1 00:00:00,000 --> 00:00:01,380 In this lesson, 2 00:00:01,380 --> 00:00:04,022 we're going to discuss the different Linux boot components. 3 00:00:04,022 --> 00:00:06,270 Now, that we've configured the kernel, 4 00:00:06,270 --> 00:00:09,300 we need to learn about how the kernel is loaded into memory 5 00:00:09,300 --> 00:00:12,510 and how the operating system is actually going to start up. 6 00:00:12,510 --> 00:00:15,600 We're also going to configure the boot process to our liking, 7 00:00:15,600 --> 00:00:17,190 ensuring that the Linux workspace 8 00:00:17,190 --> 00:00:20,070 operates as intended from the very beginning. 9 00:00:20,070 --> 00:00:21,780 Now, have you ever wondered what it takes 10 00:00:21,780 --> 00:00:24,960 to get your system ready to run applications and programs? 11 00:00:24,960 --> 00:00:28,260 To begin with, we have to understand the Linux boot process, 12 00:00:28,260 --> 00:00:29,580 because this is very important 13 00:00:29,580 --> 00:00:31,380 for us to be able to configure Linux 14 00:00:31,380 --> 00:00:33,000 and resolve startup issues, 15 00:00:33,000 --> 00:00:35,250 as well as to identify various components 16 00:00:35,250 --> 00:00:36,780 that make up the boot process 17 00:00:36,780 --> 00:00:39,480 and configure them whenever we need to. 18 00:00:39,480 --> 00:00:40,410 Now, in Linux, 19 00:00:40,410 --> 00:00:43,110 there are going to be six different stages of booting. 20 00:00:43,110 --> 00:00:45,450 This is called the booting process. 21 00:00:45,450 --> 00:00:47,665 So, what is the booting process? 22 00:00:47,665 --> 00:00:50,160 Well, booting is a process of starting 23 00:00:50,160 --> 00:00:51,660 or restarting a computer 24 00:00:51,660 --> 00:00:55,350 and loading an operating system for the user to access. 25 00:00:55,350 --> 00:00:56,773 In the boot process, 26 00:00:56,773 --> 00:00:59,070 a booting environment reads a small program 27 00:00:59,070 --> 00:01:02,550 that's stored in read-only memory known as ROM. 28 00:01:02,550 --> 00:01:05,310 This program then executes various operations 29 00:01:05,310 --> 00:01:08,370 inside your RAM or your random-access memory, 30 00:01:08,370 --> 00:01:10,740 and that's going to bootstrap the operating system 31 00:01:10,740 --> 00:01:12,660 and make it available for use. 32 00:01:12,660 --> 00:01:15,210 Linux like most other operating systems 33 00:01:15,210 --> 00:01:18,030 has to be booted before it can function. 34 00:01:18,030 --> 00:01:19,860 Now, there are various options associated 35 00:01:19,860 --> 00:01:21,870 with the boot process that you're going to be able 36 00:01:21,870 --> 00:01:24,480 to configure as well if you need to. 37 00:01:24,480 --> 00:01:27,690 A boot loader is a small program that's stored in ROM 38 00:01:27,690 --> 00:01:30,060 and that's going to load the kernel from a storage device, 39 00:01:30,060 --> 00:01:31,650 like a hard disc drive. 40 00:01:31,650 --> 00:01:34,680 And then, it's going to start up the operating system. 41 00:01:34,680 --> 00:01:36,960 When a computer is powered up or restarted, 42 00:01:36,960 --> 00:01:40,140 the basic input/output system performs initial tests 43 00:01:40,140 --> 00:01:43,560 and then transfers control over to the Master Boot Record. 44 00:01:43,560 --> 00:01:46,590 And this is where the boot loader is going to reside. 45 00:01:46,590 --> 00:01:47,700 Boot loaders are able 46 00:01:47,700 --> 00:01:50,400 to protect the boot process with a password, 47 00:01:50,400 --> 00:01:51,270 and that's going to prevent 48 00:01:51,270 --> 00:01:53,610 unauthorized booting of your system. 