MacBook Pro stuck on GRUB minimal Bash at startup
Use Startup Manager

When you use Startup Manager to select a startup disk, your Mac starts up from that disk once, then returns to using the disk selected in Startup Disk preferences.


Press and hold the option key immediately after turning on or restarting your Mac.
Release the Option key when you see the Startup Manager window.


  If your Mac is protected by a firmware password, you can release the key when you're asked to enter the password.

Select your startup disk, then click the arrow under its icon, or press Return.


  If you press and hold the control key during this step, your selection is saved in Startup Disk preferences, so it persists until you change it.





If your Mac is using OS X Lion 10.7.3 or later, you can also use this method to start up from your Time Machine backup disk. Startup Manager identifies your Time Machine backup as ”EFI Boot.”

References
The above answer was copied from "How to select a different startup disk", https://support.apple.com/en-us/HT202796, January 29, 2019.