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Thursday, July 17, 2008

Linux runlevel configuration tool to start service

Q: How do changing run levels affect us or our users?I replied with following answer: Ans: Well if you are moving to higher run levels, you may make additional services available to users, while moving to a lower run level will causes to services (daemons) to become unavailable. On production server run level 3 is the normally used and rarely changed. However some administrative tasks require the administrator to move system to run level 1 i.e single user mode.

Use the init command to change rune levels:

# init 1

Runlevel and usage:

  • Runlevel 0 is halt
  • Runlevel 1 is single-user
  • Runlevels 2-5 are multi-user (some distro uses RUN level 5 to start X [KDE/Gnome])
  • Runlevel 6 is for rebooting system

Typing init 3 will move system to run level 3:

# init 3

On most Linux server system default run level is 3 and on most Linux Desktop system default run level is 5. The default run level is defined by the initdefault line at the top of /etc/inittab. So if you wanna change default run level open file /etc/inittab and edit entry initdefault:

# vi /etc/inittab

Set initdefault to 5, so that you can boot to X next time when Linux comes up:


Save file and reboot to see changes.

getty is the program which opens a tty port, prompts for a login name and password (via /bin/login command). Your console displays a login/password prompt at run levels 1 through 6. You can use ALT+F1...ALT+F6 keys to switch console (use CTRL+ALT+F1..F6 under X windows).

However my experience so far is that students get confused with ssh and KDE/Genome Desktop login. SSH logins are handled by sshd server which starts at run level 2/3. KDE/Genome Desktop login are handled by GDM/XDM/KDM display manager which starts at run level 5 (however Debian Linux and some other distro can start them from run level 2 via special rc.d script located in /etc/init.d/gdm)

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