1.1 Fedora TextbooksIndex

• Chapter 11 (pages 417 – 424, 430-1, 435)
• Chapter 15 (pages 543, 551)
• Boot Process
• Boot Loaders: Specifically Grub
  • see also the LInux LOader: lilo
• Run Levels: display, change, etc.
• Startup Files

1.2 The InternetIndex

• Wikipedia: Booting
• We are using GRUB Legacy
  • For the latest GRUB< see GRUB Reference
• GRUB Legacy Tutorial
• GRUB Legacy Manual
• Wikipedia Upstart: https://en.wikipedia.org/wiki/Upstart
• Wikipedia Systemd: https://en.wikipedia.org/wiki/Systemd

5.1 A sample GRUB Legacy configuration file (version 0.9x)Index

All versions of GRUB use a configuration file under /boot/grub to display a menu of alternative operating systems to boot. Without a configuration file, GRUB starts in interactive mode and you have to specify and type everything yourself. With a configuration file, you can select from a menu list of choices.

In GRUB Legacy, each choice in the configuration file starts with a “title” keyword followed by a descriptive name:

# Sample GRUB menu.lst or grub.conf file (GRUB Legacy version 0.9x)
timeout=30
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
default=0

# this sdb disk has partition sdb1 for /boot and sdb2 for / (ROOT)
# (Note how pathnames do *not* start with /boot on a separate partition!)
title Fedora 2.6.31 on second disk
    root (hd1,0)
    kernel /vmlinuz-2.6.31.5-127.fc12.i686.PAE ro root=/dev/sdb2
    initrd /initramfs-2.6.31.5-127.fc12.i686.PAE.img

# this sdc disk has partition sdc1 for / (ROOT) and uses a /boot directory
# (not a separate /boot partition, so pathnames must start with /boot)
title Old Fedora 2.6.23 on third disk
    password sesame
    root (hd2,0)
    kernel /boot/vmlinuz-2.6.23.15.137.fc8 ro root=/dev/sdc1
    initrd /boot/initrd-2.6.23.15.137.fc8.img

# this first disk partition has a Windows boot loader in the first sector
title Windows on first disk
    rootnoverify (hd0,0)
    makeactive
    chainloader +1

Notes:

• The part of the file before the first “title” section is for general GRUB configuration options - see below for the keywords
  • timeout: how long the menu waits before selecting the default
  • default: which “title” section to boot when the time-out happens
    • count from zero!
  • splashimage: a pretty picture (optional) to hide boot messages
  • hiddenmenu: hide the GRUB menu (and show the picture instead)
• The “title” sections for Linux kernels have these keywords:
  • root: lets you set this prefix for all following GRUB pathnames, similar to “cd” in the shell
    • this is NOT the Linux ROOT; this is the GRUB ROOT and uses GRUB device names such as (hd0,0)
    • this is prepended to all incomplete pathname references, e.g. incomplete /foo becomes complete path (hd1,0)/foo
  • kernel: the Linux kernel image followed by optional kernel options
  • initrd: the initial RAM disk to be used by the kernel at boot time
  • password: require the given password to boot this menu entry
• Windows may not be able to boot if it isn’t the first partition of the first disk! See the GRUB documentation for ways to work around this. We don’t handle Windows bugs in this course.
• Do not move or remove the GRUB configuration files! GRUB expects the files to be in a particular location on disk, and moving them may cause GRUB to fail to find them, requiring a re-install of GRUB.
  • you can edit the files in-place as long as you do not delete or move them

5.2 grub.conf keywordsIndex

• Global options (used before any “title” sections):
  • default n [opt.]: The nth “title” boot menu item, starting at 0, will be booted. Otherwise, the first (zeroth) “title” item is booted.
  • timeout 20 [opt.]: Number of seconds before booting into default.
  • password sesame [opt.]: Disable interactive editing and protect all boot menus that include the lock option
• Individual menu options (used within the “title” sections):
  • title: Menu title of this boot menu
  • kernel: Location of Linux kernel file, followed by options
  • lock [opt.]: Requires global password to boot this menu entry
  • password [opt.]: require the given password to boot this menu entry

Note: To find out about additional GRUB commands, type help at the GRUB shell prompt, and see the above Resources.

