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My root partition was on /dev/sda1 and I changed the root partition to /dev/sda2. I updated the entry in /etc/fstab with the UUID of /dev/sda2.

But, I am not sure, how would the grub know that the root partition has changed to the new UUID i.e. /dev/sda2? Where else should changes be made apart from /etc/fstab, so that the grub can locate the root partition and hence boot the system?

Since, I want to learn stuff, I would also like to know the steps other than a simple command update-grub.

Do I have to change things somewhere else?

I am just running Linux.

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  • Interesting to get a downvote without any explanation. – infoclogged Dec 12 '18 at 14:51
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Assuming you are using grub2, the most important configuration file is grub.cfg (usually located in /boot/grub directory for bios based systems and /boot/efi/EFI/<os name>/ for efi systems). When computer starts grub follows instructions from that file. This is just a text file, so you can edit it by hand, or use some automatic tool (a shell script) grub-mkconfig (a.k.a. grub2-mkconfig) which creates configuration according to rules defined in a /etc/default/grub file and /etc/grub.d/ directory.

Anyway, if you've changed root partition there are a few things to take care of. First of all linux kernel parameters: grub directly is not affected by root partition, but it needs to pass correct path to the kernel, so look for a line like this

linux /vmlinuz-4.19.5 root=/dev/sda1

and change it to /dev/sda2.

It can also be passed in UUID way:

linux /vmlinuz-4.19.5 root=UUID=abcdefgh-ijkl-mnop-qrst-uvwxyz

You can test your root UUID with grub's command:

grub-probe --target=fs_uuid /

Second thing to check/change is a kernel file itself. It can reside on separate partition (usually /boot), or on root (/) one. In later case you need to search for a line

set root='hd0,gpt1'

and change gpt1 to gpt2. (I assumed here that you have only one disk hd0 and you are using GUID partition table, a standard nowadays).

Depending on you configuration you may also need to change filesystem UUID and a hint string to find proper path to the kernel. It could look like

search --no-floppy --fs-uuid --set=root --hint-ieee1275='ieee1275//disk@0,gpt1' --hint-bios=hd0,gpt1 --hint-efi=hd0,gpt1 --hint-baremetal=ahci0,gpt1  abcd-efgh
search --no-floppy --fs-uuid --set=root abcd-efgh

New UUID you already know, use grub-probe --target=hints_string / to find proper value of hints-string.

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    For what it's worth, on EFI installations grub.cfg is located under: /boot/efi/EFI/fedora/grub.cfg (in my case I use fedora, but insert your distro there). The command would also be grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg. Great answer though! :) – bgregs Dec 12 '18 at 16:22

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