Grub File Overview (Demo)
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Video Transcription
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>> Hello Cybrarians.
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>> Welcome back to the Linux+ course here at Cybrary.
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>> I am your instructor, Rob Goelz.
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In today's lesson we'll be talking about Grub file.
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Upon completion of today's lesson,
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you'll be able to locate the Grub file and we'll
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also discuss the contents of Grub files and directories.
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Demo time.
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I don't know about everyone else here,
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but I like to learn by getting my hands in,
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getting my hands dirty, possibly breaking things.
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We're going to see a lot of images of things blowing up,
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things getting destroyed here
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as we go through the course and that's going to be
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our cue to cut over to
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the demo environment and learn some stuff.
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Without further ado, we're going to go
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ahead and cut over to CentOS here.
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Here we are in our CentOS distribution.
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This is actually an older CentOS distribution
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known as CentOS 6,
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and it's a legacy distribution that we
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need to use because,
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well, Grub is the legacy bootloader.
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What we're going to do is we're going to navigate
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into the boot Grub directory,
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which is where all the configuration files are going to
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be that we need to look at for looking at Grub.
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Specifically, we're going to look at
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a file here called grub.conf.
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Now we can see all the files in the directory
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by running the list command and I actually
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like using ls-al that allows us to see
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a long listing of the files and it
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also allows us to skinny hidden files.
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Let's go ahead and run that.
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Inside of here, we can see that grub.com file,
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but we also notice that menu.lst is pointing to grub.com.
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Menu.lst is the same link to grub.conf,
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and that's because Grub historically
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wants to use menu.list as the default file,
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but in this distribution, in CentOS 6,
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the grub.com files, the one that
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actually holds all the information we want to see.
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Now, I know from past experience that
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unfortunately in order to get into that file,
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we have to run a pseudo command,
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pseudo edit in order to get into grub,conf,
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and that's because that file is not viewable
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by anybody who does not have a privileged user account.
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In order to get privileged on my user account,
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in order to edit that file or view it,
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I need to run pseudo.
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I'm going to run pseudoedit so we can
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actually edit the file if you wanted to.
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We're not going to do that today,
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but let's just go have a look.
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There's pseudoedit grub.conf,
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and then we're going to go ahead and specify
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my password so we can get in and
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escalate privileges and now we can
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see the contents of this file.
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The first thing we want to look
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at when we look at Grub is
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the devices and device
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naming because it's a little bit wacky.
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What we notice here is we see things like hd 0,
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0 or down here again,
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we see hd 0, 0.
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In Grub legacy,
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all devices start with zero,
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and Grub actually can't distinguish between
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a scuzzy drive or an IDE drive,
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so it just generically uses the device identifier of HD,
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hard drive, for all devices.
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The syntax for naming that device is device type,
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drive number, and partition number.
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Here we see hd 0, 0.
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Well, what that really represents is
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the first partition on the first hard disk,
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which in this system is dev/sda 1.
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Now, there are also
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global settings that are
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used to identify operating parameters.
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Well, what are our global settings?
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Well, these are the things that we can see
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like default over here.
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Default equals 0.
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Well, it's zero because it's used to determine
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the default OS we're going to load
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and it's a zero-based index,
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which means that the most recent loaded OS
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we could use is actually zero,
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which means one is the first one,
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but because the index starts with zero,
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it's always going to be zero.
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We can also see the timeout,
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timeout equals 5,
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which is right below the default value here.
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A timeout sets the time in seconds that
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Grub will wait for operator intervention.
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When the Grub menu comes up,
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it's going to sit there for five seconds
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and wait in case we want to go
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in and edit anything or
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change the way that the boot operates.
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We can also see the splash image right
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below that and this is used
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to define the image or background
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used in the Grub menu displays,
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so if you want to change the
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background and do something fancy,
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you could do that by specifying
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a different path through a different image file there.
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Now, on this system we don't have a password specified,
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but you could also have a password line that contains
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a plain text or encrypted password and that would be
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necessary if you wanted to go in and edit
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any Grub settings or
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execute commands during the Grub boot process.
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You would need to specify
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that password before you
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could get in and change anything.
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The last thing we want to talk about here
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is this area right here.
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We're going to talk first of all about the title.
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The title is what is used to
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define the available operating system,
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and we only have one here.
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But the title that we're looking at is
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this most recent one 2.6.32,
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and that actually specifies the kernel version.
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Now the root down here is used to specify
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the location of Grub's root directory or slash boot,
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which is where the operating
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system is going to be loaded,
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and as I said previously, hd 0,
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0 is actually dev/sda 1.
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The kernel line down here,
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it actually specifies the kernel file
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to load and how to mount the kernel,
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so vmlinuz is weird.
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What that actually means is that this is
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a compressed kernel file and we can
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see that it's vmlinuz is 2.6.32, etc.
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This is the kernel file,
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the compressed kernel file
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that we're going to load and then
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we have information on
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loading it and how are we going to do so.
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We have information about the UUID that's
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used to identify the device it's going to load on.
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We have information about lux,
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which we'll learn about later in
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the language we're using,
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and that's a localization thing.
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We'll learn about that later as well.
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Then finally we see down here initrd,
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some of the initrd settings.
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Well, initrd specifies
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the initrd file that uses initial RAM disk,
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and that is a file system that has
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executables and drivers we need to boot the kernel.
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With that being said,
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>> in this lesson we covered where to
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>> find Grub and Grub2 files and CentOS,
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and we also discussed
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the contents of Grub file and directories.
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Thanks for being here and I hope
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to see you in the next lesson.
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