49 00:01:53,610 --> 00:01:56,610 The boot loader can also use three main components 50 00:01:56,610 --> 00:01:59,340 that are going to work together to systematically load up 51 00:01:59,340 --> 00:02:02,430 the operating system in three different stages. 52 00:02:02,430 --> 00:02:05,280 The first component is the boot sector program 53 00:02:05,280 --> 00:02:08,160 and this is loaded by a boot environment on startup, 54 00:02:08,160 --> 00:02:12,300 but it's main function is to load up the second boot loader. 55 00:02:12,300 --> 00:02:13,980 The second stage boot loader 56 00:02:13,980 --> 00:02:15,600 is going to load the operating system, 57 00:02:15,600 --> 00:02:17,880 and this contains a kernel loader. 58 00:02:17,880 --> 00:02:20,340 On the other hand, the boot loader installer 59 00:02:20,340 --> 00:02:23,040 is going to control the installation of drive sectors 60 00:02:23,040 --> 00:02:25,920 and can only be run when booting from a drive. 61 00:02:25,920 --> 00:02:27,420 It's going to coordinate the activities 62 00:02:27,420 --> 00:02:30,180 of the boot sector and the boot loader. 63 00:02:30,180 --> 00:02:32,040 Now, there are several boot options 64 00:02:32,040 --> 00:02:35,280 where you can boot Linux from a basic input/output system 65 00:02:35,280 --> 00:02:37,950 known as BIOS or the UEFI, 66 00:02:37,950 --> 00:02:42,330 the Unified Extensible Firmware Interface or UEFI. 67 00:02:42,330 --> 00:02:44,850 Now, when a computer with BIOS is powered on, 68 00:02:44,850 --> 00:02:47,400 the BIOS firmware is the first thing to run, 69 00:02:47,400 --> 00:02:48,450 and this is going to enable it 70 00:02:48,450 --> 00:02:51,120 to test the various hardware components in a computer, 71 00:02:51,120 --> 00:02:52,680 as well as run a boot loader, 72 00:02:52,680 --> 00:02:55,260 so the operating system can start up. 73 00:02:55,260 --> 00:02:57,210 The BIOS has access to the ports 74 00:02:57,210 --> 00:02:59,940 that are used by the basic hardware input devices, 75 00:02:59,940 --> 00:03:02,010 things like the mouse and the keyboard 76 00:03:02,010 --> 00:03:04,950 And users can also load up a BIOS interface 77 00:03:04,950 --> 00:03:07,320 instead of going into an operating system. 78 00:03:07,320 --> 00:03:08,340 This is done if they need 79 00:03:08,340 --> 00:03:10,860 to make various hardware level changes. 80 00:03:10,860 --> 00:03:15,150 On the other hand, UEFI is going to run faster than the BIOS. 81 00:03:15,150 --> 00:03:17,520 It also operates with a greater amount of memory 82 00:03:17,520 --> 00:03:19,380 and it gives you access to storage drives 83 00:03:19,380 --> 00:03:21,360 of various different sizes. 84 00:03:21,360 --> 00:03:23,970 It'll also allow you to access more hardware types 85 00:03:23,970 --> 00:03:26,550 and has much more improved security protections 86 00:03:26,550 --> 00:03:28,590 than standard BIOS does. 87 00:03:28,590 --> 00:03:30,810 One security feature that both the BIOS 88 00:03:30,810 --> 00:03:34,350 and UEFI will have is the ability to set a password. 89 00:03:34,350 --> 00:03:36,870 If this password is not provided at boot time, 90 00:03:36,870 --> 00:03:39,270 the system will simply not boot. 91 00:03:39,270 --> 00:03:42,630 Now, because the BIOS and UEFI are firmware, 92 00:03:42,630 --> 00:03:44,250 they do differ based on the different 93 00:03:44,250 --> 00:03:46,500 hardware manufacturers that make them. 94 00:03:46,500 --> 00:03:48,330 And the process of setting up this password 95 00:03:48,330 --> 00:03:49,800 is not going to be consistent 96 00:03:49,800 --> 00:03:52,800 across all BIOS or UEFI firmwares. 