1.4. SysV Init Runlevels
The SysV init runlevel system provides a standard process for
controlling which programs init launches or halts
when initializing a runlevel. SysV init was chosen because it is easier
to use and more flexible than the traditional BSD-style init process.
The configuration files for SysV init are located in the
/etc/rc.d/ directory. Within this directory, are
the rc, rc.local,
rc.sysinit, and, optionally, the
rc.serial scripts as well as the following
The init.d/ directory contains the scripts used by
the /sbin/init command when controlling services. Each of
the numbered directories represent the six default runlevels configured
by default under Red Hat Linux.
Runlevels are a state, or mode, defined by the
services listed in the SysV
directory, where <x> is the number of
The idea behind SysV init runlevels revolves around the fact that
different systems can be used in a different ways. For example, a
server runs more efficiently without the drag on system resources
created by the X Window System. Other times, a system administrator
may need to operate the system at a lower runlevel to perform
diagnostic tasks, like fixing disk corruption in runlevel 1, when no
other users can possibly be on the system.
The characteristics of a given runlevel determines which services
are halted and started by init. For instance,
runlevel 1 (single user mode) halts any network services, while
runlevel 3 starts these services. By assigning specific services to be
halted or started on a given runlevel, init can
quickly change the mode of the machine without the user manually
stopping and starting services.
The following runlevels are defined by default for Red Hat Linux:
0 — Halt
1 — Single-user text mode
2 — Not used (user-definable)
3 — Full multi-user text mode
4 — Not used (user-definable)
5 — Full multi-user graphical mode (with an X-based
6 — Reboot
In general, users operate Red Hat Linux at runlevel 3 or runlevel 5 —
both full multi-user modes. Users sometimes customize runlevels 2 and
4 to meet specific needs. since they are not used.
The default runlevel for the system is listed in
/etc/inittab. To find out the default runlevel
for a system, look for the line similar to the one below near the top
The default runlevel listed in the example above is five, as the number
after the first colon indicates. To change it, edit
/etc/inittab as root.
Be very careful when editing /etc/inittab. Simple
typos can cause the system to become unbootable. If this happens,
either use a boot diskette, enter single-user mode, or enter rescue
mode to boot the computer and repair the file.
For more information on single-user and rescue mode, see the chapter
titled Rescue Mode in the
Red Hat Linux Customization Guide.
It is possible to change the default runlevel at boot-time by
modifying the arguments passed by the boot loader to the kernel. For
information on changing the runlevel at boot time, see Section 2.10 Changing Runlevels at Boot Time.
1.4.2. Runlevel Utilities
One of the best ways to configure runlevels is to use an
initscript utility. These tools are designed to
simplify the task of maintaining files in the SysV init directory
hierarchy and relieves system administrators from having to directly
manipulate the numerous symbolic links in the subdirectories of
Red Hat Linux provides three such utilities:
/sbin/chkconfig — The
/sbin/chkconfig utility is a simple
command-line tool for maintaining the
/etc/rc.d/init.d directory hierarchy.
/sbin/ntsysv — The
ncurses-based /sbin/ntsysv utility
provides an interactive text-based interface, which some find
easier to use than chkconfig.
Services Configuration Tool — The
graphical Services Configuration Tool
(redhat-config-services) program is a flexible
GTK2-based utility for configuring runlevels.
Please refer to the chapter titled Controlling Access to
Services in Red Hat Linux Customization Guide for more
information regarding these tools.