Boot process

How Linux typically starts up on a PC

you switch on the computer

BIOS is shown and the devices are check

the boot loader is executed

The boot loader is typically grub, and there is an older and smaller called lilo.

the init program is started

The Linux kernel starts the init program. Nothing else. If the init process quits, the Linux kernel panics. Init is responsible for all further services and programs that are started.

the init script are executed

This can be SysV init scripts or upstart or systemd. These init scripts typically start services. Services are grouped into runlevels. Find out your current runlevel with the command

runlevel

Change your runlevel, in this case to 3, with the command

init 3

E.g. in SUSE the runlevels are defines as

• 0: reboot
• 1: rescue system, single user
• 2: all services that run in (1) plus services needed for multi-user login
• 3: all services that run in (2) plus network services
• 4: undefined
• 5: all services that run in (3) plus graphical display

Some notable services are:

• alsasound

Used for the alsa sound system

• cups

Used for printing, it is the common Unix printing system

• dbus

Used to call public functions in running programs. Should be abstracted to users.

• esound

Used for enlightenment sound system

• ntp

Network Time Protocol to keep your clock in sync with the real time via the network

• autofs

To automount devices as soon as they are plugged in, e.g. USB disks and CD ROMs

• udev

The udev services have a lot of names, e.g. boot.udev under SUSE. They create the device handles in the /dev folder, e.g. when you plug in a USB disk or attach a web cam

• xdm

The x display manager shows a graphical login screen and starts a session when you log in. It keeps running during the session so when you end your session, you will be able to log in again.

• cifs xfs
• cron ypbind
• powerd
• gpm network
• sshd

Service that allows to control a computer via the network

See also

• http://wiki.linuxquestions.org/wiki/Boot_process