In Linux and Solaris Operating Sytems the Run levels are considered as more important.

If the Runlevels are used improperly then there will be problem in booting up and using the system.

Linux Run Levels
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0 - halt (Do NOT set initdefault to this)
1 - Single user mode
2 - Multiuser, without NFS
3 - Full multiuser mode
4 - unused
5 - X11
6 - reboot (Don’t set this to default init level)

init run level

The idea behind operating different services at different runlevels essentially revolves around the fact that different systems can be used in a different ways. Some services cannot be used until the system is in a particular state, or mode, such as ready for more than one user or has networking available.

The Init Program

The init process reads the file “/etc/inittab” and uses this file to determine how to create processes. Read the init man page for more information. Also note that init is always running and can dynamically do things and run processes based upon various signals. The administrator can also cause it to dynamically change system processes and runlevels by using the telinit program or editing the “/etc/inittab” file.

Solaris Run Levels
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0 - powerdown
1 - ok - single user
2 - multi user
3 - multi user with NFS share
4 - unavailable
5 - powerdown
6 - reboot

Run Levels And Start Scripts

When a machine boots, it looks in the /etc/rc?.d directories and if it finds a script that starts with an upper-case S, it calls that script with a start argument. As a machine shuts down, it looks in the /etc/rc?.d directories and if it finds a script that starts with an upper-case K, it calls that script with a stop argument. But it is not as simple as that…

Run levels in Sun Solaris

Know the Run levels on Sun Solaris Operating System
The following lists the 6 Run Levels in Sun Solaris. These are often different mostly from the linux run levels