Linux on Macbook, dual-boot, without rEFind
Note: The introduction of Ubuntu 18 has made these instructions obsolete. In other words, the Ubuntu 18 installer will automatically create files in the proper locations for Ubuntu to appear in the Startup Manager.


You can still employ step 7, if you want an Ubuntu icon to appear in the Startup Manager.


  Note: These instructions are for Mac models with 64 bit EFI firmware. In other words, if your Mac has Mountain Lion (OS X 10.8) or a newer version OS X (macOS) installed, then you can use these instructions. 


Instead of using rEFInd, you can use the Startup Manager to boot most Linux distributions. The Startup Manager is contained in the the firmware that is already installed in your Mac. After installing a Linux distribution, you will need to move a few files around in order for the Startup Manager to recognize and boot the Linux distribution.

Most Linux distributions install grub and other files in the first EFI System partition on a Mac computer. Normally, this is the first partition on a Mac computer. The process of making a Linux distribution appear on the Startup Manager is a fairly trivial. The steps are given below for the Ubuntu Linux distribution. If you wish to install a different Linux distribution, post a command and I will verify if these instructions will work.


Install Ubuntu.
When finished, restart and hold down the option key to
invoke the Startup Manager. Select to boot from the OS X (or MacOS)
volume.
Open a Terminal application window.
Mount the EFI System partition by entering the command given
below.

diskutil mount disk0s1

Create a folder named Boot in the folder /Volumes/EFI/EFI. You
can use the Finder application or enter the command given below.

mkdir /Volumes/EFI/EFI/Boot

Copy the file grubx64.efi from the folder
/Volumes/EFI/EFI/ubuntu to the folder /Volumes/EFI/EFI/Boot.
Next, rename this file bootx64.efi. This step can be accomplished
by using the the Finder application or by entering the command given
below.

cp /Volumes/EFI/EFI/ubuntu/grubx64.efi /Volumes/EFI/EFI/Boot/bootx64.efi

(Optional) Download a collection of icons from the sourceforge web
site Mac icns. Use the Finder application to open the
downloaded file mac-icns.dmg, then enter the command below to copy
the Ubuntu icon file os_ubuntu.icns to the EFI System partition.

cp /Volumes/mac-icns/os_ubuntu.icns /Volumes/EFI/.VolumeIcon.icns


This will add the following Ubuntu icon to the Startup Menu. 




  Note: When finished, you can use the Finder application to eject the mac-icns volume.

Use the Finder application or enter the command below to unmount
the EFI System partition labeled EFI.

diskutil unmount disk0s1