Get out of booting into Ubuntu on multi-OS Mac I installed Ubuntu on my iMac (Retina 21" late 2015) alongside macOS and rEFInd. Now, I believe the iMac boots automatically to Ubuntu in console mode. I tried a lot of combinations of keys, but without any result(option, command+R, command+option+P+R, etc.).
What should i do ?
If you properly install Ubuntu 18 Desktop, then you should be able to hold down the option at startup until the Startup Manager appears. You then can select the icon representing macOS which should boot macOS. If you hold down the control while selecting the icon, then macOS will become the new default operating system to boot. 

The Startup Manager should also show another icon which represents Ubuntu. Since the Startup Manager can be used to boot either macOS or Ubuntu, the use of rEFInd is optional.


  If macOS does not appear in the Startup Manager, then you probably erased macOS when you installed Ubuntu. 


If you choose to install rEFInd, then you should install rEFInd to separate EFI, FAT, or JHFS+ formatted partition. If you do this properly, then you can select rEFInd from the Startup Manager. In other words, the Startup Manager can be use to enable/disable rEFInd.


  Note: You should not need to use the bless command or disable System Integrity Protect (SIP) to install Ubuntu or rEFInd.