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
directories:
init.d/
rc0.d/
rc1.d/
rc2.d/
rc3.d/
rc4.d/
rc5.d/
rc6.d/ |
The init.d/ directory contains the scripts used by
the /sbin/init command when controlling services. Each of
the numbered directories represent the six runlevels configured
by default under Red Hat Enterprise Linux.
The idea behind SysV init runlevels revolves around the idea that
different systems can be used in different ways. For example, a server
runs more efficiently without the drag on system resources created by
the X Window System. Or there may be times when a system administrator
may need to operate the system at a lower runlevel to perform
diagnostic tasks, like fixing disk corruption in runlevel 1.
The characteristics of a given runlevel determine 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 under Red Hat Enterprise 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
login screen)
6 — Reboot
In general, users operate Red Hat Enterprise 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 following near the top
of /etc/inittab:
The default runlevel listed in this example is five, as the number
after the first colon indicates. To change it, edit
/etc/inittab as root.
 | Warning |
|---|
| | 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, refer to the
chapter titled Basic System Recovery in the
Red Hat Enterprise Linux System Administration 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, refer to Section 2.8 Changing Runlevels at Boot Time.
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
/etc/rc.d/.
Red Hat Enterprise 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
(system-config-services) program is a flexible
utility for configuring runlevels.
Refer to the chapter titled Controlling Access to
Services in the Red Hat Enterprise Linux System Administration Guide for more
information regarding these tools.