97 00:03:52,800 --> 00:03:55,860 Another boot option includes booting from an ISO 98 00:03:55,860 --> 00:03:57,420 which is an image file. 99 00:03:57,420 --> 00:03:59,670 This is where you need to write that ISO file 100 00:03:59,670 --> 00:04:02,400 to an optical disc or a USB thumb drive, 101 00:04:02,400 --> 00:04:04,800 and then insert that media into the computer 102 00:04:04,800 --> 00:04:07,470 and instruct your boot environment like the UEFI 103 00:04:07,470 --> 00:04:11,130 or the BIOS to boot from that media device instead. 104 00:04:11,130 --> 00:04:14,190 Another way you can boot up is by using PXE, 105 00:04:14,190 --> 00:04:16,769 which is a Preboot eXecution Environment. 106 00:04:16,769 --> 00:04:18,690 This is part of the UEFI standard, 107 00:04:18,690 --> 00:04:19,769 and it enables a client 108 00:04:19,769 --> 00:04:21,600 to retrieve the necessary boot loader 109 00:04:21,600 --> 00:04:24,780 and system files from a server over the network. 110 00:04:24,780 --> 00:04:28,320 Now, the client configures UEFI to boot from the PXE. 111 00:04:28,320 --> 00:04:30,750 And during the startup process, it's going to search 112 00:04:30,750 --> 00:04:33,510 for a Dynamic Host Configuration Protocol server 113 00:04:33,510 --> 00:04:37,890 or DHCP server that also acts as your PXE server. 114 00:04:37,890 --> 00:04:40,890 Clients can also acquire boot data over a network 115 00:04:40,890 --> 00:04:44,190 from the content delivery protocols like HTTP, 116 00:04:44,190 --> 00:04:46,110 the Hypertext Transfer Protocol 117 00:04:46,110 --> 00:04:49,200 or FTP the File Transfer Protocol. 118 00:04:49,200 --> 00:04:51,810 These are typically going to be faster, more reliable, 119 00:04:51,810 --> 00:04:55,530 and more secure than using a standard TFTP protocol 120 00:04:55,530 --> 00:04:58,189 like you'd normally use in PXE environments. 121 00:04:58,189 --> 00:05:02,610 Finally, booting from NFS or a Network File System 122 00:05:02,610 --> 00:05:05,130 allows a client to mount an NFS share 123 00:05:05,130 --> 00:05:08,490 as its root file system, and then operate over the network. 124 00:05:08,490 --> 00:05:10,500 This share must be prepared ahead of time though, 125 00:05:10,500 --> 00:05:12,510 and stored on an NFS server 126 00:05:12,510 --> 00:05:14,940 that the client can retrieve the files from. 127 00:05:14,940 --> 00:05:18,000 Therefore, the client does not store the data locally, 128 00:05:18,000 --> 00:05:19,920 but instead, it stores everything 129 00:05:19,920 --> 00:05:22,120 on that network file server. 130 00:05:22,120 --> 00:05:23,820 Another thing we have to think about 131 00:05:23,820 --> 00:05:25,710 is how is a hard disk read from 132 00:05:25,710 --> 00:05:28,380 or written to by an operating system. 133 00:05:28,380 --> 00:05:29,250 Now, the information 134 00:05:29,250 --> 00:05:31,320 about how a hard disc has been partitioned 135 00:05:31,320 --> 00:05:34,080 is going to be stored inside your Master Boot Record, 136 00:05:34,080 --> 00:05:36,120 known as the MBR. 137 00:05:36,120 --> 00:05:38,010 The Master Boot Record is a sector 138 00:05:38,010 --> 00:05:39,780 that the BIOS is going to read in 139 00:05:39,780 --> 00:05:42,480 and start when the machine is first booted. 