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/
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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.
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Warning |
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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.
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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.
