I am currently running 16.04, and am going to additionally install 18.04 side-by-side in separate partitions. I also have Windows on this machine in another partition. I'm using UEFI boot.
How is grub going to work in this scenario? Currently grub adds a bunch of menu items for Ubuntu 16.04, based on the grub scripts, like the default Ubuntu option, but also a bunch of other options such as any older kernel versions, a few entries with custom boot parameters that I added, options to boot into console/recovery mode, etc.
However, with 16.04 and 18.04, won't my grub entries only reflect whatever install I happen to run update-grub on? I guess the "other" install will be picked up by looking at the EFI entries or whatever (in the same way as Windows is), but will have only a single generic entry?
sudo update-gruband it will then add 18.04 to its grub. But you will not normally then see 16.04's grub menu as you are booting from 18.04. You can change default boot, or use configfile boot stanza to boot other boot menu or many grub configurations changes. help.ubuntu.com/community/MaintenanceFreeCustomGrub2Screengrub.cfgon 18.04 is the relevant file to be adjusted to take all boot entries. You'll need what kernel version 16.04 is using (may be older than for 18.04). You'll also need the root partition number for 16.04 and its UUID. You can find latter details fromlsblk -o name,mountpoint,UUIDand kernel details fromuname -r/boot/grub/grub.cfgfile you'll see under the first menu entry (for Ubuntu) towards the end of that section it shows the kernel version of vmlinuz, which is the image it uses to boot from. It may not be the same version as the one for 18.04 when you install it unless you're up to date. If you select 16.04 in Grub it has to be the latest version that currently resides in 16.04's/bootfolder.