140 00:05:42,480 --> 00:05:45,180 The Master Boot Record contains a small program 141 00:05:45,180 --> 00:05:46,890 that reads the partition tables 142 00:05:46,890 --> 00:05:48,990 and it checks which partitions are active 143 00:05:48,990 --> 00:05:52,140 and then reads the first sector of that partition. 144 00:05:52,140 --> 00:05:54,990 This boot sector contains another small program 145 00:05:54,990 --> 00:05:56,310 that's going to read the first part 146 00:05:56,310 --> 00:05:59,250 of the operating system that's stored on that partition, 147 00:05:59,250 --> 00:06:01,080 assuming it's active and bootable, 148 00:06:01,080 --> 00:06:03,480 and then it's going to start it up. 149 00:06:03,480 --> 00:06:05,670 If you're using a GUID Partition Table, 150 00:06:05,670 --> 00:06:07,650 also known as the GPT, 151 00:06:07,650 --> 00:06:10,650 this is a newer version of a Master Boot Record. 152 00:06:10,650 --> 00:06:12,840 And it basically makes up for the shortcomings 153 00:06:12,840 --> 00:06:14,940 that were inside of a Master Boot Record, 154 00:06:14,940 --> 00:06:17,130 because it is an older technology. 155 00:06:17,130 --> 00:06:18,930 Just like a Master Boot Record, 156 00:06:18,930 --> 00:06:21,420 a GPT is a partition structure, 157 00:06:21,420 --> 00:06:24,240 but it's going to employ a more modern design, 158 00:06:24,240 --> 00:06:26,670 and it's part of the UEFI standard 159 00:06:26,670 --> 00:06:29,640 instead of relying on the older BIOS standards. 160 00:06:29,640 --> 00:06:31,080 Every partition on a drive 161 00:06:31,080 --> 00:06:36,030 is assigned a globally unique identifier or a GUID, or GUID. 162 00:06:36,030 --> 00:06:38,940 This allows it to distinguish it from every other partition 163 00:06:38,940 --> 00:06:41,790 on every other drive theoretically. 164 00:06:41,790 --> 00:06:45,360 Other than formatting a partition as MBR or GPT, 165 00:06:45,360 --> 00:06:48,060 you can also format a partition as raw. 166 00:06:48,060 --> 00:06:50,250 Now, a raw partition enables users 167 00:06:50,250 --> 00:06:51,960 and applications to read from 168 00:06:51,960 --> 00:06:54,990 or write to a block storage device directly 169 00:06:54,990 --> 00:06:57,150 without using the system's cache. 170 00:06:57,150 --> 00:06:59,490 This is useful in situations where a software 171 00:06:59,490 --> 00:07:01,290 like a database management system 172 00:07:01,290 --> 00:07:04,656 or DBMS has its own caching mechanism. 173 00:07:04,656 --> 00:07:07,770 Another thing we need talk about is RAM disk. 174 00:07:07,770 --> 00:07:12,360 There's this thing known as the initial RAM disk or initrd. 175 00:07:12,360 --> 00:07:14,610 And this refers to the root file system 176 00:07:14,610 --> 00:07:18,295 that's temporarily loaded in the memory upon system boot. 177 00:07:18,295 --> 00:07:22,350 This initial RAM disk is going to load alongside the kernel 178 00:07:22,350 --> 00:07:25,020 and it's going to control all the functionality. 179 00:07:25,020 --> 00:07:26,400 This initial RAM disk 180 00:07:26,400 --> 00:07:29,280 will enable the system to be started in two phases. 181 00:07:29,280 --> 00:07:30,570 During the first phase, 182 00:07:30,570 --> 00:07:33,180 the system is booted with a minimal set of modules 183 00:07:33,180 --> 00:07:34,800 that are required to load the main 184 00:07:34,800 --> 00:07:37,050 or the permanent root file system. 