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101.2 Boot the system

1. Boot process - part 1

This video is the first about the boot process. How does this boot process work? The computer is turned on, as is the BIOS or UEFA. Recently, some simple hardware tests For example, whether there is a hard disc at all, how it is addressed, and so on. The buyer then looks in the master boot record (MBR, for short), which is in the first sector of the hard disk, to see if it can find a so-called bootloader. A bootloader, formerly also called a bootstrap loader, is a programme that is responsible for starting the Linux kernel. There are different bootloaders. The most common and up-to-date should be Grab Two, but Grub Legacy or Grub One can also be used. Maybe you can call it Grub One too. I think it's still very widespread today. By the way, Grub stands for "grand unified bootloader." Lilo is another well-known bootloader that is no longer widely used today. LILO stands for Linux loader. After the bootloader has started the kernel, it starts the so-called initial RAM disk, or initrd for short. Init RD is a temporary file system that is only used during the boot process. Init Rd creates a file system image on a reserved area of main memory that only contains files and directories needed for the system to start. Init Road then starts the init process. The Init process is the first process that is started on a Linux system, and therefore it always has Process ID 1, or PID 1, for short, init itself. Then it starts all other programs, and as soon as it has been started, the initial run disc is no longer required and is therefore switched off. So if we take a look at the currently running processes on a Linux system and maybe display them in a tree view with the PS 3 command—we will talk more about it later—then we can see exactly which processes are running here. And the P options mean that we can see the process ID in brackets with the options P. Let me scroll up a little bit. In the first place, with the process ID one, we see the process SystemD. System D is the successor to INIT. We will talk about SystemD in a separate video. We have installed the current Ubuntu version, in which this version is no longer used, but System D. And accordingly, the first process that is started on the System D system is also the System D process, which is given the process ID One. As I said, we will come to a detailed explanation of what System D and System are in a moment. At this point, I would install an older Ubuntu with the help of LXC containers. So I have LXC installed on my system, and I want to launch an Ubuntu 14. Since the Linux container, or LXE for short, does not appear in the Epic exam, I will not go into it in much detail here I have just downloaded the Ubuntu 14 image here and started it as a container, which is much faster than I had to create a new virtual machine. Now, okay, now I'm locked in an Ubuntu 14 container. And now I wanted to use the command pstree with the option P again, but I saw that my Ubuntu 14.04 did not know this command command not found.I tried to install it, but it didn't work either. Okay, so we have to choose another command, which is PS with the options e and F. With these options, we can see the processes that are running at the moment but, unfortunately, not in a tree view. But I think that helps too. We can see here that blood has been in it. As previously stated, it has always been the PID. And here you can see it; processor D is in it. Okay, so I am going straight out of here, so I'm back to my current Ubuntu. Perhaps you have already seen it when you boot a Linux system. If you have not just set it so that you only see a background image but also what the system does when booting, you can see that many tests are carried out. There will be hardware that is recognised and started, and so on. However, nobody can read along that quickly. Maybe you could read the word "error" somewhere when booting up, but the startup process is far too fast to be able to read what exactly it was about. In this case, we can, for example, use the command to send a message. We take a look at the main page mandmesg Dmesg prints or controls the kernel ring buffer. DMSG is used to examine or control the kernel ring buffer. The default action is to display all messages from the kernel ring buffer. D messages read out the so-called kernel ring buffer. The kernel ring buffer stores kernel messages as well as items that are locked during boot. The kernel ring buffer is 16,392 bytes by default. When it's full, the oldest messages just drop out. I run Dmsg, and we see a lot of information here that we can look at here, so that we can then understand again what actually happened during the boot. And now, for example, if you want to search for an error or a warning, you can pass the message to Grep. We will talk about that later again, but we've already done that. In another case, we can use the message followed by the grip error. For example, the I option here means that no distinction is made between upper and lower case. Now only the locks on which the system reported an error are displayed. So you can now understand exactly where errors have occurred. Another option is to use the journalCTL command to query the system via Djornal. If called without parameters, it will show the full contents of the genre, starting with the oldest entry collected genreCTL shows all system messages on a system. Therefore, of course, this only works with current Linux distributions that also use Systemd. I just briefly mentioned that system as the successor to this unit. As a result, this Vinit is available in current versions. Everything that happens in this system is simply locked by CTL. The output is made here, page by page. Through space. We go one picture further, and so we can go through the pages until the very bottom. You can leave this with queue. If you look closely, we see that there are messages from the kernel, the network manager, and so on. Let me show this. Here we have messages from accession DeepestDemon, system Dartkit, and so on. Here is a kernel message. And if you just want to see the kernel messages, which are the most important ones in most cases, then you can use genre CTL with a key option. In this case, of course, KS K stands for kernel, so that only the messages from the kernel are displayed here and here. As you see, we have only the kernel messages.

