Run Levels and the Boot Process

Linux systems can be booted to certain "run levels" that are appropriate for particular purposes. For instance, run level 3 is the normal multi-user level. Run level 3 is normally set in /etc/inittab as the default run level. Run level 5 is a multi-user mode with xdm as the controlling login application for X11 logins.

The directory /etc/rc.d/init.d contains code that can be used when the machine boots to any given run level. Any script here is invoked by means of a symbolic link from the directory that matches the run level of interest.

The processes to be invoked at a given level are listed in a subdirectory in /etc/rc.d whose name indicates the associated level. For instance, /etc/rc.d/rc3.d pertains to run level 3.

Each file in each run level's directory is a link to a script in /etc/rc.d/init.d that starts or stops a particular daemon. Removing a link from a run level's directory prevents the invocation of the associated daemon at that run level at boot time.

For instance, if 'named' is not needed in multi-user mode (run level 3) (this is true of most stand-alone systems on site), simply remove the link in /etc/rc.d/rc3.d that points to ../init.d/named. The original script in /etc/rc.d/init.d should be left (a script may be used at other run levels or you may want to turn it on later by re-establishing the link).

This document is maintained by {helpdesk@jlab.org}