1 00:00:01,890 --> 00:00:04,530 Hello, everyone, and welcome to this video. 2 00:00:04,920 --> 00:00:08,640 So in this video, we are going to see the Linux boot process. 3 00:00:08,670 --> 00:00:16,380 Now, every one of you must be knowing that whenever we press the power button, our computer gets started. 4 00:00:16,680 --> 00:00:25,140 But what's actually happening behind the hood in the background is very important to know and understand. 5 00:00:25,680 --> 00:00:34,140 That is the reason we are going to understand the Linux boot process and how a system actually starts. 6 00:00:34,830 --> 00:00:40,230 So it follows six high level steps in the boot process. 7 00:00:40,410 --> 00:00:45,870 And we are going to understand each one of them, one by one, in-depth. 8 00:00:46,380 --> 00:00:50,400 So let's begin with the first one, which is BIOS. 9 00:00:50,610 --> 00:00:58,350 So BIOS stands for basic input output system, which executes the MBR. 10 00:00:58,980 --> 00:01:07,800 The second one is MBR, which stands for Master Boot Record, which then executes the grub. 11 00:01:08,910 --> 00:01:17,340 The full form of grub is Grand Unified Bootloader, which then in turn execute the kernel. 12 00:01:18,620 --> 00:01:26,540 Next is the kernel, which is also known as the heart of the computer, which executes the spin and 13 00:01:26,540 --> 00:01:27,980 the in IT programs. 14 00:01:28,610 --> 00:01:37,310 Next is in it, which is also known as initialization, which executes the run level programs into the 15 00:01:37,310 --> 00:01:38,570 Linux system. 16 00:01:39,140 --> 00:01:41,570 The next one is the run levels. 17 00:01:41,690 --> 00:01:49,640 Now, run level are the programs which are executed from this directory, which is slash etsi, slash 18 00:01:49,640 --> 00:01:58,840 RC, dot D, slash RC one dot rd, rc two dot rd, which has been demonstrated here with RC star dot 19 00:01:58,850 --> 00:01:59,210 D. 20 00:01:59,870 --> 00:02:06,200 Now run levels are those levels in which the Linux system will boot. 21 00:02:06,230 --> 00:02:12,740 For example, there are multiple run levels which start from one level zero to run level six, which 22 00:02:12,740 --> 00:02:16,130 we are going to understand in the later part of the lesson. 23 00:02:18,820 --> 00:02:22,180 Now let's quickly understand what is bias. 24 00:02:22,390 --> 00:02:29,080 As we have learned, the full form of bias is basic input output system. 25 00:02:29,990 --> 00:02:33,140 Now, what does actually the BIOS does? 26 00:02:33,380 --> 00:02:36,020 It performs the integrity checks. 27 00:02:36,110 --> 00:02:40,940 It also searches, loads and executes the bootloader program. 28 00:02:41,210 --> 00:02:49,100 Now, whenever you start your computer bios quickly checks for all the CD, DVDs, SD cards and the 29 00:02:49,100 --> 00:02:52,880 hard disk drives which have been attached to your computer. 30 00:02:53,420 --> 00:02:56,090 It also checks the boot sequence. 31 00:02:56,120 --> 00:03:02,300 Now, if you want to modify the boot sequence, you can press the keys, which is F2, L or F2. 32 00:03:02,300 --> 00:03:05,540 And it may also vary based on your system. 33 00:03:07,100 --> 00:03:12,140 The next one is the MBR, which stands for the Master Boot Record. 34 00:03:12,290 --> 00:03:18,950 Now the NBA is located into the first sector of the bootable disk. 35 00:03:19,310 --> 00:03:28,130 It is usually into this path which is slash dev slash SDA, where dev steps for device or sometimes 36 00:03:28,130 --> 00:03:31,200 it's in slash dev slash HD. 37 00:03:31,670 --> 00:03:35,420 This is the first sector of the bootable disk. 38 00:03:36,020 --> 00:03:43,650 Now it is always less than 512 bytes in size and it has three major components. 39 00:03:43,670 --> 00:03:50,450 As you can see onto the right hand side of the screen, the first four, four, six bytes are being 40 00:03:50,450 --> 00:03:54,110 assigned to the primary bootloader info. 41 00:03:54,350 --> 00:04:02,540 The next 64 bytes have been assigned to the partition table and the last two bytes are given to the 42 00:04:02,540 --> 00:04:04,940 grub, which we are going to see the next. 43 00:04:06,020 --> 00:04:10,970 So what does GRUB stands for and what is the importance of it? 44 00:04:11,210 --> 00:04:15,020 So GRUB stands for Grand Unified Bootloader. 45 00:04:15,590 --> 00:04:21,650 Now it chooses the kernel image to load and it executes it later on. 46 00:04:22,280 --> 00:04:30,350 It is like a splash screen onto your computer which execute the default kernel images from your computer. 47 00:04:30,500 --> 00:04:36,020 You can have multiple kernel images like kernel one, kernel two, kernel three. 48 00:04:36,020 --> 00:04:41,270 That can be different operating systems like Kali, Bharat or RedHat. 49 00:04:41,570 --> 00:04:48,950 So it loads and executes the kernel and gives you the opportunity to load into a specific operating 50 00:04:48,950 --> 00:04:49,550 system. 51 00:04:50,860 --> 00:04:56,290 The next is the colonel, which is also known as the heart of the computer. 