2. Boot process - part 2

API requires that commands be passed to the bruteloader at startup, among other things. We will simulate that in a moment. As already mentioned, the current bootloader is Grub Two. Grub Two has different modes. Once the standard mode is selected, that is, the menu mode, then the command line, which is in this case called the command line interface, or CLI mode for short, Then there is the rescue mode, and there is the edit mode. The menu mode shows us a selection menu when booting, in which we select which operating system should be started. Surely you have seen this before. However, if only one operating system is installed, the selection menu remains hidden and is not displayed. However, by pressing the Shift key during booting, you can force the menu to be displayed. You should press the shift key immediately after the BIOS and hold it down until the menu appears. Incidentally, it works best if you have switched off the virtual machine and then switch it on again, because at least I usually miss the right time when restarting. I would now simulate that. At this point, I will first turn off the virtual machine. Power off. Okay, the machine has switched off. Now I started again and tried to press the shift key as quickly as possible, which is sometimes not easy with virtual machines. Okay, I start, and I try to push the shift key as soon as possible. Okay, so it worked. So here I am in menu mode. We can now select which system we want to launch. In my case, however, I only installed Ubuntu up here. We can see that we then also know which bootloader we are using here, namely Grub 204. By pressing the e key, we get into edit mode. This is used when we want to change certain start parameters. So that's exactly what API requires for the exam, and we know how to change the start parameters. Okay, I'll press the E button now. And now we can, for example, enter another Linux kernel. Go a little bit down here. It is the case that we normally always have two kernel versions stored because if we have updated a kernel and it is faulty and the system no longer starts, then we can simply change the boot parameters in this way and enter the other kernel here and then restart the system. with this kernel. We also see the reference to the initial RAM disc here. There are always two of these on a Linux system, and we also have various other options here. But for the exam, we don't need to know what all these options mean. It is important to know that changes we make in this way only affect this one system start.That makes sense, of course, because this mode is actually only intended to be able to intervene if the system no longer starts with control and X. The system started with these changes made.I haven't made a change now, and I won't, but I choose Control X. And now the system starts again. If we had just pressed Escape, the Edit mode would have simply been exited and we would have ended up in the selection menu again. Instead of pressing the E key in the mode selection menu to get to the edit mode, you can switch to the CLI mode with the C key. This mode is comparable to the normal shell, but here there are only the commands available that can be used to start and analyse the start parameters. The CLI mode is most suitable if you want to analyse the complete startup behavior, which is of course much more extensive than what we were able to do in the Edit mode. We don't need to know more about this topic for the Epic exam, and because of that, we'll see you in the next lesson.

