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Hey guys and welcome back.

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So in the previous nugget we had talked about the init tab file which is what the init process

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would originally consult when beginning to boot up.

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Within that nugget however we also mentioned the concept of some type of setup scripts.

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These are ultimately programs that run that will execute multiple commands or start multiple

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processes and for the purposes of the Lpik2 examination we have to understand what these

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boot scripts are and how they may differ depending on the type of distribution you have.

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Now one thing to mention when we're talking about this which is definitely relevant is

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how Cysfianet actually operates.

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So Cysfianet is not the most efficient way to start up your system and in fact this is

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one of the reasons why Cysfianet was ultimately replaced but the way it would operate it would

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operate any linear fashion.

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So what do I mean when I say linear?

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Well it would start the init process and it would tell if I could write correctly the

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init process.

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So that could be process number one and then after that was done it would begin process

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number two, whatever that may be and then process number three and then number four and then

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number five and then six.

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You kind of get the drift after number one had kicked off and done its thing.

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Number two followed and then number three and then number four, number five and when

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you happen to have lots of processes that have to be run.

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This is quite slow, quite sluggish and definitely inefficient so just something to be aware of.

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This is how the process actually happens when you're using a Cysfianet system.

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Now how does Cysfianet determine which processes to run and in which order i.e. what should

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I run and what should I run first and then second and then third and not just in some

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random order because of course certain processes rely on other processes so the order is indeed

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importance.

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So this is what we want to talk about primarily in this nugget right here.

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We have two different locations where init is going to look.

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If we happen to be looking on a red hat based distribution the location is going to be different

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than if we are looking on a Debian based distribution.

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So if you happen to be using a Debian based system the location where the init process

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is going to look is going to look in the exe directory and then the init.d directory.

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However if we happen to be using a red hat based system again we look in the exe directory

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but we then look in the rc.d file or rather rc.d directory my apologies.

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So let's talk about two important scripts that you would find on a red hat based system.

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So the first script I want to talk to you about is in the exe directory.

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It's going to be in rc.d because we're talking about red hat systems and the first script

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is going to be rc.sys init.

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What this one is all about it's designed to effectively load the system to a very simple

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state that is quite honestly the same as run level 1 a single user mode and all this script

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is going to do is going to perform actions such as mounting file systems and enabling

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devices such as RAID devices and logical volume manager and starting up some essential

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system services.

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Now for real world purposes this script right here happens to be what is known as a bash

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script i.e. it is written using the born again shell and you are very very unlikely to modify

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the contents of this bash script but as an administrator you may have to be able to look

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into this file and do a little bit of detective work in the event that you have some issues

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booting up your system.

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The next one again in the exe directory is going to be in rc.d and this one is just called

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rc.

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Now whenever we are on run levels 1 to 6 this script is going to be executed.

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Now this script itself will not do anything on its own you actually have to pass a particular

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value to this script such as 1 or such as 5.

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Recall what we saw in the init tab file in the previous nugget.

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Within this file here this is where you would specify a particular argument.

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Now what this actually means say for example we had l3 which would be the id and then run

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level 5 and then the weights action and then we could specify exe rc.d.rc and then we could

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provide an argument number 5.

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We are basically giving the argument 5 to this script.

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For run level 5 we are going to use the weights action i.e. allow it to complete and this

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script is ultimately going to find additional script to help us load into the correct specified

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run level in this case here run level 5.

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We will actually find directories within this directory and it is going to follow a very

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particular format.

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The format would look something like say this rc0.d and rc1.d and maybe rc5.d get the drift.

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These are directories which actually correlate to a particular run level i.e. within this

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directory here this would be files relating to run level 0 whereas in rc5.d this would

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be a directory for run level 5.

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Now within these directories what you are going to find are symbolic links.

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Now these symbolic links ultimately relate to the starting of processes as well as the

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killing of processes which might sound a little bit dramatic but ultimately starting processes

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and stopping processes.

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Now these symbolic links are going to follow a very particular format.

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If the symbolic link happens to begin with an s then this is going to point to a process

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relating to starting a process and if the symbolic link begins with a k you can guess

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this actually relates to killing processes.

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So all these symbolic links do is they point to scripts and these scripts handle these

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tasks i.e. starting things or stopping things.

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But if we happen to see a symbolic link which was k and then maybe the number 9 and then

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maybe i don't know nginx we can infer by the formatting of this symbolic link of what it's

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actually doing we see a k so we know this sim link is pointing to a script which is going

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to kill a particular process.

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We get the number here which is the numerical number remember 6 feet and it operates in

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a sequential manner.

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Process 1 will go before process 2 in this case here we have process 9 so after process

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8 begins this particular one will execute and it's going to kill the nginx process.

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So like i say on modern systems the sysv init program is ultimately outdated and has been

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replaced with system d but nevertheless we can get a rough idea of what this looks like

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even on a modern system so what i'll do is i'll go into the etsy directory and if i happen

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to go into maybe say rc3.d what this is is scripts relating to run level 3 whereas if

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i happen to go in let's say rc5.d this would be scripts relating to run level 5 now if

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i do an ls we can just see a whole bunch of water sim links if i do an ls-l for the long

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listing we can actually see where these sim links are pointing to now notice because this

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is a debian based system these are actually pointing to init.d whereas this would be different

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within a red hat based system.

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Now notice we have the value k this means that this is relating to killing processes

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whereas the other ones here these are all beginning with s these are for starting processes

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and the next one is the value the process number now you might notice that these all appear

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to be starting at the same time we are now on a modern system debaist system and things

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can actually operate in parallel so we're no longer hamstrung by the older implementation

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of sysvian.net so that is what i'm saying that the example here is not exactly perfect

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because we are talking about older outdated technology which frustratingly is still a

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big part of the lpik 2 examination of course but that is what you're seeing when you see

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these discrepancies we are ultimately mapping old technology onto new technology so if i

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happen to open up one of these scripts right here so i'll say nano s01 acpid and hit enter

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what you are looking at here is a bash script this is ultimately just a file full of programming

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logic to do certain things such as loading particular modules and starting and stopping

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particular processes so on and so forth and ultimately this is what all of these files

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look like they're all bash scripts again we go into maybe say s1 cron same type of deal

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we are just running a script to stand up the cron service a whole bunch of programming

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logic within here now if we wanted to control a particular action maybe we wanted to change

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the rsync from starting up what we could actually do is we could rename the sim link now the

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sim link happens to be s01 rsync what if we happen to change that so i said s01 rsync

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and i changed it to k01 rsync what i need to do of course is to do super user privileges

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because i'm in the etsy directory let me just type in my passwords and now if i do an ls-l

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we can see here rsync has a sim link beginning with k so what this means is that when we

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happen to drop into our particular run level this is run level 5 instead of starting the

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rsync program we would happen to stop it now the issue is here is that whilst we can manage

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these processes we can ultimately invoke scripts to start or to kill particular applications

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and actions this is not the most friendly way to manage such a thing just by renaming sim

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links things can be confusing and by the way on older systems whereby we had to use this

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particular action i.e. when we were using sys the init-based systems whereby the processes

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were not parallel they were sequential and the order of operations mattered much much more

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by renaming sim links if we happen to maybe use the wrong number we could actually change the

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order of operations and if some processes relied on another process standing up and we got

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the order wrong then well we could definitely cause some issues but the good news was for system

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the based administrators they did actually have the ability to use some particular tools to simplify

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this action now what exactly were those tools how were they used well that's what we're going to be

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talking about in the very next nugget so i hope this has been informative for you and i'd like to

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thank you for viewing