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101.2.-boot-the-system.md

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101.2. Boot the system

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Candidates should be able to guide the system through the booting process.

Key Knowledge Areas

  • Provide common commands to the boot loader and options to the kernel at boot time.
  • Demonstrate knowledge of the boot sequence from BIOS to boot completion.
  • Check boot events in the log files.

Terms

  • BIOS
  • bootloader
  • kernel
  • init
  • /var/log/messages
  • dmesg

BIOS

BIOS is Basic Input Output System and does the first steps of the PC bootup. For example is does a POST (Power On Self Test) and decides which hardware should boot the system.

bootloader

Bootloader can be GRUB (1&2) or LILO which are great for disks less than 2TB.

/etc/lilo.conf
/boot/grub/grub.cfg
/boot/grub/menu.lst

Kernel

Kernel parameters (sometimes called boot parameters) supply the kernel with information about hardware parameters that it might not determine on its own - say single user mod boot (S)

init

When the kernel finishes loading, it usually starts /sbin/init. This program remains running until the system is shut down. It is always assigned process ID 1.

first process, process in charge, a big family tree of commands:

pstree

init is being replaced in many distros (say ubuntu with upstart) but still is in exam and has its own section.

dmesg

Funny fact: During the bootup, only The Kernel is running so it should record and keep its own logs!

dmesg command will show the full data from kernel ring buffer up to now. But

cat /var/log/dmesg

will show only the data during the boot

/var/log/messages

After the init process comes up, syslog daemon will log messages. It has timestamps and will persist during restarts.

  • Kernel is still logging its own messages in dmesg
  • in some systems it might be called /var/log/syslog
  • there are many other logs at /var/log