3. SysVinit

A brief note before we go into practice. These Vinit systems are normally outdated and are hardly used today. For this, you would have to install an older Ubuntu system. The last Ubuntu system that this vendor used was, for example, version six. But today there are some Linux distributions that still use this unit. One of these systems is Linux or OS X. That is an actual installation of the PC Linux OS. I have downloaded and installed this Linux only for this one lesson to show you how this system works. Okay, as I said, this video is about the unitsystem, or just Init, as already mentioned in the lesson. It is the first process that is started, whether before or after the last in. And this process always has Process ID 1. We can take a look at that with a PS 3. With the PS three command, we get an overview of all running processes in a tree view. We see here above that all processes originate from Init. Init is the first process. So with the option P, we can also display the process IDs. These are the process IDs. and let's again scroll up. And here, you can see Init has the process ID. One unit is part of the Syssystem that is used on Unix systems. System V could be written into this unit. V stands for five. in this case. As I said, Init is responsible for starting all processes when the computer is switched on and the kernel takes over for the first time. The first thing it does after starting the Init Ramfs is look for the Init command. It searches for four and finds it. has been in it. As I said, INIT is then responsible for starting all programmes and services that the system needs. Init proceeds here in a fixed order. The first service, A, has begun. When the start is complete, it starts service B, then service C, then service D, and so on. From today's point of view, the latter is the disadvantage because if a process has to be started or if a process that has to be started causes problems, these must be solved before further processes can be started. That would, of course, slow down the boot process. I just said that in its beginning, the relevant services come one after the other in a specific order. I would like to briefly show what it looks like when Init takes over for the first time. It first looks at a file called Initap. We can find it on Etsy. So let's take a look at that file. The diamonds here or the hash signs here indicate that these are just comments that will not be carried out. So this line here is important. This line says that the system's standard run level is run level five. So the system starts at Run Level 5. What are run levels? Perhaps a very brief explanation in advance to be able to follow this lesson further. We will talk about run level in more detail in a later video. Run levels are states in which Linux systems are operated. There are seven different run levels from run level zero to run level six, and sometimes they differ a little from distribution to distribution. Run level zero means that the system is switched off. The command init zero can be used to switch to running level zero on the console. So if I enter "init zero," the system would be switched off. Run level six means that the system will be restarted. In our case, we see here that the system is using run level five. Run level five means that multiuser login is possible, that network access is available, and that a graphical user interface is available. Run number five is used by default on desktop systems and servers. On the other hand, run level three would be the standard because then you would have multiuser login network access, but no graphical user interface, and that would be run level three. So if I would enter init three into the console, then the system would start in round five. So we're dealing with graphical design and a graphical desktop. I'm in a graphical desktop; if I make this a little smaller than you see, I have a graphical desktop; if I then use the command in it three, the system will reboot and enter the run level three state, and we will not have any graphical elements. Okay, so we see in the tapfile that the system starts at run level five. It would now look in the directory to see which processes needed to be started and in what order. The directory names differ, partly depending on the Linux distribution. In DBN distributions, the directory would be at cinna d; in Red Hat distributions, it would be at CRC dot d. You should definitely remember this for the exam. Let me leave this file. So keep that in mind. We have Etsy in it dot d or we have at CRC d. So you can see in my case that both directories exist; the content of this directory is here, as are various subdirectories. It is obvious that RC 0 d stands for run level zero, RC 1, D for run level one, RC 2, D for run level two, and so on. I don't know why. Here is an RC 7D. Normally there are only seven run levers, so from zero to six. Maybe that's a special thing here with the PC Linux OS. I'm not familiar with this Linux distribution. I only downloaded and installed it because I had to show you how this VNET works. So maybe you can investigate why there is an RC 7D, but I think it is not important for the exam. Keep in mind that normally there are three run levels: 00:26 So we saw in the Etsy init tab file that the system starts at level five. So let's take a look at that or at the contents of the RC. There are files that start with an SSS followed by a number. Also, S stands for start at K. This means that when the system is started init looks at the Etsy in it tap file. The initiated file says that the system should start at run level five. So Init looks up the RC 5D directory and looks for scripts that begin with an S. So these are all scripts that are supposed to start within it. Based on the numbers, it knows which script should be started first, what follows afterwards, et cetera. Unfortunately, it doesn't always go from one to 99, and sometimes a few numbers are missing. So it looks like nine is the lowest number here, which is why I initial the script here, followed by 1013, 14, 1517, 1820, and so on. We also see that the files are only linked here. We will talk about that in detail later. But as you can see here, this is a link. This file is linked to when the system is to be shut down. It looks for the K files and checks what should be stopped and killed first. So in this case and so on with thecommand check config list we can see which processesare installed here on the system and in whichrun level they should be started. Check the config-minus list, and here you can see the processes. Let's look maybe for the line network we seein run level zero network is switched off. This is of course logical, because "run level zero" means that the computer is switched off. The network is turned off in run level one because no multiuser login is normally possible at that level, so the network is turned off. In this case, however, as I said before, due to the fact that there are definitely differences between the various distributions, the network is also switched on in run levels three to five. Because running level six means restarting the system, the network is, of course, turned off. And as I said before, we simply ignore Run Level 7. Here there are two options for stopping or starting a process in the ZSV unit system. Let's take the example of the NGINX Web Server. I just installed the web server, and now we want to see the status of this web server. And a possible command could be "Etsyinit D" and then NGINX status. and we see. Here angel. Next has been stopped, and we now want to begin and index. So we would use "here" to get the information starting with X. And again, we check the status. And now we see that Endo Next is running with these process IDs here. The other way to start and stop a process in thisVenice systems would be as follows service and then NGINX. Now we want to start it again. So the service engine is okay here on the right side, and then again to check the service and index status, it is running. The following command can also be used to restart the Postfix service. Enter next restart and you can see here stopping okay. Starting okay. In this way, the individual services existing within its system are stopped and started. The next video is about Upstart. Upstart is the successor to Initiate.