185 00:07:37,050 --> 00:07:38,430 In the second phase, 186 00:07:38,430 --> 00:07:40,980 the main root file system is going to be mounted. 187 00:07:40,980 --> 00:07:42,900 And then, the previously mounted 188 00:07:42,900 --> 00:07:45,990 initial RAM disk file system can be removed. 189 00:07:45,990 --> 00:07:47,700 And this way, the user space 190 00:07:47,700 --> 00:07:50,010 boot process can continue forward. 191 00:07:50,010 --> 00:07:52,110 The initial RAM disk is going to be useful 192 00:07:52,110 --> 00:07:53,910 because there are potential variables 193 00:07:53,910 --> 00:07:56,370 that can complicate the boot process. 194 00:07:56,370 --> 00:07:58,500 For example, the kernel needs to find 195 00:07:58,500 --> 00:08:01,020 and load the necessary device driver modules, 196 00:08:01,020 --> 00:08:03,750 as well as the actual route file system itself. 197 00:08:03,750 --> 00:08:05,190 There's also the possibility 198 00:08:05,190 --> 00:08:07,230 that the route file system uses one 199 00:08:07,230 --> 00:08:09,420 of several advanced storage mechanisms 200 00:08:09,420 --> 00:08:12,060 like LVM the Logical Volume Manager, 201 00:08:12,060 --> 00:08:14,760 or NFS the Network File System. 202 00:08:14,760 --> 00:08:16,590 And these have different mounting requirements 203 00:08:16,590 --> 00:08:18,442 than a standard partition would. 204 00:08:18,442 --> 00:08:21,720 The Linux initial RAM disk image is an archive file 205 00:08:21,720 --> 00:08:23,520 that contains all the essential files 206 00:08:23,520 --> 00:08:26,460 that are required for booting up the operating system. 207 00:08:26,460 --> 00:08:29,760 It can be built or customized to include additional modules 208 00:08:29,760 --> 00:08:34,470 or remove unnecessary modules, or update existing modules. 209 00:08:34,470 --> 00:08:35,789 Typically, this image 210 00:08:35,789 --> 00:08:38,532 is going to be stored in the /boot directory. 211 00:08:38,532 --> 00:08:40,950 Now, to make our initial RAM disk, 212 00:08:40,950 --> 00:08:44,070 we're going to use the make initial RAM disk command. 213 00:08:44,070 --> 00:08:45,960 The make initial RAM disk command 214 00:08:45,960 --> 00:08:48,390 is used to create the initial RAM disk image 215 00:08:48,390 --> 00:08:50,700 for preloading the kernel modules. 216 00:08:50,700 --> 00:08:55,050 This command is written as mkinitrd. 217 00:08:55,050 --> 00:08:56,760 Now, there's a couple of different options 218 00:08:56,760 --> 00:08:59,010 for the make initial RAM disk command. 219 00:08:59,010 --> 00:09:03,480 This includes the --preload equals module name option, 220 00:09:03,480 --> 00:09:05,040 and this is used to load a module 221 00:09:05,040 --> 00:09:06,900 into that initial RAM disk image 222 00:09:06,900 --> 00:09:09,270 before loading the other modules. 223 00:09:09,270 --> 00:09:14,070 Another option we have is --width equals the module name. 224 00:09:14,070 --> 00:09:15,540 This option is going to be used 225 00:09:15,540 --> 00:09:18,330 to load a module in the initial RAM disk image 226 00:09:18,330 --> 00:09:20,580 after the loading of the other modules. 227 00:09:20,580 --> 00:09:22,830 The -f option is going to be used 228 00:09:22,830 --> 00:09:26,040 to overwrite an existing initial RAM disk image file. 229 00:09:26,040 --> 00:09:28,140 And the -nocompress option 230 00:09:28,140 --> 00:09:31,740 is used to disable the compression of that initial image. 