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- [Instructor] It can be essential

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or at least advantageous

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to modify how your kernel boots.

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So the Linux kernel supports

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what are called kernel command-line parameters,

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and GRUB is responsible for passing those parameters.

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When GRUB starts up the kernel,

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it will tell the kernel

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where to find the parameters

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that GRUB got out of its config file.

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So the kernel will process those command line arguments,

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and if there's any it doesn't recognize,

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it pretty much just ignores those,

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so you can make up new ones.

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Sometimes user space will take advantage of that

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by looking for special arguments

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that were passed at the kernel

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that could affect how the user space program works.

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You can look at what the kernel command line was

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by looking at the output of the dmesg command

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or in the file /proc.cmdline.

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There is documentation for many other kernel parameters.

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You can look in the kernel source tree

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underneath the Documentation subdirectory

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in the file aptly called kernel-parameters.txt.

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In mine, I found about 1,000 kernel parameters

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are documented there.

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And here's a fancy little grep that started the line.

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There's a tap character

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and then, you know, a letter or a digit.

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So anyway, that found a little over 1,000 for my kernel.

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And if you're looking at the kernel source,

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and you want to get an idea

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of what handles various of these parameters,

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you can look at what's getting registered

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with the function with_setup().

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That's associated with kernel parameters, for the most part.

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So like I said, there's over 1,000.

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There's all kinds of different ones.

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For example, maybe to work

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around a hardware bug, like cachesize,

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or maybe to disable certain CPU feature,

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or maybe there's a security risk, like clearcpuid.

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And of course, no guarantee

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that messing with things like this is going to work.

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You might cause your kernel to misbehave or not even boot.

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So, of course, you got to be careful.

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crashkernel's used to reserve some memory

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in case you have a crash

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for a special crashkernel to come up.

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And the way that works is the memory

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that your original kernel was using is preserved,

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and the crashkernel can save

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that other memory out to a file.

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So you can examine the state of memory

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when your kernel crashed for debugging.

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The enforcing kernel parameter is used to enable

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or disable SELinux.

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ftrace is used to start a kernel tracer

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as early as possible

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to get some trace information that could be helpful

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to help debug what's going on in the kernel early on.

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You can set a init to run a program instead of /sbin/init,

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the process that started up

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when the kernel starts the first process.

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And if you're using an init ramdisk

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or init/ramfs,

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you can set what program to run for init within that image.

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So there's a lot of parameters,

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and you might find when you do a web search

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for a problem you're having,

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that the advice is to set a kernel parameter.