Linux boot process with systemd

Question

I am trying to understand the Linux boot process, but when I read about it on Google, a path similar to the below one is stated :

BIOS -> MBR -> GRUB -> Kernel -> init -> fstab

For instance, here : https://www.tecmint.com/linux-boot-process/

Can someone explain to me what is different in newer versions in this process?

Note - When I say newer I mean for instance, RHEL 7 and Ubuntu 16/17/18

For instance, shouldnt we swap init for systemd in that path?

• Raman Sailopal: The classic sysVinit system before systemd used to execute startup/shutdown processes one after the other sequentially where as systemd spawns processes together. The (in theory) allows for faster startup and shutdown times,
• JdeBP: Always treat comment answers with a huge sackful of salt, including ones that ignore the existence of startpar.
• JdeBP: That article by Narad Shrestha should be taken with a huge sackful of salt, too. It doesn't mention fstab anywhere in the "About /etc/fstab" section, for starters. And where it does mention it elsewhere in the article, it gets things wrong.
• JdeBP: Unsurprisingly, this WWW site does indeed have a "How does Linux boot?" question. It is unix.stackexchange.com/questions/60522 from back in 2013, closed for being too broad a topic. Reducing it to Linux alone would have made it somewhat less broad. This question at least is restricted to how Linux with systemd bootstraps differently to Linux with van Smoorenburg init+rc.
• AlexP: (1) That's not /etc/fstab but /etc/inittab. (2) Modern Linuxes such as Debian or Ubuntu boot just as they always did, except that init (PID 1) is now systemd and it does not read /etc/inittab but its own configuration files. See the excellent documentation of systemd for details.

Answer 1

Since you specified "with systemd": systemd ships a manpage describing the boot process.

man 7 bootup

However, not all distributions use systemd for the initrd, so the "bootup in the initial RAM disk" section may or may not apply in your case. The general process should still be the same, though.

Be sure to also check out the manpages mentioned in the "see also" – systemd has lots of documentation in manpage format. (Or online, if you prefer: https://www.freedesktop.org/software/systemd/man/bootup.html)