I just want to know if it is possible to install Ubuntu 18.04 on a Mac-book. I don't want to dual boot or run a virtual machine, I want to completely install Ubuntu. I tried looking around and all I could find are dual-boot and USB guides. I've heard that it is not possible to install anything other than Mac-OS on apple products and I want a second opinion. I am going to receive the laptop second hand so I don't care about voiding any warranties. The laptop was only used for the first 6 months since its original purchase and has the Mac-OS version that came with it.
Absolutely, make a bootable usb stick of any distro of your choice, boot to it and follow the steps. Assuming you are on a mac I would use dd from the terminal:
e.g. (umount the unit in this case disk2s1 before this command) dd if=/path/to/ubuntu.iso of=/dev/disk2 bs=1M
With your bootable usb stick in hand you can now boot your Macbook depending on the model to select booting device and go on