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:
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.
