1 00:00:00,005 --> 00:00:03,004 - [Instructor] GRUB is the most popular bootloader 2 00:00:03,004 --> 00:00:07,001 for Linux for servers and desktops. 3 00:00:07,001 --> 00:00:10,003 Embedded Linux devices often have a different bootloader, 4 00:00:10,003 --> 00:00:14,001 but GRUB is valuable to know about. 5 00:00:14,001 --> 00:00:16,004 There have been, of course, different versions of GRUB, 6 00:00:16,004 --> 00:00:18,003 just like everything else. 7 00:00:18,003 --> 00:00:21,001 For GRUB, there's some important config files 8 00:00:21,001 --> 00:00:23,009 underneath etc, 9 00:00:23,009 --> 00:00:27,009 so you can configure GRUB there 10 00:00:27,009 --> 00:00:31,000 and add new entries for example. 11 00:00:31,000 --> 00:00:33,001 A lot of this happens automatically 12 00:00:33,001 --> 00:00:34,007 through the way packages work. 13 00:00:34,007 --> 00:00:37,001 If you install a new kernel, 14 00:00:37,001 --> 00:00:39,003 generally your GRUB will get updated 15 00:00:39,003 --> 00:00:42,003 to boot your new kernel. 16 00:00:42,003 --> 00:00:47,000 But you can create new entries, you can customize things. 17 00:00:47,000 --> 00:00:48,002 If you want to add a new one, 18 00:00:48,002 --> 00:00:54,005 generally you'll edit 40_custom underneath etc/grub.d. 19 00:00:54,005 --> 00:00:57,000 When you make changes to GRUB configuration, 20 00:00:57,000 --> 00:01:01,003 you need to generate its config file it's going to use, 21 00:01:01,003 --> 00:01:05,003 so you'll run grub2-mkconfig. 22 00:01:05,003 --> 00:01:09,004 One of the nice features of GRUB is 23 00:01:09,004 --> 00:01:14,004 when your system starts booting, it's going to run GRUB, 24 00:01:14,004 --> 00:01:17,008 and then GRUB can pause briefly 25 00:01:17,008 --> 00:01:19,009 before it starts the kernel. 26 00:01:19,009 --> 00:01:23,007 And that can give you time to interrupt it 27 00:01:23,007 --> 00:01:27,007 so that you can temporarily change 28 00:01:27,007 --> 00:01:30,005 the kernel command line. 29 00:01:30,005 --> 00:01:33,000 If you're experimenting with things and you have a typo 30 00:01:33,000 --> 00:01:35,002 and you give it the path to a kernel 31 00:01:35,002 --> 00:01:36,005 that doesn't exist or something, 32 00:01:36,005 --> 00:01:37,009 then your system won't boot. 33 00:01:37,009 --> 00:01:42,006 So being able to interactively fix that is kind of nice. 34 00:01:42,006 --> 00:01:46,004 So GRUB has a bunch of interactive commands 35 00:01:46,004 --> 00:01:49,004 like B to boot and so forth, 36 00:01:49,004 --> 00:01:50,008 and there's a little bit of documentation 37 00:01:50,008 --> 00:01:53,009 that tell you, use B or use Control + X. 38 00:01:53,009 --> 00:01:56,002 Now the challenge here is you might not be able 39 00:01:56,002 --> 00:01:58,004 to get to a system console. 40 00:01:58,004 --> 00:02:01,002 If you have the system at your desk and you have a monitor 41 00:02:01,002 --> 00:02:03,002 and keyboard and all hooked up to it, 42 00:02:03,002 --> 00:02:05,007 then you can get the system console. 43 00:02:05,007 --> 00:02:07,004 But these days, 44 00:02:07,004 --> 00:02:09,002 most of your systems are probably in data centers 45 00:02:09,002 --> 00:02:10,006 or they're Vms, 46 00:02:10,006 --> 00:02:12,005 getting the system console might be difficult, 47 00:02:12,005 --> 00:02:15,000 but it could be handy. 48 00:02:15,000 --> 00:02:17,003 I've done it many, many times. 49 00:02:17,003 --> 00:02:20,002 So the generated GRUB config file 50 00:02:20,002 --> 00:02:24,001 should be boot/grub/grub.cfg, 51 00:02:24,001 --> 00:02:26,001 and that's still kind of hard to read, 52 00:02:26,001 --> 00:02:26,009 but if you read through it, 53 00:02:26,009 --> 00:02:29,009 you'll see lines for booting the kernel, 54 00:02:29,009 --> 00:02:32,002 you'll see the kernel command line. 55 00:02:32,002 --> 00:02:35,006 For example, you might see something like this, Linux, 56 00:02:35,006 --> 00:02:39,005 and then name of a Linux kernel file, 57 00:02:39,005 --> 00:02:45,006 and then where the root file system's supposed to come from. 58 00:02:45,006 --> 00:02:47,001 And ro is usually there, 59 00:02:47,001 --> 00:02:50,008 telling the kernel to mount the root file system read-only. 60 00:02:50,008 --> 00:02:51,007 That's kind of curious. 61 00:02:51,007 --> 00:02:53,009 Your root file system isn't read-only. 62 00:02:53,009 --> 00:02:57,003 Well, it is when GRUB boots it, 63 00:02:57,003 --> 00:03:00,000 that lets the system check it 64 00:03:00,000 --> 00:03:03,000 for consistency while nothing's writing to it. 65 00:03:03,000 --> 00:03:05,005 And then after everything checks out, 66 00:03:05,005 --> 00:03:11,003 the system daemons will remount your file system read-write, 67 00:03:11,003 --> 00:03:13,001 and it's not till after that's done 68 00:03:13,001 --> 00:03:15,009 that you can actually log in. 69 00:03:15,009 --> 00:03:17,008 So might be valuable to know. 70 00:03:17,008 --> 00:03:20,003 You could mount it read-write to start with, 71 00:03:20,003 --> 00:03:24,000 but don't do a file system check with it writeable.