LPI 101 Revision - Booting Linux

Runlevel: Both a command and a description of the sytem state. Briefly explained by O'Reilly as "System administration command. Display the previous and current system runlevels."

(from the SAG at the Linux Documentation Project) "A run level is a state of init and the whole system that defines what system services are operating. Run levels are identified by numbers."
init: (As explained by O'Reilly) init is the first process run by any Unix machine at boot time. It verifies the integrity of all filesystems and then creates other processes, using fork and exec, as specified by /etc/inittab. Which processes may be run are controlled by runlevel. All process terminations are recorded in /var/run/utmp and /var/log/wtmp. When the runlevel changes, init sends SIGTERM and then, after 20 seconds, SIGKILL to all processes that cannot be run in the new runlevel.

(from the SAG at the Linux Documentation Project) "When the kernel has started itself (has been loaded into memory, has started running, and has initialised all device drivers and data structures and such), it finishes its own part of the boot process by starting a user level program, init. Thus, init is always the first process (it's process number is always 1).
When init starts, it finishes the boot process by doing a number of administrative tasks, such as checking filesystems, cleaning up /tmp, starting various services, and starting a getty for each terminal and virtual console where users should be able to log in." ....more

Note: The IBM revision guide recommends you understand the /etc/inittab file.
telinit: "System administration command. Signal init to change the system's runlevel. telinit is actually just a link to init, the ancestor of all processes." (O'Reilly)
shutdown: Redhat has a description on using the shutdown commands.

(from the SAG at the Linux Documentation Project) "If you are running a system where you are the only user, the usual way of using shutdown is to quit all running programs, log out on all virtual consoles, log in as root on one of them (or stay logged in as root if you already are, but you should change to root's home directory or the root directory, to avoid problems with unmounting), then give the command shutdown -h now (substitute now with a plus sign and a number in minutes if you want a delay, though you usually don't on a single user system)" ....more
halt: A description is avalible at linuxcommand.org
reboot: System administration command. Close out filesystems, shut down the system, then reboot the system. Because this command immediately stops all processes, it should be run only in single-user mode. If the system is not in runlevel 0 or 6, reboot calls shutdown -nf. (O'Reilly)
poweroff: A description is avalible at linuxcommand.org
lilo: Described extensively in IBM's revision notes.
"LILO (LInux LOader) is a boot loader that can reside on your hard disk or your boot floppy. It is executed when the system boots. [....] LILO has the capability to act as a boot loader for other operating systems. It will allow you to select the operating system you wish to boot."
It's recommended that you understand the options that can be used with lilo also.
dmesg: System administration command. Display the system control messages from the kernel ring buffer. This buffer stores all messages since the last system boot or the most recent ones, if the buffer has been filled. (O'Reilly)
last: A description is avalible at linuxcommand.org

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9. Booting Linux