231 00:09:31,740 --> 00:09:34,740 In order to run the make initial RAM disk command, 232 00:09:34,740 --> 00:09:38,760 you have to type in mkinitrd, 233 00:09:38,760 --> 00:09:39,810 the options, 234 00:09:39,810 --> 00:09:43,230 the initial RAM disk image name and the kernel version. 235 00:09:43,230 --> 00:09:46,500 For example, if you want to create the initial RAM disk image 236 00:09:46,500 --> 00:09:48,960 from your current kernel version and name it 237 00:09:48,960 --> 00:09:51,993 as initrd-.img, 238 00:09:53,250 --> 00:09:58,250 you can type mkinitrd/boot/initrd-$(uname -r) 239 00:10:05,348 --> 00:10:10,348 .img $(uname -r) 240 00:10:11,160 --> 00:10:12,930 in the command prompt. 241 00:10:12,930 --> 00:10:15,840 Now, as defined by the File System Hierarchy Standard 242 00:10:15,840 --> 00:10:19,740 or FHS, the /boot directory contains files 243 00:10:19,740 --> 00:10:23,100 that are used to facilitate the Linux boot process. 244 00:10:23,100 --> 00:10:24,930 Now, some of the files and sub-directories 245 00:10:24,930 --> 00:10:27,510 within the /boot directory that you should know about 246 00:10:27,510 --> 00:10:30,630 are things like the /boot/grub directory. 247 00:10:30,630 --> 00:10:32,520 This contains configuration files 248 00:10:32,520 --> 00:10:35,100 for a type of boot loader known as GRUB. 249 00:10:35,100 --> 00:10:38,400 If you go into the /boot/grub2 directory, 250 00:10:38,400 --> 00:10:40,230 this is also a different type of boot loader, 251 00:10:40,230 --> 00:10:41,730 but it's known as GRUB 2. 252 00:10:41,730 --> 00:10:44,070 It's an improved version of GRUB. 253 00:10:44,070 --> 00:10:47,670 The /boot/efi directory contains boot files 254 00:10:47,670 --> 00:10:51,420 for an EFI system partition known as ESP, 255 00:10:51,420 --> 00:10:52,710 which is going to be required 256 00:10:52,710 --> 00:10:53,970 for partitions of a system 257 00:10:53,970 --> 00:10:56,970 that boot from the UEFI interface. 258 00:10:56,970 --> 00:10:59,820 It also will contain a boot loader, device drivers, 259 00:10:59,820 --> 00:11:03,810 and system application files that are executed by the UEFI. 260 00:11:03,810 --> 00:11:05,250 The next one we need to talk about 261 00:11:05,250 --> 00:11:09,543 is /boot/initramfs-.img. 262 00:11:10,761 --> 00:11:12,150 This is a file 263 00:11:12,150 --> 00:11:15,120 that is going to be the initial RAM file system image. 264 00:11:15,120 --> 00:11:16,350 And this is an alternative 265 00:11:16,350 --> 00:11:19,440 to the initial RAM disk image that we talked about before. 266 00:11:19,440 --> 00:11:20,880 It's going to use different methods 267 00:11:20,880 --> 00:11:22,740 to do the same basic things. 268 00:11:22,740 --> 00:11:25,140 It's going to initialize a temporary route file system 269 00:11:25,140 --> 00:11:28,830 on the boot drive and then load in those modules as needed. 270 00:11:28,830 --> 00:11:33,630 Lastly, we have /boot/vmlinuz- 271 00:11:33,630 --> 00:11:36,450 and this contains the Linux kernel itself. 272 00:11:36,450 --> 00:11:39,330 The boot loader is going to load this file into memory 273 00:11:39,330 --> 00:11:40,710 during the boot process 274 00:11:40,710 --> 00:11:44,370 to initialize the overall Linux operating system. 275 00:11:44,370 --> 00:11:46,350 Another command you need to be familiar with 276 00:11:46,350 --> 00:11:49,620 is the dracut command. 277 00:11:49,620 --> 00:11:53,370 This is used to generate an init RAM file system image, 278 00:11:53,370 --> 00:11:57,330 similar to how we use the mkinitrd command 279 00:11:57,330 --> 00:11:59,790 to generate an initial RAM disk image. 