How can I recover after deleting libc++?

Question

I stupidly deleted libc++ from my Mac (Mojave 10.14 on a 2015 MacBook Air). I had several versions and wanted to just use the newest... but now nothing works. The kernel panicked and for a while, it would panic/shutdown/boot/panic again without any interaction.

Now it doesn't even get to the panic screen! it just makes the startup sound over and over again -- even with the screen down or after pressing the power button repeatedly.

How can I get an OS installed again? Preferably macOS. All my important files were backed up so that's no issue. I don't have access to another computer, but I do have a flash drive with Ubuntu sitting around...or I could try to get to another computer to make an OS if necessary.

edit: I held down the power button for a while and now it is no longer trying to start over and over again.

Answer 1

Hold down Cmd-R at boot time to boot into recovery mode.

When you install macOS, it makes a small, usually somewhat invisible emergency/recovery boot partition on your internal drive. Holding Cmd-R at boot tells your Mac to boot from it so you can reinstall the OS.

If that doesn't work, hold down Cmd-Opt-R at boot to enter Internet Recovery mode. You will need an Internet connection via Ethernet or Wi-Fi. If the Wi-Fi network uses security, it must be one of the flavors that only requires a password. It can't be the enterprise-grade security that requires a username & password or any certificates. Your Mac will attempt to contact Apple's recovery servers and download the necessary bootable OS image from there.

Generally, reinstalling macOS over an existing install of the same version of macOS does a "repair install"; it should only overwrite the files that are part of the OS, and it should leave all your other files in place. But as always, if you have files you can't lose and you don't have a recent, tested backup, you should avoid doing this until you have a good backup.

If you can borrow another Mac, you can hold down T at boot to boot into Target Disk Mode, where your Mac just acts like an external hard drive and you can connect it to another Mac via Thunderbolt (or USB or FireWire, on older machines), and get to its hard drive contents that way.