This video is about the successor to this Venice, namely Upstart. Upstart was an Ubuntu project and was replaced by it. I say "Wolf" because it is already out of date and is hardly used anymore. But for the Epic Exam, you should at least know what Upstart is and how it works. As a reminder, Init started all processes sequentially one after the other. Upstart is able to start processes in parallel, as long as these processes are independent of each other. This significantly speeds up the system start compared to Init; the Etsy-initiated file is omitted. The Etsy Init directory contains configuration files that determine when a process is started. All files in the Etsy Init directory are started automatically. Let's look at the content of, and we can see that there are a number of files with the extension.com, indicating that they are all configuration files. We can look at such a file, for example, with Vipud, and we find a small description here. We see when this file is started and when this file is stopped. And we can also see below which file will ultimately be executed for the exam. However, we do not need to know what exactly this file ultimately says. We will not be presented with such a file during the examination. We only need to know that these configuration files are available at Upstart and that they are located at Etsy. In its directory, Upstart is backwards compatible. That means the old services that were started with S and stopped with K can still be used, even if they are now called Upstart. The first process with the process IDone is still called Init, and Init is still located in the Spin directory. Let me blank the screen. So-called Init jobs can now be started or stopped with the Initctl command. We remember with this Vinit system we used the service command, for example, servicepost fix start" and "add up start." We use Initctl with the Initctl list. We can display a list of all init jobs and also see the corresponding state in its CTL list. If we now, for example, want to see the status of Udef, we use image status Udef, and we see Udef is running and has the process ID. We can terminate Udef with the following command: initctl stop udef udef stop waiting, and Udef will be terminated. We can test it by running initctlstatus udef udef stop waiting. We start Udef again with init udef, and Udef is running again with the process ID 1872.

5. system

This video is about systemd. Systemd is the successor to Upstart and is used by default in almost all Linux distributions today. As a reminder, it started each process one after the other in a specified order. Upstart started processes that are not dependent on each other in parallel. And System D also starts interdependent processes in parallel, making the system start a little faster. Systemd also uses files similar to Upstart. These are located in two directories, once in the etsysystemd system and once in the libsystemd system. These files are also called units. Let's go to the Etsy system, to the System folder. And here we see the corresponding units. Let's take a look at the other folder. The same here. If two units with the same name are in both directories, then the unit in the Etsy Systemd system directory is preferred. The unit in the Lipsystem D System directory is then ignored. You just have to remember that the units are in two different directories. One is the directory etsy system v system, and the other is the lip system d system. The directory system Etsy system takes precedence over the Lip system B system. It's important to know for the exam to start and stop processes, and we use the system CTL command as a reminder. With these initial systems, we use the command service; with an upstart, we use init CTL, and with system systems, we use systemctl. With SystemCTL list units, we can display all units of the system and the corresponding stages of SystemCTL list units. Here on the left side we have the unit name. Then here is the load status. Is this unit loaded? Here is the active status: Is this unit active or not? That's not interesting here, and here we have a little description. With space, you can go a page down until the end. Here we have some active waiting in the queue. You can quit, and yeah, let's take a look at the status of a unit. I would prefer to install a little webserver before the web server engine. Don't worry if you do not know this command now. I will not explain it now because we learned this in detail in a later video. So let me please just install Engineix; it's installed now. Now we can take a look at the status of Engineix. Is Engineix running or not? We can check this with the following command system. We see that the status is active, and we also see that in comparison to this init or upstart, we receive significantly more information. It is important to know for the exam. In the case of system CTL, the command status, start, stop, or restart comes directly after system CTL. and the same here with CTL. With this V-I-T system, sorry, it is different because then we have service, then mechanics, and then status. You have to remember this. You have to learn this for the exam. It is very important. If we now want to stop Engineix, we use the following command I think you know it already systemctl stopengineix, so we check the status with System CTLstatus Engineix again, and the stage is inactive. We see it here as inactive debt. and we see it here on the little circle. Here it is green. Here it is white. So we can see it on the first look. Okay, let me restart the engine with Pseudo System and check the stages again, and we can see that NGINX is now active. Of course, there is also the command system, CTL RestartEngineX. Restart is a combination of stop followed by start now, and Linux has been stopped and started again in the background. It should be active again. Yes, that's it. You can see that it has been active since 11 seconds ago. If you want to change configurations for Systemd, you would have to do this in the main configuration file of Systemd. This is called System.com and is located in the Etsy systemd directory. Okay, please make a note of this for the exam. The main configuration file of Systemd is the fileSystem, and it is placed in Etsy Systemd. Let's take a quick look at this file, systemConf. So this is how the file looks. You can change many different things here. But, again, you won't have to know exactly what the file contains in the exam. At most, you will be asked what the file is called and where it is located. Okay, that's it. With this lesson, we have now completed most of the work with this V and with Upstart and Systems. In the next lesson, we will get to know the topics of run level and boot targets in more detail.

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