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We explained how SysV init works on x86 based machines. Startup procedures may vary on other architectures and distributions. Other systems may use the BSD-style init, where startup files are not split up into multiple /etc/rc<LEVEL>.d directories. It might also be possible that your system uses
/etc/rc.d/init.d instead of /etc/init.d.
4.2.5. Init run levels
The idea behind operating different services at different run levels essentially revolves around the fact that different systems can be used in different ways. Some services cannot be used until the system is in a particular state, or mode, such as being ready for more than one user or having networking available.
There are times in which you may want to operate the system in a lower mode. Examples are fixing disk corruption problems in run level 1 so no other users can possibly be on the system, or leaving a server in run level 3 without an X session running. In these cases, running services that depend upon a higher system mode to function does not make sense because they will not work correctly anyway. By already having each service assigned to start when its particular run level is reached, you ensure an orderly start up process, and you can quickly change the mode of the machine without worrying about which services to manually start or stop.
Available run levels are generally described in /etc/inittab, which is partially shown below:
#
# inittab This file describes how the INIT process should set up
# the system in a certain run-level.
# Default run level. The run levels are:
# 0 - halt (Do NOT set initdefault to this)
# 1 - Single user mode
# 2 - Multiuser, without NFS
# (The same as 3, if you do not have networking)
# 3 - Full multiuser mode
# 4 - unused
# 5 - X11
# 6 - reboot (Do NOT set initdefault to this)
#
id:5:initdefault:
<--cut-->
#
# inittab This file describes how the INIT process should set up
# the system in a certain run-level.
# Default run level. The run levels are:
# 0 - halt (Do NOT set initdefault to this)
# 1 - Single user mode
# 2 - Multiuser, without NFS
# (The same as 3, if you do not have networking)
# 3 - Full multiuser mode
# 4 - unused
# 5 - X11
# 6 - reboot (Do NOT set initdefault to this)
#
id:5:initdefault:
<--cut-->
Feel free to configure unused run levels (commonly run level 4) as you see fit. Many users configure those run levels in a way that makes the most sense for them while leaving the standard run levels as they are by default. This allows them to quickly move in and out of their custom configuration without disturbing the normal set of features at the standard run levels.
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 file (or if you have simply forgotten your password), boot into single-user mode.
When you are working in text mode because you didn't get presented a graphical login screen on the console of your machine, you can normally switch to console 7 or up to have a graphical login. If this is
not the case, check the current run level using the command who -r. If it is set to something else than the original default from /etc/inittab, chances are that the system does not start up in graphical mode by default. Contact your system administrator or read man init in that case. Note that switching run levels is done preferably using the telinit command; switching from a text to a graphical console or vice versa does not involve a run level switch.
The discussion of run levels, scripts and configurations in this guide tries to be as general as possible. Lots of variations exist. For instance, Gentoo Linux stores scripts in /etc/run levels. Other systems might first run through (a) lower run level(s) and execute all the scripts in there before arriving at the final run level and executing those scripts. Refer to your system documentation for more information. You might also read through the scripts that are refered to in /etc/inittab to get a better comprehension of what happens on your system.
4.2.5.1. Tools
The chkconfig or update-rc.d utilities, when installed on your system, provide a simple command-line tool for maintaining the /etc/init.d directory hierarchy. These relieve system administrators from having to directly manipulate the numerous symbolic links in the directories under /etc/rc[x].d.
In addition, some systems offer the ntsysv tool, which provides a text-based interface; you may find this easier to use than chkconfig's command-line interface. On SuSE Linux, you will find the yast and insserv tools. For Mandrake easy configuration, you may want to try DrakConf, which allows among other features switching between run levels 3 and 5. In Mandriva this became the Mandriva Linux Control Center.
Most distributions provide a graphical user interface for configuring processes, check with your system documentation.