280 00:11:59,790 --> 00:12:02,160 In fact, on some distributions, 281 00:12:02,160 --> 00:12:05,460 the make initial RAM disk is a compatibility wrapper 282 00:12:05,460 --> 00:12:08,520 that'll actually call the dracut command. 283 00:12:08,520 --> 00:12:11,610 When you want to create an initial RAM file system image, 284 00:12:11,610 --> 00:12:13,500 you can use the dracut command 285 00:12:13,500 --> 00:12:18,500 by typing in dracut /boot.initramfs-$(uname -r) 286 00:12:26,553 --> 00:12:31,553 .img $(uname -r). 287 00:12:32,580 --> 00:12:34,650 This boot process is going to be repeated 288 00:12:34,650 --> 00:12:36,720 each time your computer is started 289 00:12:36,720 --> 00:12:40,230 by loading up the operating system from a storage device. 290 00:12:40,230 --> 00:12:42,750 It's going to involve a series of sequential steps 291 00:12:42,750 --> 00:12:46,410 that can be divided into the BIOS or UEFI initialization, 292 00:12:46,410 --> 00:12:49,830 the boot loader, the kernel, and the initial RAM disk 293 00:12:49,830 --> 00:12:52,260 or initial RAM file system initiation, 294 00:12:52,260 --> 00:12:54,570 and finally, your boot scripts. 295 00:12:54,570 --> 00:12:57,330 First, the processor's going to check for the BIOS 296 00:12:57,330 --> 00:13:01,170 or UEFI firmware, and if it finds it, it's going to execute it. 297 00:13:01,170 --> 00:13:02,490 This is also where your power 298 00:13:02,490 --> 00:13:05,580 on self-test or post is going to occur. 299 00:13:05,580 --> 00:13:08,220 Second, the BIOS or UEFI is going to check 300 00:13:08,220 --> 00:13:10,950 for bootable media from the internal storage devices 301 00:13:10,950 --> 00:13:15,300 or peripherals like USB thumb drives and DVD ROMs. 302 00:13:15,300 --> 00:13:17,820 By doing this, it's going to locate a valid device 303 00:13:17,820 --> 00:13:19,620 to boot up your system from. 304 00:13:19,620 --> 00:13:23,400 Then, the BIOS or UEFI is going to load the primary boot loader 305 00:13:23,400 --> 00:13:27,030 from the Master Boot Record or the GUID Partition Table. 306 00:13:27,030 --> 00:13:29,100 It's going to do this into your memory. 307 00:13:29,100 --> 00:13:32,730 And then, it's going to load the partition table along with it. 308 00:13:32,730 --> 00:13:33,990 The user will then be prompted 309 00:13:33,990 --> 00:13:36,570 by the GRUB 2 boot loader or other boot loader 310 00:13:36,570 --> 00:13:39,660 to select the operating system that they want a boot from. 311 00:13:39,660 --> 00:13:41,400 If the user doesn't respond, 312 00:13:41,400 --> 00:13:44,413 then the default operating system is going to be booted up. 313 00:13:44,413 --> 00:13:47,280 Next, the boot loader determines the kernel 314 00:13:47,280 --> 00:13:50,040 and locates the corresponding kernel binary. 315 00:13:50,040 --> 00:13:53,220 It'll then upload the respective initial RAM disk image 316 00:13:53,220 --> 00:13:55,110 into memory and transfer control 317 00:13:55,110 --> 00:13:57,450 of the boot process to the kernel. 318 00:13:57,450 --> 00:13:58,680 Then, the kernel 319 00:13:58,680 --> 00:14:01,140 will configure the available hardware drivers, 320 00:14:01,140 --> 00:14:02,640 including your processors, 321 00:14:02,640 --> 00:14:05,310 input/output systems, and storage devices. 322 00:14:05,310 --> 00:14:07,800 It'll decompress the initial RAM disk image 323 00:14:07,800 --> 00:14:10,320 and mount it to load the necessary drivers. 