5.3 GRUB Device and file Naming Convention - (hd0,0)/grub/menu.lstIndex

A full GRUB path name specification is a device name (often a partition name) followed by a file system pathname inside that partition (relative to the start of the partition), e.g. (hd0,0)/grub/menu.lst where /grub/menu.lst is relative to the start of the partition (hd0,0). (This is similar to DOS/Windows and its drive letters.)

• Device names are identified by enclosing parentheses ()
• Hard disks are named hd, e.g (hd0), (hd1)
• Floppy disks are named fd, e.g. (fd0), (fd1)
• Disks and Floppies are numbered sequentially, starting at 0 (ZERO!)
• Partition numbers are given after commas, e.g. (hd0,0), (hd0,1)
• Partitions are also numbered sequentially, starting at 0 (ZERO!)
  • WARNING! Linux numbers disks starting at a! GRUB starts at ZERO.
  • WARNING! Linux numbers partitions starting at 1 (ONE)! GRUB starts at ZERO.
  • Linux sda1 is GRUB (hd0,0); Linux sdc2 is GRUB (hd2,1)
• Examples:
  • (hd0) – First recognized hard disk (any type: SCSI, ATA, USB, etc.); usually corresponds to /dev/sda
  • (fd0) – First recognized floppy diskette; usually corresponds to /dev/fd0
  • (hd0,0) – First partition on first recognized hard disk; usually corresponds to /dev/sda1
  • (hd1,4) – First logical drive on second recognized hard disk; usually corresponds to /dev/sdb5
• The GRUB “root” command can pre-set the device name for all following pathnames
  • e.g. “root (hd0,2)” followed by “kernel /linux” is the same as “kernel (hd0,2)/linux”

5.4 Effect of separate /boot partition on GRUB pathnamesIndex

GRUB configuration files are stored under directory /boot/grub in Linux. /boot/grub is the Linux pathname, not necessarily the GRUB pathname, because GRUB pathnames are composed of a partition name followed by a pathname inside the partition, e.g. (hd0,0)/some/path/name where /some/path/name is relative to the start of the partition.

Since partitions are mounted on directories in Linux, and those directory pathnames prefix the names inside the partition for Linux pathnames, a GRUB pathname and a Linux absolute pathname will always differ, except for the one partition that is mounted on ROOT (because prefixing an absolute pathname with / [ROOT] doesn’t change it).

5.5 Installing GRUBIndex

Your Linux installation already installed GRUB for you. If you have to re-install GRUB, you may find (or be able to install) the “grub-install” script that will do it for you. You can also try it manually, if the GRUB setup files are already stored under /boot/grub:

• As root, type grub at the command line: this will load GRUB and display the GRUB shell prompt “grub>”
• Type setup with two GRUB pathname arguments: the boot sector installation device and the device containing the GRUB setup files
  • the GRUB setup files (e.g. grub.conf) must be on the given partition
• To put GRUB into the MBR of the first disk:   setup (hd0) (hd0,0)
• Exit GRUB with the quit command when done

Most versions of the GRUB shell have TAB command completion, where you can type part of a device or pathname and GRUB will give you all the possible completions. The command-line GRUB in Fedora 12 is broken, and does not have this feature. The GRUB that runs at boot time is not broken and does have the TAB completion feature.

BE CAREFUL. GRUB does not ask you for confirmation! If you type the wrong thing, you will overwrite your boot sector with garbage and be unable to boot your system. You will need to boot a “live” CD and repair.

5.6 Booting without a GRUB menu - interactive GRUB commandsIndex

If grub.conf is not present or has been moved, GRUB cannot display a menu. In this case it will display the GRUB interactive shell, similar to what you get by typing grub at a Linux command line.

