Linux System Boot Runlevels Detail

Mode	Directory	Run Level Description
0	/etc/rc.d/rc0.d	Halt
1	/etc/rc.d/rc1.d	Single-user mode
2	/etc/rc.d/rc2.d	Not used (user-definable)
3	/etc/rc.d/rc3.d	Full multi-user mode (no GUI interface)
4	/etc/rc.d/rc4.d	Not used (user-definable)
5	/etc/rc.d/rc5.d	Full multiuser mode (with GUI interface)
6	/etc/rc.d/rc6.d	Reboot

Based on the selected runlevel, the init process then executes startup scripts located in subdirectories of the /etc/rc.d directory. Scripts used for runlevels 0 to 6 are located in subdirectories /etc/rc.d/rc0.d through /etc/rc.d/rc6.d, respectively.

Determining the Default Boot runlevel

The default boot runlevel is set in the file /etc/inittab with the initdefault variable. When set to 3, the system boots up with the text interface on the VGA console; when set to 5, you get the GUI. Here is a snippet of the file (delete the initdefault line you don't need):

# Default runlevel. The runlevels used by RHS are:
# 0 - halt (Do NOT set initdefault to this)
# 1 - Single user mode
# 2 - Multiuser, without NFS (The same as 3, if you do not have networking)
# 3 - Full multiuser mode
# 4 - unused
# 5 - X11
# 6 - reboot (Do NOT set initdefault to this)
# 
id:3:initdefault:                         # Console Text Mode
id:5:initdefault:                         # Console GUI Mode

Note the following:

• Most home users boot up with a Windows like GUI (runlevel 5)
• Most techies will tend to boot up with a plain text-based command-line-type interface (runlevel 3)
• Changing initdefault from 3 to 5, or vice-versa, has an effect upon your next reboot. See the following section on how to get a GUI login all the time until the next reboot.
• Of course, don't set the initdefault value to 6 or your system will constantly reboot. Setting it to 0 will never allow it to start!