324 00:14:10,320 --> 00:14:12,990 If the system implemented any virtual devices, 325 00:14:12,990 --> 00:14:15,870 such as the LVM or software RAID, 326 00:14:15,870 --> 00:14:18,120 those will also be initialized. 327 00:14:18,120 --> 00:14:20,580 The kernel will then mount the main root partition 328 00:14:20,580 --> 00:14:23,970 and release unused memory back into the system. 329 00:14:23,970 --> 00:14:25,710 To set up the user environment, 330 00:14:25,710 --> 00:14:28,320 the systemd program is going to run. 331 00:14:28,320 --> 00:14:32,010 It's going to become Process ID 1 on that system. 332 00:14:32,010 --> 00:14:35,280 Then, the systemd or the system Daemon program 333 00:14:35,280 --> 00:14:37,950 is going to search for the default.target file, 334 00:14:37,950 --> 00:14:38,940 which contains details 335 00:14:38,940 --> 00:14:41,400 about the services that need to be started. 336 00:14:41,400 --> 00:14:43,080 It'll then mount the file system 337 00:14:43,080 --> 00:14:46,500 based on the /etc/fstab file 338 00:14:46,500 --> 00:14:49,440 and begin the process of starting up at services. 339 00:14:49,440 --> 00:14:51,420 On most systems, the target either 340 00:14:51,420 --> 00:14:55,800 be a multiuser.target or graphical.target file. 341 00:14:55,800 --> 00:14:57,750 If the graphical mode was selected, 342 00:14:57,750 --> 00:15:00,990 then a display manager like XDM or KDM 343 00:15:00,990 --> 00:15:02,070 is going to be started up 344 00:15:02,070 --> 00:15:04,890 and the login window will be displayed on the screen. 345 00:15:04,890 --> 00:15:06,360 The user can then enter a username 346 00:15:06,360 --> 00:15:08,550 and password to log into the system. 347 00:15:08,550 --> 00:15:09,810 And then, the system 348 00:15:09,810 --> 00:15:13,050 will authenticate the user based on those credentials. 349 00:15:13,050 --> 00:15:14,550 If the user is valid, 350 00:15:14,550 --> 00:15:17,730 the various profile files are going to be executed. 351 00:15:17,730 --> 00:15:19,410 Finally, the shell has started 352 00:15:19,410 --> 00:15:22,288 and the system is ready for the user to begin their work. 353 00:15:22,288 --> 00:15:24,060 Now, everything I just said 354 00:15:24,060 --> 00:15:27,030 assumes that everything went right in the boot up process, 355 00:15:27,030 --> 00:15:29,310 but there's times things go wrong. 356 00:15:29,310 --> 00:15:31,170 And this is why we have a mechanism 357 00:15:31,170 --> 00:15:34,170 for the system to detect if there's been a fatal error, 358 00:15:34,170 --> 00:15:35,460 and it's going to respond to that 359 00:15:35,460 --> 00:15:37,860 by doing what's called a kernel panic. 360 00:15:37,860 --> 00:15:39,720 Now, a fatal error typically results 361 00:15:39,720 --> 00:15:43,290 in the system becoming unstable or totally unusable. 362 00:15:43,290 --> 00:15:45,660 There is software that handles these kernel panics 363 00:15:45,660 --> 00:15:47,970 and those display an error message to the user 364 00:15:47,970 --> 00:15:50,220 and dump the current state of the kernel memory 365 00:15:50,220 --> 00:15:53,070 to a storage device for later debugging as well. 366 00:15:53,070 --> 00:15:55,110 Depending on how your system is configured, 367 00:15:55,110 --> 00:15:56,610 the panic handler will either 368 00:15:56,610 --> 00:15:58,410 reboot the system automatically, 369 00:15:58,410 --> 00:16:00,180 or it'll shut itself down 370 00:16:00,180 --> 00:16:02,680 and wait for the user to turn on the system again.