Difference between revisions of "Boot process"

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* ntp
 
* ntp
 
: Network Time Protocol to keep your clock in sync with the real time via the network
 
: Network Time Protocol to keep your clock in sync with the real time via the network
* autofs         boot.udev                 xdm
+
* 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
 
* cifs              xfs
 
* cron                        ypbind
 
* cron                        ypbind

Revision as of 18:04, 11 January 2014

How Linux typically starts up on a PC

you switch on the computer

BIOS is shown and the devices are check

the master boot record is executed

the boot loader is executed

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

the Linux kernel is executed

the initrd is loaded/executed

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

X Windowing system is started

login manager is started

user session incl. Desktop environment is started

See also