Help File Library: Using SysVinit
Written By:
Tigger
The /etc/inittab file contains the info needed for the system to boot and
what to do in certain cases. The default runlevel is set here and the
system runlevel is determined by this setting or if you have passed a
specific runlevel at the lilo prompt ('linux 3' will start the system in
runlevel 3).
All of the files in the rc#.d directories are just links to a script in
/etc/rc.d/init.d. Upon initialization of the system, rc.sysinit is run
and spawns off the appropriate runlevel by respawning the script
/etc/rc.d/rc # (where # is the runlevel # that it determined from the
default in /etc/inittab or from being passed to it from the lilo
prompt). 'rc' runs the K* links first and then the S* links. As you can
see, S seems to stand for start and K seems to stand for kill (you Unix
and Linux vets will forgive me if I am wrong ;) )
Basically, 'rc' runs the link as follows:
/etc/rc.d/rc#.d/K##service stop
for the K ones and
/etc/rc.d/rc#.d/S##service start
for the S ones.
The reserved runlevels are 0, 1, and 6. The user runlevels are 2 to 5. Most
distributions usually have the lower runlevels as text modes and the
higher ones as X modes (i.e., they load xdm, gdm, or kdm to give you a GUI
login screen). Runlevels 2 to 5 are not always the same from one
distribution to another.
When you issue the halt command or shutdown -h now, rc runs in the
rc0.d directory. A system halt is also known as switching to runlevel
0. When you issue the reboot command or shutdown -r now, it does the same
but rc runs in the rc6.d directory. A reboot is known as switching to
runlevel 6. Those of us, who through tinkering may have accidentally
messed something up and need to boot into 'linux single' mode or runlevel
1, know that this is the 'fix what you messed up' mode. The most popular
reason for this is to change back your 'root' password to something you
know.
This whole process is referred to as SysVinit. Most distributions either
init this way or have the ability to do so. All the distributions based
on Red Hat init this way and Debian is only different in the location of
the service scripts. The actual script for the individual services are
in /etc/rc.d/init.d for Red Hat and variants or /etc/init.d for Debian and
variants.
Many distributions have their own GUI tools to edit the different
runlevels and you should check with your individual distribution if you do
not want to do it manually.
You can add the runlevel links automatically with a program called
chkconfig (at the CLI). This will work as long as the original
startup/shutdown script is in /etc/rc.d/init.d with the rest of them.
Usage is as follows:
chkconfig --add service
chkconfig --del service
Or you can do it really manually as follows:
cp service /etc/init.d
Select which runlevel you want it run in and go to that directory
cd /etc/rc.d/rc#.d
Now create the link to run it and remember that rc selects them in the
order that they are displayed when you list the directory.
ln -s /etc/rc.d/init.d/service S99service
Most scripts you add will be at the end and that is what the 99 means. I
hope this sheds some light on the SysVinit process for all of you.
