manjaro

vim /boot/grub/custom.cfg

menuentry "Manjaro" {
set isofile="/manjaro-xfce-17.1.7-stable-x86_64.iso"
set dri="free"
search --no-floppy -f --set=root $isofile
probe -u $root --set=abc
set pqr="/dev/disk/by-uuid/$abc"
loopback loop $isofile
linux (loop)/boot/vmlinuz-x86_64 img_dev=$pqr img_loop=$isofile driver=$dri
initrd (loop)/boot/initramfs-x86_64.img
}

update-grub
or
grub-mkconfig -o /boot/grub/grub.cfg

refer:https://forum.manjaro.org/t/booting-manjaro-iso-using-grub2/3132

linux mint

vim /boot/grub/custom.cfg

menuentry "linux mint" {
insmod loopback
loopback loop (hd0,5)/linuxmint-18.3-xfce-64bit.iso
linux (loop)/casper/vmlinuz boot=casper iso-scan/filename=/linuxmint-18.3-xfce-64bit.iso
initrd (loop)/casper/initrd.lz
}
update-grub
or
grub-mkconfig -o /boot/grub/grub.cfg

refer:http://www.mintos.org/skill/mint-hd-install.html

xUbuntu

menuentry "xUbuntu" {
insmod loopback
set isofile="/xubuntu-18.04-desktop-amd64.iso"
loopback loop (hd0,1)$isofile
linux (loop)/casper/vmlinuz boot=casper iso-scan/filename=$isofile noeject noprompt quiet splash
initrd (loop)/casper/initrd.lz
}

装完Windows，丢失Ubuntu启动项

sudo mount /dev/sda8 /mnt

sudo grub-install --root-directory=/mnt /dev/sda