Star Discover How a UNIX Machine Boots
Linked by David Adams on Sat 17th May 2008 03:39 UTC, submitted by IdaAshley
General Unix Ever wonder what makes a computer tick or how a UNIX server does what it does? Discover what happens when you push the power button on your computer. This article discusses the different boot types, managing the AIX bootlist and the AIX boot sequence. After reading this article, you will better understand what exactly happens when your server starts.
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RE[2]: Comment by sonic2000gr
by parentaladvisory on Sun 18th May 2008 19:37 UTC in reply to "RE: Comment by sonic2000gr"
parentaladvisory
Member since:
2006-12-18

Doc Pain wrote: "The alternative to the runlevels is the use of an rc script and the rc.d/ entries, such as it is the case in the FreeBSD OS. Refer to "man boot", "man loader", "man init" and "man rc" for further education."

I have seen these rcX directories on some Linux distrobutions, and my debian installation has a init.d/ and several rcX.d/ direcotries in /etc.
In these rc0,1,2,3,4,5,6.d/ direcories are symlinks to scripts in /etc/init.d/, and it seems to me at least that this system uses both "rc.d/ entries" and runlevels, so I dont really get the destinction between rc-directories ans runlevels...
Care to explain? ;)

Edited 2008-05-18 19:38 UTC

Reply Parent Bookmark Score: 1

sonic2000gr Member since:
2007-05-20

I have seen these rcX directories on some Linux distrobutions, and my debian installation has a init.d/ and several rcX.d/ direcotries in /etc.
In these rc0,1,2,3,4,5,6.d/ direcories are symlinks to scripts in /etc/init.d/, and it seems to me at least that this system uses both "rc.d/ entries" and runlevels, so I dont really get the destinction between rc-directories ans runlevels...
Care to explain? ;)


Have a look at your /etc/inittab file. You will find a line that says something like:

id:2:initdefault:

This means that when you system, starts normally, it enters runlevel 2. Now move a few lines down, and you will see this:

l2:2:wait:/etc/init.d/rc 2

This basically means the rc script will execute the scripts in /etc/rc2.d

Now, have a look at the scripts in /etc/rc2.d:

An example:

S20ssh -> ../init.d/ssh

this starts the ssh server. Obviously it is just a link to a script in init.d, but the rc script reads this from /etc/rc2.d. The name and number are significant too. The 20 signifies the order of the script. For example this:

S19nis -> ../init.d/nis

executes before ssh. The "S" in the name means the rc script will call this script with a "start" argument. Essentially, S20ssh is like writing:

/etc/init.d/ssh start

If it had a "K" instead of an "S" it would be called with a "stop" argument.

Have a look at your /etc/rc1.d scripts. These are called when you switch to single user mode (runlevel 1). You will see that quite a few services are stopped (or "K"illed) when entering runlevel 1:

K80nfs-kernel-server -> ../init.d/nfs-kernel-server

this calls /etc/init.d/nfs-kernel-server stop, so NFS sharing is stopped when you enter single user mode.

There are also two "special" (or transient) runlevels, namely 0 (for shutdown) and 6 (for reboot). Have a look at the scripts there too.

After all, it is not a difficult system ;)

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