The interactive GRUB shell has its own prompt (“grub>”) and its own built-in commands, some with the same names as BASH shell commands. Be clear on when you are typing into the GRUB shell and when you are typing into BASH. The same command name may do different things, depending on whether GRUB executes it or BASH executes it.

Most versions of the GRUB shell have TAB command completion, where you can type part of a device or pathname and GRUB will give you all the possible completions. The command-line GRUB in Fedora 12 is broken, and does not have this feature enabled. The GRUB that runs stand-alone at boot time is not broken and does have the TAB completion feature.

Type “help” for a partial list of commands you can use. Some examples:

• cat - display a text file
• geometry - display information about disk partitions
• find - find which device (partition) contains a pathname
• md5crypt - prompt for and encrypt a password

At the GRUB shell prompt, GRUB commands can be entered to query the system, see text files, and load and then boot a kernel image. Example:

• grub> find /grub/grub.conf
• grub> cat (hd0,0)/grub/grub.conf
• grub> kernel (hd0,0)/vmlinuz ro root=/dev/sda2
• grub> initrd (hd0,0)/initrd
• grub> boot

Note: The GRUB shell can also be accessed by following directions (usually by pressing c) when the GRUB menu is presented. [Esc] brings you from the GRUB shell back to the menu. Pressing e allows you to edit boot menu entries of grub.conf. Pressing b boots the edited menu item. Some versions of GRUB allow typing a to go directly to a GRUB “kernel” line and edit the options.

6.1 Booting Single User Mode (Maintenance Mode)Index

Single-user mode is a “half-up” state used for system repair and maintenance (especially for resetting the ROOT password!). The system only brings up a minimal number of services. The GUI is not started. Networking may not be enabled. Not all disks may be mounted. The console terminal gets a shell running as the ROOT user.

To boot into single-user mode, add the word “single” as an option to a “kernel” line in GRUB and then boot that entry, e.g.:

• kernel (hd0,0)/vmlinuz ro root=/dev/sda3 single

6.2 Booting into a different Run LevelIndex

(Run Levels are summarized below.) To start Linux with a different run level than the default, add the run level digit to the kernel options, e.g. to boot Run Level 3 (multi-user without the X11 GUI window system):

• kernel (hd0,0)/vmlinuz ro root=/dev/sda3 3

6.3 Confirming the startup of individual servicesIndex

To be prompted at boot time before each system service starts up:

• kernel (hd0,0)/vmlinuz ro root=/dev/sda3 confirm

6.4 Fixing a kernel panic (missing ROOT file system)Index

If you moved your ROOT file system to a different partition but forgot to change the GRUB configuration file, your kernel will “panic” and tell you it can’t find the root. You can reboot into the GRUB shell and edit the root= option on the “kernel” line to point to the correct ROOT partition:

• kernel (hd0,0)/vmlinuz ro root=/dev/sdb2
• FoxTrot Kernel Panic

7.1 Run Level CommandsIndex

• To display previous and current system Run Levels (separated by a space) use the runlevel command:
  • “N” will be printed as the previous Run Level after booting
• To change Run Levels use the telinit command:
  • telinit 1   # Switch to single-user mode (but use “shutdown now” instead)
  • telinit 3   # Switch to multi-user console mode (no GUI)
  • telinit 5   # Switch to full multi-user mode with X11 GUI
  • telinit 6   # Reboot (but use “shutdown -r now” instead)

7.2 Run Level Services and chkconfigIndex

You can see which services are started and stopped in each Run Level using the chkconfig command. If that command is not available, look at the file symlinks under the Run Level directories /etc/rc?.d. Each symlink starts with either an “S” (start this service) or a “K” (kill this service) followed by a two-digit sequence number followed by the name of the service, e.g.

• /etc/rc3.d/K15httpd   # kill the HTTP daemon
• /etc/rc3.d/S55sshd   # start the SSH daemon

Each of those symlinks points to a script file responsible for killing or starting the indicated service. You can change what services get killed or started using the chkconfig command that adds or removes symlinks from the Run Level directories (or you can add/remove symlinks manually).