Init Runlevels

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. There are times in which you may want to operate the system at a lower mode, such as testing a networking problem in runlevel 2 or leaving the system in runlevel 3 without an X session running. In these cases, running services that depend upon a higher system mode to function doesn't make sense because they won't work correctly anyway. By already having each service assigned to start when its particular runlevel is reached, you ensure an orderly start up process and can quickly change the mode of the machine without worrying about which services to manually start or stop.

Generally, Red Hat Linux operates in runlevel 3 — full multi-user mode. The following runlevels are defined in Red Hat Linux:

• 0 — Halt

• 1 — Single-user mode

• 2 — Multi-user mode, without networking

• 3 — Full multi-user mode

• 4 — Not used

• 5 — Full multi-user mode (with an X-based login screen)

• 6 — Reboot

The default runlevel for a system to boot to and stop is configured in /etc/inittab. For more information on /etc/inittab, see the section called SysV Init.

If your machine gets into a state where it will not boot due to a bad /etc/inittab or will not let you log in because you have a corrupted /etc/passwd (or if you have simply forgotten your password), boot into single-user mode by typing linux single at the LILO boot: prompt. A very bare system will boot, and you will have a command shell from which you can fix things.