2.6.18-8.el5.img • title Windows • rootnoverify (hd1,0) • chainloader +1

  9. Boot Loaders – GRUB (cont) • At the splash screen • Hit ‘a’ and type “ single” to boot to single-user mode • Hit ‘c’ to enter command-line mode • To boot other OSes that aren’t in grub.conf • Display system information • Press TAB to see possible commands

  10. Kernel Initialization • A program itself • /vmlinuz or /boot/vmlinuz • Two-stage loading process • initrd (init RAM disk) • A transient root filesystem in RAM before a real root filesystem is available • Eg. It is used to install file system modules into the kernel • The real root filesystem • Device detection and configuration • You tell the kernel what to expect • The kernel probes the H/W itself • Kernel threads creation • Eg. init (a user process), kjournald, kswapd

  11. Single-user mode • A manual mode after kernel initialization and before running startup scripts • “single” is passed to init, sulogin is run instead • Most system processes are not running • Users can’t log in, except root • / is mounted read-only • $ mount -o rw,remount / • Check/repair the disk if there are disk problems • $ fsck -y /dev/sda1 • Run ‘exit’ to exit single-user mode

  12. Startup/Init Scripts • After Kernel initialization, a process called init is created with PID 1 • init runs startup scripts (normal shell scripts) to perform specific tasks, eg. • Setting the hostname, time zone, etc • Checking and mouting the disks • Configuring network interfaces • Starting up daemons and network services

  13. Startup/Init Scripts (cont) • Startup scripts (rc files) are run based on run levels • 0 the level in which the system is completely shut down • 1 single-user mode • 2 multiuser mode w/out NFS • 3 full multiuser mode • 4 unused • 5 X11 • 6 reboot level • Starts with run level 0 to the default run level (usually 3) • /etc/inittab tells init what to do at each level • To find out which run level the system is current in • $ runlevel

  14. Startup/Init Scripts (cont) • init runs the scripts from /etc/rc.d/rc[0-6].d/ • /etc/rc.d/rc0/K25sshd → /etc/init.d/sshd • /etc/rc.d/rc3/S55sshd → /etc/init.d/sshd • Each server/daemon provides a master script • Stored in /etc/init.d • Understands the arguments: start, stop, restart • /etc/init.d/sshd start • run level 0 → 3 • /etc/rc.d/rc3/S* start • run level 3 → 0 • /etc/rc.d/rc0/K* stop • Pretty ugly!

  15. Startup/Init Scripts (cont) • Use chkconfig instead. Eg. • $ chkconfig --add sshd • $ chkconfig --del sshd • Before that, need to add/modify /etc/init.d/sshd • # chkconfig: 2345 55 25 • sshd should be started/stopped at run level 2, 3, 4 and 5 with the start priority of 55 and the stop priority of 25

  16. Reboot & Shutdown • To reboot • $ shutdown -r now • $ reboot • $ telinit 6 • To halt • $ shutdown -h now • $ halt • $ telinit 0 • $ poweroff

  17. Reboot & Shutdown (cont) • To shutdown gracefully • $ shutdown -h +15 “Shutdown in 15 mins”

  18. References • LAH • Ch 2: Booting and Shutting Down • Grub manual • http://www.gnu.org/software/grub/manual/grub.html

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