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- [Instructor] Let's talk a little bit more about booting.

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So GRUB passes control to the kernel,

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giving it its command line arguments

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and telling it where in memory GRUB loaded

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the initial RAM file system,

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if there is one.

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That initial RAM file system or initial RAM disk

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might have a root file system on it

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and that might get mounted first.

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And an init from that could run,

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perhaps loading some loadable kernel modules

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that were in that image,

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that might be required

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to be able to mount your real root file system.

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If your kernel was not built with support

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for say the kind of device

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that holds your root file system,

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then that support could be loaded via the initramfs.

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The Linux kernel then kind of by hand

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creates the very first process.

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So that process will have process ID one.

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And in the kernel source code, it looks for a program

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and a couple of places with the name init,

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and that's what it's going to start.

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And if it doesn't find something called init,

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then it'll try to start a shell.

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In my experience, if it can't find init,

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it's not going to be able to find a shell

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because something was seriously wrong.

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And although you might have the name init,

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it might be a link.

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Like these days, init probably is a link

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to the systemd program.

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So systemd will be the very first process.

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And the init program, like systemd,

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is then responsible for starting up everything else.

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So it's the ancestor of all processes,

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maybe the great-grandfather or great-great-grandfather,

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but it's back there as an ancestor.

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So it starts up Daemons, it starts up whatever you need

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to be able to log in and so forth.

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So of course, as Linux has evolved,

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how all that work has evolved.

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It used to be services underneath etc/rc.d,

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but these days, most Linux systems use systemd,

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and there's lots of service files

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and you can go look at some

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by looking at etc/systemd/system.

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And there's other places on your system

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where the services might be,

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but that's configured just start up stuff,

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like your web server.

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So these are user-space services, remember.

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These are not parts of the kernel.

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The kernel's job was to get systemd running

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and then systemd takes it from there.