52 00:04:56,530 --> 00:05:05,320 It execute the has been slash in it and there are multiple system programs which get executed into the 53 00:05:05,330 --> 00:05:05,860 step. 54 00:05:06,220 --> 00:05:14,290 Now the kernel mounts the root file system into the grub configuration file, which has been specified. 55 00:05:14,950 --> 00:05:22,600 It also executes the initialized programs, which is located into the S-pen folder, which is the system 56 00:05:22,600 --> 00:05:23,380 in folder. 57 00:05:23,860 --> 00:05:28,450 And finally, it loads the file system of the operating system. 58 00:05:29,710 --> 00:05:31,690 Next is the init. 59 00:05:31,870 --> 00:05:35,110 So init executes the run level programs. 60 00:05:35,140 --> 00:05:38,570 Now you may think what are the run levels? 61 00:05:38,590 --> 00:05:42,760 You're going to understand the run levels in next couple of seconds. 62 00:05:43,120 --> 00:05:48,700 So init decides in which run level your system needs to be booted. 63 00:05:48,970 --> 00:05:54,370 It also decides the programs to be loaded when your computer starts. 64 00:05:54,640 --> 00:05:58,270 So coming to the run level, how many are there? 65 00:05:58,390 --> 00:06:01,420 As you can see onto your screen in the right hand side. 66 00:06:02,960 --> 00:06:10,320 There are different rent levels which starts from one level zero run level one to run level six. 67 00:06:10,340 --> 00:06:17,780 Now, whenever your system boots, it goes into a fixed run level, which is the run level three, which 68 00:06:17,780 --> 00:06:20,240 is multi user plus networking. 69 00:06:20,750 --> 00:06:23,870 Let's understand all the run levels one by one. 70 00:06:24,110 --> 00:06:27,230 So what does the run level zero stands for? 71 00:06:27,260 --> 00:06:34,180 It stands for shut down or halt, which means onto your computer into a terminal. 72 00:06:34,190 --> 00:06:39,290 If you type run level zero, then your computer is going to shut down. 73 00:06:39,410 --> 00:06:41,650 Now, you cannot type the command run level. 74 00:06:41,660 --> 00:06:48,920 You have to take the command, which is in it zero, which gives the instruction to your computer that 75 00:06:48,920 --> 00:06:52,580 the user wants to go into the run level, which is zero. 76 00:06:52,580 --> 00:06:57,410 So the command is in it, which stands for initialize the run level zero. 77 00:06:57,980 --> 00:07:00,050 The next is run level one. 78 00:07:00,530 --> 00:07:08,660 Now if you type in it one into your terminal, you're going to go into a single user mode without networking. 79 00:07:08,660 --> 00:07:14,840 That means only one user can now use the system and there will be no Internet. 80 00:07:15,810 --> 00:07:24,240 Next is the level two, which is multiple users with no networking run level three is the default run 81 00:07:24,240 --> 00:07:30,370 level into which any operating system boot, which is the multi user plus networking. 82 00:07:30,390 --> 00:07:35,220 Now this is the level which every one of us gets by default. 83 00:07:35,220 --> 00:07:41,640 Booted in within multiple users can be switched as well as we have the network access. 84 00:07:42,120 --> 00:07:49,260 Next one is the run level four, which is undefined and which is used for research and development purposes. 85 00:07:49,620 --> 00:07:52,080 The next is the run level five. 86 00:07:52,110 --> 00:08:01,230 Now a run level five works exactly as run level three, which is multi user plus networking with X11 87 00:08:01,230 --> 00:08:02,370 protocol support. 88 00:08:02,580 --> 00:08:11,310 Now X11 protocol is a display GUI protocol which is used to enhance the operating system. 89 00:08:11,790 --> 00:08:17,820 And the last one is the run level six, which is used to reboot or restart the system. 90 00:08:17,820 --> 00:08:25,230 So onto your terminal if you type in it six, which means you are given the instruction to initialize 91 00:08:25,230 --> 00:08:28,920 the run level six, your computer will automatically restart. 92 00:08:29,460 --> 00:08:38,520 So now we have completed the run levels, which is the level 0123, four, five, six respectively. 93 00:08:39,240 --> 00:08:46,380 Now, as we have also understood, the run levels and the programs that are executed from this directory, 94 00:08:46,380 --> 00:08:54,930 which is slash ETSI, slash RC D slash RC one dot D, RC two point D and so on. 95 00:08:55,890 --> 00:09:02,400 We have also understood how the system executes program based on fixed run levels. 96 00:09:02,430 --> 00:09:05,610 We have also understood the directories of these programs. 97 00:09:06,150 --> 00:09:14,310 Now, one important thing to remember over here is the programs that start with SSE into those directories 98 00:09:14,310 --> 00:09:20,460 are used for start up and the programs which start with K are used for shutdown. 99 00:09:20,490 --> 00:09:21,520 Now why K? 100 00:09:21,540 --> 00:09:23,760 Because K stands for kill. 101 00:09:24,000 --> 00:09:25,950 I hope you guys understood that. 102 00:09:25,950 --> 00:09:31,710 What are the run levels and how the Linux boot process functions? 103 00:09:32,310 --> 00:09:36,660 If you have any questions or queries, you can always post it into the Q&A. 104 00:09:36,690 --> 00:09:37,500 Thank you.