Linux systems today generally use eight runlevels. Runlevels define what
services or processes should be running on the system. The init process can
run the system in one of eight runlevels. The system runs only one of the
eight runlevels at a time. The main runlevels are from 0 - 6. Here's what
each runlevel is for
Runlevel 0: Halt System - To shutdown the system Runlevel 1: Single user
mode Runlevel 2: Basic multi user mode without NFS Runlevel 3: Full multi
user mode (text based) Runlevel 4: unused Runlevel 5: Multi user mode with
Graphical User Interface Runlevel 6: Reboot System
Runlevels 1 and 2 are generally used for debugging purposed only, and are
not used during normal operations. Most desktop linux distributions boot
into runlevel 5, which starts up the Graphical Login Prompt. This allows the
user to use the system with X-Windows server enabled. Most servers boot into
runlevel 3, which starts the text based login prompt.
Linux runlevels can be changed on the fly using the init tool. If you want
to switch from text based operations to the Graphical Interface, you just
have to type in 'telinit 5' in the root prompt. This will bring up the
Graphical Interface in your system.
Each runlevel can be configured by the system administrator. The "/etc/inittab"
file has information on which runlevel to start the system at and lists the
processes to be run at each runlevel.
Each runlevel has its own directory structure where you can define the order
in which the services start. These directories are located in the /etc/rc.d/
directory, under which you have rc1.d, rc2.d, rc3.d…. rc6.d directories
where the number from 0 through 6 that corresponds to the runlevel. Inside
each directory are symbolic links that point to master initscripts found in
/etc/init.d or /etc/rc.d/init.d.
You can also change the runlevel at boot time. If your system uses LILO as
the boot manager, you can append the runlevel to the boot command : LILO:
linux 3 or LILO: linux 5
If your system uses GRUB, you can change the boot runlevel by pressing the
`e' key to edit the boot configuration. append the runlevel to the end of
the boot command as shown: kernel /vmlinuz ro root=/dev/hda1 5
Vinu Thomas is a consultant on Web design and Internet Technologies. His
website is http://www.vinuthomas.com.
You can read more articles on Linux @
http://www.vinuthomas.com/sections-listarticles-6.html
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